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UBC Theses and Dissertations

Post-clearcutting forest floor nitrogen dynamics and regeneration response in the Coastal Western Hemlock… Martin, Wayne Lloyd 1985

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POST-CLEARCUTTING FOREST FLOOR'NITROGEN DYNAMICS AND REGENERATION RESPONSE IN THE COASTAL WESTERN HEMLOCK WET SUBZONE By WAYNE LLOYD MARTIN . S c . , The V i r g i n i a P o l y t e c h n i c I n s t i t u t e a S t a t e U n i v e r s i t y , 1979 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF FOREST SCIENCES We accept t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA October 1985 (c) Wayne L . Martin, 1985 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t of the requirement of the advanced degree at the U n i v e r s i t y of B r i t i s h C o l u m b i a , I agree tha t the l i b r a r y s h a l l make i t f r e e l y a v a i l a b l e for r e f e r e n c e and s t u d y . I f u r t h e r agree that p e r m i s s i o n for e x t e n s i v e c o p y i n g of t h i s t h e s i s for s c h o l a r l y purposes may be g r a n t e d by the head of my department by h i s or her r e p r e s e n t a -t i v e s . I t i s unders tood t h a t c o p y i n g or p u b l i c a t i o n of t h i s t h e s i s for f i n a n c i a l g a i n s h a l l not be a l l o w e d without my w r i t t e n permi s s i o n . The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P l a c e V a n c o u v e r , Canada V6T 1W5 ABSTRACT The o b j e c t i v e of t h i s s tudy was to d e s c r i b e the dynamics of p o s t - c l e a r c u t t i n g f o r e s t f l o o r N on mesic s i t e s in the wet subzone of the C o a s t a l Western Hemlock b i o g e o c l i m a t i c zone . P o s t -c l e a r c u t t i n g r e l e a s e of N was examined by measuring the f o l l o w i n g parameters for a chronosequence of f i v e s i t e s rang ing i n age from an o l d - g r o w t h f o r e s t to a 2 6 - y r - o l d s t a n d : f o r e s t f l o o r N c a p i -t a l ; r a t e of i_n s i t u m i n e r a l i z a t i o n of f o r e s t f l o o r ; r a t e of decompos i t i on of c e l l u l o s e ; c o n c e n t r a t i o n s of i n o r g a n i c - N and t o t a l - N in s o i l s o l u t i o n ; and a d s o r p t i o n of i n o r g a n i c - N by ion exchange r e s i n bags. E f f e c t s of the p o s t - c l e a r c u t t i n g r e l e a s e of N on t r e e growth were a s se s sed by measuring f o l i a r N l e v e l s and h e i g h t growth .of A b i e s a m a b i l i s (Dougl . ) F o r b e s advanced r e g e n e r a t i o n . The f o l l o w i n g s i n k s f o r the m i n e r a l i z e d f o r e s t f l o o r N were i n v e s t i g a t e d : a c c u m u l a t i o n i n p l a n t biomass; s o l u t i o n t r a n s f e r of i n o r g a n i c - N and t o t a l - N from the f o r e s t f l o o r to s torage in the upper m i n e r a l s o i l . Based on the d i f f e r e n c e between the o l d - g r o w t h and the 6-y e a r - o l d f o r e s t f l o o r N c a p i t a l s , p o s t - c l e a r c u t t i n g m o b i l i z a t i o n -1 of f o r e s t f l o o r N was e s t i m a t e d to be 950 k g . N . h a . A l t h o u g h not s i g n i f i c a n t (P=.05) , r a t e s of c e l l u l o s e decompos i t ion were about 3 t imes h i g h e r in the young c l e a r c u t s than i n the o l d - g r o w t h s t a n d . I n d i c a t o r s of N a v a i l a b i l i t y , i n c l u d i n g iri s i t u r a t e s of m i n e r a l i z a t i o n , s o i l water c o n c e n t r a t i o n s and ion exchange r e s i n bags , r e v e a l e d h igher l e v e l s of n i t r a t e i n the f o r e s t f l o o r a n d / o r m i n e r a l s o i l i n the 3- and 6 - y e a r - o l d c l e a r c u t s than in i i the o ther s i t e s . N i t r a t e c o n c e n t r a t i o n s d e c l i n e d to p r e - c l e a r c u t l e v e l s by 8-10 years a f t e r c l e a r c u t t i n g . %N and N content of c u r r e n t n e e d l e s , and 1983 h e i g h t i n c r e -ment of a m a b i l i s f i r r e g e n e r a t i o n r e f l e c t e d the p a t t e r n of N a v a i l a b i l i t y . -1 Of the observed change in f o r e s t f l o o r N (kg .N .ha ) over the 10 year p o s t - c l e a r c u t t i n g p e r i o d , p l a n t biomass accumula t ion accounted for 105 and s o l u t i o n t r a n s f e r from the f o r e s t f l o o r for 187. T o g e t h e r , t h i s amounts to 1/3 of the observed -1 d e c l i n e of 950 k g . N . h a P o t e n t i a l r a t e s of d e n i t r i f i c a t i o n in the 5- and 1 2 - y e a r - o l d c l e a r c u t f o r e s t f l o o r s were 2-5 t imes g r e a t e r than in the o l d -growth s t a n d . The p o t e n t i a l r a t e s were s u f f i c i e n t l y h i g h to p r o v i d e a p l a u s i b l e e x p l a n a t i o n for the p o s t - c l e a r c u t t i n g d e c l i n e i n f o r e s t f l o o r N . i i i TABLE OF CONTENTS A b s t r a c t . i i T a b l e of Contents i v L i s t of T a b l e s x i i L i s t of F i g u r e s x v i Acknowledgements xx Chapter 1. I n t r o d u c t i o n 1.1 I n t roduc t ion 1 1.2 O b j e c t i v e s and hypotheses 5 Chapter 2. D e s c r i p t i o n of the .Study A r e a 2 . 1 L o c a t i o n 8 2.2 C l i m a t e 10 2.3 Geology 12 2.4 S o i l s d e s c r i p t i o n 13 2 . 5 V e g e t a t i o n 15 Chapter 3. Changes i n F o r e s t F l o o r Mass , N C o n t e n t , D e c o m p o s i t i o n , M i n e r a l i z a t i o n , N i t r o g e n A v a i l a b i l i t y and R e g e n e r a t i o n Response A f t e r C l e a r c u t t i n g : E s t a b l i s h i n g a P a t t e r n 3.1 I n t r o d u c t i o n 26 3.2 L i t e r a t u r e Review 3.2.1 Decompos i t ion 27 i v 3 . 2 . 1 . 1 The d i f f i c u l t y of measur ing decompos i t ion in mor humus 27 3 . 2 . 1 . 2 Changes i n d e c o m p o s i t i o n wi th s tand age 28 3 . 2 . 1 . 3 F a c t o r s c o n t r o l l i n g changes in decompos i t i on 30 3 . 2 . 2 M i n e r a l i z a t i o n and i m m o b i l i z a t i o n 35 3 .2 .4 C o n c l u s i o n s 39 3 Methods 3 .3 .1 F o r e s t f l o o r N c a p i t a l 40 3 .3 .2 Rates of d e c o m p o s i t i o n 44 3 . 3 . 2 . 1 I_n s i t u m i n e r a l i z a t i o n of f o r e s t f l o o r 44 3 . 3 . 2 . 2 Decompos i t ion of c e l l u l o s e 45 3 . 3 . 3 I n d i c e s of N a v a i l a b i l i t y 46 3 . 3 . 3 . 1 Ion exchange r e s i n s 46 3 . 3 . 3 . 2 S o i l s o l u t i o n c h e m i s t r y 47 3 . 3 . 3 . 3 F o l i a r a n a l y s i s 48 3 . 3 . 3 . 4 Height growth response 50 4 R e s u l t s and D i s c u s s i o n 3 .4 .1 Changes in the f o r e s t f l o o r a f t e r c l e a r -c u t t i n g : chronosequence s i t e s 51 3 . 4 . 1 . 1 Depth 51 3 . 4 . 1 . 2 Bulk d e n s i t y 53 3 . 4 . 1 . 3 Mass 55 3 . 4 . 1 . 4 Percent N 62 3 . 4 . 1 . 5 T o t a l N 65 3 . 4 . 1 . 6 Ev idence of p o s t - c l e a r c u t t i n g f o r e s t f l o o r v changes from other s i t e s 68 3 . 4 . 1 . 7 Remeasurement of young c l e a r c u t s . . . . . . 72 3 . 4 . 2 . Rates of d e c o m p o s i t i o n 74 3 . 4 . 2 . 1 Rate of decompos i t i on as measured by the use of c e l l u l o s e s t r i p s 75 3 . 4 . 2 . 2 I_n s i t u m i n e r a l i z a t i o n of f o r e s t f l o o r : an index of m i n e r a l i z a t i o n r a t e s a f t e r c l e a r -c u t t i n g 80 3 . 4 . 2 . 3 F a c t o r s c o n t r o l l i n g r a t e s of decompos i t ion and N m i n e r a l i z a t i o n 85 3 . 4 . 3 I n d i c e s of n i t r o g e n a v a i l a b i l i t y 95 3 . 4 . 3 . 1 S o i l water c h e m i s t r y 96 3 . 4 . 3 . 2 Ion exchange r e s i n s 99 3 . 4 . 3 . 3 KC1 e x t r a c t i o n s of the upper m i n e r a l -s o i l 104 3 . 4 . 3 . 4 F o l i a r a n a l y s i s 107 3 . 4 . 3 . 5 Height growth response 114 3 . 4 . 3 . 6 F o l i a r N and other i n d i c e s of n i t r o g e n a v a i l a b i l i t y 122 3 . 4 . 3 . 7 G r a p h i c a l a n a l y s i s of f o l i a r n i t r o g e n 126 3 . 5 . C o n c l u s i o n s 128 Chapter 4. S i n k s for F o r e s t F l o o r N A f t e r C l e a r c u t t i n g 4.1 I n t r o d u c t i o n 131 4.2 L i t e r a t u r e Review 4 .2 .1 N i t r o g e n a c c u m u l a t i o n i n v e g e t a t i o n . . . . . 131 v i 4 .2 .2 I m m o b i l i z a t i o n in s l a s h a n d / o r s t u m p s . . . 137 4 .2 .3 M i n e r a l s o i l s t o r a g e 139 4 .2 .3 .1 T h e o r e t i c a l b a s i s for s torage 139 4 . 2 . 3 . 2 Methods for e s t i m a t i n g changes • in s torage 142 4 . 2 . 3 . 3 E s t i m a t e s of n i t r o g e n c a p i t a l i n the m i n e r a l s o i l " 142 4.3 Methods 4 .3 .1 U n d e r s t o r y accumula t ion 145 4 .3 .2 O v e r s t o r y a c c u m u l a t i o n 146 4 .3 .3 M i n e r a l s o i l s torage 148 4.4 R e s u l t s and D i s c u s s i o n 4.4.1 Biomass a c c u m u l a t i o n 149 4 . 4 . 1 . 1 U n d e r s t o r y and s e e d l i n g biomass 149 4 . 4 . 1 . 2 . O v e r s t o r y 154 . 4 . 4 . 2 M i n e r a l s o i l s torage 164 4 . 4 . 2 . 1 M i n e r a l s o i l N r e s e r v e s 164 4 . 4 . 2 . 2 Storage of N in the m i n e r a l s o i l a f t e r c l e a r c u t t i n g 167 4.5 C o n c l u s i o n s 169 Chapter 5. P o s t - C l e a r c u t t i n g F o r e s t F l o o r and M i n e r a l S o i l S o l u t i o n F l u x 5.1 I n t r o d u c t i o n 171 5.2 L i t e r a t u r e Review 5.2.1 Models and c o n t r o l l i n g mechanisms of movement of n i t r o g e n i n the s o i l p r o f i l e . . . . 172 v i i 5.3 Methods 5.3.1 Expor t of n i t r o g e n i n s o l u t i o n 177 5 .3 .2 E v a l u a t i o n of the methodology 181 5.4 R e s u l t s and D i s c u s s i o n 5.4.1 F o r e s t f l o o r s o l u t i o n c h e m i s t r y 182 5 .4 .1 .1 C o n c e n t r a t i o n s l e a v i n g the f o r e s t f l o o r 182 5 . 4 . 1 . 2 F l u x e s from the f o r e s t f l o o r 184 5 .4 .2 M i n e r a l s o i l s o l u t i o n c h e m i s t r y 184 5 .4 .3 .1 C o n c e n t r a t i o n s l e a v i n g the r o o t i n g zone 184 5 . 4 . 2 . 2 F l u x e s from the r o o t i n g zone 189 5 .4 .3 P a t t e r n s of s o l u t i o n c h e m i s t r y wi th s o i l depth 194 5.5 C o n c l u s i o n s 198 Chapter 6. P o s t - C l e a r c u t t i n g D e n i t r i f i c a t i o n Losses 6.1 I n t r o d u c t i o n 200 6.2 L i t e r a t u r e Review 6.2.1 Importance of den i t r i f i c a t ion 202 6 .2 .2 Proces s of d e n i t r i f i c a t i o n 202 6 .2 .3 Other pathways of gaseous N l o s s 203 6 .2 .4 F a c t o r s c o n t r o l l i n g d e n i t r i f i c a t i o n r a t e s 205 6 .2 .5 F a c t o r s p e r t i n e n t to m o d e l l i n g d e n i t r i f i c a t i o n i n f o r e s t s o i l s 206 v i i i 6 . 2 . 6 Methods of e s t i m a t i n g d e n i t r i f i c a t i o n . . . 207 6 .2 .7 I_n s i t u d e n i t r i f i c a t i o n r a t e s v s . d e n i t r i f i c a t i o n p o t e n t i a l s 208 6 .2 .8 E s t i m a t e s of d e n i t r i f i c a t i o n i n f o r e s t s o i l s 209 6 .2 .8 .1 O r g a n i c v s . m i n e r a l s o i l 212 6 . 2 . 8 . 2 V a r i a t i o n between f o r e s t t ypes 213 6.3 Methods 6.3.1 P o t e n t i a l d e n i t r i f i c a t i o n l o s s e s 214 6.4 R e s u l t s and D i s c u s s i o n 6.4.1 Ev idence from s o i l core s taken from the chronosequence 217 6 .4 .1 .1 Exper iments u s i n g s i e v e d f o r e s t f l o o r . . . 218 6 . 4 . 1 . 2 Exper iments u s i n g cores 222 6 . 4 . 1 . 3 D i f f e r e n c e s in p o t e n t i a l r a t e s for the chronosequence 222 6 . 4 . 1 . 4 A c t u a l d e n i t r i f i c a t i o n l o s s e s a f t e r c l e a r c u t t i n g 226 6 .4 .2 Ev idence of r a p i d d e n i t r i f i c a t i o n under p a r t i a l l y a n a e r o b i c c o n d i t i o n s 231 6 .4 .3 I n f e r e n c e s from other e x p e r i m e n t a l data 234 6.5 C o n c l u s i o n s 237 Chapter 7. O v e r a l l Model of P o s t - C l e a r c u t t i n g N Dynamics 7.1 I n t r o d u c t i o n 238 i x 7.2 L i t e r a t u r e Review 7.2.1 Geochemica l and b i o l o g i c a l i n p u t s 241 7 .2 .2 P r e c i p i t a t i o n i n p u t s 242 7 .2 .3 B iogeochemica l i n p u t s 243 7.3 Methods 7 .3 .1 Inputs to the f o r e s t f l o o r and m i n e r a l s o i l 246 7 .3 .1 .1 P r e c i p i t a t i o n 246 7 . 3 . 1 . 2 T h r o u g h f a l l 248 7 . 3 . 1 . 3 Aboveground l i t t e r f a l l 249 7 . 3 . 1 . 4 Belowground l i t t e r f a l l 251 7 . 3 . 1 . 5 S l a s h 252 7 . 3 . 1 . 6 N i t r o g e n f i x a t i o n 254 7 .3 .2 Outputs from the f o r e s t f l o o r and m i n e r a l s o i l 254 7 .3 .2 .1 E s t i m a t i n g p l a n t uptake 254 7 . 3 . 2 . 2 I m m o b i l i z a t i o n 255 7 . 3 . 2 . 3 L e a c h i n g l o s s e s 256 7 . 3 . 2 . 4 D e n i t r i f i c a t i o n 256 7 .3 .3 Net d i f f e r e n c e between i n p u t s and outputs 256 7 .3 .4 Compartments 256 7 . 3 . 5 V a l i d i t y of approach 256 7.4 R e s u l t s and D i s c u s s i o n 7 .4 . N i t r o g e n budgets f o r the p o s t - c l e a r c u t t i n g chronosequence 257 7.4.1 O l d - g r o w t h 257 7 .4 .2 Three to four y e a r s a f t e r c l e a r c u t t i n g . . 260 x 7 .4 .3 Four to ten years a f t e r c l e a r c u t t i n g . . . . 262 7 .4 .4 Twenty - s ix y e a r s a f t e r c l e a r c u t t i n g 267 7.5 C o n c l u s i o n s 272 Chapter 8. S i l v i c u l t u r a l I m p l i c a t i o n s 8.1 I n t r o d u c t i o n 273 8 . 2 . C o n t r o l l i n g f o r e s t f l o o r d e c l i n e 274 8.3 The e f f e c t of f o r e s t f l o o r N r e l e a s e on e a r l y h e i g h t growth 275 Chapter 9. Summary and C o n c l u s i o n s 282 L i t e r a t u r e C i t e d 287 Appendices Appendix 1. S o i l d e s c r i p t i o n s 315 Appendix 2a . F a c t o r s c o n t r o l l i n g r a t e s of decompos i t i on 324 Appendix 2b. F a c t o r s a f f e c t i n g m i n e r a l i z a t i o n of N 325 Appendix 2c . Height increment and f o l i a r N 325 Appendix 2d. He ight growth and o ther i n d i c e s of N a v a i l a b i l i t y 328 Appendix 3. Comparison of biomass e q u a t i o n s 329 Appendix 4. Watershed v s . l y s i m e t e r approach 331 x i Appendix 5. F o r e s t f l o o r and m i n e r a l s o i l l e a c h a t e : c o l l e c t i o n dates and f l u x e s 336 Appendix 6. D e n i t r i f i c a t i o n -A n a l y s i s of v a r i a n c e 340 Appendix 7. T a b l e s of o r i g i n a l data not p r o v i d e d in t e x t (by r e p l i c a t e ) 342 x i i LIST OF TABLES Page T a b l e 2.1 Comparison of c l i m a t i c data from the F l e e t R i v e r 11 study v e r s u s 3 CWHb v a r i a n t s taken from K l i n k a et a l . (1979) . T a b l e 2.2 Some d e s c r i p t i v e s i t e v a r i a b l e s p e r t a i n i n g to the 14 chronosequence i n v e s t i g a t e d . T a b l e 2.3 D e s c r i p t i o n of s tands i n terms of d i a m e t e r , dens- 16 i t y and h e i g h t for the s tands in the chronosequence . T a b l e 2.4 Spec i e s c o m p o s i t i o n of the o v e r s t o r y p r i o r to 18 c l e a r c u t t i n g and e s t imates of i n d i v i d u a l s p e c i e s above-ground biomass i n the o l d growth stands i n v e s t i g a t e d . T a b l e 3.1 F o r e s t f l o o r v a r i a b l e s for each of the 15 c h r o n o - 56 sequence s t a n d s . T a b l e 3.2 F o r e s t f l o o r biomass and n i t r o g e n content for 59 v a r i o u s s tands i n western N o r t h A m e r i c a . T a b l e 3.3 Remeasurement of f o r e s t f l o o r depth i n the young 73 c l e a r c u t s . T a b l e 3.4 The percentage of t imes r o t t i n g wood was observed 79 at the s u r f a c e of the f o r e s t f l o o r , based on 3 sample p l o t s per age c l a s s and 50 o b s e r v a t i o n s per p l o t . The extent of f o r e s t f l o o r d e c l i n e based on remeasure-ment and f o r e s t f l o o r T o t a l N content a f t e r the 2 year p e r i o d are i n d i c a t e d . T a b l e 3.5 I_n s i t u i n c u b a t i o n s of s i e v e d FH l a y e r m a t e r i a l 81 in p o l y e t h y l e n e bags i n the f o r e s t f l o o r i n the c h r o -nosequence s t a n d s . T a b l e 3.6 Comparison of a i r temperature and f o r e s t f l o o r 88 temperature between the o l d - g r o w t h s tand and a 1 0 - y r - o l d c l e a r c u t between September 1982 and September 1983. T a b l e 3.7 M o i s t u r e content (% by mass) of the FH l a y e r 91 d u r i n g the summer and f a l l of 1982. T a b l e 3.8 C o r r e l a t i o n c o e f f i c i e n t (r ) between d i f f e r e n t 94 parameters measured i n the i_n s i t u i n c u b a t i o n e x p e r i -ments of FH m a t e r i a l for a l l s tands combined for the summer and f a l l of 1982. T a b l e 3.9 Weighted average c o n c e n t r a t i o n s (ppm) of n i t r a t e - , 97 ammonium-, o r g a n i c - N and t o t a l - N i n water c o l l e c t e d over a two year p e r i o d . x i i i T a b l e 3.10 Q u a n t i t y of n i t r a t e and ammonium (ueq n i t r a t e - or 100 ammonium-N per gram of dry r e s i n ) adsorbed by exchange r e s i n s i n the f o r e s t f l o o r and at 30 cm i n the m i n e r a l s o i l i n the chronosequence s t a n d s . T a b l e 3.11 I n t e r s t a n d comparison of q u a n t i t i e s of n i t r a t e 101 and ammonium adsorbed by ion exchange r e s i n s . T a b l e 3.12 Average K C l - e x t r a c t a b l e n i t r a t e - , ammonium- and 105 m i n e r a l - N c o n c e n t r a t i o n s and amounts in the upper m i n e r a l s o i l f o r 3 r e p l i c a t e s i n each age c l a s s i n the c h r o n o s e -quence . T a b l e 3.13 Percent n i t r o g e n , N:P r a t i o and n i t r o g e n content 108 of c u r r e n t years f o l i a g e of a m a b i l i s f i r c o l l e c t e d in the f a l l of 1982. T a b l e 3.14 F a l l f o l i a r n i t r o g e n c o n c e n t r a t i o n s (%) for A b i e s "111 s p p . r e p o r t e d in the l i t e r a t u r e for c u r r e n t and 1 - y r - o l d n e e d l e s . T a b l e 3.15 N i t r o g e n c o n t e n t i n f o l i a g e of A b i e s spp . r e - 113 p o r t e d in the l i t e r a t u r e (mg.N per 100 need les ) and the F l e e t R i v e r d a t a . T a b l e 3.16 N:P r a t i o s i n c o n i f e r o u s f o l i a g e r e p o r t e d in the 115 l i t e r a t u r e and the F l e e t R i v e r d a t a . T a b l e 3.17 A b s o l u t e and r e l a t i v e h e i g h t increment i n 1983 121 f o r a m a b i l i s f i r advance r e g e n e r a t i o n in the c h r o n o s e -quence examined. T a b l e 3.18 C o r r e l a t i o n c o e f f i c i e n t s (r) of 1983 h e i g h t i n - 124 crement and % f o l i a r N in c u r r e n t need les (1982) w i t h s e l e c t e d i n d i c e s of n i t r o g e n a v a i l a b i l i t y . T a b l e 4.1 N i t r o g e n a c c u m u l a t i o n in u n d e r s t o r y biomass for 134 v a r i o u s stands in western N o r t h A m e r i c a . T a b l e 4.2 T o t a l aboveground t r e e and f o l i a r biomass and 135 n i t r o g e n content i n v a r i o u s s tands in western N o r t h A m e r i c a . T a b l e 4.3 M i n e r a l s o i l n i t r o g e n content for v a r i o u s s tands 144 i n western North A m e r i c a . T a b l e 4.4 N i t r o g e n c o n t e n t i n the u n d e r s t o r y biomass for 153 the chronosequence examined in 1981. T a b l e 4 .5 Average d e n s i t y , biomass and n i t r o g e n content of 155 t r e e s w i t h stems < 2.5cm b a s a l stem diameter or dbh, f o r the chronosequence of s tands examined. T a b l e 4.6 Regres s ion e q u a t i o n s used to e s t i m a t e above- 157 ground biomass components f o r t r e e s p e c i e s i n the s tands x i v sampled. A l l equat ions are of the form l n Y = (a + b l n x ) . T a b l e 4.7 Aboveground t r e e biomass by s p e c i e s f o r the 159 the r e p l i c a t e s i n t e n s i v e l y sampled a l o n g the c h r o n o s e -quence . T a b l e 4.8 Biomass of o v e r s t o r y s p e c i e s f o r the c h r o n o - 161 sequence of s i t e s . T a b l e 4.9 N i t r o g e n content in the o v e r s t o r y by component for 162 the chronosequence s i t e s . T a b l e 4.10 Average c o n c e n t r a t i o n s and amounts of t o t a l - N 165 (TKN) i n the top 30 cm of m i n e r a l s o i l in each of the age c l a s s e s examined in the chronosequence in the summer of 1982. T a b l e 5.1 E s t i m a t e s of net s o i l d r a i n a g e based on: 1. l y s i - 180 meter l e a c h a t e volumes c o l l e c t e d at the base of the f o r e s t f l o o r wi th z e r o - t e n s i o n l y s i m e t e r s , and 2. a w a t e r b a l a n c e . T a b l e 5.2 N i t r o g e n f l u x e s i n s o l u t i o n from the f o r e s t f l o o r 186 and r o o t i n g zone. T a b l e 6.1 F a t e of f o r e s t f l o o r N over the f i r s t 10 y e a r s 201 p o s t - c l e a r c u t t i n g . T a b l e 6.2 E s t i m a t e s of d e n i t r i f i c a t i o n r a t e s in f o r e s t s o i l s -210 under v a r i o u s c o n d i t i o n s u s i n g d i f f e r e n t methods for v a r i o u s s u b s t r a t e s . T a b l e 6.3 D e n i t r i f i c a t i o n p o t e n t i a l s i n d i f f e r e n t s u b s t r a t e s 211 i n f o r e s t s o i l s . T a b l e 6.4 Monthly d e n i t r i f i c a t i o n l o s s e s in the f o r e s t f l o o r 229 and the m i n e r a l s o i l of a 5 - y e a r - o l d c l e a r c u t . E s t i m a t e s are based on ' c o n t r o l ' r a t e s e x h i b i t e d in the l a b o r a t o r y a t 30 deg C , a d j u s t e d f o r f i e l d temperature assuming a Q l 0 of 1.6. T a b l e 6.5 Monthly d e n i t r i f i c a t i o n l o s s e s i n the f o r e s t f l o o r 230 and m i n e r a l s o i l in the o l d - g r o w t h s tand (See T a b l e 6.4 for d e t a i l s ) . T a b l e 6.6 C o r r e l a t i o n between oxygen c o n c e n t r a t i o n and n i - 235 t r o u s o x i d e p r o d u c t i o n i n the presence and absence of n i t r a t e . Means are an average a c r o s s a l l s t a n d s . T a b l e 7.1 N i t r o g e n i n p u t s i n p r e c i p i t a t i o n in western N o r t h 244 A m e r i c a . T a b l e 7.2 T h r o u g h f a l l and l i t t e r f a l l i n some western N o r t h 245 American f o r e s t s . x v T a b l e 7.3 Inputs of m i n e r a l - N and t o t a l - N in p r e c i p i t a t i o n 247 for the chronosequence examined. T a b l e 7.4 E s t i m a t e s of v a r i o u s t r a n s f e r s i n t o and out of the 258 f o r e s t f l o o r and upper m i n e r a l s o i l . x v i LIST OF FIGURES Page F i g u r e 2.1 L o c a t i o n of the s tudy area and chronosequence 9 s i t e s on southwestern Vancouver I s l a n d . F i g u r e 2.2 V e g e t a t i o n and s o i l of the o l d - g r o w t h s t a n d . Mean 20 depth of f o r e s t f l o o r was 24 cm. Photo of roadcut was taken about 100 m downslope from p l o t s in the o l d - g r o w t h s t a n d . F i g u r e 2.3 V e g e t a t i o n and s o i l of the 3 - y r - o l d c l e a r c u t . 22 Mean depth of f o r e s t f l o o r was 27 cm. Note absence of f i n e s l a s h which had become p a r t of the f o r e s t f l o o r . F i g u r e 2.4 V e g e t a t i o n and s o i l of the 6 - y r - o l d c l e a r c u t . 23 Mean depth of f o r e s t f l o o r was 19 cm. Note the presence of H y p o c h a e r i s sp . in the photo of the s o i l . However, f i r eweed was the dominant u n d e r s t o r y s p e c i e s p r e s e n t . F i g u r e 2.5 V e g e t a t i o n and s o i l of the 1 0 - y r - o l d c l e a r c u t . 24 Mean depth of the f o r e s t f l o o r was 16 cm. Note dominance of f i r e w e e d . F i g u r e 2.6 V e g e t a t i o n and s o i l of the 2 6 - y r - o l d c l e a r c u t . 25 Mean depth of f o r e s t f l o o r was 21 cm. Average he ight of dominant a m a b i l i s f i r was 16 to 17 m. F i g u r e 3.1 F o r e s t f l o o r depth (cm) f o r the chronosequence of 52 s tands examined on southwestern Vancouver I s l a n d . -3 F i g u r e 3.2 Bulk d e n s i t y (g.cm ) of the f o r e s t f l o o r for the 54 chronosequence of s t a n d s . -1 F i g u r e 3.3 Mass of f o r e s t f l o o r ( t . h a ) f or the c h r o n s e - 58 quence of s tands examined. F i g u r e 3.4 F o r e s t f l o o r N (%) f o r the chronosequence of 63 s tands examined. -1 F i g u r e 3.5 F o r e s t f l o o r N content (kg .ha ) for the 66 chronosequence of s tands examined. F i g u r e 3.6 G e n e r a l p a t t e r n of d e c l i n e in f o r e s t f l o o r depth 70 a f t e r c l e a r c u t t i n g . F i g u r e 3.7 G e n e r a l p a t t e r n of d e c l i n e i n f o r e s t f l o o r N 71 a f t e r c l e a r c u t t i n g . F i g u r e 3.8 Mass l o s s (%) of c e l l u l o s e paper incubated i n 76 the f o r e s t f l o o r . F o r e s t f l o o r mass and mass l o s s f o r the chronosequence are p r o v i d e d for c o m p a r i s o n . x v i i F i g u r e 3.9 S o i l pH w i t h depth for the chronosequence of 86 s tands examined. F i g u r e 3.10 P o s t - c l e a r c u t t i n g r e l a t i v e and a b s o l u t e h e i g h t 117 increment of a m a b i l i s f i r r e g e n e r a t i o n on the 1978 (3-y r - o l d ) s i t e (Means and S . D . ) . F i g u r e 3.11 P o s t - c l e a r c u t t i n g he ight increment for a m a b i l i s 118 f i r r e g e n e r a t i o n f o r the 1975 ( 8 - y r - o l d ) s i t e . F i g u r e 3.12 P o s t - c l e a r c u t t i n g r e l a t i v e h e i g h t increment of 119 a m a b i l i s f i r r e g e n e r a t i o n for the 1971 ( 1 2 - y r - o l d ) s i t e . F i g u r e 3.13 P o s t - c l e a r c u t t i n g he ight increment of a m a b i l i s 120 f i r r e g e n e r a t i o n f o r the 1978 ( 5 - y r - o l d ) and 1971 (12-y r - o l d ) s i t e s combined (Means and S . D . ) . F i g u r e 3.14 G r a p h i c a l r e p r e s e n t a t i o n of the response of 127 n a t u r a l r e g e n e r a t i o n of a m a b i l i s f i r t o the p o s t - c l e a r -c u t t i n g "assart e f f e c t " . -1 F i g u r e 4.1 Biomass of important u n d e r s t o r y s p e c i e s (kg .ha ) 151 f o r the chronosequence of stands examined. -1 -1 F i g u r e 5.1 S o l u t i o n f l u x ( k g . N . h a . y r ) of n i t r a t e - , 185 ammonium-, o r g a n i c - and t o t a l - N l e a v i n g the f o r e s t f l o o r for the chronosequence of stands examined. - ' 1 - 1 F i g u r e 5.2 S o l u t i o n f l u x ( k g . N . h a . y r ) of n i t r a t e - , 190 ammonium-, o r g a n i c - and t o t a l - N l e a v i n g the upper 60 cm of m i n e r a l s o i l f o r the chronosequence of s tands examined. F i g u r e 5.3 Changes i n weighted c o n c e n t r a t i o n s (ppm) of 195 ammonium-, n i t r a t e - and t o t a l - N w i t h s o i l depth f o r the chronosequence of s tands examined ( '0 ' depth i s f o r e s t f l o o r l e a c h a t e ) . F i g u r e 6.1 C u m u l a t i v e p r o d u c t i o n of n i t r o u s ox ide (nmoles . 219 g"1 ) from n i t r a t e - and argon-amended, s i e v e d (<2mm) FH l a y e r m a t e r i a l taken from a 5 - y r - o l d and an o l d - g r o w t h s t a n d . -1 -1 F i g u r e 6.2 Rate of n i t r o u s ox ide p r o d u c t i o n (nmoles .g . h r ) 220 from n i t r a t e - and argon-amended, s i e v e d (<2mm) FH l a y e r m a t e r i a l taken from a 5 - y r - o l d and an o l d - g r o w t h s t a n d . F i g u r e 6.3 E f f e c t of s u c r o s e a d d i t i o n on c u m u l a t i v e n i t r o u s 221 ox ide p r o d u c t i o n (nmoles .g ) from n i t r a t e - and a r g o n -amended, s i e v e d (<2mm) FH l a y e r m a t e r i a l taken from a 5-y r - o l d and an o l d - g r o w t h s t a n d . -1 -1 F i g u r e 6.4 Rates of n i t r o u s ox ide p r o d u c t i o n (nmoles .g . h r )223 from c o r e s of f o r e s t f l o o r taken from a 5 - y r - o l d c l e a r -cut (n=4). Cores were a d j u s t e d to 200% m o i s t u r e content x v i i i by mass and i n c u b a t e d at 30 deg C . -1 -1 F i g u r e 6.5 Rates of n i t r o u s ox ide p r o d u c t i o n (nmoles .g , h r ) 224 from c o r e s of f o r e s t f l o o r taken from a 1 2 - y r - o l d c l e a r -cut (n=4). Cores were a d j u s t e d to 200% m o i s t u r e content by mass and i n c u b a t e d at 30 deg C . -1 -1 F i g u r e 6.6 Rates of n i t r o u s ox ide p r o d u c t i o n (nmoles .g . h r ) 225 from c o r e s of f o r e s t f l o o r taken from an o l d - g r o w t h s tand (n=4). Cores were a d j u s t e d to 200% m o i s t u r e c o n -tent by mass and incubated at 30 deg C . F i g u r e 6.7 E f f e c t of n i t r a t e - a d d i t i o n under s e m i - a n a e r o b i c 233 and a e r o b i c c o n d i t i o n s , and the e f f e c t of argon addi t i o n in absence of n i t r a t e on n i t r o u s ox ide p r o d u c t i o n . F i g u r e 7.1 A s imple model of annual input s and o u t p u t s of N 239 to the f o r e s t f l o o r and m i n e r a l s o i l for the c h r o n o -sequence of s tands examined. F i g u r e 7.2 S imple model o u t l i n i n g compartments and t r a n s f e r s 259 to the f o r e s t f l o o r and m i n e r a l s o i l in the o l d - g r o w t h s i t e . U n i t s are kg.N.ha" 1 f or compartments and k g . N . h a ' 1 . y r " ' f or t r a n s f e r s . F i g u r e 7 . 3 - S i m p l e model o u t l i n i n g compartments and t r a n s f e r s 2'61 to the f o r e s t f l o o r and m i n e r a l s o i l i n the 1978 ( 3 - y r -o ld ) s i t e . U n i t s are k g . N . h a for compartments and k g . N . ha"1 . y r" ' f o r t r a n s f e r s . F i g u r e 7.4 S imple model o u t l i n i n g compartments and t r a n s f e r s 263 to the f o r e s t f l o o r and m i n e r a l s o i l in the 1975 ( 6 - y r -o ld ) s i t e . U n i t s are k g . N . h a for compartments and k g . N . ha"', . y r " 1 f o r t r a n s f e r s . F i g u r e 7.5 S imple model o u t l i n i n g compartments and t r a n s f e r s 266 to the f o r e s t f l o o r and m i n e r a l s o i l in the 1971 ( 1 0 - y r -o ld ) s i t e . U n i t s are k g . N . h a for compartments and k g . N . ha"' . y r " ' f o r t r a n s f e r s . F i g u r e 7.6 S imple model o u t l i n i n g compartments and t r a n s f e r s 268 to the f o r e s t f l o o r and m i n e r a l s o i l i n the 1955 ( 2 6 - y r -o ld ) s i t e . U n i t s are k g . N . h a for compartments and k g . N . ha"' . yr" ' f o r t r a n s f e r s . F i g u r e 7.7 A . Observed d i f f e r e n c e s i n f o r e s t f l o o r N between 270 chronosequence s i t e s . B . Net d i f f e r e n c e between i n p u t s and output s based on an N budget for each s i t e . F i g u r e 8.1 P o s t - c l e a r c u t t i n g h e i g h t growth of "poor ly" grow- 276 ing a m a b i l i s f i r r e g e n e r a t i o n in the Courtney area at low, medium and h i g h e l e v a t i o n s for x e r i c , mesic and h y g r i c s i t e s w i t h i n the CWH and CDFb (upper) (Kimmins u n p u b l i s h e d ) . x i x F i g u r e 8.2 P o s t - c l e a r c u t t i n g h e i g h t growth of "wel l" grown 277 a m a b i l i s f i r r e g e n e r a t i o n i n the Courtney a r e a at low, medium and h i g h e l e v a t i o n s for x e r i c , mesic and h y g r i c s i t e s w i t h i n the CWH and CDFb (upper) (Kimmins unpub-1i s h e d ) . xx ACKNOWLEDGEMENTS I would l i k e to thank D r . J . P . Kimmins, my graduate s u p e r v i -s o r , chairman of my s u p e r v i s o r y committee , f or h i s g u i d a n c e , a i d i n g e n e r a t i n g i d e a s , and c r i t i c a l review of the t h e s i s ; D r s . T . M . B a l l a r d , M . C . F e l l e r , K . K l i n k a , G . F . Weetman, and J . W o r r a l l for a d v i c e and c r i t i c a l review of the t h e s i s . ; M r . Min Tsze and M r s . Eva Tsze for t h e i r a s s i s t a n c e i n the l a b o r a t o r y wi th c h e m i c a l a n a l y s e s ; M r . B a r r y Wong and M r . John Emanuel for t h e i r a s s i s -tance in computer a n a l y s e s ; Mr . Robin Dory and M r . Ken Hart of P a c i f i c F o r e s t P r o d u c t s L t d . and M r . A r t Walker of B r i t i s h Colum-b i a F o r e s t P r o d u c t s , who p r o v i d e d a s s i s t a n c e in s i t e s e l e c t i o n . I thank my wife T h e r e s e , who p r o v i d e d encouragement and moral support d u r i n g d i f f i c u l t t imes . The p r o j e c t was funded through g r a n t s to D r . J . P . Kimmins from the Canadian F o r e s t S e r v i c e f o r development of FORCYTE- a computer s i m u l a t i o n model . I would l i k e to thank the U n i v e r s i t y of B r i t i s h C o l u m b i a , the Department of F o r e s t S c i e n c e s and the Canadian F o r e s t S e r v i c e for p e r s o n a l f i n a n c i a l support d u r i n g the s t u d y . x x i CHAPTER 1 I n t r o d u c t i o n 1 . 1 I n t r o d u c t i o n Many o l d - g r o w t h c o n i f e r o u s f o r e s t s i n b o r e a l and temperate c l i m a t e s accumulate deep f o r e s t f l o o r s which may c o n t a i n the major p r o p o r t i o n of the s i t e n u t r i e n t c a p i t a l of a few important n u t r i e n t s . F o l l o w i n g a d i s t u r b a n c e t h a t removes the o v e r s t o r y , there i s o f t en a marked a c c e l e r a t i o n of m i n e r a l i z a t i o n and a r e d u c t i o n in f o r e s t f l o o r depth and mass. The r e s u l t i n g f l u s h of n u t r i e n t s which o f t en o c c u r s a f t e r c l e a r c u t t i n g as a r e s u l t of i n c r e a s e d m i n e r a l i z a t i o n , sometimes r e f e r r e d to as the "assar t e f f e c t " (Romell 1935; Tamm 1 950;. Tamm and P e t t e r s s o n 1969), can promote r a p i d e a r l y growth of p l a n t a t i o n s . L a b o r a t o r y m i n e r a l i z a t i o n s t u d i e s by Tamm and P e t t e r s s o n (1969) suggest that the o r g a n i c l a y e r s may p r o v i d e a l a r g e p r o -p o r t i o n of the i n c r e a s e in m i n e r a l - N a f t e r c l e a r c u t t i n g . F a i l u r e of r e g e n e r a t i o n to "capture" the " a s s a r t e f f e c t " may r e s u l t in reduced t r e e growth (Tamm and P e t t e r s s o n 1969; Nyland et a l . 1979; Weetman 1980), e s p e c i a l l y on n u t r i e n t - p o o r s i t e s , and t h i s can l e a d to "capture" of the s i t e by heath v e g e t a t i o n (Damman 1971). However, i n s p i t e of the importance of the "assar t e f f e c t " in d e t e r m i n i n g the growth of young s t a n d s , the e f f e c t of a d e l a y in p l a n t i n g a f t e r c l e a r c u t t i n g on t r e e n u t r i t i o n and h e i g h t growth has not been w e l l documented. Except on the most f e r t i l e s i t e s , r e g e n e r a t i o n s t r a t e g i e s s h o u l d be adopted which ensure tha t crop t r e e s are a b l e to b e n e f i t from the r e l e a s e of 1 n u t r i e n t s . In order to d e v e l o p such s t r a t e g i e s , the t i m i n g and magnitude of the n u t r i e n t f l u s h and i t s importance in the n u t r i -t i o n of v a r i o u s s p e c i e s need to be q u a n t i f i e d a c r o s s a wide range of s i t e s . The e f f e c t of the n u t r i e n t f l u s h on e a r l y t r e e growth can l e a d f o r e s t e r s to m i s i n t e r p r e t the l o n g - t e r m f e r t i l i t y and p r o -d u c t i v i t y of a s i t e . N u t r i e n t demanding s p e c i e s may show good growth d u r i n g the e a r l y y e a r s a f t e r c l e a r c u t t i n g , but growth may d e c l i n e once n u t r i e n t a v a i l a b i l i t y drops to p r e - d i s t u r b a n c e l e v e l s . An analogy would be the s h o r t - t e r m growth response a f t e r a s i n g l e f e r t i l i z a t i o n . As a r e s u l t of the "assar t e f f e c t " , c o n v e r s i o n of o l d - g r o w t h s tands to second-growth p l a n t a t i o n s u s u a l l y i n v o l v e s a l o s s of f o r e s t f l o o r o r g a n i c m a t t e r , and may i n v o l v e a l o s s of nu tr i en t s - , as has been documented i n n o r t h e a s t e r n hardwoods (Dominski 1971; Cov ington 1981). A l t h o u g h temperature i n c r e a s e s have o f t en been r e c o r d e d i n f o r e s t s o i l s a f t e r c l e a r c u t t i n g (eg. Timmer and Weetman 1969) or t h i n n i n g (eg. P iene 1978), the magnitude of the temperature i n c r e a s e i s g e n e r a l l y s m a l l e r than tha t needed to account f o r the observed i n c r e a s e s i n r a t e s of decompos i t i on or m i n e r a l i z a t i o n ( G a d g i l and G a d g i l 1975, 1978). T h i s has l e d to the study of the b i o l o g i c a l f a c t o r s tha t are c a u s i n g changes in f o r e s t s o i l s a f t e r c l e a r c u t t i n g . Increase s i n the numbers of b a c t e r i a (Sundman and Niemela 1978), and decreases i n the numbers of fung i (Baathe 1980) have been r e c o r d e d in f o r e s t s o i l s 7-10 y e a r s a f t e r c l e a r c u t t i n g i n S c a n d i n a v i a . In p a r t i c u l a r , s u p p r e s -s i o n of l i t t e r decompos i t i on by m y c o r r h i z a l f u n g i has been shown 2 to occur in radiata pine stands (Gadgil and Gadgil 1978). The large inputs of organic matter (such as dead fine root biomass) after c l e a r c u t t i n g , may provide a stimulatory effect in promoting the breakdown of the generally more re c a l c i t r a n t humus layer- the presence of a more readily decomposable material in close proxi-mity from which decomposers can obtain energy. For example, the addition of straw has been shown to increase nitrogen a v a i l a -b i l i t y and improve the n u t r i t i o n of Pinus banksiana Lamb, stands, although the exact nature of the effect was not established (Weetman and Algar 1974). However, a variable period of immobi-l i z a t i o n may be expected to precede mineralization, the length depending on the "quality" of the added carbonaceous material. Assessment of the long-term potential of d i f f e r e n t s i t e s to produce desired yields over many rotations requires- an under-standing of the sinks for the N mobilized from the forest floor during the post-clearcutting period. Some sinks represent a tem-porary immobilization or storage ( i . e . microbial biomass and mineral s o i l f i x a t i o n ) , whereas others result in permanent loss from the ecosystem in question. It i s the l a t t e r losses ( i . e . d e n i t r i f i c a t i o n , leaching and harvested products) which are of greatest concern when examining the long-term productivity of forest s i t e s . In most studies of ecosystem nutrient budgets, the focus of research has been on n i t r a t e losses in streamwater (Likens et a l . 1970; F e l l e r 1974; Vitousek et a l . 1979), although a few studies have estimated total-N fluxes from the mineral s o i l rooting zone (eg. Gessel and Cole 1965). Losses in harvested materials have also received considerable attention over the past three decades (Rennie 1955; Weetman and Webber 1972; Kimmins and 3 F e l l e r 1976; F e l l e r and Kimmins 1984; Weetman and A l g a r 1983). However, d e n i t r i f i c a t i o n in f o r e s t s o i l s has o n l y r e c e n t l y been s t u d i e d , and l i t t l e i n f o r m a t i o n e x i s t s as to i t s importance r e l a t i v e to o t h e r s i n k s for m i n e r a l i z e d N . C o n s i d e r i n g a l l the c o s t s a s s o c i a t e d wi th f o r e s t f e r t i l i z a -t i o n , i t i s becoming i n c r e a s i n g l y important to conserve o r g a n i c matter and n u t r i e n t s in order to reduce the need for f e r t i l i z e r amendments; amendments that w i l l be necessary for the maintenance of p r o d u c t i v i t y of f u t u r e r o t a t i o n s on medium to poor s i t e s , and p o s s i b l y a l s o on good s i t e s . S i t e n u t r i e n t management i s thus becoming a s i g n i f i c a n t aspect of o v e r a l l management. Because of the c o m p l e x i t y of f o r e s t ecosystems, o p t i m i z a t i o n of s i t e n u t r i e n t management i s d i f f i c u l t . A v a l u b l e t o o l i n overcoming t h i s d i f f i c u l t y i s the use of computer s i m u l a t i o n models which can p r o v i d e a r a p i d and dynamic a n a l y s i s of the consequences for s i t e n u t r i e n t management and f u t u r e y i e l d of a wide v a r i e t y of a l t e r n a t i v e f o r e s t management p r a c t i s e s . Such s i m u l a t i o n models , eg . FORTNITE (Aber et a l . 1982) and FORCYTE (Kimmins and S c o u l l a r 1984), w i l l improve our u n d e r s t a n d i n g of ecosystem proces ses and our a b i l i t y to m a i n t a i n a n d / o r improve the p r o d u c t i v i t y of f u t u r e r o t a t i o n s . The o r i g i n a l idea for the t h e s i s came from the need to t e s t the v a l i d i t y of some of the p r e d i c t i o n s of the FORCYTE model c o n c e r n i n g the magnitude and d u r a t i o n of p o s t - h a r v e s t N l o s s e s . One important q u e s t i o n that t h i s t h e s i s was des igned to answer was: Are the n i t r a t e l e a c h i n g l o s s e s p r e d i c t e d by FORYCTE-10 e x c e s s i v e ? T h i s q u e s t i o n was r a i s e d a f t e r s e v e r a l model runs 4 p r e d i c t e d n i t r a t e l e a c h i n g l o s s e s exceed ing those p u b l i s h e d in the l i t e r a t u r e , and r e p r e s e n t s an important t e s t of the mode l ' s v a l i d i t y s i n c e i t s y i e l d p r e d i c t i o n s are s e n s i t i v e to s i t e N l o s s e s . N i t r o g e n was the element s t u d i e d because i t i s the most l i m i t -i n g element for f o r e s t s in the P a c i f i c Northwest (Cole and Rapp 1981). The i n f l u e n c e of N f e r t i l i z e r a d d i t i o n s on t r e e growth has been c l e a r l y demonstrated for Pseudotsuga m e n z i e s i i ( M i r b . ) F r a n c o ( D o u g l a s - f i r ) ( G e s s e l and Walker 1956) and for A b i e s  a m a b i l i s (Dougl . ) Forbes ( G a l l a g h e r 1964). E f f e c t s of N f e r t i l i -z e r a d d i t i o n on growth of Tsuga h e t e r o p h y l l a ( R a f . ) S a r g . have v a r i e d somewhat ( D e B e l l et a l . 1975; Webster et a l . 1976). In a d d i t i o n , the f a c t tha t n i t r o g e n c y c l e s between the s i t e and the atmosphere makes i t p o t e n t i a l l y more s u s c e p t i b l e to l o s s e s than o t h e r n u t r i e n t s . . There i s i n c r e a s i n g ev idence tha t gaseous n i t r o g e n l o s s e s such as d e n i t r i f i c a t i o n might be much more p r e v a -l e n t i n f o r e s t s o i l s than p r e v i o u s l y thought (Robertson and T i e d j e 1984). 1.2 O b j e c t i v e s and hypotheses The o v e r a l l o b j e c t i v e of the t h e s i s s tudy was to q u a n t i f y the dynamics and fa te of p o s t - c l e a r c u t f o r e s t f l o o r n i t r o g e n and i t s importance in the n u t r i t i o n of young r e g e n e r a t i n g s tands of a m a b i l i s f i r and western hemlock in the wet subzone of the Coas -t a l Western Hemlock b i o g e o c l i m a t i c zone (CWHb; K r a j i n a 1969). S p e c i f i c o b j e c t i v e s were as f o l l o w s : 1 . To document changes i n f o r e s t f l o o r d e p t h , mass, p e r c e n t N, N 5 c o n t e n t , N a v a i l a b i l i t y and r e g e n e r a t i o n response (eg. f o l i a r N and h e i g h t growth) a f t e r c l e a r c u t t i n g on a mesic s i t e by s t u d y i n g an age sequence of s tands (a c h r o n o s e q u e n c e ) . T h i s p a t t e r n then a l l o w e d me to t e s t whether t h e r e i s a change in f o r e s t f l o o r N c a p i t a l ( a l s o d e p t h , mass and percent N ) , the magnitude of the change and when i t o c c u r r e d a f t e r c l e a r c u t t i n g . Does a m a b i l i s f i r h e i g h t growth r e f l e c t the changes in f o r e s t f l o o r N c a p i t a l and N a v a i l a b i l i t y a f t e r c l e a r c u t t i n g on these mesic s i t e s ? 2. To e s t a b l i s h the magnitude of s i n k s f o r the m o b i l i z e d n i t r o g e n on t h i s chronosequence . Can a c c u m u l a t i o n of N in p l a n t biomass and l e a c h i n g l o s s e s account f o r most of the change i n f o r e s t f l o o r N a f t e r c l e a r c u t t i n g , or are o ther s i n k s or l o s s e s such as m i n e r a l s o i l s torage or d e n i t r i f i c a t i o n important? . D e n i t r i f i c a -t i o n was added as a p o t e n t i a l l y important l o s s a f t e r p a r t of the s tudy had been comple ted . The above q u e s t i o n s were expressed i n terms of the f o l l o w i n g hypotheses . 1. There shou ld be a s i g n i f i c a n t r e l e a s e of f o r e s t f l o o r N a f t e r c l e a r c u t t i n g and i t shou ld be r e f l e c t e d in the growth response of advanced r e g e n e r a t i o n of a m a b i l i s f i r . 2-. The p o s t - c l e a r c u t t i n g r e l e a s e of f o r e s t f l o o r N can be accounted f o r i n terms of p l a n t uptake and s o l u t i o n t r a n s f e r to the m i n e r a l s o i l . The f i r s t two years of r e s e a r c h r e s u l t e d i n a r e j e c t i o n of h y p o t h e s i s 2. An a d d i t i o n a l h y p o t h e s i s was then t e s t e d : 3. that gaseous l o s s e s of n i t r o g e n , v i a d e n i t r i f i c a t i o n , can be an important proces s in p o s t - c l e a r c u t t i n g f o r e s t f l o o r N 6 dynamics and can e x p l a i n the d i s c r e p a n c y i n the i n i t i a l f o r e s t f l o o r N budget . The t h e s i s i s o r g a n i z e d i n t o 9 c h a p t e r s as f o l l o w s : 1 . I n t r o -d u c t i o n , 2. D e s c r i p t i o n of the Study A r e a , 3. Changes in F o r e s t F l o o r Mass ( i . e . o r g a n i c m a t t e r ) , N C o n t e n t , D e c o m p o s i t i o n , M i n -e r a l i z a t i o n , N a v a i l a b i l i t y and Height Growth A f t e r C l e a r c u t t i n g : E s t a b l i s h i n g a P a t t e r n , , 4. S i n k s for F o r e s t F l o o r N i t r o g e n A f t e r C l e a r c u t t i n g , 5. P o s t - C l e a r c u t t i n g F o r e s t F l o o r and M i n e r a l S o i l S o l u t i o n F l u x , 6. P o s t - C l e a r c u t t i n g D e n i t r i f i c a t i o n L o s s e s , 7. O v e r a l l Model of P o s t - C l e a r c u t t i n g N Dynamics , 8. S i l v i c u l t u r a l I m p l i c a t i o n s and 9. C o n c l u s i o n s . Chapter 3 e s t a b l i s h s the p a t t e r n of f o r e s t f l o o r d e c l i n e and i t s e f f e c t on f o l i a r N and h e i g h t growth of a m a b i l i s f i r r egener -a t i o n . A f t e r q u a n t i f y i n g the changes in f o r e s t f l o o r mass, N c o n t e n t , e t c . , the next 3 c h a p t e r s set out to q u a n t i f y the f a t e of t h i s f o r e s t f l o o r N in p l a n t biomass and m i n e r a l s o i l s torage (Chapter 4 ) , l e a c h i n g l o s s e s (Chapter 5 ) , and d e n i t r i f i c a t i o n l o s s e s (Chapter 6 ) . In Chapter 7, a n u t r i e n t budget i s deve loped from both da ta c o l l e c t e d i n the study as w e l l as i n f o r m a t i o n i n the l i t e r a t u r e . Chapter 8 d i s c u s s e s some of the i m p l i c a t i o n s t h a t the p o s t - c l e a r c u t t i n g changes in f o r e s t f l o o r N have for s i t e p r o d u c t i v i t y and r e g e n e r a t i o n response . 7 CHAPTER 2 D e s c r i p t i o n of the Study Area 2.1 L o c a t i o n The study area i s l o c a t e d on southwestern Vancouver I s l a n d , B r i t i s h C o l u m b i a , about 15-20 km southwest of Lake Cowichan, in the Wetter M a r i t i m e C o a s t a l Western Hemlock B i o g e o c l i m a t i c sub-zone ( F i g u r e 2 . 1 ) . The g e n e r a l area was s e l e c t e d a f t e r s e v e r a l weeks of r e c o n n a i s s a n c e and d i s c u s s i o n wi th numerous i n d i v i d u a l s i n d i c a t e d tha t a chronosequence of uns la shburned s i t e s was p r e s e n t on t h i s p a r t of Vancouver I s l a n d . The s tudy was conducted on a s e r i e s of s i t e s that were judged to be s i m i l a r in n u t r i e n t s t a t u s ( trophotope) and m o i s t u r e s t a t u s ( h y g r o t o p e ) , but which v a r i e d i n time s i n c e c l e a r c u t t i n g . The s e l e c t i o n of p l o t s was based on the combinat ion of u n d e r s t o r y v e g e t a t i o n (which i s b e l i e v e d to r e f l e c t the s i t e m o i s t u r e and n u t r i e n t r e g i m e s ) , and p h y s i c a l s i t e p a r a m e t e r s . A l t h o u g h s u c e s -s i o n a l ground v e g e t a t i o n had invaded my c l e a r c u t s i t e s (eg . E p i l o b i u m a n g u s t i f o l i u m L . ) , h i g h - l e a d y a r d i n g had not d e s t r o y e d the presence of those s p e c i e s used to i d e n t i f y a mesic s i t e . The f i r s t three p l o t s were in an uneven-aged o l d - g r o w t h (300-400 y e a r s ) f o r e s t . T r i p l e t s of p l o t s were l o c a t e d i n areas that had been c l e a r c u t i n 1978, 1975, 1971 and 1955. At the s t a r t of the s t u d y , these s i t e s were 3 - , 6 - , 10- and 2 6 - y e a r s - o l d p o s t -c l e a r c u t t i n g , r e s p e c t i v e l y . With the e x c e p t i o n of the 1955 c l e a r -c u t s ( 2 6 - y e a r - o l d a t the s t a r t of the s t u d y ) , a l l of the p l o t s 8 F i g u r e 2.1 L o c a t i o n o f the s t u d y a r e a and chronosequence s i t e s on: A. Vancouver I s l a n d ( s o u t h w e s t e r n B r i t i s h C o l u m b i a ) , B. Cowichan Lake - ( s o u t h w e s t e r n Vancouver I s l a n d ) , C. Caycuse a r e a , D. F l e e t R i v e r a r e a and E. Gordon R i v e r - H a r r i s Creek a r e a . 9 were l o c a t e d in the upper p a r t of the F l e e t R i v e r , south of Lake Cowichan. The o l d - g r o w t h and 6 - y r - o l d s i t e s were in c l o s e p r o x i -mity ( i . e . 1 km), wh i l e the 3- and 1 0 - y r - o l d s i t e s were spaced about 4 km a p a r t . The 1955 c l e a r c u t s (probab ly yarded u s i n g a steam donkey as opposed to h i g h - l e a d y a r d i n g on the o ther s i t e s ) were l o c a t e d in two a r e a s , about 15-20 km to the west near Gordon R i v e r and the other on branch C11 of Caycuse M a i n , south of Caycuse on the southern shore of Lake Cowichan ( F i g u r e 2 . 1 ) . The sample p l o t s were s i t u a t e d at an upper mids lope p o s i t i o n on a v a l l e y s i d e h i l l at 550-650 m e l e v a t i o n . Percent s lope was g e n e r a l l y i n the 10-30% range . A s p e c t s for the F l e e t R i v e r p l o t s were W to NW, except for the 3 - y r - o l d c l e a r c u t which was due N . Aspec t s for the 2 6 - y r - o l d c l e a r c u t s were W to SW (Table 2 . 2 ) . A c c o r d i n g to K l i n k a et a l . (1979) the 610 m e l e v a t i o n l e v e l i s the d i v i d i n g l i n e between the submontane and montane v a r i a n t s i n the CWHb. S i n c e the p l o t s were l o c a t e d at a p p r o x i m a t e l y the boundary, v e g e t a t i o n and c l i m a t e were i n t e r m e d i a t e between those of the submontane and montane v a r i a n t s in the a r e a . 2.2 C l i m a t e The c l i m a t e i s d e f i n e d as C f b / c , a c c o r d i n g to Koppen ( i n Trewartha 1968), i m p l y i n g a humid, c o o l , mesothermal c l i m a t e , w i th no d i s t i n c t dry season, and a c o o l to s h o r t c o o l summer. K l i n k a et a l . (1979) have d e s i g n a t e d f i v e b i o g e o c l i m a t i c v a r i a n t s w i t h i n the CWHb, and T a b l e 2.1 compares some c l i m a t i c v a r i a b l e s of the F l e e t R i v e r w i t h those of the f o l l o w i n g v a r -i a n t s : 1. West Vancouver I s l a n d Submontane Wetter M a r i t i m e (WVISWM), 2. West Vancouver I s l a n d Montane Wetter M a r i t i m e 10 T a b l e 2.1 Comparison of c l i m a t i c d a t a f o r the F l e e t R i v e r study a r e a v e r s u s 3 CWHb v a r i a n t s t a k e n from K l i n k a e t a l . (1979). C l i m a t i c v a r i a b l e WVISWM. WVIMWM EVIMWM F l e e t R i ver Mean annual p r e c i p i t a t i o n (mm) 3819 3941 1892 2117 (852) (412) (751) Mean temp, of c o l d e s t month (deg C) 4.6 0. 1 -2 . 3 3. 1 Index of c o n t 1 n e n t a 1 1 t y * 1 2 15 7 Mean p r e c i p i t a t i o n ( A p r i l - S e p t ) (mm) 876 967 376 503 (79) ( 121 ) P e r c e n t of t o t a l ( A p r 1 l - S e p t ) 23 24.5 20 23 Mean p r e c i p i t a t i o n of d r i e s t month 91 102 36 76 (17) (20) (10) Mean annual temperature (deg C) 7 . 1 5.0 5.4 7.3 ( .9) (.5) (.4) P o t e n t i a l e v a p o t r a n s p l r a t l o n (mm) 502 436 486 670.5 (8.1) (15.0) (23.0) A c t u a l e v a p o t r a n s p l r a t l o n (mm) 502 433 381 444 (8.1) (8.1) (43.1) R a t i o a c t u a l : p o t e n t 1 a l (%) 100 99 79 66 D r a i n a g e ( o r water s u r p l u s ) (mm)** 3317 3508 1512 1673 D r a i n a g e (% o f t o t a l p r e c i p i t a t i o n ) 87 89 80 79 * Index of cont1nenta11ty= (1.7*(mean J u l y temperature-mean January t e m p e r a t u r e ) / s i n e (degrees l a t i t u d e ) - 20.4) * d r a i n a g e was e s t i m a t e d from a water b a l a n c e . Ep was e s t i m a t e d u s i n g the Pre 1st1ey-Tay1 or e q u a t i o n (Campbell 1977) f o r the F l e e t R i v e r , w h i l e Ep was e s t i m a t e d v i a the method of W i l s o n and Rouse ( 1972) 1n KI inka e t a l . 1979 f o r the v a r i a n t s . Both e s t i m a t e s used the T h o r n t h w a i t e (1957) p r o c e d u r e . (WVIMWM), 3 . E a s t Vancouver I s l a n d Montane Wetter M a r i t i m e (EVIMWM). From t h i s compar i son , i t can be seen t h a t the F l e e t R i v e r s t a n d s are most s i m i l a r to the EVIMWM v a r i a n t w i th r e s p e c t to t o t a l p r e c i p i t a t i o n and d r a i n a g e . However, t emperatures appear to be h i g h e r than those of the EVIMWM. T h i s c o u l d have been due e i t h e r to h i g h e r than average temperatures d u r i n g the year of o b s e r v a t i o n , or to the l o c a t i o n of the s i t e s at the lower e l e v a -t i o n boundary of t h i s v a r i a n t . The h i g h e r temperatures r e s u l t in a lower p e r c e n t a g e of the t o t a l p r e c i p i t a t i o n o c c u r i n g as snow as compared t o the average for the EVIMWM v a r i a n t . A n e g l i g i b l e amount of snow f e l l d u r i n g the second year of the s t u d y , when p r e c i p i t a t i o n amounted to 2117 mm. The es t imate of 1889 mm p r e c i -p i t a t i o n i n the f i r s t year of the s tudy may have been low s i n c e 0 . 6 - 0 . 9 m of snow (about 7.5mm,water e q u i v a l e n t ) f e l l in the area at the e l e v a t i o n of the p l o t s d u r i n g the winter months; t h i s was not r e c o r d e d by the p r e c i p i t a t i o n c o l l e c t o r s t h a t were u s e d . 2.3 Geology A c c o r d i n g to a g e o l o g i c a l map of Vancouver I s l a n d ( M u l l e r and Carson 1969 i n Nor thco te 1981), the F l e e t R i v e r a r e a , in which the 3 - , 6 - , and 1 0 - y r - o l d and o l d - g r o w t h s i t e s were l o -c a t e d , l i e s i n a rock u n i t c a l l e d "In land I n t r u s i o n s " , formed in the middle t o l a t e J u r a s s i c P e r i o d . T h i s rock f o r m a t i o n i s des -c r i b e d as " l a r g e b a t h o l i t h s of g r a n o d i o r i t e to q u a r t z d i o r i t e c o m p o s i t i o n w i t h l o c a l s t o c k s of q u a r t z monzonite and g r a n i t e d i f f e r e n t i a t e s " . The three 2 6 - y r - o l d p l o t s were l o c a t e d in the Karmutsen F o r m a t i o n which i s of T r i a s s i c age or o l d e r , and i s d e s c r i b e d as " s l i g h t l y metamorphosed b a s a l t i c l a v a s c o n s i s t i n g of 12 massive and a m y g d a l o i d a l f lows , p i l l o w l a v a s , and a s s o c i a t e d b r e c c i a " . Based on coarse fragments taken from the s tudy p l o t s , the l i t h o l o g y of the F l e e t R i v e r area appears to be g r a n o d i o r i t e , whereas the rock types of the 2 6 - y r - o l d p l o t s are d i f f e r e n t . Two of the l a t t e r s tands near Gordon R i v e r are growing on s o i l s d e r i v e d from what appears to be a v e r y s i l i c a - r i c h rock based on i t s v e r y l i g h t c o l o u r (Walmsley et a l . 1980), whereas the o ther 2 6 - y r - o l d p l o t near Caycuse i s u n d e r l a i n by t i l l s d e r i v e d from d a r k e r r o c k s . T h i s g e o l o g i c a l d i f f e r e n c e , which might confound the i n t e r p r e t a t i o n s of changes in m i n e r a l s o i l N content w i t h time s i n c e c l e a r c u t t i n g , was u n f o r t u n a t e but u n a v o i d a b l e as no b e t t e r s i t e s were a v a i l a b l e . Comparison of the 2 6 - y r - o l d s i t e da ta w i t h t h a t from the o ther p l o t s must be made w i t h t h i s i n mind . The e f f e c t that parent m a t e r i a l s have on s i t e n u t r i e n t s t a t u s i s d i s c u s s e d in s e c t i o n 4 . 4 . 2 . 2.4 S o i l D e s c r i p t i o n One 50 x 50 x 80 cm deep s o i l p i t was d e s c r i b e d i n one of each of the 3 - , 6- and 1 0 - y r - o l d , and o l d - g r o w t h s i t e s , w h i l e one p i t was d e s c r i b e d in each of the 2 6 - y r - o l d s i t e s s i n c e they were s e p a r a t e d by f a i r l y l a r g e d i s t a n c e s r e l a t i v e to those i n the F l e e t R i v e r a r e a . S o i l d e s c r i p t i o n s ( a c c o r d i n g to Walmsley et a l . 1980) are p r e s e n t e d i n Appendix 1. Inc luded i n the d e s c r i p t i o n are h o r i z o n d e p t h s , c o l o u r , pH ( i n c a l c i u m c h l o r i d e ) and t e x t u r e of the B h o r i z o n ( u s i n g the hydrometer method) . Based on the f i e l d des -c r i p t i o n the s o i l s were c l a s s i f i e d as sandy or s a n d y - s k e l e t a l H u m o - F e r r i c Podzo l s ( C . S . S . C . 1978) and the dominant humus 13 T a b l e 2.2 Some d e s c r i p t i v e s i t e v a r i a b l e s p e r t a i n i n g to the chronosequence i n v e s t i g a t e d . * ** *** **** S i t e / Aspect S lope S o i l S o i l pH <2mm C l e a r c u t , % Texture F r a c t i o n B h o r i z o n FH B Mean s • D 3 - y r - o l d N10 E 1 5-30 LS 3.4 4.2 33.7 3 .0 6 - y r - o l d N30-50 W 20- 30 LS 3.2 4.5 26.9 2 .4 1 0 - y r - o l d N30-50 W 10- 20 s 3.7 4.7 30.0 3 .5 2 6 - y r - o l d w-sw 1 o- 30 S-LS 3.6 4.6 28. 3 6 .4 o l d - g r o w t h N40-50 W 20- 30 LS 3.6 4.3 18.1 1 .4 * Age of c l e a r c u t in 1981. ** ISS c l a s s i f i c a t i o n based on hydrometer method for < 2mm f r a c t i o n of the B h o r i z o n . * * * s o i l pH ( i n CaC12 : 1:1 r a t i o of s o i l to .01M CaC12) of the FH l a y e r and B h o r i z o n . * * * * p e r c e n t (by mass) of t o t a l s o i l f r a c t i o n < 2mm based on 25 m i n i p i t s per s tand for each of the 15 s tands (3 per age c l a s s ) . 1 4 form(s) l ignohumimor and orthihumimor ( K l i n k a et a l . 1981). A few s e l e c t v a r i a b l e s are g i v e n in T a b l e 2 .2 , of which on ly percent by mass of the <2 mm f r a c t i o n v a r i e s much between s i t e s . T h i s v a l u e was based on the average for the upper 30 cm of m i n e r a l s o i l of 25 m i n i - p i t s , r a t h e r than modal s o i l p i t s from which the o ther data were t a k e n . F u r t h e r d e t a i l s of s o i l p h y s i c a l and c h e m i c a l p r o p e r t i e s w i l l be d i s c u s s e d in s e c t i o n 4 . 4 . 2 . 2.5 V e g e t a t i o n T a b l e 2.3 p r o v i d e s a d e s c r i p t i o n of the o v e r s t o r y in terms of d i a m e t e r , h e i g h t of dominants , b a s a l area and d e n s i t y for the 15 s tands i n v e s t i g a t e d (3 r e p l i c a t e s of 5 s tand a g e s ) . Stand d e n s i t y of stems > 2.5cm at b a s a l stem diameter (bsd) ranged from 255 to 1083 in the 3 - , 732 to 1720 in the 6 - , • and 2643 to 2994 stems per ha in the 10 -yr -61d c l e a r c u t s ; stems > 2.5cm at d i a -meter at b r e a s t h e i g h t (dbh) were 1400 to 4000 per ha i n the 26-y r - o l d s t a n d s . One of the 2 6 - y e a r - o l d s tands had been spaced to 1400 stems per ha i n 1972, 9 y e a r s be fore the s t a r t of the s t u d y . I t was i n c l u d e d i n the study because there was no other 2 6 - y r - o l d p l o t a v a i l a b l e . The canopy had s t i l l not c o m p l e t e l y c l o s e d (75-80%) in t h i s s tand 10 y e a r s a f t e r the s p a c i n g t rea tment . The expected i n c r e a s e in f o r e s t f l o o r depth due the i n c o r p o r a t i o n of s l a s h from the s p a c i n g treatment might be o f f s e t by the r e l a -t i v e l y slow r e - e s t a b l i s h m e n t of a c l o s e d canopy and a s s o c i a t e d -1 r a t e s of l i t t e r f a l l . S i m i l a r f o r e s t f l o o r N content s (kg .ha ) were observed for the three 2 6 - y r - o l d p l o t s . Based on the h e i g h t s of dominant and co-dominant western hemlock and the s i t e index c u r v e s of Barnes ( i n D i l w o r t h 1979) 15 T a b l e 2.3 D e s c r i p t i o n of s tands in terms of d i a m e t e r , b a s a l a r e a , d e n s i t y and h e i g h t for the s tands in the c h r o n o -sequence. * ** *** **** S i t e / Average Tree of B . A . D e n s i t y He ight C l e a r c u t d iameter average (cm) B . A . (m2/ha) (stems/ha) (m) 3 - y r - o l d - l 3 .9 4. 1 1 .4 1 083 1.3-3 . 0 3 - y r - o l d - 2 3.9 3.9 0.3 • 255 1.4-2.1 3 - y r - o l d - 3 3.3 3.4 0.8 955 1 .5-2 .6 6 - y r - o l d - 1 2.1 4.4 1 .3 860 1 .7-2 .5 6 - y r - o l d - 2 3.4 2.8 0.4 732 1 .2 -2 .9 6 - y r - o l d - 3 3.3 3.4 1 .6 1 720 1 .5-2 .6 1 0 - y r - o l d - 1 4.5 5.0 5.5 2834 4 . 6 - 6 . 5 1 0 - y r - o l d - 2 5.0 5. 1 6.0 2994 3 . 2 - 3 . 5 1 0 - y r - o l d - 3 5.5 5.7 6.8 2643 3 . 5 - 6 . 0 ***** 2 6 - y r - o l d - 1 13.4 16.3 29. 1 1 400 17.4 2 6 - y r - o l d - 2 11.4 13.2 42. 1 3057 16.5 2 6 - y r - o l d - 3 12.4 15.1 73.0 4076 17.0 ****** o ld -growth-1 23.3 34.8 151.1 1 592 T H - 3 8 , T P - 2 8 o l d - g r o w t h - 2 14.3 26.3 141.7 261 1 T H - 3 4 , A A - 3 4 o l d - g r o w t h - 3 • 9.5 20.5 66.0 2006 T H - 2 7 , T P - 2 0 * - c l e a r c u t age in 1981. * * - average diameter (cm) of a l l stems > 2 . 5 cm dbh in the case of 1955 and o l d - g r o w t h stands and > 2.5 cm at b a s a l stem diameter i n the o t h e r s * * * - b a s a l area i s based on dbh in the 1955 and o l d - g r o w t h s tands and b a s a l stem d iameter i n the o t h e r s . * * * * _ r a n g e of h e i g h t s for dominants in the 3 - , 6- , and 10-y r - o l d c l e a r c u t s ; average h e i g h t of dominants i n the 2 6 - y r - o l d and o l d - g r o w t h s t a n d s . H e i g h t s g iven for the 2 6 - y r - o l d c l e a r c u t s are f o r a m a b i l i s f i r , whereas those in the younger c l e a r c u t s i n c l u d e both a m a b i l i s f i r and western hemlock. * * * * * _ s tand had been spaced to 1400 stems per ha i n 1972 * * * * * * A £ - a m a b i l i s f i r ; TH - western hemlock TP - western redcedar 16 s i t e index at 100 y e a r s of the uneven-aged o l d - g r o w t h s tand was e s t i m a t e d to be about 24-34 m (34 m in 300-1, 31 m i n 300-2 and 24 m i n 300-3 ( i . e . 3rd p l o t in o l d - g r o w t h ) ) . U s i n g s i t e index equat ions p u b l i s h e d in the F o r e s t r y Handbook for B r i t i s h Columbia ( F o r e s t r y Undergraduate S o c i e t y 1983), s i t e index at 100 years for western hemlock was e s t i m a t e d to be 23-33 m. Fewer l a r g e i n d i v i d u a l s were a v a i l a b l e for h e i g h t measurements in 300-3, as the s tand appeared more broken up. O r l o c i ' s (1965) e s t i m a t e s of s i t e index at 100 y r s for the mesic s i t e in the CWHb for the montane and submontane v a r i a n t s are western hemlock- 32 m, wes-t e r n redcedar (Thuja p i i c a t a Donn.) - 30 m, D o u g l a s - f i r - 38 m and a m a b i l i s f i r - 29 m. In c o n t r a s t , K l i n k a (1976) e s t i m a t e d s i t e index at 100 y r s for submesic to mesic s i t e s ( R h y t i d i a d e l - phus l o r e u s - V a c c i n i u m a laskaense - Tsuqa h e t e r o p h y l l a type) for western hemlock and D o u g l a s - f i r to be 23 m as compared to 21 m for a m a b i l i s f i r in the montane v a r i a n t of the CWHb in the U . B . C . Research F o r e s t . T a b l e 2.4 summarizes the v a r i a b i l i t y in o v e r s t o r y t r e e spe-c i e s c o m p o s i t i o n p r i o r to c l e a r c u t t i n g in the F l e e t R i v e r s tands based on s tand t y p i n g a c c o r d i n g to P a c i f i c F o r e s t P r o d u c t s L t d . ( p e r s . comm.). The t y p i n g may not a c c u r a t e l y r e f l e c t the o r i g i n a l c o m p o s i t i o n in the c l e a r c u t sample p l o t s due to the l a r g e s i z e of the typed b l o c k s of t i m b e r , as compared to the sample p l o t s ; i t merely p r o v i d e s a q u a l i t a t i v e i n d i c a t i o n of the p r o b a b l e s p e c i e s c o m p o s i t i o n p r i o r to c l e a r c u t t i n g . One of the three o l d - g r o w t h b l o c k s of t imber examined had a lower board foot volume ( i . e . 18 v s . 27-33 M ) based on maps p r o v i d e d by P a c i f i c F o r e s t P r o d u c t s L t d . ( p e r s . comm.). The o l d -17 T a b l e 2.4 S p e c i e s c o m p o s i t i o n of the o v e r s t o r y p r i o r to c l e a r c u t t i n g and e s t i m a t e s of i n d i v i d u a l s p e c i e s above-ground biomass i n the o l d growth s tands i n v e s t i g a t e d . * '** *** **** S i t e / Type Volume (M) Biomass ( t . h a - 1 ) C l e a r c u t 1000 T o t a l AA TH TP b o a r d f e e t o l d - g r o w t h 1 H - ( B ) / 3 27 514 45 386 83 o l d - g r o w t h 2 H - ( B ) / 3 27 641 143 1 45 353 o l d - g r o w t h 3 B - ( H ) / 2 18 355 64 191 100 3 - y r - o l d B - C - ( H ) / 3 30 6 - y r - o l d B - ( C ) / 3 33 1 0 - y r - o l d B - C - H / 3 30 * - r e f e r s to c l e a r c u t age in 1981. * * -Type r e f e r s to the name g i v e n by P a c i f i c F o r e s t Products to b l o c k s of t imber w i t h i n which the sample p l o t s f e l l ( i . e . a c t u a l volumes and s p e c i e s c o m p o s i t i o n may d i f f e r due to w i t h i n - b l o c k v a r i a t i o n ) p r i o r to c l e a r c u t t i n g . H - western hemlock, B - a m a b i l i s f i r , C - western red c e d a r , and the number r e f e r s to the volume c l a s s . * * * - b o a r d f o o t volumes (M) taken from maps p r o v i d e d by P a c i f i c F o r e s t P r o d u c t s for the b lock of t imber in quest i o n . * * * * _ biomass e s t i m a t e d u s i n g r e g r e s s i o n s of G h o l t z et a l . (1979) . 1 8 growth p l o t l o c a t e d w i t h i n t h i s b l o c k had a lower biomass when e s t imated u s i n g r e g r e s s i o n e q u a t i o n s of G h o l t z et a l . (1979) . Ken Hart ( p e r s . comm.), d i s t r i c t f o r e s t e r w i t h P a c i f i c F o r e s t P r o -3 d u c t s , e s t imated volumes to be about 550 to 700 m for these same b l o c k s of t i m b e r . Hence, w i t h the e x c e p t i o n of t h i s one p l o t ( i . e . 300-3) , the o ther F l e e t R i v e r p l o t s appear to have had very s i m i l a r volumes p r i o r to c l e a r c u t t i n g . The s i t e s were s e l e c t e d based on the expected u n d e r s t o r y complement of s p e c i e s for the moss ( R h y t i d i a d e l p h u s l o r e u s (Hedw.) W a r n s t . ) - A l a s k a b l u e b e r r y (Vacc in ium a laskaense Howel l ) a s s o c i a t i o n c h a r a c t e r i s t i c of the m e s i c , mesotrophic s i t e s in the CWHb ( K l i n k a et a l . 1979). Other mosses ( R h y t i d i o p - s i s r o b u s t a (Hedw.) B r o t h , Hylocomium sp lendens (Hedw.) B . S . G . , P l a g i o t h e c ium undulatum (Hedw.) B . S . G . ) , and shrubs (Rubus  pedatus J . E . S m i t h , G a u l t h e r i a s h a l l o n P u r s h . ) were p r e s e n t , but in l e s s e r abundance and v i g o r than R^ l o r e u s and a l a s k a e n s e . A comprehensive a n a l y s i s of the u n d e r s t o r y v e g e t a t i o n u s i n g r e l e v e ' sampl ing might have been h e l p f u l i n c h a r a c t e r i z i n g more f u l l y the s i m i l a r i t y of the s i t e s . However, a s imple c h a r a c t e r i z a t i o n of the s i t e s based on quadrat sampl ing of the biomass of 6 c a t e -g o r i e s of u n d e r s t o r y v e g e t a t i o n p r o v i d e s a measure of the s i m i -l a r i t y of the s tands wi th r e s p e c t to the u n d e r s t o r y v e g e t a t i o n . These data are p r e s e n t e d i n the r e s u l t s s e c t i o n i n Chapter 4. A v i s u a l comparison of the v e g e t a t i o n and s o i l s of the s tands a l o n g the chronosequence i s p r e s e n t e d i n F i g u r e s 2.2 to 2 . 6 . 19 I Figure 2.2 Vegetation and s o i l of the old-growth stand. Mean depth of f o r e s t f l o o r was 24 cm. Photo of roadcut was taken about 100 m downslope from p l o t s i n the old-growth stand. 20 Figure 2.2 cont. Note presence of the mosses (Rhytidiadelphus  l o r e u s and Hylocomium splendens) i n the old-growth stand. 21 Figure 2.3 Vegetation and s o i l of the 3-yr-old c l e a r c u t . Mean depth of the f o r e s t f l o o r was 27 cm. Note absence of f i n e s l a s h which had become p a r t of the f o r e s t f l o o r . 22 Figure 2.4 Vegetation and s o i l of the 6-yr-old c l e a r c u t . Mean depth of f o r e s t f l o o r was 1 9 cm. Note the presence of Hypochaeris sp. i n photo of s o i l . However, fireweed was the dominant understory species present at t h i s stage. 23 Figure 2.5 Vegetation and s o i l of the 1 0 - y r - o l d c l e a r c u t . Mean depth of f o r e s t f l o o r was 16 cm. Note dominance of fireweed. 24 I Figure 2.6 Vegetation and s o i l of the 26-yr-old c l e a r c u t . Mean depth of f o r e s t f l o o r was 21 cm. Average height of dominant am a b i l i s f i r was 16 to 17 m. 25 CHAPTER 3 Changes i n F o r e s t F l o o r Mass , N C o n t e n t , Decompos i t i on , M i n e r a l i z a t i o n , N i t r o g e n A v a i l a b i l i t y and Regenera t ion Response A f t e r C l e a r c u t t i n g : E s t a b l i s h i n g a P a t t e r n 3.1 I n t r o d u c t i o n A l t h o u g h i t has been r e p o r t e d for s e v e r a l decades that there are changes i n decompos i t ion and m i n e r a l i z a t i o n of o r g a n i c matter a f t e r a d i s t u r b a n c e (Romell 1935), the i n f o r m a t i o n necessary to a c c u r a t e l y p r e d i c t the l o s s of f o r e s t f l o o r mass and N content a f t e r c l e a r c u t t i n g s t i l l does not e x i s t . T h e . t i m i n g and ex tent of f o r e s t f l o o r m i n e r a l i z a t i o n a f t e r a d i s t u r b a n c e p l a y an important r o l e in d e t e r m i n i n g the e s t a b l i s h -ment and e a r l y growth of n a t u r a l r e g e n e r a t i o n or p l a n t e d seed-l i n g s . F a i l u r e to e s t a b l i s h a new c r o p u n t i l a f t e r the p o s t -d i s t u r b a n c e p e r i o d of a c c e l e r a t e d m i n e r a l i z a t i o n has ended can r e s u l t i n growth "check" (Weetman 1980), or i n p l a n t a t i o n f a i l -ure due to c o m p e t i t i o n from brush s p e c i e s t h a t e s t a b l i s h e d d u r i n g t h i s p e r i o d of in t ense m i n e r a l i z a t i o n and i n c r e a s e d n u t r i e n t a v a i l a b i l i t y . Be ing a b l e to c o n t r o l the magnitude and d u r a t i o n of the "assar t e f f e c t " i s not o n l y important for prompt r e f o r e s t a -t i o n , but a l s o to minimize l e a c h i n g l o s s e s from the s i t e ( V i t o u -sek et a l . 1979; V i t o u s e k 1981). In s p i t e of these c o n s i d e r a -26 t i o n s , o n l y over the past decade has there been a major focus on the m i c r o b i o l o g i c a l p r o c e s s e s c o n t r o l l i n g n i t r o g e n a v a i l a b i l i t y a f t e r c l e a r c u t t i n g (Sundman and Niemela 1978; Baath 1980; C l a r -holm et a l . 1981). The o b j e c t i v e s of t h i s c h a p t e r are to q u a n t i f y the magnitude, d u r a t i o n and t i m i n g of the p o s t - ' c l e a r c u t changes i n f o r e s t f l o o r mass and N c o n t e n t , d e c o m p o s i t i o n , m i n e r a l i z a t i o n and N a v a i l -a b i l i t y u s i n g a v a r i e t y of methods, i n c l u d i n g a m a b i l i s f i r f o l i a r N l e v e l s and he ight growth . 3.2 L i t e r a t u r e Review The o b j e c t i v e of t h i s s e c t i o n i s to b r i e f l y review some of the s t u d i e s tha t have i n v e s t i g a t e d d e c o m p o s i t i o n , m i n e r a l i z a t i o n , and s o i l N a v a i l a b i l i t y i n f o r e s t ecosystems, e s p e c i a l l y w i th regard to changes tha t occur a f t e r c l e a r c u t t i n g . N i t r i f i c a t i o n and c o n t r o l l i n g f a c t o r s w i l l be d i s c u s s e d wi th r e f e r e n c e to l e a c h i n g l o s s e s in Chapter 5. 3 .2.1 Decompos i t ion 3 . 2 . 1 . 1 The d i f f i c u l t y of measur ing decompos i t i on i n mor humus The e x i s t e n c e of d i f f e r e n t types ( f u n g i v e r s u s b a c t e r -i a /macro faun a) of d e c o m p o s i t i o n ( r a t h e r than d i f f e r e n c e s in r a t e s of d e c o m p o s i t i o n ) was f i r s t suggested about f i f t y years ago (Romell 1935). Romel l (1935) expanded and s y n t h e s i z e d h i s own ideas from e a r l i e r work by M u l l e r (1903,1910) d e a l i n g wi th the f o u n d a t i o n s of a b i o l o g i c a l t h e o r y of mul l and mor f o r m a t i o n , and 27 i t s importance i n u n d e r s t a n d i n g decompos i t ion and a c t i v a t i o n e f f e c t s r e s u l t i n g from d i s t u r b i n g a mor as opposed to a m u l l . Romel l (1935) emphasized that sampl ing a mor f o r e s t f l o o r r e p r e s e n t s a d r a s t i c a l t e r a t i o n of the p h y s i c a l i n t e g r i t y of the more-or l e s s permanent a r c h i t e c t u r e of r o o t s , m y c o r r h i z a l hyphae, and o r g a n i c matter i n v a r i o u s s tages of d e c o m p o s i t i o n . I n c u b a t i n g mor f o r e s t f l o o r m a t e r i a l , i n c l u d i n g many broken r o o t s , hyphae, e t c . , which r e p r e s e n t s a 'green manure' promotes m i c r o b i a l break-down. Sampling a mor f o r e s t f l o o r in an u n d i s t u r b e d s tand p r o b a b l y r e p r e s e n t s a much g r e a t e r d i s t u r b a n c e than sampl ing the same f o r e s t f l o o r a f t e r c l e a r c u t t i n g , where the i n f l u e n c e of r o o t s / m y c o r r h i z a e on s t r u c t u r e would be g r e a t l y weakened. T h i s suggests that compar i sons of r e l a t i v e r a t e s of decompos i t i on of f o r e s t f l o o r or m i n e r a l s o i l o r g a n i c matter in uncut and c l e a r c u t s tands may be b i a s e d i f they are based on a d i s r u p t i v e sampl ing techn i q u e . 3 . 2 . 1 . 2 Changes i n d e c o m p o s i t i o n w i t h s tand age Stand age i n f l u e n c e s the decompos i t i on r a t e , both i n terms of i t s e f f e c t on m i c r o c l i m a t e and in terms of the q u a l i t y of mater-i a l s a v a i l a b l e f o r d e c o m p o s i t i o n . The r e l a t i v e importance of each f a c t o r w i l l depend on the r e g i o n a l c l i m a t e or env ironmenta l m o d i f i c a t i o n t h a t the o v e r s t o r y produces , and the amount of management or m a n i p u l a t i o n tha t the s tand i n c u r s d u r i n g i t s development . Hea l (1979) o u t l i n e d a p a t t e r n of decompos i t i on for even-aged c o n i f e r p l a n t a t i o n s based on the observed amounts of 28 o r g a n i c matter and n u t r i e n t s accumulated over t i m e , and the response to p a r t i c u l a r management p r a c t i c e s : 1. an i n i t i a l p u l s e of decompos i t i on and n u t r i e n t r e l e a s e as a r e s u l t of death of ground v e g e t a t i o n and the d i s t u r -bance c r e a t e d by s i t e p r e p a r a t i o n and y a r d i n g . 2. a g r a d u a l d e c l i n e in these parameters as i n p u t s of ground v e g e t a t i o n d e c l i n e , as c o n i f e r l i t t e r i n c r e a s e s , and as s o i l temperature v a r i a t i o n and moi s ture content d e c l i n e as the canopy c l o s e s . 3. at t h i n n i n g or j u v e n i l e s p a c i n g , a p u l s e of decomposi-t i o n and n u t r i e n t r e l e a s e as a r e s u l t of the input of green n e e d l e s , green stemwood and f i n e r o o t s , and a s s o c i a t e d with i n c r e a s e s i n temperature v a r i a t i o n and m o i s t u r e . 4. a f u r t h e r d e c l i n e in decompos i t i on wi th s tand age as the p r o p o r t i o n of woody m a t e r i a l in the f o r e s t f l o o r cont inues to i n c r e a s e . 5. a f i n a l pu l se of d e c o m p o s i t i o n at h a r v e s t ; g r e a t e r than that a f t e r t h i n n i n g due to the l a r g e i n p u t s of f r e s h l i t -t e r . The q u a l i t y of the l i t t e r w i l l depend on the age of the s t a n d , s i n c e u t i l i z a t i o n l e v e l s and the abundance of d e c a y i n g logs ( i . e . m o r t a l i t y ) w i l l be a f f e c t e d by stand age or t ime s i n c e the l a s t d i s t u r b a n c e . Hea l (1979) noted that t h e r e has been l i t t l e a c t u a l documentat ion of the d e t a i l s of t h i s p a t t e r n . Due to the i n h e r e n t problems a s s o c i a t e d w i t h measuring de-c o m p o s i t i o n r a t e s i n mor humus, i n v e s t i g a t o r s have sometimes i n f e r r e d changes i n d e c o m p o s i t i o n based on d i f f e r e n c e s i n accumu-l a t i o n s of o r g a n i c matter i n s tands of d i f f e r e n t ages . The p r o b -29 lems w i t h t h i s method are that i n p u t s in terms of l i t t e r f a l l a l s o change wi th s tand age, and v a r i a b i l i t y between s i t e s o f t en mask t r e n d s w i t h s tand age . For example, Cov ing ton (1981) observed a d e c l i n e in f o r e s t -1 f l o o r o r g a n i c matter of 30.7 t . h a (55%) d u r i n g the f i r s t f i f t e e n y e a r s a f t e r c l e a r c u t t i n g f o l l o w e d by a s teady accumula-t i o n to w i t h i n 5% of an asymptot i c v a l u e by 64 y e a r s in a chrono-sequence of n o r t h e a s t e r n hardwood s t a n d s . A 3 year es t imate of p o s t - c l e a r c u t t i n g o r g a n i c matter d e c l i n e based on an e m p i r i c a l l y d e r i v e d r e g r e s s i o n e q u a t i o n was ex tremely c l o s e to D o m i n s k i ' s -1 (1971) o b s e r v a t i o n of o r g a n i c matter l o s s of 10.8 t . h a on a cut and h e r b i c i d e d watershed at Hubbard Brook . Page (1974) has observed between 20-70% d e c l i n e s i n f o r e s t f l o o r depth a f t e r c l e a r c u t t i n g on a v a r i e t y of s o i l s (eg . podzol-s, b r u n i s o l s and g l e y s o l s ) dominated by P i c e a mariana ( M i l l . ) B . S . P . (b lack spruce) a n d / or A b i e s balsamea L . (balsam f i r ) ; the d e c l i n e s were g e n e r a l l y g r e a t e r under b l a c k spruce than under balsam f i r s t a n d s , and on p o d z o l s than on e i t h e r b r u n i s o l s or g l e y s o l s . 3 . 2 . 1 . 3 F a c t o r s c o n t r o l l i n g changes i n decompos i t i on C l e a r c u t t i n g r e s u l t s in a "green manuring" e f f e c t (Tamm 1950), which may be enhanced on s i t e s w i th a c i d i c mor f o r e s t f l o o r s , i f s u f f i c i e n t s l a s h i s l e f t on the s i t e to smother mor v e g e t a t i o n ( i . e . e r i c a c e o u s genera such as C a l l u n a and V a c c i n i u m , e t c . ) . I t has been suggested (Romel l 1935; Tamm 1950) that the removal of t h i s v e g e t a t i o n r e l e a s e s the mor f o r e s t f l o o r from the 30 i n h i b i t i n g i n f l u e n c e of the m y c o r r h i z a e a s s o c i a t e d w i t h the root systems. T h i s phenomena has been demonstrated i n r a d i a t a p ine l i t t e r d e c o m p o s i t i o n exper iments ( G a d g i l and G a d g i l 1975,1978) where r a t e s were compared in the presence and absence of r a d i a t a p ine s e e d l i n g s under c o n t r o l l e d e n v i r o n m e n t a l c o n d i t i o n s . The amount of woody s l a s h m a t e r i a l s l e f t on the s i t e a f t e r c l e a r c u t -t i n g may i n f l u e n c e the degree to which any r e s i d u a l v e g e t a t i o n ( i . e . e r i c a c e o u s or n a t u r a l r e g e n e r a t i o n ) can i n h i b i t the r e l e a s e of N, and hence c o n t r o l the magnitude and t i m i n g of changes in decompos i t i on r a t e s and a v a i l a b i l i t y of n i t r o g e n . Only r e c e n t l y has the e f f e c t of d i f f e r e n t l e v e l s of s i t e treatment and u t i l i z a -t i o n l e v e l on decompos i t i on and n u t r i e n t a v a i l a b i l i t y a f t e r c l e a r c u t t i n g been examined (Vi tousek and Matson 1985). U t i l i z a -t i o n l e v e l seemed to have much l e s s of an impact than the e f f e c t c l e a r c u t t i n g had on N m i n e r a l i z a t i o n r a t e s in some h a r v e s t e d l o b l o l l y p i n e s t a n d s . S l a s h i n p u t s a l s o e f f e c t m i n e r a l i z a t i o n / i m m o b i l i z a t i o n p a t -t e r n s . I m m o b i l i z a t i o n p e r i o d s tend to vary h i g h l y wi th the " q u a l i t y " of the added m a t e r i a l s (Aber et a l . 1983), wi th s h o r t p e r i o d s of i m m o b i l i z a t i o n be ing observed for f i n e r o o t s , and much longer p e r i o d s for l a r g e r o o t s and stumps. T h i s w i l l be d i s c u s s e d f u r t h e r i n Chapter 4. Numerous s t u d i e s ( S u c h t i n g and Chr i s tmann 1935; M o l l e r 1954; Hart 1961; Witkamp 1971; Dominski 1971; Marks and Bormann 1972) have sugges ted that removal of the o v e r s t o r y l e a d s to h i g h e r decompos i t i on r a t e s as a r e s u l t of h i g h e r t e m p e r a t u r e s . Fewer s t u d i e s have a c t u a l l y documented an i n c r e a s e i n temperature a f t e r c l e a r c u t t i n g (Timmer and Weetman 1969; G l a v a c and Koenies 1978a) 31 or p a r t i a l c u t t i n g ( spac ing) (Piene 1978). G l a v a c and Koenies (1978a) noted h i g h e r maximum (8-10 deg C) and lower minimum (2-4 deg C) s o i l t emperatures i n a c l e a r c u t as compared to an uncut Norway spruce (P icea a b i e s L . ) s t a n d . Piene (1978) observed a c u r v i l i n e a r r e l a t i o n s h i p between temperature and h i g h e r decompo-s i t i o n r a t e s of balsam f i r need les in spaced s t a n d s . S i m i l a r l y , i t has been sugges ted that more f a v o u r a b l e moi s -t u r e c o n d i t i o n s occur a f t e r c l e a r c u t t i n g (Heal 1979), but most s t u d i e s in which needles have been i n c u b a t e d in s tands of d i f f e r -ent age have f a i l e d to observe more f a v o u r a b l e m o i s t u r e c o n d i -t i o n s and h i g h e r decompos i t i on r a t e s in young c l e a r c u t s . Decom-p o s i t i o n r a t e s of needles of a m a b i l i s f i r (Edmonds 1979), Norway spruce (Berg and Staaf 1980) and D o u g l a s - f i r ( F e l l e r et a l . 1983) were found to be g r e a t e s t under c l o s e d c a n o p i e s . . T h i s i s supported by W i l l et a l . ' s (1983) s tudy which i n d i c a t e d that decompos i t i on r a t e s of r a d i a t a p i n e l i t t e r f a l l i n u n b u r i e d l i t t e r b a g s was s l i g h t l y g r e a t e r (not s i g n i f i c a n t ) i n uncut , as compared to recen t c l e a r c u t s . S t u d i e s in Norway spruce s tands (Glavac and Koenies 1978a) have p r o v i d e d ev idence t h a t m o i s t u r e content of the L l a y e r w i l l d e c l i n e a f t e r c l e a r c u t t i n g , w h i l e the moi s ture content of the FH and Ah l a y e r s w i l l i n c r e a s e a f t e r c l e a r c u t t i n g . I n c u b a t i o n s t u d i e s have o f t e n n e g l e c t e d v a r i a t i o n in m o i s t u r e content between s o i l l a y e r s and r e s u l t a n t e f f e c t s on decompos i t i on r a t e s . The change i n the l i g n i n and element content of l i t t e r i n young c l e a r c u t s compared to tha t of mature s tands sh ou ld promote more r a p i d t u r n o v e r i n the former ( W i t t i c h 1930). U s i n g l i t t e r -32 bags , mass l o s s by decompos i t ion of E p i l o b i u m a n g u s t i f o l i u m l eaves was e s t i m a t e d to be 60% of t h e i r i n i t i a l mass w i t h i n 12 months ( F e l l e r et a l . 1 9 8 3 ) . In c o n t r a s t , Edmonds (1984) observed t h a t i t took 6 y e a r s for a m a b i l i s f i r needle l i t t e r to l o se 64.3% of i t s mass, and that net m i n e r a l i z a t i o n d i d not occur u n t i l a f t e r 4 years of d e c o m p o s i t i o n . Higher n u t r i e n t c o n c e n t r a -t i o n s in some p i o n e e r spec ie s ' such as a n g u s t i f o l i u m probab ly promotes h i g h e r decompos i t ion and m i n e r a l i z a t i o n r a t e s in the f o r e s t f l o o r in the e a r l y years a f t e r c l e a r c u t t i n g . Data which support the above i n c l u d e that of V i t o u s e k and Matson (1985). L a b o r a t o r y i n c u b a t i o n s of s o i l taken from young c l e a r c u t s and u n d i s t u r b e d s i t e s at cons tant temperature and moi s ture i n d i c a t e d 2 t imes g r e a t e r m i n e r a l i z a t i o n of n i t r o g e n in the young c l e a r c u t s •examined. T h i s suggests that s u b s t r a t e q u a l i t y ( i . e . u n d e r s t o r y ) a n d / o r m i c r o b i o l o g i c a l d i f f e r e n c e s may i n f l u e n c e the p a t t e r n of f o r e s t f l o o r m i n e r a l i z a t i o n , as much as temperature and mois ture f a c t o r s . An a d d i t i o n a l f a c t o r a s s o c i a t e d w i t h types of l i t t e r f a l l i s the i n f l u e n c e that n u t r i e n t s , c a r b o h y d r a t e s or o r g a n i c compounds (such as p o l y p h e n o l s ) in t h r o u g h f a l l or canopy wash have on decompos i t i on r a t e s in some ecosys tems . Seasonal p a t t e r n s of decompos i t i on 'have been r e l a t e d to c a r b o h y d r a t e and n u t r i e n t l e v e l s in t h r o u g h f a l l (Gosz 1984). The presence of both p o t e n t i a l i n h i b i t o r s ( i . e . water s o l u b l e o r g a n i c s (Rice 1974; Heng and Goh 1984) and s t i m u l a t o r s ( i . e . n u t r i e n t s (Ro l f e et a l . 1978) and c a r b o h y d r a t e s (de Boo i s and Jansen 1976)) i n t h r o u g h f a l l make g e n e r a l i n t e r p r e t a t i o n s about the e f f e c t of c l e a r c u t t i n g and a s s o c i a t e d changes i n t h r o u g h f a l l d i f f i c u l t . F u r t h e r study i s 33 needed to q u a n t i f y i f , and which ones , are i m p o r t a n t i n the changes in s o i l m i c r o f l o r a and N a v a i l a b i l i t y a f t e r c l e a r c u t t i n g . Changes i n the type of decompos i t i on have been, i n f e r r e d from changes in the abundance of s o i l m i c r o f l o r a a f t e r c l e a r c u t t i n g i n S c a n d i n a v i a (Niemela and Sundmannn 1977; Sundman and N-iemela 1978; Baath 1980). Niemela and Sundman (1977) o b s e r v e d the most marked changes in f r e q u e n c i e s of b a c t e r i a at about 7 y e a r s a f t e r c l e a r c u t t i n g wi th p o p u l a t i o n s r e t u r n i n g to c o n t r o l ( i . e . mature f o r e s t ) l e v e l s a f t e r about 13 y e a r s in n o r t h e r n spruce f o r e s t s o i l s i n F i n l a n d . M i n e r a l s o i l b a c t e r i a p o p u l a t i o n s seemed to undergo g r e a t e r changes than f o r e s t f l o o r p o p u l a t i o n s in the s tands s t u d i e d (Niemela and Sundman 1977). A s i m i l a r p a t t e r n was observed for the t o t a l biomass of i n v e r t e b r a t e f a u n a , w i t h a maximum at 7 years and r e t u r n i n g to c o n t r o l l e v e l s w i t h i n 13 y e a r s a f t e r c l e a r c u t t i n g . In c o n t r a s t , Baath (1980) showed tha t f l u o r e s c e i n d i a c e t a t e a c t i v e (FDA) and t o t a l funga l biomass decreased throughout the s o i l p r o f i l e a f t e r c l e a r c u t t i n g i n Swedish f o r e s t s . The decrease in f u n g a l biomass was not r e l a t e d to amounts of f e l l i n g r e s i d u e s l e f t on the s i t e . M i n e r a l s o i l m o i s t u r e content d i d not change a p p r e c i a b l y a f t e r c l e a r c u t t i n g , a l t h o u g h f u n g a l biomass decreased throughout the s o i l p r o f i l e , i m p l y i n g tha t m o i s t u r e c o n t e n t was not the reason for the d e c l i n e i n f u n g a l b iomass . The most ob-v i o u s reason for the d e c l i n e in f u n g a l biomass i s the r e d u c t i o n i n root biomass , and root exudates which p r o v i d e among other t h i n g s the energy source for m y c o r r h i z a l fung i and some non-m y c o r r h i z a l fungi i n the r h i z o s p h e r e ( G a d g i l and G a d g i l 1978; 34 R i c h a r d s 1974). At the moment t h e r e i s no method to d i s t i n g u i s h between m y c o r r h i z a l and n o n - m y c o r r h i z a l f u n g i , but the two groups may d i f f e r a p p r e c i a b l y in t h e i r r e a c t i o n to c l e a r c u t t i n g . The i n f l u e n c e that the presence of m y c o r r h i z a e has on decom-p o s i t i o n of o r g a n i c matter was f i r s t s t u d i e d by G a d g i l and G a d g i l (1975). More r e c e n t l y , G a d g i l and G a d g i l (1978) e v a l u a t e d the i n f l u e n c e t h a t the presence of m y c o r r h i z a l r o o t s has on decom-p o s i t i o n of P i n u s r a d i a t a l i t t e r , by comparing mass l o s s of shaded l i t t e r in a c l e a r f e l l e d area and i n a mature s t a n d . Both f i e l d and l a b o r a t o r y exper iments i n d i c a t e d t h a t the presence of m y c o r r h i z a l r o o t s i n h i b i t e d the decompos i t i on of o r g a n i c m a t t e r , which r e s u l t e d in g r e a t e r a c c u m u l a t i o n of f o r e s t f l o o r o r g a n i c matter i n the presence of m y c o r r h i z a l r o o t s . A l t h o u g h the death of m y c o r r h i z a l r o o t s does not n e c e s s a r i l y remove the f u n g i , the v i g o u r and ex tent of hypha l growth may be such ' t h a t the i n h i b i t o r y mechanism i s weakened. Hence, the removal of the m y c o r r h i z a l r o o t s a f t e r c l e a r c u t t i n g shou ld r e s u l t in an i n c r e a s e in d e c o m p o s i t i o n r a t e s and , i n many c a s e s , n i t r o g e n a v a i l a b i l i t y . Appendix 2a p r o v i d e s a more d e t a i l e d account of s t u d i e s that have i n v e s t i g a t e d the e f f e c t s of d i f f e r e n t f a c t o r s on decomposi -t i o n . 3 .2 .2 M i n e r a l i z a t i o n and i m m o b i l i z a t i o n The oppos ing f o r c e s of m i n e r a l i z a t i o n ( c o n v e r s i o n of o r g a n i c N to i n o r g a n i c ammonium ions ) and i m m o b i l i z a t i o n r e s u l t i n what i s commonly r e f e r r e d to as net m i n e r a l i z a t i o n , or net i m m o b i l i z a -t i o n depending on the d i r e c t i o n of changes i n m i n e r a l n i t r o g e n l e v e l s i n the s o i l . I m m o b i l i z a t i o n r e s u l t s i n the i n c o r p o r a t i o n 35 of m i n e r a l n i t r o g e n i n t o m i c r o b i a l t i s s u e s , most of which i s l a t e r m i n e r a l i z e d , but some i s c o n v e r t e d to s t a b l e humus forms (Stevenson 1982). I m m o b i l i z a t i o n has a l s o been used in r e f e r e n c e to v e g e t a t i o n (V i tousek and Matson u n p u b l . ) . The dynamic na ture of i n o r g a n i c l e v e l s of ammonium and n i t r a t e in the s o i l has l e d many i n v e s t i g a t o r s to use i n c u b a t i o n s r a t h e r than d i r e c t e x t r a c -t i o n s to e v a l u a t e r a t e s of m i n e r a l i z a t i o n (Tamm 1964). Appendix 2b o u t l i n e s v a r i o u s f a c t o r s which have been i n v e s t i g a t e d in o r d e r to b e t t e r unders tand m i n e r a l i z a t i o n r a t e s . Heal et a l . (1982) have p o i n t e d out 3 f e a t u r e s which are important i n d i f f e r e n t i a t i n g n i t r o g e n dynamics in a g r i c u l t u r a l v e r s u s f o r e s t s o i l s : 1. fung i possess a wide range of b i o l o g i c a l and b i o c h e m i c a l mechanisms whereby they compete f o r n i t r o g e n . They vary both i n d i v e r s i t y and a b i l i t y to u t i l i z e n i t r o g e n and f u n c t i o n w i t h a wide range of c e l l u l a r N l e v e l s . The n i t r o g e n content of m i c r o b i a l t i s s u e s v a r i e s w i d e l y but w i l l be of the o r d e r of 6-13% f o r b a c t e r i a and 3-6% for fung i (Stevenson 1982). Turnover of e x t r a c e l l u l a r p o o l s of N can occur when C:N r a t i o s are h i g h , 2. s a p r o p h y t i c and m y c o r r h i z a l fung i bypass the p r o c e s s of a m m o n i f i c a t i o n by a b s o r b i n g s o l u b l e o r g a n i c N (Bowen and Smith 1981), 3. humus format ion a c t u a l l y r e p r e s e n t s an i m m o b i l i z a t i o n of n i t r o g e n (Handley 1954). A l t h o u g h these ideas have been proposed b e f o r e , the second p o i n t i s r e l a t i v e l y u n s u b s t a n t i a t e d , and needs f u r t h e r i n v e s t i g a -t i o n . The o n l y ev idence i n the f i e l d tha t t r e e s can u t i l i z e s o l u b l e o r g a n i c N as a major source of n i t r o g e n was tha t of van C l e v e and White (1980) who observed that the t u r n o v e r of s o l u b l e 36 o r g a n i c - N was the major pathway i n the f o r e s t f l o o r of b i r c h s tands in A l a s k a . A d d i t i o n a l ev idence for mycorrh izae b y p a s s i n g a m m o n i f i c a t i o n i s based on the e x i s t e n c e of c e l l u l o l y t i c enzymes i n m y c o r r h i z a e ( A . E . L i n k i n s p e r s . comm. in Aber et a l . 1983). Wi th r e f e r e n c e to p o i n t 1, Berg and Ekbohm (1983), have observed net m i n e r a l i z a t i o n at C:N r a t i o s of 66 and 109 i n c l e a r c u t s , and they suggest tha t there are no f i x e d C:N q u o t i e n t s for i m m o b i l i -z a t i o n and r e l e a s e of n i t r o g e n . Youngberg (1978) and van C l e v e ( i n Heal et a l . 1982) have both observed n e g a t i v e c o r r e l a t i o n s between C:N r a t i o s and N uptake in p o t t e d s e e d l i n g s . Given the m i n e r a l i z a t i o n / i m m o b i l i z a t i o n c y c l e that r e s u l t s from f l u c t u a t i o n s in decomposer p o p u l a t i o n s , i t has been sug-ges ted that i t i s the frequency in f l u c t u a t i o n s i n these f a c t o r s which i n f l u e n c e the q u a n t i t i e s of m i n e r a l i z e d n u t r i e n t s (Piene 1978). The i n c r e a s e in m i n e r a l i z a t i o n r a t e s a f t e r r e w e t t i n g a i r -d r i e d s o i l s ( A r s j a d and Giddens 1966) suggests that microorgan i sm p o p u l a t i o n s are very r e s i l i e n t , and tha t f l u c t u a t i o n s in c o n d i -t i o n s are l i k e l y to l e a d to g r e a t e r m i n e r a l i z a t i o n . An i n c r e a s e i n m i c r o b i a l biomass a long wi th m i n e r a l i z e d - N at the end of 6 week s o i l i n c u b a t i o n s ( C l a r h o l m et a l . 1981; V i t o u s e k and Matson u n p u b l i s h e d ) , suggests t h a t i n c u b a t i o n s of o r g a n i c - N i n which c o n d i t i o n s are h e l d cons tant may underes t imate a c t u a l r a t e s of m i n e r a l i z a t i o n in the f i e l d . On the o ther hand, due to the r e l a -t i v e l y h i g h temperatures used i n a s s e s s i n g m i n e r a l i z a t i o n r a t e s i n the l a b o r a t o r y , as compared to those observed i n the f i e l d , Powers (1982) b e l i e v e s t h a t l a b o r a t o r y e s t i m a t e s of in. s i t u m i n e r a l i z a t i o n are p r o b a b l y h i g h . 37 For the same reasons as mentioned e a r l i e r for d e c o m p o s i t i o n , chronosequences have been used to e s t imate the d e c l i n e in f o r e s t f l o o r N a f t e r c l e a r c u t t i n g . F o r e s t f l o o r N c a p i t a l may decrease a f t e r c l e a r c u t t i n g even though m i n e r a l i z a t i o n may remain the same, due to the absence of l i t t e r f a l l N i n p u t s . However, i f the d e c l i n e in f o r e s t f l o o r N c a p i t a l i s l a r g e r e l a t i v e to annual i n p u t s of l i t t e r f a l l N , then the d i f f e r e n c e between the cut and uncut f o r e s t can be main ly a t t r i b u t e d to g r e a t e r m i n e r a l i z a t i o n a f t e r c l e a r c u t t i n g . Few s t u d i e s have q u a n t i f i e d the change in f o r e s t f l o o r N -1 c a p i t a l ( k g . N . h a ) a f t e r c l e a r c u t t i n g . C o v i n g t o n (1981) has -1 e s t i m a t e d a d e c l i n e in f o r e s t f l o o r N c a p i t a l of 808 k g . N . h a (55%) d u r i n g the f i r s t f i f t e e n y e a r s a f t e r c l e a r c u t t i n g in n o r t h e a s t e r n hardwoods in USA. For s i m i l a r s i t e s , Dominski (1971) -1 r e c o r d e d a d e c l i n e of about 170 k g . N . h a d u r i n g the f i r s t three y e a r s a f t e r c l e a r c u t t i n g and h e r b i c i d i n g . In Norway spruce s tands i n Sweden, N y k v i s t (1974) observed a d e c l i n e in f o r e s t f l o o r N of -1 188 d u r i n g the f i r s t year and 560 k g . N . h a d u r i n g the f i r s t 4 y e a r s a f t e r c l e a r c u t t i n g ; these r e p r e s e n t d e c l i n e s of 16 and 47% d u r i n g the same time p e r i o d s . Hence, a d e c l i n e of about 50% d u r i n g the f i r s t 5-15 y e a r s a f t e r c l e a r c u t t i n g seems p o s s i b l e on my s i t e s . The t i m i n g and the d u r a t i o n of the d e c l i n e seems to v a r y between g e o g r a p h i c a l a r e a s . T h i s might be r e l a t e d to amounts of s l a s h l e f t on the s i t e s a f t e r c l e a r c u t t i n g . The ex ten t to which m i n e r a l i z a t i o n r a t e s change a f t e r c l e a r c u t t i n g have been s t u d i e d u s i n g e i t h e r in s i t u m i n e r a l i z a t i o n of s o i l samples or s o i l m o n o l i t h s . Glavac and Koenies (1978a) have observed about a 278% i n c r e a s e i n N 38 m i n e r a l i z a t i o n d u r i n g a 5 month summer p e r i o d i n a c l e a r c u t v s . an uncut Norway spruce s tand u s i n g s o i l m o n l i t h s excavated from the uncut s t a n d . However, they d i d not observe an i n c r e a s e i n N m i n e r a l i z a t i o n i n beech (Fagus s y l v a t i c a L . ) f o r e s t s o i l u s i n g s i m i l a r methodology (Glavac and Koenies 1978b). There i s some e v i d e n c e that the type of f o r e s t f l o o r (eg . moder v s . raw humus) may i n f l u e n c e the magnitude of the i n c r e a s e i n N m i n e r a l i z a t i o n a f t e r c l e a r c u t t i n g . C l a u s n i t z e r ( i n E l l e n b e r g 1978) noted a p r o p o r t i o n a t e l y g r e a t e r i n c r e a s e i n N m i n e r a l i z a t i o n in a moder than in "sour raw humus" f o r e s t f l o o r s a f t e r c l e a r c u t t i n g s p r u c e ( - f i r ) s t a n d s . T h i s suggests tha t i t may be the F r a t h e r than the H l a y e r which undergoes g r e a t e s t m i n e r a l i z a t i o n a f t e r c l e a r c u t t i n g . Hence, the need for c l a s s i f i c a t i o n of s i t e s in terms of humus forms as w e l l as v e g e t a t i o n . V i t o u s e k and Matson ( u n p u b l . ) have a l s o documented an i n c r e a s e in in_ s i t u m i n e r a l i z a t i o n r a t e s in the upper m i n e r a l s o i l (0-15cm) a f t e r c l e a r c u t t i n g ; r a t e s i n c r e a s e d from 22-24 be fore stem--1 -1 o n l y h a r v e s t i n g to 94 i n the f i r s t and 65 k g . N . h a . y r in the second year a f t e r h a r v e s t i n g . Most of the m i n e r a l N was i n the ammonium form before and i n the n i t r a t e form a f t e r c l e a r c u t t i n g . A p p r o x i m a t e l y h a l f of the m i n e r a l i z e d N o c c u r r e d i n the f o r e s t f l o o r . Based on the above s t u d i e s , and g iven the n a t u r e of the s o i l s in the F l e e t R i v e r s i t e s , one might expect about a 2-4 t imes i n c r e a s e i n N m i n e r a l i z a t i o n a f t e r c l e a r c u t t i n g , w i t h much of m i n e r a l i z e d N o c c u r r i n g i n the f o r e s t f l o o r . 39 3 .2 .4 C o n c l u s i o n s As c l e a r c u t t i n g has the p o t e n t i a l to a l t e r so many f a c t o r s such as t emperature , m o i s t u r e , a v a i l a b l e s u b s t r a t e s , p l a n t up-t a k e , e t c . , i t i s ex tremely d i f f i c u l t to i s o l a t e the important f a c t o r s r e s p o n s i b l e for i n c r e a s e d decompos i t i on and m i n e r a l i z a -t i o n a f t e r c l e a r c u t t i n g . The e x c a v a t i o n and d i sp lacement of s o i l or s o i l m o n o l i t h s from an uncut s tand to a nearby c l e a r c u t has been used to p r e d i c t s h o r t - t e r m (6 mos) changes in N m i n e r a l i z a -t i o n r a t e s a f t e r c l e a r c u t t i n g (Glavac and Koenies 1978a,b) . T h i s method does not so lve the problem of d i s t u r b a n c e when sampl ing mor humus in an uncut " c o n t r o l " s t a n d , but does i n s u r e a g r e a t e r s i m i l a r i t y i n the f o r e s t f l o o r and m i n e r a l s o i l between the cut and uncut s t a n d s . An a d d i t i o n a l weakness of the method i s that i t does not c o n s i d e r d i f f e r e n c e s i n p o p u l a t i o n s of m i c r o f l o r a and fauna between cut and uncut s i t e s . A combinat ion of methods appears to be the best approach in u n d e r s t a n d i n g and p r e d i c t i n g changes i n n i t r o g e n a v a i l a b i l i t y (Tamm 1964) and was the approach taken in my s t u d y . To my knowledge no p u b l i s h e d s t u d i e s have used r e g e n e r a t i o n as an index or b ioas say of the a v a i l a b i l i t y of N a f t e r c l e a r c u t t i n g . Based on p u b l i s h e d s t u d i e s from other g e o g r a p h i c a l a r e a s , a p o s t - c l e a r c u t t i n g d e c l i n e of about 50% in f o r e s t f l o o r mass or N c a p i t a l i s e x p e c t e d . The t i m i n g and d u r a t i o n of the d e c l i n e seems to vary between the few l o n g - t e r m s t u d i e s that have been c o n d u c t e d . 40 3.3 Methods 3.3.1 F o r e s t f l o o r mass and n i t r o g e n c a p i t a l As was o u t l i n e d in Chapter 2, 15 s tands were i n t e n s i v e l y sampled (3 r e p l i c a t e s in each of 5 s tand age c l a s s e s ) for f o r e s t f l o o r n i t r o g e n content in the summer of 1981. L o c a t i o n and s i t e s e l e c t i o n were d i s c u s s e d in Chapter 2. The s i z e of each p l o t ranged from 25 x 25 m to a maximum of 50 x 50 m, the l a t t e r area be ing the d e s i r e d s i z e p r o v i d i n g that the d e s i g n a t e d area was homogeneous wi th r e s p e c t to u n d e r s t o r y v e g e t a t i o n and p h y s i c a l f a c t o r s such as s l o p e . A s y s t e m a t i c a l l y randomized d es i gn was used to l o c a t e 50 sampl ing p o i n t s w i t h i n the p l o t . F i v e p a r a l l e l t r a n s e c t s , u s u a l l y 50 m in l e n g t h , were e s t a b l i s h e d at 5-lOm spac ings p e r p e n d i c u l a r to the c o n t o u r , wi th the s t a r t i n g p o i n t of the f i r s t , t r a n s e c t be ing randomly d e t e r m i n e d . Each t r a n s e c t was then d i v i d e d i n t o f i v e 10 m l e n g t h s , w i t h i n each of which 2 newly generated random numbers were used to determined sampl ing p o i n t s , f or a t o t a l of 10 sampl ing p o i n t s per t r a n s e c t and 50 p o i n t s per p l o t . The c h o i c e of the number of sampl ing p o i n t s was based on e s t imates of w i t h i n - s t a n d v a r i a b i l i t y for f o r e s t f l o o r biomass and n i t r o g e n content by Quesne l and L a v k u l i c h (1980) . Arp (1984) and A r p and Krause (1984) p r o v i d e f u r t h e r a n a l y s i s of w i t h i n - s t a n d v a r i a b i l i t y for p e r c e n t N and n i t r o g e n content for f o r e s t f l o o r s . In order to ensure that f a l l e n logs ( w i n d f a l l s ) in the o l d growth were c o n s i d e r e d in the same ca tegory as f a l l e n l ogs and s l a s h i n the c l e a r c u t s , a d i s t i n c t i o n had to be made between the f o r e s t f l o o r and s l a s h c r e a t e d by l o g g i n g at each sampling p o i n t . 41 O t h e r w i s e , l o g g i n g s l a s h c o u l d have been i n c l u d e d in the f o r e s t f l o o r ca tegory i n the o l d e r c l e a r c u t s , l e a d i n g to an o v e r e s t i m a -t i o n of f o r e s t f l o o r mass and N c a p i t a l . The d i s t i n c t i o n between the two was based on the p lane of i n t e r s e c t i o n of the f a l l e n l og (or s l a s h ) and the f o r e s t f l o o r . I f the c e n t e r of g r a v i t y of the f a l l e n l o g was above t h i s p lane then i t was not c o n s i d e r e d p a r t of the f o r e s t f l o o r . Large i n t a c t l ogs were e a s i l y d i s c e r n i b l e as l o g g i n g s l a s h , whereas h i g h l y decomposed l o g s were n o t . I t was main ly the l a t t e r tha t needed a c o n s i s t e n t c r i t e r i o n as to whether to c o n s i d e r l ogs as f o r e s t f l o o r or s l a s h . For the l a c k of any way of d i s t i n g u i s h i n g between p r e - c l e a r c u t and p o s t -c l e a r c u t needle l i t t e r and o ther very f i n e l i t t e r m a t e r i a l , need les that had f a l l e n from s l a s h onto the f o r e s t f l o o r were measured as p a r t of the f o r e s t f l o o r . A l l m a t e r i a l s suspended above the f o r e s t f l o o r , i n c l u d i n g l o g s , b r a n c h e s , t w i g s , and need les were a t t r i b u t e d to to l o g g i n g s l a s h and were removed p r i o r to sampl ing the f o r e s t f l o o r . At each sampl ing p o i n t a 25 x 25 cm b l o c k of f o r e s t f l o o r was removed down to m i n e r a l s o i l and p l a c e d i n paper a n d / o r p l a s t i c bags for t r a n s p o r t to the l a b o r a t o r y . In n e a r l y a l l c a s e s , the f o r e s t f l o o r (LFH) was e a s i l y d i s t i n g u i s h a b l e from the m i n e r a l s o i l (Ae or B f ) . In those cases where the b e g i n n i n g of an Ah was p r e s e n t , on ly the o r g a n i c l a y e r s ( i . e . L , F and H) were removed. Depth measurements were taken at each of the 4 c o r n e r s of the sample p o i n t to be used l a t e r i n c a l c u l a t i n g bu lk d e n s i t y . Roots l a r g e r than 2.5 cm were l e f t in_ s i t u , but were measured for d iameter and l e n g t h to e s t imate t h e i r volume, which was sub-42 t r a c t e d from the f o r e s t f l o o r sample volume. In the l a b o r a t o r y , a l l r o o t s g r e a t e r than about 1 mm were removed p r i o r to measure-ment of f o r e s t f l o o r b iomass . Any m i n e r a l fragments >2mm were removed u s i n g a 2mm s i e v e . F o r e s t f l o o r samples were spread out on p l a s t i c to a i r dry for 3-7 days , p r i o r to b e i n g oven d r i e d at 70 deg C for 48 h o u r s . -1 Mass of f o r e s t f l o o r (kg .ha ) was c a l c u l a t e d by d i v i d i n g the 2 dry mass in grams by the area of the sample (0.0625 m ) and m u l -t i p l y i n g by 10. Bulk d e n s i t y was c a l c u l a t e d from the mass and sample volume data a f t e r c o r r e c t i o n for the volume of r o o t s and c o a r s e fragments . Mass of r o o t s and rocks were c o n v e r t e d to -3 volumes u s i n g d e n s i t i e s of 0.4 g.cm for r o o t s (Panshin and de -3 Zeeuw 1980) and 2.65g.cm for c o a r s e fragments ( H i l l e l 1980). Subsamples of f o r e s t f l o o r were ground to pass a 2 mm mesh s i z e in a W i l e y m i l l to ensure homogeniety of samples used for c h e m i c a l a n a l y s i s . The 50 subsamples per p l o t were bu lked a t random i n t o groups of 10 to reduce the t o t a l number of f o r e s t f l o o r samples f o r c h e m i c a l a n a l y s i s from 750 to 150. T h i s was based on v a r i a b i l i t y e s t i m a t e s of Quesnel and L a v k u l i c h (1980) for p e r c e n t N i n the f o r e s t f l o o r for mesic s i t e s on Vancouver I s l a n d . A R j e l d a h l d i g e s t i o n (Twine and W i l l i a m s 1971) u s i n g Se as a c a t a l y s t , and a b l o c k d i g e s t o r w i th g l a s s tubes and marbles (which ac t as r e f l u x condensers ) was used to o x i d i z e a l l o r g a n i c -N . A T e c h n i c o n a u t o a n a l y s e r was then employed to determine ammonium c o l o r i m e t r i c a l l y u s i n g the B e r t h e l o t r e a c t i o n and the f o r m a t i o n of the b lue indopheno l complex. P e r c e n t N was c a l c u -l a t e d as f o l l o w s : 43 Percent N=[((mg/L ammonium)x 0.05 L ) / mass of sample(mg)] x 100 x (AW n i t rogen) / (MW of ammonium) where 0.05 L was the volume of s o l u t i o n The average mass for the 10 bu lked samples was then m u l t i p l e d by percent N to o b t a i n 10 e s t i m a t e s of the t o t a l n i t r o g e n c a p i t a l i n the f o r e s t f l o o r . The average depth (cm) of the f o r e s t f l o o r i n each of the r e p l i c a t e s was r e g r e s s e d a g a i n s t the average -1 f o r e s t f l o o r n i t r o g e n c a p i t a l ( k g . N . h a ) in order to e s t imate n i t r o g e n c a p i t a l in the f o r e s t f l o o r in other s tands u s i n g depth measurements. Us ing average v a l u e s for both depth and n i t r o g e n c a p i t a l p r o b a b l y e l i m i n a t e s a l o t of the m i c r o s i t e v a r i a t i o n w i t h i n each s i t e that i s not d i r e c t l y a t t r i b u t a b l e to time s i n c e c l e a r c u t t i n g . Depth of the f o r e s t f l o o r was remeasured in 1983 i n a l l c l e a r c u t s sampled i n 1981 (1971, 1975, 1978 c l e a r c u t s ) . By u s i n g the second depth measurement in c o n j u n c t i o n wi th the depth and n i t r o g e n content r e g r e s s i o n , a measure was o b t a i n e d of changes in N content i n the above c l e a r c u t s over a two year p e r i o d . No resampl ing of e i t h e r percent N or bulk d e n s i t y was u n d e r t a k e n . The r e g r e s s i o n between depth and N content ( k g . N . -1 ha ) f or v a l u e s i n T a b l e 3.1 had an r - v a l u e of 0 .84 , which i s s i g n i f i c a n t at P=.01. 3 .3 .2 Rates of decompos i t ion 3 . 3 . 2 . 1 I_n s i t u m i n e r a l i z a t i o n of f o r e s t f l o o r . At each of f i v e randomly determined l o c a t i o n s i n each of the 15 s t a n d s , a sample of f r e s h FH l a y e r m a t e r i a l was removed, 44 f o r c e d to pass a 2 mm s i e v e , 10 g p l a c e d i n a 1 m i l p o l y e t h y l e n e 2 bag (Eno 1960) of a p p r o x i m a t e l y 200 cm s u r f a c e area and then r e p l a c e d i n the f o r e s t f l o o r to i n c u b a t e f o r 8 weeks. In a d d i -t i o n , a subsample of the s i e v e d FH m a t e r i a l from each l o c a t i o n was brought back to the l a b o r a t o r y to d e t e r m i n e b o t h : 1. i n i t i a l l e v e l s of KC1 e x t r a c t a b l e ammonium and n i t r a t e and 2. g r a v i m e t r i c mo i s ture content (per g dry mass) . A Weather Measure Model H311 hygrothermograph was p l a c e d i n s i d e a Stevenson screen i n the b lock of o l d - g r o w t h t imber (where a l l t h r e e o l d - g r o w t h s tands were l o c a t e d ) and in the 10 to 1 1 - y r - o l d c l e a r c u t to monitor a i r t emperature ; 3 max/min thermometers were p l a c e d i n the f o r e s t f l o o r (middle of the FH l a y e r ) in the same s tands to measure d e v i a t i o n s of f o r e s t f l o o r temperature from a i r t empera ture . In  s i t u m i n e r a l i z a t i o n r a t e s were i n v e s t i g a t e d d u r i n g the summer and f a l l of. 1982. A f t e r 8 weeks in the f i e l d , i n c u b a t e d samples were brought back to the l a b o r a t o r y and s t o r e d in a c o l d room at 4 deg C for up to two days p r i o r to e x t r a c t i o n by s h a k i n g f o r 1 hour w i t h 10 mL of 1 N KC1 per g of s o i l (Bremner 1965). Subsamples were t r e a t e d in a s i m i l a r manner to de termine i n i t i a l l e v e l s of ammonium and n i t r a t e . A f t e r e x t r a c t i o n , s o l u t i o n s were s t o r e d f r o z e n u n t i l a n a l y z e d u s i n g the T e c h n i c o n A u t o a n a l y z e r ; s torage d i d not exceed 3-4 weeks. Net m i n e r a l i z a t i o n (ppm) of n i t r o g e n per gram of f o r e s t f l o o r i n the time p e r i o d examined was c a l c u -l a t e d as the sum of the d i f f e r e n c e s between i n i t i a l and f i n a l c o n c e n t r a t i o n s of ammonium + n i t r a t e , r e s p e c t i v e l y . 4 5 3.3 .2 .2 Decompos i t ion of c e l l u l o s e One mm ny lon mesh bags c o n t a i n i n g 4.25 cm diameter d i s c s of Whatman No. 1 f i l t e r paper were i n s e r t e d i n t o the middle of the FH h o r i z o n at 5 randomly determined l o c a t i o n s w i t h i n each of the 15 s t a n d s . A f t e r a p p r o x i m a t e l y 8 months of i_n s i t u d e c o m p o s i t i o n , (September 1982 to A p r i l 1983), the c e l l u l o s e samples were r e -t u r n e d to the l a b o r a t o r y and d r i e d at 70 deg C . B i t s of f o r e s t f l o o r m a t e r i a l and funga l hyphae were removed by soak ing the c e l l u l o s e d i s c s f i r s t i n d i e t h y l e t h e r and then in benzene to remove a l i p h a t i c and aromat i c s u b s t a n c e s . The d i s c s were r e d r i e d at 70 deg C and each mass r e c o r d e d to the neares t 0.1 mg. Samples were then combusted in a muf f l e f u r n a c e at 450 deg C to measure any c o n t a m i n a t i o n of the d i s c s by m i n e r a l p a r t i c l e s . Percent .mass l o s s was c a l c u l a t e d as f o l l o w s : % MASS LOSS = (Min-Mwd)-(Mash-Mb) X100 Min where Min = i n i t i a l mass of d i s c p r i o r to i n c u b a t i o n Mwd = mass a f t e r washing and d r y i n g Mash = mass a f t e r a s h i n g Mb = average ash c o n t e n t of i n c u b a t e d d i s c s 3.3.3 I n d i c e s of N a v a i l a b i l i t y 3.3.3 .1 Ion exchange r e s i n s Two ion exchange r e s i n s , one a c a t i o n exchange r e s i n (Amber-+ -1 l i t e IRC-50 C . P . ( R C O O - H ) wi th an exchange c a p a c i t y of 10 meq.g of dry r e s i n , and moi s ture c o n t e n t (MC)=47%), and the o ther an + an ion exchange r e s i n ( A m b e r l i t e IR-45 C P . (RNH OH ) w i th an -1 3 exchange c a p a c i t y of 5 meq.g d r y r e s i n and MC=41%), (both r e s i n s b e i n g 20-50 mesh s i z e when wet ) , were used to assess the 46 a v a i l a b i l i t y of ammonium and n i t r a t e at the f o r e s t f l o o r and m i n e r a l s o i l i n t e r f a c e and at 30 cm i n the m i n e r a l s o i l . E i g h t g q u a n t i t i e s of moist r e s i n of each type were p l a c e d in s eparate ny lon "knee h igh" s t o c k i n g s w i t h the open ends doubly knot ted be fore b e i n g p l a c e d i n the s o i l at the d e s i r e d depths at the same 5 randomly l o c a t e d spots as f o r the c e l l u l o s e sample bags . At these p o i n t s , a p i t was excavated down to 30 cm in the m i n e r a l s o i l . A s m a l l t u n n e l was dug i n the u p h i l l w a l l of the p i t at t h i s depth and the bag i n s e r t e d , a f t e r which the t u n n e l and p i t were b a c k - f i l l e d . Res in bags at the humus and m i n e r a l s o i l i n t e r -face were p l a c e d under an u n d i s t u r b e d p o r t i o n of the f o r e s t f l o o r , p r i o r to b a c k - f i l l i n g the p i t . Res in bags and c e l l u l o s e sample bags were i n s t a l l e d i n the f i r s t week of September 1982. A f t e r be ing in the s o i l f or 8 months, the r e s i n bags were removed and p l a c e d i n c o l d s torage at about 4 deg C for up to 8 weeks u n t i l they were e x t r a c t e d w i t h one a l i q u o t of 100 mL of 1N KC1 ( B i n k l e y and Matson 1983). A f t e r shaking for 1 h r , the s o l u -t i o n s were f i l t e r e d through a Whatman No. 1 f i l t e r p a p e r , made up to 100 mL, and f rozen for up to 4 weeks p r i o r to be ing a n a l y z e d on the T e c h n i c o n a u t o a n a l y z e r for ammonium and n i t r a t e . 3 . 3 . 3 . 2 S o i l s o l u t i o n c h e m i s t r y Two types of water samplers were used to i n v e s t i g a t e s o i l s o l u t i o n n i t r o g e n c o n c e n t r a t i o n s : 1. z e r o - t e n s i o n l y s i m e t e r s 2 ( c o n s i s t i n g of a p l a s t i c f u n n e l 113.1 cm i n a r e a , a p l u g of ny lon g l a s s wool to f i l t e r p a r t i c u l a t e s , tygon t u b i n g , and a 4.73 L p o l y e t h y l e n e b o t t l e ) which c o l l e c t e d s o i l s o l u t i o n p e r c o l a t i n g 47 through the f o r e s t f l o o r , and 2. ceramic cup s o i l water samplers (about 7 cm in d iameter to which a t e n s i o n of -33 c e n t i b a r s was a p p l i e d ) , which c o l l e c t e d s o i l s o l u t i o n in the m i n e r a l s o i l at a 60 cm .depth . Three of each type of l y s i m e t e r were i n s t a l l e d at randomly l o c a t e d p o s i t i o n s w i t h i n each of the 15 s tands i n the f a l l of 1981. An a d d i t i o n a l 5 ceramic cup s o i l water samplers were i n -s t a l l e d at 15 cm in the m i n e r a l s o i l in May of 1982 in the 1978, 1975, and 1971 c l e a r c u t s , whi l e 2 ceramic cups were i n s t a l l e d at 60 cm in the m i n e r a l s o i l in May 1982 in a s tand c l e a r c u t in the summer of 1981 ( i . e . a 1 - y r - o l d c l e a r c u t ) . Water samples were c o l l e c t e d on average every 38 days over a two year p e r i o d , ending in October 1983. A f t e r bein.g t r a n s p o r t e d to the l a b o r a t o r y , water samples were s t o r e d f rozen in 125 mL p l a s t i c b o t t l e s u n t i l a n a l y z e d for' ammonium, n i t r a t e and t o t a l p e r s u l f a t e n i t r o g e n (TPN). TPN was determined a f t e r a u t o c l a v i n g a mix ture of 5 mL of s o i l water wi th 7.5 mL of o x i d i z i n g reagent (a s o l u t i o n c o n s i s t i n g of 6.7 g of potass ium p e r s u l f a t e and 3.0 g of sodium h y d r o x i d e d i s s o l v e d in 1 L of d i s t i l l e d water) for 30 minutes in b o r o s i l i c a t e screw-, cap c u l t u r e tubes ( D ' E l i a et a l . 1977). A T e c h n i c o n a u t o a n a l y z e r was used to determine l e v e l s of n i t r a t e a f t e r r e d u c t i o n to n i t r i t e by h y d r a z i n e , and the f o r m a t i o n of an azo compound u s i n g s u l f a n i l a m i d e and N - ( 1 - n a p h t h y l ) - e t h y l e n e d i a m i n e (Keeney and Nelson 1982); ammonium was determined u s i n g the indophenol b lue method (Keeney and Ne l son 1982). 48 3 . 3 . 3 . 3 F o l i a r a n a l y s i s Need le s of Ab ie s a m a b i l i s were chosen over those of Tsuqa  h e t e r o p h y l l a as a b i o a s s a y of n i t r o g e n a v a i l a b i l i t y due t o : 1. the h i g h e r n u t r i t i o n a l requirements of A ^ a m a b i l i s ( K r a j i n a et a l . 1982), 2. the g e n e r a l l y g r e a t e r numbers and biomass of A .  amabi1 i s on the c l e a r c u t s examined and 3. the de terminent nature of growth of A_^  a m a b i l i s which a l l o w e d for e a s i e r i d e n t i f i c a t i o n of a n n u a l growth and age c l a s s of n e e d l e s . Branches were c l i p p e d from t r e e s in randomly l o c a t e d p l o t s in November a n d / o r December of 1982. A p p r o x i m a t e l y 10-15 t r e e s were sampled w i t h i n each of the r e p l i c a t e s of the 1978, 1975, 1971, and 1955 c l e a r c u t s . One branch was taken from the t h i r d whorl from the top of t r e e s i n the 1975 and 1971 c l e a r c u t s , from the second, whorl i n the 1978 c l e a r c u t s , and from the upper t h i r d of the crown in the 1955 c l e a r c u t s . In a d d i t i o n , c u r r e n t f o l i a g e was taken from the upper-most whorl of sampled t r e e s in the 1975 c l e a r c u t , s i n c e the t h i r d whorl was in the middle of the crown in those i n d i v i d u a l s . Needle N c o n c e n t r a t i o n s have been f r e q u e n t l y found t o i n c r e a s e w i th i n c r e a s i n g sampl ing h e i g h t w i t h i n the t r e e ( T u r n e r et a l . 1978). The second whorl was chosen i n the 1978 c l e a r c u t s i n c e i t h e l d both the c u r r e n t y e a r ' s f o l i a g e and the f o l i a g e from the f i r s t year i n which the r e g e n e r a t i o n showed p o s t - c l e a r c u t i n c r e a s e d h e i g h t growth. Branches were s t o r e d w i t h needles i n t a c t in a c o l d room at 4 deg C p r i o r to be ing s e p a r a t e d i n t o age c l a s s e s ( c u r r e n t , 1-y e a r , 2 - y e a r - and in some cases 3-year o l d n e e d l e s ) . A f t e r be ing d r i e d a t 70 deg C for 48 h o u r s , needles were s e p a r a t e d from t w i g s , mass of 100 need les were determined and 100 mg samples 49 were weighed out i n t o pyrex t e s t tubes to be wet d i g e s t e d u s i n g a K j e l d a h l procedure (Twine and W i l l i a m s 1972), f o l l o w e d by d e t e r -m i n a t i o n of ammonium u s i n g the indopheno l b lue method on a T e c h -n i c o n A u t o a n a l y z e r (Keeney and Nelson 1982). 3 . 3 . 3 . 4 Height growth response of a m a b i l i s f i r a f t e r c l e a r c u t t i n g Annual h e i g h t increment s i n c e c l e a r c u t t i n g based on i n t e r -whorl d i s t a n c e s of i n d i v i d u a l a m a b i l i s f i r r e g e n e r a t i o n was measured to the neares t cm on the < 2m t r e e s and to the neares t 5 cm on the > 2m t r e e s . Measurements were taken on a l l t r e e s i n randomly l o c a t e d 5 m r a d i u s c i r c u l a r p l o t s i n each of the 6 - y r -o l d r e p l i c a t e s . S i m i l a r measurements were made i n on ly one r e p l i c a t e in each of the 1971 and 1978 s tands because of the v i s u a l l y - a s s e s s e d low degree of v a r i a t i o n between r e p l i c a t e s of these age c l a s s e s . A t o t a l of 30 to 40 t r e e s were sampled i n each c l e a r c u t age c l a s s . Because c u r r e n t h e i g h t growth had not yet t e r m i n a t e d i n e a r l y J u l y when measurements were taken on the 1978 and 1971 c l e a r c u t s , a subsample of t r e e s was marked and remeasured in l a t e August to a d j u s t upwards the c u r r e n t (1983) h e i g h t growth e s t i m a t e s . R e l a -t i v e h e i g h t increment was c a l c u l a t e d as h e i g h t increment d i v i d e d by t o t a l h e i g h t i n 1983. The reasons f o r u s i n g " r e l a t i v e " h e i g h t growth were: 1. to min imize the e f f e c t of t r e e s i z e on the a v e r -age v a l u e s and a s s o c i a t e d s t a n d a r d d e v i a t i o n s of annua l h e i g h t increment w i t h i n the same p l o t , and 2. to make b e t w e e n - s i t e comparisons of annual h e i g h t increment f o r t r e e s of d i f f e r e n t s i z e more i n t e r p r e t a b l e , e s p e c i a l l y w i t h r e s p e c t to t r e n d s for N a v a i l a b i l i t y a f t e r c l e a r c u t t i n g . 50 3.4 R e s u l t s and D i s c u s s i o n D i s c u s s i o n of the p a t t e r n of d e c l i n e in f o r e s t f l o o r d e p t h , o r g a n i c matter content and n i t r o g e n c a p i t a l a f t e r c l e a r c u t t i n g w i l l be based on : 1. the 1981 da ta on the f o r e s t f l o o r s of the chronosequence s i t e s ( i . e . the 1978, 1975, 1971 and 1955 c l e a r -c u t s and the o l d - g r o w t h s t a n d s ) , 2. an examinat ion of f o r e s t f l o o r depths on a d d i t i o n a l s i t e s , and 3. data from a 1983 remea-surement of f o r e s t f l o o r depths i n the chronosequence s i t e s . A f t e r d i s c u s s i n g the observed p a t t e r n of d e c l i n e , the temporal p a t t e r n s of the v a r i o u s n i t r o g e n a v a i l a b i l i t y i n d i c e s w i l l be presented as ev idence of p o s t - c l e a r c u t p a t t e r n s of decompos i t i on and' n i t r o g e n m i n e r a l i z a t i o n . 3.4.1 Changes i n the f o r e s t f l o o r a f t e r c l e a r c u t t i n g : c h r o n o s e -quence s i t e s . 3 .4 .1 .1 Depth F i g u r e 3.1 i l l u s t r a t e s the 1981 p a t t e r n of change i n f o r e s t f l o o r depth (FFD) for the chronosequence s i t e s . The s tands r e p r e -sented t imes of 0 ( i . e . p r e - c l e a r c u t ) , 3, 6, 10, and 26 years a f t e r c l e a r c u t t i n g . The d e c l i n e i n FFD was about 33%, which i s c o n s i d e r a b l y l e s s than the a p p r o x i m a t e l y 50% l o s s of FFD a f t e r c l e a r c u t t i n g balsam f i r s tands growing on p o d z o l s i n Newfoundland (Page 1974). However, the a b s o l u t e magnitude of the d e c l i n e observed by Page (1974) i s a p p r o x i m a t e l y h a l f of t h a t measured on my chronosequence s i t e s because of the great depth of f o r e s t f l o o r i n humid west coas t o l d - g r o w t h f o r e s t s . 51 Ox £ O X h-0_ LU O cr o o LU Q: o o • m • a -o L n -o a 300-yr-old 0 -'T-IS 20 25 i 30 TIME SINCE CLEARCUTTING (years) Figure 3.1 Forest f l o o r depth (cm) f o r the chronosequence of stands examined on southwestern Vancouver I s l a n d (Means and S.E.; n=3 r e p l i c a t e s per age c l a s s ; n=50 measurements per r e p l i c a t e ) . 3 . 4 . 1 . 2 Bulk d e n s i t y A s s o c i a t e d w i t h changes in f o r e s t f l o o r depth were non-s i g n i f i c a n t (P=.05) changes in bulk d e n s i t y (BD) ( F i g u r e 3 . 2 ) . A l t h o u g h the BD v a l u e s for any g i v e n s i t e were v a r i a b l e and t h e r e were no s i g n i f i c a n t b e t w e e n - s i t e d i f f e r e n c e s , there does appear to be an e x p l a i n a b l e t r e n d wi th t ime: -1 1. BD d e c l i n e d s l i g h t l y (0-3 y e a r s ) (.14 to .13 g.cm ) due to l a r g e a d d i t i o n s of needle and twig l i t t e r from l o g g i n g s l a s h ; t h i s needle and twig l i t t e r was i m p o s s i b l e to separate from the p r e - c l e a r c u t L l a y e r l i t t e r m a t e r i a l 3 years a f t e r c l e a r c u t t i n g . 2. A g r a d u a l (3 to 10 y e a r s ) i n c r e a s e in BD due to the l o s s of the L and at l e a s t p a r t of the F h o r i z o n , which tend to be more decomposable than the h i g h e r BD humus . l a y e r . T h i s s m a l l -3 i n c r e a s e of .01 g.cm in BD c o u l d have been due to i n c r e a s e d mix ing of the upper m i n e r a l s o i l and humus l a y e r s as was sug-ges ted by U g o l i n i (1982) , f o l l o w i n g the c l e a r c u t t i n g of a mature s tand of a m a b i l i s f i r i n Washington . For the most p a r t , there was l i t t l e v i s u a l e v i d e n c e of mix ing of the H and m i n e r a l s o i l l a y e r s in.my p l o t s . No measures of ash content were t a k e n , however. U s u a l l y a s s o c i a t e d w i t h an i n c r e a s e i n BD of the f o r e s t f l o o r i s a decrease i n BD of the m i n e r a l s o i l . Page (1974) noted a -3 d e c l i n e in BD at 2 .5 cm i n the m i n e r a l s o i l of 0.1 to 0.2 g.cm a f t e r c l e a r c u t t i n g balsam f i r s tands in Newfoundland. He sug-ges ted t h a t the i n c o r p o r a t i o n of o r g a n i c matter i n t o the m i n e r a l s o i l was due to d i s t u r b a n c e d u r i n g the l o g g i n g o p e r a t i o n a n d / o r i n c r e a s e d a c t i v i t y of s o i l a n i m a l s . Earthworm and c e n t i p e d e a c t i -v i t y i n the f o r e s t f l o o r and upper m i n e r a l s o i l was observed 53 in i O CD (/) z: L U O o ro 300-y r -oLd -n D m o o . 0 5 — r ~ 10 15 20 25 - 1 30 TIME SINCE CLEARCUTTING (years) F i g u r e 3 .2 Bulk d e n s i t y (g.cm D) o f the f o r e s t f l o o r f o r the chronosequence o f s tands (Means and S . E . ) . about 5-10% of the time in the young F l e e t R i v e r c l e a r c u t s whi le t a k i n g f o r e s t f l o o r a n d / o r m i n e r a l s o i l samples . T h i s suggests tha t at l e a s t some mix ing was be ing caused by s o i l a n i m a l s . On the o ther hand, such s o i l an imal a c t i v i t y was r a r e l y seen in my o l d - g r o w t h s t a n d s . Other s t u d i e s have noted the presence of earthworms in the s o i l s of c o n i f e r s tands on the western p a r t of Vancouver I s l a n d ( D i c k i n s o n 1984). 3 . There i s l i t t l e ev idence for a change in BD between 10 and 26 years a f t e r c l e a r c u t t i n g g i v e n the l a r g e v a r i a t i o n between p l o t s in the 2 6 - y r - o l d age c l a s s ; some of t h i s v a r i a t i o n i s due to the spac ing treatment that one of the 2 6 - y r - o l d stands r e c e i v e d 9-10 y e a r s be fore the s t a r t of the study (Table 3 . 1 ) . G r i e r et a l . ' s (1974) study suggests a d e c l i n e , in f o r e s t f l o o r BD due to the i n c r e a s i n g p r o p o r t i o n of woody l i t t e r f a l l to t o t a l l i t t e r f a l l wi th i n c r e a s i n g s tand age . 3 . 4 . 1 . 3 Ma s s Changes in the mass of f o r e s t f l o o r (FFM) a f t e r c l e a r -c u t t i n g were dependent on changes in both depth and bulk d e n s i t y . A l t h o u g h n o n - s i g n i f i c a n t d i f f e r e n c e s in BD were observed between s i t e s , the s l i g h t l y h i g h e r BD i n the 1 0 - y r - o l d c l e a r c u t compen-sa ted for the reduced f o r e s t f l o o r depth and r e s u l t e d in f o r e s t f l o o r s of s i m i l a r mass in both the 6- and 1 0 - y r - o l d c l e a r c u t s -1 ( i . e . 210-220 t . h a . Hence, depth measurements, a l t h o u g h easy to make, are not always an a c c u r a t e p r e d i c t o r of f o r e s t f l o o r mass. The d e c l i n e i n f o r e s t f l o o r mass between 0 and 6 (or 10) -1 y e a r s a f t e r c l e a r c u t t i n g was about 112 t . h a . , or about 35% of 55 T a b l e 3.1 F o r e s t f l o o r v a r i a b l e s for each of the i n t e n s i v e l y sampled s tands in the chronosequence . * ** S i t e / Depth BD Mass N T o t a l - N c l e a r c u t cm -3 g.cm t . h a "1 % kg . ha -1 *** 3 - y r - o l d - 1 23.4 0.13 293 0 .854 2498 bcde 3 - y r - o l d - 2 32.7 0.13 375 0 .876 3283 de 3 - y r - o l d - 3 24.5 0. 13 349 0 .803 2799 de 6 - y r - o l d - 1 15.6 0. 14 175 0 .619 1 084 a 6 - y r - o l d - 2 22.5 0.15 266 0 .675 1 793 abc 6 - y r - o l d - 3 17.6 0.13 196 0 .747 1467 a b c 1 0 - y r - o l d - 1 14.4 0.13 187 0 .773 1 446 ab 1 0 - y r - o l d - 2 18.4 0.14 236 0 . 775 1829 abc 1 0 - y r - o l d - 3 14.5 0.15 214 0 .717 1 535 abc **** 2 6 - y r - o l d - 1 27.6 0.10 288 0 .681 1 963 abc 2 6 - y r - o l d - 2 20. 1 0.14 251 0 .812 2041 abc 2 6 - y r - o l d - 3 14.0 0.18 234 0 .810 1898 abc o l d - g r o w t h - 1 18.3 0.14 255 0.742 1890 abc o l d - g r o w t h - 2 25.6 0.13 318 0.849 2701 de o l d - g r o w t h - 3 27.1 0.14 399 0.648 2588 cde * - r e f e r s to c l e a r c u t age i n 1981. * * - BD=bulk d e n s i t y , OM=organic mat t er , N=percent n i t r o g e n * * * - l e t t e r s r e f e r to v a l u e s b e l o n g i n g to the same s u b s e t . Subse t s w i th no l e t t e r s i n common are s i g n i f i c a n t l y d i f f e r e n t at P = .05 u s i n g Duncans MRT. * * * * _ a n a d d i t i o n a l 2 6 - y r - o l d s i t e was sampled but was l a t e r r e j e c t e d as f a r as f u r t h e r da ta c o l l e c t i o n was c o n c e r n e d . 5 6 -1 the p r e - c l e a r c u t o l d - g r o w t h f o r e s t f l o o r mass of 339 t . h a ( F i g u r e 3 . 3 ) . T h i s compares c l o s e l y wi th the 33% d e c l i n e in f o r e s t f l o o r depth a f t e r c l e a r c u t t i n g . There i s no comparable p a t t e r n of p o s t - c l e a r c u t t i n g changes in f o r e s t f l o o r mass for mixed s tands of west coas t c o n i f e r s . However, T a b l e 3.2 p r o v i d e s v a r i o u s e s t imates of f o r e s t f l o o r -1 mass ( t . h a ) f o r some t r u e f i r s t a n d s , which i n c l u d e : 1. 48 in a 2 3 - y r - o l d a m a b i l i s f i r s tand (Vogt et a l . 1983), 2. 150 in a 1 8 0 - y r - o l d a m a b i l i s f i r s tand (Vogt et a l . 1983), 3. 53.5 in a 1 7 5 - y r - o l d a m a b i l i s f i r s tand (Turner and S inger 1976), a l l three of the above in Washington, 4. 103.9 in a mature s u b a l p i n e f i r s tand i n C o l o r a d o ( S n e l l et a l . 1974), 5. 55.7 in a mature sub-a l p i n e f i r s tand i n . t h e i n t e r i o r of B . C . (Kimmins 1974), 6. 92.2 in a balsam f i r s tand in the n o r t h e a s t e r n U . S . (Lang et a l . 1981), 7. 105 i n a A . mariana s tand in Japan (Tsutsumi 1971) and 8. 43 in an A_j_ f i rma s tand i n Japan (Ando: in C o l e and Rapp 1980). The above e s t imates i n d i c a t e c o n s i d e r a b l e v a r i a b i l i t y between s tands of s i m i l a r age . S i m i l a r l y , e s t i m a t e s of f o r e s t -1 f l o o r in Washington vary from about 15 to 150 t . h a (Table 3 . 2 ) . -1 Abee (1973) r e p o r t e d an e s t imate of 50.9 t . h a for an o l d - g r o w t h D o u g l a s - f i r s t a n d , whi le lower v a l u e s of between 14.3 to 28 .5 , as w e l l as h i g h e r v a l u e s of 85.9 to 170.8 were r e p o r t e d for Doug las -f i r s tands i n Washington and Oregon (Wooldridge 1961). Due to the g r e a t e r frequency of f i r e at lower e l e v a t i o n s , such as i n the C o a s t a l D o u g l a s - f i r and C o a s t a l Western Hemlock dry subzones , i t i s u n l i k e l y t h a t f o r e s t s w i l l have had the time to have accumulated f o r e s t f l o o r s as t h i c k as those observed in 57 8 £ O CD m • P 3 s m cc m CD X o 3 CD O ^ CD CO CD H , H O O c+ H i O c+ & CD o o o CD. CD d CD O CD m CO n m r> r~ m > 73 n c —i d z CD 0= CD Q 1 (0 FOREST FLOOR MASS (t.ha" 1) a -cn -o ' cn' o ' 100.0 200.0 i 300.0 400.0 j a a i T a b l e 3.2 F o r e s t f l o o r mass and n i t r o g e n c o n t e n t f o r v a r i o u s stands i n western N o r t h America. Dominant t r e e sp. Age L o c a t i o n Mass N i t r o g e n Source -1 -1 y r t .ha fo kg. ha A b i e s amabl11s 23 WA 48 1 43 686 Vogt e t a l . 1983 A b i e s amabl11s 23 WA 666 G r i e r unpub.(Cole and Rapp A. amabl11s/ Tsuga m e r t e n s l a n a 175 WA 54 1 21 650 Turner and Si n g e r 1976 A. amabl?1s/ T. m e r t e n s l a n a o l d WA 651 W i l l i a m s and Dryness 1967 A. amabl11s 180 WA 150 1 25 1871 Vogt et a l . 1983. A. g r a n d I s 60 WA 49 0 76 372 Gessel and Klock 1982 A. c o n c o l o u r NM 81 1 10 883 Wo1lum 1973 A. 1 a s l o c a r p a CO 104 0 58 598 Snel1 et a l . 1977 • A. nob111s 130 OR 884 G r i e r unpub. ( C o l e and Rapp A. balsamea NH 92 2 50 2300 Lang et a l . 1981 A. l a s l o c a r p a / 110-P. g l a u c a 350 BC 56 1 51 842 K1mm1ns 1974 A. marlana Japan 105 1 16 1218 Tsutsumi 1971 A. f1rma 97-145 Japan 43 1 12. 484 Ando ( C o l e and Rapp 1980) P s e u d o t s u g a / m e n z l e s l 1 3-good BC 1 0 10 < 1 F e l l e r e t a l . 1983 n II 3-poor BC 2 0 10 < 2 •I it II II 6-good BC 2 0 05 < 1 •I H n ii 6-poor BC 5 0 08 4 •I ti II n 11-good BC 12 0 08 9 •I ii II II 9-poor BC 8 0 06 5 •I it II II 19-good BC 40 0 10 40 II . . n II 19-poor^ BC 15 0 13 20 it ti n II 48-good BC 80 0 15 120 it it II n 48-poor BC 35 0 13 45 it n •i ti 73-good BC 1 10 0 13 140 it n n II 74-poor BC 30 0 08 23 H II II n 15-20 BC 32 0 86 276 Webber 1977 II II 22 WA 21 0 87 178 Turner 1975 n II 30 WA 17 0 92 154 II II II ti 36 WA 23 0 77 175 it i. it II 42 WA 17 0 95 163 ti II if it 73 WA 96 0 60 574 II II •t II 95 WA 81 0 52 423 ti it Table 3.2 continued Dominant tree sp. Age Location Mass Nitrogen -1 t.ha % kg.ha Pseudotsuga menz1es 1 1 o l d WA 51 0 81 414 WA 29 1 15 327 II ti WA 14 1 35 192 H ii WA 28 35 96 it ii OR 39 0 94 389 II n OR 23-86 .63-1 95 169 -1306 n WA OR 19-171 .70-1 40 176 -1810 Tsuga/ heterophyl1 a 180 OR 2090 old-growth coniferous (mull) WA 72 1 93 1393 (mor) WA 158 1 . 10-1 .46 2040 Source Abee 1973 B a l d (WooldMdge 1961) Tarrant and M i l l e r 1963 Youngberg (WooldMdge 1961) Williams and Dyrness (WooldMdge 1961) GMer unpub.(Cole and Rapp 1980) Gessel and B a l d 1965 wetter and c o o l e r a r e a s . T h i s might account for the g e n e r a l l y lower e s t i m a t e s for f o r e s t f l o o r mass in D o u g l a s - f i r s tands r e p o r t e d i n T a b l e 3 .2 . Based on sampl ing in Washington , G e s s e l and B a l c i (1955) r e p o r t e d an average v a l u e for a m u l l to be 72 -1 -1 t . h a , as compared to 158 t . h a f o r a mor. F o r e s t f l o o r s of -1 110 and 30 t . h a were r e p o r t e d for 73 to 7 4 - y r - o l d f i r e - o r i g i n D o u g l a s - f i r s tands on good and poor s i t e s , r e s p e c t i v e l y , in the d r i e r subzone of the CWH of c e n t r a l Vancouver I s l a n d ( F e l l e r et a l . 1983). Given the v a r i a b i l i t y i n the above da ta and the a b -sence of knowledge about s i t e c o n d i t i o n s in most c a s e s , i t i s d i f f i c u l t to comment on a g e - r e l a t e d between-stand d i f f e r e n c e s in f o r e s t f l o o r mass p u b l i s h e d in the l i t e r a t u r e . The chronosequences examined by T u r n e r (1975) and F e l l e r et a l . (1983) p r o v i d e the best i n d i c a t i o n of a g e - r e l a t e d p a t t e r n s of f o r e s t f l o o r a c c u m u l a t i o n . The p a t t e r n documented by F e l l e r et a l . 1983) f o r a good s i t e was s i m i l a r to that found by Turner -1 ( (1975); i n C o l e and Johnson (1980)) of c l o s e to 100 t . h a of o r g a n i c mat ter a c c u m u l a t i o n in about 75 years p o s t - l o g g i n g . T h i s suggests t h a t r a p i d a c c u m u l a t i o n of f o r e s t f l o o r mass can occur a f t e r l o g g i n g and s l a s h b u r n i n g . The p a t t e r n of d e c l i n e and subse-quent r e - a c c u m u l a t i o n of f o r e s t f l o o r N a f t e r c l e a r c u t t i n g has r e c e i v e d l e s s a t t e n t i o n . A f t e r the c l e a r c u t t i n g of n o r t h e a s t e r n hardwoods, C o v i n g t o n (1981) observed a r e t u r n to w i t h i n 5% of the p r e c u t FFM in 64 -1 y e a r s a f t e r a d e c l i n e of 30.7 t . h a . In my s t u d y , the i n c r e a s e i n FFM between 10 and 25 y e a r s p o s t - c l e a r c u t t i n g was about 40-45 -1 t . h a . T h i s i s s l i g h t l y h i g h e r than those r e p o r t e d by o ther s t u d i e s , perhaps due to the d i f f e r e n t l o g g i n g methods and l e v e l s 61 of u t i l i z a t i o n employed in the young c l e a r c u t s than i n the 2 6 - y r -o l d s t a n d s . 3 . 4 . 1 . 4 Percent n i t r o g e n F i g u r e 3.4 shows the t r e n d i n f o r e s t f l o o r percent n i t r o g e n (FFPN) a f t e r c l e a r c u t t i n g . An a n a l y s i s of v a r i a n c e i n d i c a t e d no s i g n i f i c a n t d i f f e r e n c e s (P=.05) i n FFPN between c l e a r c u t s . Other s t u d i e s have i n d i c a t e d i n c o n s i s t e n t t r e n d s i n FFPN wi th s tand development ( F e l l e r et a l . 1983), or no d i f f e r e n c e s (Cov ington 1981). Popovic (1974) noted a decrease in t o t a l - N (ppm) from 10,950 (adjacent uncut spruce) to 9400 in a 1 - y r - o l d c l e a r c u t for " n i t r i f y i n g " a r e a s , and an i n c r e a s e from 13,500 to 13,875 in " n o n - n i t r i f y i n g " a r e a s . FFPN c o n c e n t r a t i o n s were somewhat lower than those r e c o r d e d by Vogt et a l . d 9 8 3 ) and T u r n e r and S i n g e r (1976) for a m a b i l i s f i r , Lang et a l . (1981) for balsam f i r , Kimmins (1974) for sub-a l p i n e f i r / w h i t e s p r u c e , Wollum (1973) for white f i r , Tsutsumi (1971) for A b i e s mayriana and Ando: in Co le and Rapp (1980) for A . f i rma in J a p a n . The r e l a t i v e l y h i g h c o n c e n t r a t i o n s for FFPN for the t r u e f i r s tands i n . T a b l e 3 . 2 , c o u l d be due to a l e s s e r tendency to r e c y c l e i n t e r n a l l y N p r i o r to l e a f f a l l i n t r e e s w i th p r o l o n g e d needle r e t e n t i o n (Schwab 1979). G i v e n the biomass of f o l i a g e r e l a t i v e to annual l i t t e r f a l l i n such t r e e s , the need for i n t e r n a l l y r e c y c l i n g would be l e s s than i n t r e e s p e c i e s where 20 to 30% of the f o l i a g e c a p i t a l i s l o s t as l i t t e r f a l l each y e a r . In the case of balsam f i r they c o u l d be due to the l a r g e i n p u t s of N in p r e c i p i t a t i o n i n s u b a l p i n e environments in the n o r t h -62 ON LU O O LY. V-\-Z LU U cr LU o_ CD 00 o ' CO CD " 300-y r -oLd CD . 10 I 15 20 25 30 TIME SINCE CLEARCUTTING (years ) F i g u r e 3.4 F o r e s t f l o o r N ( p e r c e n t ) f o r the chronosequence o f s t a n d s examined (Means and S.E.). e a s t e r n U . S . (Love t t et a l . 1981). However, the F l e e t R i v e r FFPN c o n c e n t r a t i o n s are w i t h i n the range of v a l u e s observed by T u r n e r (1975) and F e l l e r et a l . (1983) for second-growth D o u g l a s - f i r s t a n d s . T h i s may r e f l e c t the low n u t r i e n t s t a t u s of the parent m a t e r i a l from which the s o i l on the o l d - g r o w t h and young c l e a r c u t chronosequence s i t e s i s de -r i v e d . These are igneous i n t r u s i v e s , such as g r a n o d i o r i t e s , which a c c o r d i n g to Walmsley et a l . (1980) , ( a l l o ther t h i n g s be ing equa l ( i . e . seepage i n p u t s ) ) u s u a l l y deve lop i n t o s o i l s w i th a submesotrophic n u t r i e n t regime. Parent m a t e r i a l p r o b a b l y i n f l u e n c e s s o i l N s t a t u s through the f o r m e r ' s a f f e c t s on s o i l pH, c a t i o n exchange c a p a c i t y ( i . e . ammonium), and s o i l an imal a c t i v -i t y . E l l e n b e r g (1978) r e p o r t s a s t r o n g c o r r e l a t i o n between s o i l N r e s e r v e s and s o i l pH in some European s o i l s . However, s i m i l a r FFPN and s o i l pH v a l u e s in the 2 6 - y r - o l d and F l e e t R i v e r stands suggest tha t the d i f f e r e n c e s in N a v a i l a b i l i t y may not have been e n t i r e l y due to d i f f e r e n c e s in parent m a t e r i a l s . Another e x p l a n a t i o n for the r e l a t i v e l y low FFPN of the study s i t e s may be the s tand h i s t o r y and s p e c i e s c o m p o s i t i o n . The mature s tands c o n t a i n e d a s i g n i f i c a n t component of western hem-l o c k , western r e d c e d a r , as w e l l as a m a b i l i s f i r . Based on S c h m i d t ' s (1957) f i n d i n g s , which suggest that a m a b i l i s f i r on ly becomes a major component of the codominant and dominant crown c l a s s e s 600 to 800 years a f t e r the l a s t f i r e , the f o r e s t f l o o r has been b u i l d i n g up for many c e n t u r i e s . The i n c r e a s i n g p r o p o r -t i o n of woody m a t e r i a l i n p u t s caused by i n d i v i d u a l t r e e death due to d i s e a s e a n d / o r windthrow through the s t a n d ' s h i s t o r y , and the l a c k of a major d i s t u r b a n c e such as f i r e , p r o b a b l y e x p l a i n the 64 l a r g e a c c u m u l a t i o n of f o r e s t f l o o r . The presence of m i s t l e t o e -i n f e c t e d western hemlock in the o l d - g r o w t h s tands and the h i g h i n c i d e n c e of r o t t e n wood support the above s c e n a r i o . 3 . 4 . 1 . 5 T o t a l n i t r o g e n T a b l e 3.1 presented data on average d e p t h , bu lk d e n s i t y , mass, percent n i t r o g e n , and t o t a l n i t r o g e n for each of the s tands i n t e n s i v e l y sampled a long the chronsequence . Duncan's MRT (P=.05) -1 for t o t a l N (kg .ha ) i n d i c a t e d tha t none of the r e p l i c a t e s were s i g n i f i c a n t l y d i f f e r e n t from other r e p l i c a t e s of the same age; a l t h o u g h there are s i g n i f i c a n t d i f f e r e n c e s between a g e s . F i g u r e 3.5 i l l u s t r a t e s the p a t t e r n of d e c l i n e and a c c u m u l a t i o n of f o r e s t f l o o r n i t r o g e n c a p i t a l a f t e r c l e a r c u t t i n g based on the c h r o n o -sequence. Means are an average of 3 r e p l i c a t e s at each p o i n t in t ime a f t e r c l e a r c u t t i n g . Duncans MRT (P=.05) i .nd ica ted t h a t two groups were e v i d e n t : 1. o l d - g r o w t h and 3 - y r - o l d c l e a r c u t s , 2. 6 - , 10- and 2 6 - y r - o l d c l e a r c u t s . T h i s i s v i r t u a l l y the same p a t t e r n as for FFM because of i n s i g n i f i c a n t changes i n FFPN a l o n g the chronosequence . There are few r e p o r t s in the l i t e r a t u r e of s tands h a v i n g -1 more than 2000 kg .ha of f o r e s t f l o o r N . Two e x c e p t i o n s are a -1 1 8 0 - y r - o l d western hemlock s tand i n Oregon (2090 kg .ha ) ( G r i e r : i n C o l e and Rapp 1981) and a balsam f i r s tand i n New Hampshire -1 (2300 kg .ha ) (Lang et a l . 1981). As d i s c u s s e d e a r l i e r , the absence of f i r e s may be an important de terminant of the v e r y l a r g e a c c u m u l a t i o n s of f o r e s t f l o o r N in the s tudy a r e a . Based on the d i f f e r e n c e between t ime zero ( o l d - g r o w t h s tand) 65 o o • o ro o o • a ro o o a 300-yr-oLd 0 - r 25 10 15 20 30 TIME SINCE CLEARCUTTING (years) Figure 3 . 5 Forest f l o o r N content (kg.ha ) f o r the chronosequence of stands examined (Means and S.E.). and s i x y e a r s a f t e r c l e a r c u t t i n g (1975 c l e a r c u t ) on the chronose -quence s i t e s , the magnitude of the d e c l i n e of f o r e s t f l o o r N -1 a f t e r c l e a r c u t t i n g i s e s t i m a t e d to be a p p r o x i m a t e l y 950 kg .ha The d e c l i n e appears to occur be.tween 3 and 6 y e a r s a f t e r c l e a r -c u t t i n g based on the chronosequence s i t e s . The f o l i a r n i t r o g e n content of the o v e r s t o r y was e s t i m a t e d to -1 be a p p r o x i m a t e l y of 234 k g . N . h a for the F l e e t R i v e r o l d - g r o w t h s i t e s . The d i f f e r e n c e between the f o r e s t f l o o r n i t r o g e n c a p i t a l in the o l d - g r o w t h and 1978 c l e a r c u t s , 3 years a f t e r c l e a r c u t t i n g -1 was a p p r o x i m a t e l y 200 k g . N . h a . T h i s suggests t h a t very l i t t l e a c t i v i t y o c c u r r e d in the f o r e s t f l o o r d u r i n g the f i r s t three y e a r s a f t e r c l e a r c u t t i n g . One of the few comparable p u b l i s h e d s t u d i e s was conducted at Hubbard Brook in a n o r t h e a s t e r n U . S . hardwood f o r e s t , where there was a d e c l i n e in f o r e s t f l o o r n i t r o --1 " • gen c o n t e n t of 808 kg .ha (55%) d u r i n g the f i r s t 15 y e a r s a f t e r -1 c l e a r c u t t i n g (Covington 1981) and 171 k g . N . h a d u r i n g the f i r s t three y e a r s a f t e r c l e a r c u t t i n g (Dominski 1971). However, the immediate p o s t - l o g g i n g d e c l i n e in f o r e s t f l o o r N that was r e p o r t e d by Dominski (1971) was not observed in the F l e e t R i v e r s t u d y . T h i s might have been due to d i f f e r e n c e s i n the amount of s l a s h l e f t on my s i t e s as compared to those a t Hubbard Brook a n d / o r to the d i f f e r e n c e s in the C:N r a t i o of the s l a s h and f o r e s t f l o o r . The C:N r a t i o has been proposed as the major reason f o r the l a r g e d i f f e r e n c e s in n i t r a t e l o s s e s a f t e r c l e a r c u t t i n g at Hubbard Brook v e r s u s those of o t h e r watersheds ( S o l l i n s and M c C o r i s o n 1981). Another comparable s tudy at Garpenberg , Sweden, was conducted over a . 0 to 4 y e a r s p o s t - l o g g i n g p e r i o d ( N y k v i s t 1974). T h i s 67 study found a p o s t - c l e a r c u t t i n g d e c l i n e i n f o r e s t f l o o r N of 188 -1 d u r i n g the f i r s t year and 560 kg .ha (47%) d u r i n g the f i r s t 4 -1 y e a r s . Wi th s l a s h i n c l u d e d , a d e c l i n e of 256 and 798 k g . N . h a was observed for the same two time p e r i o d s . No s t a t i s t i c a l t e s t s were performed on the d a t a , due to the l a c k of r e p l i c a t i o n s . As was observed in the Dominski (1971) s t u d y , the p o s t - c l e a r c u t t i n g d e c l i n e in f o r e s t f l o o r N was immediate , w h i l e in the F l e e t R i v e r study there appears to be a s l i g h t d e l a y of a few y e a r s . Hence, t h e r e appears to be a s l i g h t l y s m a l l e r percentage d e c l i n e in the F l e e t R i v e r s i t e s f o r e s t f l o o r N c a p i t a l (40%) as compared to o ther e s t imates of p o s t - c l e a r c u t t i n g f o r e s t f l o o r d e c l i n e s of 55% in n o r t h e a s t e r n hardwoods (Covington 1981) and 47% in Norway spruce s tands in Sweden (Nykv i s t 1874). The magnitude of the d e c l i n e s in terms of a b s o l u t e amounts of f o r e s t -1 f l o o r N (kg .ha ) l o s t are f a i r l y c l o s e (808 v s . 950) for the n o r t h e a s t e r n hardwood study and the F l e e t R i v e r s tudy; the Swe-d i s h study o n l y looked at the d e c l i n e up to 4 years a f t e r c l e a r -c u t t i n g . 3 . 4 . 1 . 6 E v i d e n c e of p o s t - c l e a r c u t t i n g f o r e s t f l o o r changes from o ther s i t e s on southern Vancouver I s l a n d . The p a t t e r n of f o r e s t f l o o r d e c l i n e e s t a b l i s h e d for the chronosequence s i t e s was t e s t e d by sampl ing a d d i t i o n a l s i t e s in the same a r e a . A r e g r e s s i o n e q u a t i o n of n i t r o g e n content on f o r e s t f l o o r depth deve loped f o r the 15 chronosequence s i t e s was used to p r e d i c t n i t r o g e n content from depth measurements for seventeen a d d i t i o n a l p o i n t s i n t ime a f t e r c l e a r c u t t i n g , 8 of 68 which were p r e v i o u s l y unsampled s i t e s w i th s i m i l a r u n d e r s t o r y v e g e t a t i o n , topography and s tand treatment h i s t o r y , but v a r y i n g i n age s i n c e c l e a r c u t t i n g , and 9 were remeasurements of c h r o n o s e -quence s i t e s . The p a t t e r n of d e c l i n e e x h i b i t e d by the 17 a d d i -t i o n a l p o i n t s was s i m i l a r to that shown in F i g u r e 3 . 5 . For exam-p l e , the a d d i t i o n a l o l d - g r o w t h stands sampled i n d i c a t e d an a v e r --1 age f o r e s t f l o o r N of 2520 kg .ha , as compared to 2393 for the chronosequence s tands ; comparable data p a i r s were 2386 and 2670 -1 kg .ha for the 3 - y r - o l d s i t e s s t u d i e d , and 1945 and 1448 -1 -1 k g . N . h a for 5- and 6 - y r - o l d s i t e s , and 1478 and 1603 k g . N . h a for 8- and 1 0 - y r - o l d s i t e s . Based on the a d d i t i o n a l 3 - y r - o l d s i t e s , there appears to be a somewhat e a r l i e r onset of the d e c l i n e , and based on the 5 and 8 - y r - o l d v a l u e s i t c o n t i n u e s to d e c l i n e u n t i l at l e a s t 8 y e a r s a f t e r c l e a r c u t t i n g . The v a l u e s for f o r e s t f l o o r N are very s i m i l a r , s u p p o r t i n g the assumption that the 15 chronosequence s i t e s are r e p r e s e n t a t i v e of the g e n e r a l p a t t e r n of p o s t - c l e a r c u t t i n g f o r e s t f l o o r d e c l i n e in t h i s area and perhaps in t h i s subzone. However, the average d e c l i n e f o r the r e g i o n appears to s t a r t a l i t t l e e a r l i e r and l a s t s l i g h t l y l onger than the 15 chronosequence s i t e s sugges t . Based on the combined chronosequence and a d d i t i o n a l sampl ing (32 data p o i n t s ) , Duncans MRT (P=.05) i d e n t i f i e d 2 subsets w i th r e s p e c t to depth ( F i g u r e 3.6) ( 3 - , 3 0 0 - , 5 - and 2 6 - y r - o l d ) and (5-, 2 6 - , 6 - , 1 2 - , 8 - , a n d 1 0 - y r - o l d ) , and t h r e e subse t s w i t h r e s p e c t to N content ( F i g u r e 3.7) (3-,300 - y r - o l d ) , (26- , 5 - , 1 2 - , 1 0 - and 8 - y r -o l d ) and ( 1 2 - , 1 0 - , 8 - and 6 - y r - o l d ) . Due to s m a l l d i f f e r e n c e s in %N between chronosequence s i t e s , the p a t t e r n s for depth and N 69 O -i O 1 — X a h- Ln -CL LU Q LY. o O a -o 1 oa 1 Li_ 1- o 01 IT) -LU LY. O L_ a a ^ — 300-yr -oLd 10 15 20 - r 25 30 80 T I M E S I N C E C L E A R C U T T I N G (years) Figure 3 . 6 General p a t t e r n of d e c l i n e i n f o r e s t f l o o r depth a f t e r c l e a r -c u t t i n g (Means and S.E.; n=32, 1 5 values from the chronosequence and 1 7 a d d i t i o n a l values of f o r e s t f l o o r depth, of which 9 were a remea-surement of f o r e s t f l o o r depth 2 years a f t e r i n i t i a l measurement). o. o ro o o. LO O o. o O O LO O O O . 300 - y r - o l d rVrh --rh 15 20 25 "~i 30 80 TIME SINCE CLEARCUTTING (years) Figure 3 - 7 General p a t t e r n of d e c l i n e i n f o r e s t f l o o r N (kg.ha ) a f t e r c l e a r c u t t i n g (Means and S.E.; n = 3 2 ; see Figure 3.6 f o r d e t a i l s ) . content are v e r y s i m i l a r i n F i g u r e s 3.6 and 3 . 7 , r e s p e c t i v e l y . The e s t imate of f o r e s t f l o o r d e c l i n e a f t e r c l e a r c u t t i n g based on -1 the 32 sample p l o t s was 1018 k g . N . h a , as compared to 950 -1 k g . N . h a for the 15 chronosequence s i t e s . 3 . 4 . 1 . 7 Remeasurement of young c l e a r c u t s As a f u r t h e r t e s t of the observed p a t t e r n of d e c l i n e i n f o r e s t f l o o r N c o n t e n t a f t e r c l e a r c u t t i n g , the 1971, 1975, and 1978 c l e a r c u t s sampled in 1981 were remeasured for depth d u r i n g the summer of 1983 (Tab le 3 . 3 ) . The remeasurement da ta were used i n the o v e r a l l d a t a s e t , which was d i s c u s s e d above . L o o k i n g at the remeasurement da ta on t h e i r own i n d i c a t e s a s i m i l a r p a t t e r n i n s p i t e of the s m a l l sample s i z e (3 ) , and v a r i a t i o n in s t o c k i n g l e v e l s .of advance -and p o s t - l o g g i n g r e g e n e r a t i o n . . U s i n g the same r e g r e s s i o n as b e f o r e , ( i . e . FFPN = 23.27 + 94.88 * depth (cm)) the d e c l i n e in FFPN between 1981 and 1983 was e s t i m a t e d as the d i f f e r e n c e in FFPN i n each of the 3 - , 6- and 1 0 - y r - o l d c l e a r c u t s i n 1981 and 1983 ( T a b l e 3 . 3 ) . The c u m u l a t i v e d e c l i n e was e s t i -mated as the sum of the d i f f e r e n c e s between 1981 and 1983 for the -1 3 to 5 and 6 to 8 year p e r i o d s , which amounted to 958 k g . N . h a U s i n g i n d i v i d u a l e q u a t i o n s between depth and N content for each -1 p l o t , I e s t i m a t e d a d e c l i n e of 860 k g . N . h a . for the same time p e r i o d s . T h i s i m p l i e s t h a t the v a r i a t i o n between s i t e s for BD d i d not e f f e c t the p r e d i c t i o n of f o r e s t f l o o r N d e c l i n e by more than 12%. The absence o f a l a r g e d e c l i n e ( i . e . 161 v s . 1186 and 778 -1 k g . N . h a ) i n f o r e s t f l o o r n i t r o g e n on s i t e 3 - y r - o l d - 1 i s perhaps an anomaly. I t i s p o s s i b l e that on t h i s p l o t the major d e c l i n e 72 T a b l e 3.3 Remeasurement of f o r e s t f l o o r depth in the young c l e a r c u t s . T o t a l - N content was p r e d i c t e d u s i n g a r e g r e s s i o n e q u a t i o n between depth and t o t a l - N . See t e x t for d e t a i l s . (S tandard e r r o r s for depth measurements are i n b r a c k e t s , n=50) S i t e / C l e a r c u t Depth (cm) 1981 1 983 ** T o t a l - N d i f f e r e n c e kg .N .ha -1 3 - y r - o l d - 1 3 - y r - o l d - 2 3 - y r - o l d - 3 *** 23.4 (3 .2) 21 .7 (2. 1 ) - 1 . 7 -161 32.7 (4 .2) 20.2 (2 .4) - 1 2 . 5 -1186 (-708) 28.2 (3 .5) 20.0 (2 .4) - 8 . 2 -778 6 - y r - o l d - 1 6 - y r - o l d - 2 6 - y r - o l d - 3 15.6 (1 .6) 22.5 (3 .4) 17.6 (2 .7) 14.8 (1 .8) - 0 . 8 16.3 (2.1) - 6 . 2 16.7 (2.2) - 0 . 9 -76 -588 (-250) -85 1 0 - y r - o l d - 1 1 0 - y r - o l d - 2 1 0 - y r - o l d - 3 14.4 (1 .8) 18.4 (2 .0) 14.5 (1 .3) 18.0 (2.6) +3.6 21.9 (2 .3) +3.5 15.7 (1.9) +1.2 + 342 +332 (+263) + 114 * - age of c l e a r c u t in 1981. * * - c a l c u l a t e d as the d i f f e r e n c e in t o t a l - N as p r e d i c t e d i n 1983 minus that p r e d i c t e d i n 1981, where t o t a l N = 23.27 + 94.88 * depth (cm). * * * - v a l u e s for t o t a l N i n b r a c k e t s are an average for the r e p l i c a t e s i n each age c l a s s . 73 o c c u r r e d p r i o r to sampl ing in 1981. There was an u n u s u a l l y l a r g e amount of advance r e g e n e r a t i o n on t h i s s i t e , as w e l l as s e v e r a l l a r g e downed western hemlock, which had not been yarded due to t h e i r advanced s t a t e of decay . These two f a c t o r s are c o n s i s t e n t w i t h the t h e o r y tha t these l a r g e downed t r e e s had f a l l e n p r i o r to c l e a r c u t t i n g c a u s i n g a d i s t u r b a n c e l a r g e enough to open the canopy s u f f i c i e n t l y to r e s u l t in a d e c l i n e in the f o r e s t f l o o r and the subsequent e s t a b l i s h m e n t of the r e g e n e r a t i o n p r i o r to c l e a r c u t t i n g . The w e l l e s t a b l i s h e d advance r e g e n e r a t i o n and V a c -c i n i u m spp. would have moderated the d e c l i n e a f t e r c l e a r c u t t i n g , a c c o u n t i n g f o r the s m a l l e r d e c l i n e observed in t h i s p l o t . In a d d i t i o n , the s m a l l number of l a r g e merchantable l o g s removed from t h i s s i t e at the time of l o g g i n g (based on the number of cut s tumps) , as w e l l as the presence of the l a r g e downed i n d i v i d u a l s p r o b a b l y r e s u l t e d in l e s s d i s t u r b a n c e of the f o r e s t f l o o r at the t ime of h a r v e s t . T a b l e 3.3 i n d i c a t e s an average i n c r e a s e of 2.8 cm in f o r e s t f l o o r depth d u r i n g the 1981 and 1983 p e r i o d i n the 1 0 - y r - o l d (1971) c l e a r c u t . T h i s amounts to an "apparent" i n c r e a s e of about -1 -1 263 k g . N . h a or about 35 t . h a of o r g a n i c m a t t e r . T h i s f a i r l y l a r g e i n c r e a s e i n N c a p i t a l p r e d i c t e d by the r e g r e s s i o n e q u a t i o n u s i n g measured depths of f o r e s t f l o o r in 1981 and 1983 i s thought to be due to the absence of a %N and a BD term i n the e q u a t i o n . The i n c r e a s e i n depth would a c t u a l l y occur in the L l a y e r , but the e q u a t i o n i s based on the f o r e s t f l o o r as a whole . The r e a l i n c r e a s e would be c o n s i d e r a b l y l e s s . T h i s i l l u s t r a t e s one of the. problems a s s o c i a t e d w i t h the use of r e g r e s s i o n e q u a t i o n s of t h i s n a t u r e . 74 3 .4 .2 Rates of decompos i t ion The l a s t s e c t i o n examined the d e c l i n e i n f o r e s t f l o o r n i t r o -gen a f t e r c l e a r c u t t i n g . In t h i s s e c t i o n , s i m i l a r p o s t -c l e a r c u t t i n g changes i n r a t e s of d e c o m p o s i t i o n , m i n e r a l i z a t i o n and n i t r i f i c a t i o n w i l l be d i s c u s s e d . 3 . 4 . 2 . 1 Rate of decompos i t i on as measured by the use of c e l l u l o s e s t r i p s . A l though be tween- s i t e d i f f e r e n c e s were not s i g n i f i c a n t (P=.05) because of the s m a l l sample s i z e (n=3 r e p l i c a t e s per age c l a s s ) , p e r c e n t mass l o s s va lues were a p p r o x i m a t e l y 2 .5 , 3.0 and 2.3 t imes g r e a t e r in the 5- (1978), 8- (1975) , and 12 -year -o l d c l e a r c u t s (1971), r e s p e c t i v e l y , than in the o l d - g r o w t h s i t e s ( F i g u r e 3 . 8 ) . W i t h i n - s i t e v a r i a t i o n was about t w i c e as great in the young c l e a r c u t s as compared to the 2 6 - y r - o l d and o l d - g r o w t h s t a n d s , based on the observed s t a n d a r d d e v i a t i o n s . Inc luded i n -1 F i g u r e 3.8 are the p a t t e r n of f o r e s t f l o o r mass ( t . h a ) and -1 the change in mass ( t . h a ) (mass of f o r e s t f l o o r in the o l d -growth s tand minus tha t i n each of the o ther s tands examined) p l o t t e d a g a i n s t time s i n c e c l e a r c u t t i n g . The p e r i o d of g r e a t e s t percentage l o s s of c e l l u l o s e (5-12 years a f t e r c l e a r c u t t i n g ) more or l e s s p a r a l l e l s the p e r i o d of lowest f o r e s t f l o o r mass and the p e r i o d of g r e a t e s t f o r e s t f l o o r mass l o s s (3-8 y e a r s a f t e r c l e a r -c u t t i n g ) . B i n k l e y (1982) observed an i n c r e a s e i n decompos i t i on of c e l -l u l o s e s t r i p s in low and medium e l e v a t i o n s l a s h b u r n e d s i t e s a f t e r c l e a r c u t t i n g , but not in the h i g h e l e v a t i o n stands d u r i n g 75 Figure 3 .8 Mass l o s s (%) of c e l l u l o s e paper incubated i n the f o r e s t f l o o r (Means and S.E.). Forest f l o o r mass and mass l o s s f o r the chronosequence are provided f o r comparison. 7 6 the summer months (June 7 to August 7 ) . H i s medium e l e v a t i o n s tands had a percentage l o s s of c e l l u l o s e s t r i p s of about 10% p r i o r to c l e a r c u t t i n g and 40-45% a f t e r c l e a r c u t t i n g , as compared to v a l u e s of 11% and 25-33% i n my study ( for an 8 month time p e r i o d between September and A p r i l ) . L e s s e r w i t h i n - s i t e v a r i a t i o n in B i n k l e y ' s (1982) study might have been due to the s l a s h b u r n i n g treatment and i t s homogenizing e f f e c t in terms of s l a s h accumula-t i o n , m o i s t u r e and temperature reg imes . A l though not d i r e c t l y comparab le , F e l l e r et a l . (1983) n o t i c e d somewhat g r e a t e r percentage l o s s of biomass of P o l y s t i c h u m muni - tum L . f ronds a f t e r 24 mos in 3- and 6 - y r - o l d c l e a r c u t s than in o l d e r s tands (11 to 7 3 - y r - o l d ) . In c o n t r a s t , percentage biomass l o s s a f t e r 24 months of D o u g l a s - f i r needles was g r e a t e r in the o l d e r s t a n d s . E a r l i e r s t u d i e s have i n d i c a t e d a s i m i l a r p a t t e r n for needles of a m a b i l i s f i r (Edmonds 1979) and Norway spruce (Berg and Staaf 1980), as those observed for D o u g l a s - f i r by F e l l e r et a l . (1983) . I n c r e a s e s in mass of c e l l u l o s e s t r i p s were observed for some of the samples in the o l d - g r o w t h , 5- and 12-, but not in the 28-y e a r - o l d c l e a r c u t s . An i n c r e a s e in mass suggests an i n i t i a l p e r i o d of c o l o n i z a t i o n by f u n g i or b a c t e r i a (dur ing which n u t r i e n t s are i m m o b i l i z e d ) , p r i o r to decompos i t i on and mass l o s s of c a r b o n . F e l l e r et a l . (1983) and Edmonds (1979) have i n d i c a t e d tha t i n c r e a s e s or decreases in N content were h i g h l y v a r i a b l e d u r i n g decompos i t i on of D o u g l a s - f i r and a m a b i l i s f i r n e e d l e s , r e s p e c t i v e l y , i n s tands of d i f f e r e n t ages . D u r i n g the i n i t i a l sampl ing of the f o r e s t f l o o r in the o l d -growth s tand in 1981, a l a y e r of dark gray to b lack humus w i t h 77 prominent ye l low fung i was f r e q u e n t l y observed between the L l a y e r and a dark red l a y e r of w e l l decomposed r o t t i n g wood. The frequent exposure of t h i s dark r e d l a y e r a t the s u r f a c e in the c l e a r c u t s suggested tha t i t was l o s s of t h i s upper , ye l low f u n g i -dominated dark l a y e r (F or FH) which accounted for much of the d e c l i n e in f o r e s t f l o o r weight a f t e r c l e a r c u t t i n g . As a t e s t of t h i s i n t e r p r e t a t i o n , the f o r e s t f l o o r was c l a s s i f i e d i n t o : 1. r o t t i n g wood (dark brown to red) or 2. humus (gray to b l a c k ) , based on the c o l o u r of the s u r f a c e a c c o r d i n g to my e y e s i g h t d u r i n g the 1983 remeasurement of the f o r e s t f l o o r in the 1978, 1975 and 1971 c l e a r c u t s . T a b l e 3.4 p r e s e n t s da ta on the p e r c e n -tage of the f o r e s t f l o o r s u r f a c e c l a s s i f i e d as r o t t i n g wood (approximates percent cover ) and the net change in f o r e s t f l o o r n i t r o g e n d u r i n g the two year p e r i o d between depth measurements. The s m a l l sample s i z e and the l a r g e v a r i a b i l i t y r e s u l t e d in what appears to be no r e a l t r e n d w i t h t ime s i n c e c l e a r c u t t i n g . How-e v e r , w i t h i n the 3 - y r - o l d r e p l i c a t e s , the r e p l i c a t e w i t h the lowest percent of exposed r o t t i n g wood ( i . e . 24%) had -the s m a l -l e s t d e c l i n e i n f o r e s t f l o o r depth between 1981 and 1983. I f the t h r e e 3 - y r - o l d r e p l i c a t e s have not reached the same p o i n t i n the temporal p a t t e r n of p o s t - h a r v e s t f o r e s t f l o o r d e c l i n e , then the percentage of exposed r o t t i n g wood would be expected to be d i f f e r e n t . U s i n g t h i s i n t e r p r e t a t i o n , the 6 - y r -o l d #2 p l o t had not yet reached as low a p o i n t on the f o r e s t f l o o r d e c l i n e curve as the o ther 6 - y r - o l d p l o t s ; i t had a lower percentage of exposed r o t t i n g wood, but a h i g h e r weight l o s s - i t s t i l l had a r e s e r v e of decomposable m a t e r i a l t h a t c o u l d be l o s t 78 T a b l e 3.4 The percentage of t imes r o t t i n g wood was observed at the s u r f a c e of the f o r e s t f l o o r , based on 3 sample p l o t s per age c l a s s , and 50 o b s e r v a t i o n s per p l o t . The ex tent of f o r e s t f l o o r d e c l i n e based on remeasurement of f o r e s t f l o o r depth and the amount of f o r e s t f l o o r N c a p i t a l p r e d i c t e d by a r e g r e s s i o n equat ion at the end of the two year p e r i o d are a l s o i n d i c a t e d . * ** *** **** S i t e / Percent RW Net Change N i t r o g e n C a p i t a l C l e a r c u t k g . N . h a 1 3 - y r - o l d - 1 24 -161 2082 3 - y r - o l d - 2 36 -1 186 1 940 3 - y r - o l d - 3 38 -778 1921 6 - y r - o l d - 1 30 -76 1427 6 - y r - o l d - 2 20 -588 1 570 6 - y r - o l d - 3 30 -85 1608 1 0 - y r - o l d - 1 44 + 342 1731 1 0 - y r - o l d - 2 44 + 332 2101 l O - y r - o l d - 3 1 6 + 114 1513 * - r e f e r s to age of c l e a r c u t i n 1981. * * - RW= r o t t i n g wood * * * - Net change r e f e r s to the d e c l i n e i n f o r e s t f l o o r N c a p i t a l between 1981 and 1983 based on a r e g r e s s i o n e q u a t i o n between N c a p i t a l and measured depths . * - r e f e r s to t o t a l f o r e s t f l o o r N c a p i t a l p r e d i c t e d in 1983 ( a f t e r the d e c l i n e ) based on a r e g r e s s i o n e q u a t i o n . 79 and i t had a 6.2 cm d e c l i n e i n f o r e s t f l o o r depth between 1981 and 1983, as compared to o n l y 0.8 and 0.9 cm d e c l i n e s in the o ther two 6 - y r - o l d p l o t s . Based on the r e s u l t s from d e c o m p o s i t i o n s t u d i e s , i t appears that branches and bark decompose more s l o w l y than do needles and twigs (Foge l and Cromack 1977; F e l l e r et a l . 1983), and that decompos i t i on r a t e s are r e l a t e d to l i g n i n c o n t e n t , w i th h igher l i g n i n content be ing found i n branches and bark than i n n e e d l e s . Given the above, the so c a l l e d " r o t t i n g wood" which I i d e n t i f i e d as one group shou ld have a h i g h e r l i g n i n content than "humus" ( de f . i m p l y i n g the dominant presence of f i n e o r g a n i c m a t e r i a l s : eg . or th ihumimor: K l i n k a et a l . 1981). I t c o u l d be most ly the "humus" p o r t i o n of the f o r e s t f l o o r which undergoes m o b i l i z a t i o n a f t e r c l e a r c u t t i n g . C l a u s n i t z e r ( i n E l l e n b e r g 1978) has i n d i c a t e d a g r e a t e r tendency for "moder" types of f o r e s t f loor-- . - to show pronounced i n c r e a s e s i n N m i n e r a l i z a t i o n a f t e r c l e a r c u t t i n g , as compared to " a c i d or raw humus" t y p e s . F u r t h e r work needs to be done to f o l l o w the p o s t - c l e a r c u t t i n g d e c l i n e of d i f f e r e n t f o r e s t f l o o r types (eg. l ignohumimor v s . or th ihumimor and humimor v s . hemimor) ( K l i n k a et a l . 1981). 3 . 4 . 2 . 2 I_n s i t u m i n e r a l i z a t i o n of f o r e s t f l o o r : an index of m i n e r a l i z a t i o n r a t e s a f t e r c l e a r c u t t i n g . T a b l e 3 .5 p r o v i d e s some r e s u l t s for s i e v e d (< 2mm) and i n c u b a t e d FH l a y e r m a t e r i a l i n p o l y e t h y l e n e bags (Eno 1960) for 8 week p e r i o d s d u r i n g the summer (June and J u l y ) and f a l l (Septem-ber and October ) of 1982. As T a b l e 3.5 i n d i c a t e s , no s i g n i f i c a n t (P=.05) between-s tand d i f f e r e n c e s were observed i n the summer 80 T a b l e 3.5 In s 1 t u I n c u b a t i o n s o f s i e v e d FH l a y e r m a t e r i a l In p o l y e t h y l e n e bags In the f o r e s t f l o o r In the chronosequence stan d s . S i t e / * E x t r a c t e d Net M i n e r a l i z a t i o n M o i s t u r e C l e a r c u t N 1 t r a t e - N Ammon1um-N N l t r a t e - N Ammonium-N T o t a l - N Content % Summer 1982 * * Mean S. D. Mean S.D. Mean S D. Mean S.D. Mean S.D. Mean S.I o l d - g r o w t h 0.30 0. .09 84.4 32.7 -.90 1 . 04 40.4 33.8 39.4 32.9 218 48 4 - y r - o l d 0.34 0. . 1 1 77 . 1 32.9 - .56 0. . 36 38.7 34 . 1 38. 1 33.0 248 84 7 - y r - o l d 0.36 0. , 27 76.5 25.0 - .70 0. . 56 34 .6 18.9 33.9 22 . 1 204 72 1 1 - y r - o l d 0.50 0 59 65.3 24 .0 - .61 0. . 77 29 .0 17.7 28 . 7 17.2 187 66 Average 0.37 0. . 32 75.8 59.0 - .70 0, . 72 35 . 7 27 .6 35.0 4.0 214 70 F a l l 1982 ****** o l d - g r o w t h 0 81b . 18 44 .8 14 . 1 0. . 54ab . 25 -2 .6 14. .7 -2 .0 14. 3 120b 63 4 - y r - o l d 1 . 11a .38 56 .5 30. . 4 0. 81a . 38 17. .7 14 .5 18 .4 12. 9 179a 75 7 - y r - o l d 1 . 08a . 25 51 . 5 18 . 9 0. . 79a . 23 14 .5 18 . 4 15 .3 17 . 6 164a 57 1 1 - y r - o l d 0 77b . 23 36 .8 16, . 1 0. 34b .32 5 .6 16 . 3 6 . 1 15 . 8 96b 30 2 7 - y r - o l d 1 08a .41 56 .8 16 , O 52c . 70 7 4 25. .8 7 .0 24. 9 95b 21 Average 0 .95 .32 48 . 2 23 , .0 0. 32 0 .65 7 . 1 19. .5 9 .0 7. 2 123 57 00 * - C l e a r c u t age i n 1982. ** - Means f o l l o w e d by the same l e t t e r a r e not s i g n i f i c a n t l y d i f f e r e n t at P=.05 u s i n g Duncans MRT. Means f o l l o w e d by no l e t t e r s a r e the same. F i v e samples were i n c u b a t e d per r e p l i c a t e w i t h 3 r e p l i c a t e s per age c l a s s . *** - E x t r a c t e d r e f e r s to KC1 e x t r a c t e d c o n c e n t r a t i o n at the end of the 8 week I n c u b a t i o n p e r i o d . **** - Net m i n e r a l i z a t i o n r e f e r s to KC1 e x t r a c t e d at the end minus at the b e g i n n i n g of the i n c u b a t i o n p e r i o d . ***** - M o i s t u r e c o n t e n t by mass (%) at the b e g i n n i n g of the i n c u b a t i o n p e r i o d . ****** - S i g n i f i c a n t F - t e s t (P=.05): but no d i f f e r e n c e s between ages u s i n g Duncans MRT. d a t a . However, i n c u b a t i o n s c a r r i e d out in the f a l l produced h i g h e r l e v e l s of n i t r a t e i n samples taken from the 4- (1978) and 7 - y e a r - o l d (1975) c l e a r c u t s . A l t h o u g h the 2 7 - y e a r - o l d (1955) c l e a r c u t s ( f a l l samples) e x h i b i t e d r e l a t i v e l y h i g h v a l u e s for f i n a l e x t r a c t e d n i t r a t e , m i n e r a l i z e d n i t r a t e was the lowest of the f a l l samples . T h i s suggests tha t e i t h e r l i t t l e or no n i t r i f i -c a t i o n , or tha t d e n i t r i f i c a t i o n o c c u r r e d d u r i n g the i n c u b a t i o n p e r i o d . The r e l a t i v e l y h i g h i n i t i a l . v a l u e s for e x t r a c t e d n i t r a t e in the 2 7 - y r - o l d stands c o u l d have been due to the s i e v i n g d i s -turbance . Other s t u d i e s have r e p o r t e d s i m i l a r r e s u l t s when f i n a l ex-t r a c t e d v a l u e s are c o r r e c t e d f o r i n i t i a l c o n c e n t r a t i o n s of m i n e r a l - N (Powers 1980), and i t i s sometimes advocated tha t on ly the f i n a l e x t r a c t e d c o n c e n t r a t i o n be used in comparing between s i t e s . However, f i n a l e x t r a c t e d v a l u e s (as opposed to m i n e r a l i z e d v a l u e s ) tended to p r o v i d e fewer d i f f e r e n c e s between s t a n d s , i m p l y i n g l e s s d i f f e r e n t i a t i n g power. In the case of ammonium, s i g n i f i c a n t (P=.05) d i f f e r e n c e s were not observed between s t a n d s . M i n e r a l i z e d ( n i t r a t e + ammonium) ( i . e . m i n e r a l i z e d N) e x h i b -i t e d the same temporal p a t t e r n as tha t of f o r e s t f l o o r n i t r o g e n c a p i t a l d e c l i n e ( F i g u r e s 3.5 and 3 . 7 ) ; the g r e a t e s t r e d u c t i o n in f o r e s t f l o o r n i t r o g e n o c c u r r e d between 3 to 8 y e a r s a f t e r c l e a r -c u t t i n g . Fewer s i g n i f i c a n t (P=.05) d i f f e r e n c e s i n m i n e r a l i z e d N were observed between s tands as compared to tha t for n i t r a t e a l o n e . The Jj2 s i t u i n c u b a t i o n r e s u l t s are comparable to those found by Matson and V i t o u s e k (1981) . These a u t h o r s observed 82 h i g h e r m i n e r a l i z e d n i t r a t e in c l e a r c u t than i n nearby mature s tands over 30 day i n c u b a t i o n p e r i o d s both i n the f i e l d and i n the l a b o r a t o r y , under v a r y i n g temperature c o n d i t i o n s . However, they observed on ly s m a l l d i f f e r e n c e s i n m i n e r a l i z e d N between c l e a r c u t and uncut s t a n d s ; an i n c r e a s e i n m i n e r a l i z e d N was observed in a 4 - y e a r - o l d but not in a 6- or 1 1 - y e a r - o l d c l e a r c u t when f o r e s t f l o o r taken from the mature s t a n d was p l a c e d in each of the c l e a r c u t s (Matson and V i t o u s e k 1981). P o p o v i c ' s (1974) study a l s o showed a much g r e a t e r d i f f e r e n c e i n n i t r a t e m i n -e r a l i z a t i o n in the f o r e s t f l o o r between a 1 - y r - o l d c l e a r c u t (393 ppm) and an adjacent uncut spruce s tand (4 .2 ppm), as compared to a (ammonium + n i t r a t e ) m i n e r a l i z a t i o n d i f f e r e n c e of on ly 503 v e r s u s 406 ppm. The above r e s u l t s are s i m i l a r to t h a t found i n my study in the sense of g r e a t e r m i n e r a l i z a t i o n of n i t r a t e and fewer d i f f e r -ences in terms of m i n e r a l i z e d N . However, i n my study the temporal p a t t e r n i s very s i m i l a r for both m i n e r a l i z e d n i t r a t e and m i n e r a l i z e d N. A l s o , the t r e n d i s f or g r e a t e r m i n e r a l i z e d N up to at l e a s t 7 years a f t e r c l e a r c u t t i n g . F a l l da ta for f o r e s t f l o o r n i t r a t e m i n e r a l i z a t i o n v a l u e s are about t w i c e as h i g h and m i n e r a l i z e d N va lues 2.5 to 3 t imes as h i g h i n the 4 and 7 - y r - o l d c l e a r c u t s as compared to v a l u e s in the 1 1 - y r - o l d c l e a r c u t . Data i n T a b l e 3.5 suggest that between-sample v a r i a t i o n i s g r e a t e r f o r ammonium than i t i s f or n i t r a t e ; as a r e s u l t a g r e a t e r number of samples i s needed to d e t e c t s i m i l a r s i g n i f i c a n t d i f f e r e n c e s between treatments for ammonium than f o r n i t r a t e . The p a t t e r n of n i t r o g e n a v a i l a b i l i t y a f t e r a n a t u r a l d i s -turbance such as wave-form d i e b a c k of mountain hemlock was m o n i -83 t o r e d by Matson and Boone (1984) . They r e c o r d e d h i g h e r rn s i t u (about twice) and l a b o r a t o r y a n a e r o b i c N m i n e r a l i z a t i o n r a t e s in so c a l l e d ' b a r e zone' and 'young regrowth ' areas as compared to o l d ' r e g r o w t h ' and o l d - g r o w t h a r e a s . 'Bare zone 'and 'young r e -growth' areas are comparable to the 3 - y e a r - o l d c l e a r c u t s and 5-1 0 - y e a r - o l d c l e a r c u t s , w h i l e ' o l d regrowth ' areas are s i m i l a r to the 2 7 - y e a r - o l d s tands i n my s t u d y . However, c l e a r c u t s p r o b a b l y d i f f e r from these d ieback areas in many r e s p e c t s , i n c l u d i n g the e f f e c t s of l o g g i n g , and d i f f e r e n c e s in temperature regimes due to v a r i a t i o n s i n the amounts of m a t e r i a l l e f t e i t h e r s tand ing or l y i n g on the s i t e . V i t o u s e k and Matson ( u n p u b l i s h e d ) found i_n s i t u r a t e s of m i n e r a l i z a t i o n in the f o r e s t f l o o r and upper m i n e r a l s o i l to be about four t imes g r e a t e r in t h e - f i r s t year and about two t imes g r e a t e r in the second year a f t e r c l e a r c u t t i n g l o b l o l l y p ine s tands than i n an u n d i s t u r b e d 2 0 - y e a r - o l d s tand nearby . Most of t h i s m i n e r a l i z e d N was i n the n i t r a t e form on the c u t o v e r s i t e s , e s p e c i a l l y in the f i r s t y e a r . L a b o r a t o r y i n c u b a t i o n s a l s o i n d i c a t e d h i g h e r r a t e s of n i t r i f i c a t i o n in m i n e r a l s o i l s taken from c l e a r c u t s i t e s than i n the u n d i s t u r b e d s i t e s . G l a v a c and Koenies (1978a) observed about a 278% i n c r e a s e in N m i n e r a l i z a t i o n in the L , FH and Ah l a y e r s i n a c l e a r c u t as compared to an uncut spruce s tand i n Germany over a 6 month p e r i o d u s i n g s o i l m o n o l i t h s excava ted from the uncut s t a n d . However, no d i f f e r e n c e i n N m i n e r a l i z a t i o n was observed in cut and uncut beech s tands (Glavac and Koenies (1978b) . Based on r e l a t i v e l y l a r g e d i f f e r e n c e s (2 .5 to 3 t imes) in 84 m i n e r a l i z a t i o n r a t e s between 4- and 7 - y r - o l d c l e a r c u t s and 1 1 - y r -o l d c l e a r c u t s , i t appears that the p e r i o d of g r e a t e r m i n e r a l i z a -t i o n of f o r e s t f l o o r N a f t e r c l e a r c u t t i n g may p e r s i s t l onger in west coas t mixed c o n i f e r s i t e s s i m i l a r to those of my study due to the g e n e r a l l y c o o l e r temperature reg imes , as compared to those i n s o u t h e a s t e r n U . S . A . However, there i s not a l o t of data for c l e a r c u t s o l d e r than one to two y e a r s . 3 . 4 . 2 . 3 F a c t o r s c o n t r o l l i n g r a t e s of decompos i t i on and N m i n e r a -l i z a t i o n . The e f f e c t of m o i s t u r e , t emperature and i n i t i a l l e v e l s of ammonium and n i t r a t e on the p r o c e s s of m i n e r a l i z a t i o n , and the e f f e c t of m o i s t u r e and temperature on c e l l u l o s e d e c o m p o s i t i o n , in cut and uncut fores t s - i n the study a r e a were e v a l u a t e d . C o n s i d e r i n g the s m a l l d i f f e r e n c e s observed between c h r o n o -sequence s i t e s for pH ( i n .01 M c a l c i u m c h l o r i d e ) of the f o r e s t f l o o r and the m i n e r a l s o i l , pH does not seem to have been an important f a c t o r in the changes i n m i n e r a l i z a t i o n r a t e s over the chronosequence ( F i g u r e 3 . 9 ) . However, t h i s i s based on very l i m i t e d s a m p l i n g , where composi te samples were taken from p i t s dug for the purpose of s o i l d e s c r i p t i o n . Popovic (1974) has observed t h a t c l e a r c u t areas w i th r e l a -t i v e l y h i g h pH e x h i b i t e d the h i g h e s t r a t e s of ijrt s i t u m i n e r a l i z a -t i o n of n i t r a t e , but areas of h i g h e r pH in the uncut s tand d i d not show h i g h e r r a t e s of i_n s i t u m i n e r a l i z a t i o n of n i t r a t e . T h i s suggests t h a t changes i n s o i l pH were not s o l e l y r e s p o n s i b l e for the changes i n _in s i t u n i t r a t e m i n e r a l i z a t i o n a f t e r c l e a r c u t t i n g . 8 5 X Q_ o o ID' • O in" D ro' o 300-YR-OLD 0.0 o CD" O in' o o ro' o r\3. 330-TO-OLD 0.0 o (D ' O LO ' O ro" o 300-YR-OLD 0.0 o ID' O o to' o 300-YR-OLD 0.0 FH LAYER i 1 — 5.0 10.0 15.0 — I 1 1 20.0 25.0 30.0 A HORIZON 5.0 — I 1 1 1 1 10.0 15.0 20.0 25.0 30.0 B HORIZON • -1 1 — 5.0 10.0 — I 1 1 1 15.0 20.0 25.0 30.0 C HORIZON — • __! j 5.0 10.0 — i 1 15.0 20.0 25.0 30.0 TIME SINCE CLEARCUTTING years F i g u r e 3 . 9 S o i l pH o f each o f the h o r i z o n s f o r the chronosequence o f stan d s examined. 8 6 3 . 4 . 2 . 3 . 1 Temperature Between September 1982 and September 1983, the average a i r temperature on the 1 0 - y r - o l d s i t e was 1.4 deg C h i g h e r than i n the o l d - g r o w t h s t a n d , which was s i g n i f i c a n t (P=.05) u s i n g a p a i r e d t - t e s t . Between-stand d i f f e r e n c e s were as l a r g e as 2.5 deg C in the s p r i n g and summer months (Table 3 . 6 ) . FH l a y e r tem-p e r a t u r e was g e n e r a l l y s l i g h t l y lower than a i r t e m p e r a t u r e , ex-c e p t i o n s be ing in October i n the o l d - g r o w t h and August i n the 10-y e a r - o l d c l e a r c u t . Between-stand d i f f e r e n c e s in d a i l y maximum a i r temperature o f t en approached 10 deg C in the summer months ( there was l e s s of a d i f f e r e n c e in d a i l y minimum), but i n g e n e r a l the d i f f e r e n c e was l e s s than 5 deg C between the c l e a r c u t and o l d - g r o w t h s t a n d . T h e ' average annual d i f f e r e n c e in d a i l y maximum a i r temperature was on ly 3.3 deg C as compared to 1.2 deg C for a s i m i l a r va lue for the middle of the FH l a y e r . Given the number of in s t ruments used , not too much c o n f i d e n c e can be p l a c e d in the r e s u l t s . However, a i r temperature might be expected to v a r y l e s s w i t h i n the c l e a r c u t , hence one hygrothermograph p r o b a b l y p r o v i d e s a reasonab le e s t imate of the a i r t e m p e r a t u r e . Given tha t the average FH l a y e r temperature d i d not d i f f e r a l l that much from the average a i r t emperature , where da ta i s a v a i l a b l e (Table 3 . 6 ) , i t appears that the d i f f e r e n c e in average temperature between the 10 to 1 1 - y r - o l d c l e a r c u t and the o l d growth s tand i n the F l e e t R i v e r s tudy was somewhat s m a l l e r than that r e c o r d e d in some pas t s t u d i e s . F o r , example Timmer and Weetman (1969) noted about a 5-8 deg C i n c r e a s e i n mean summer s o i l t emperature a f t e r c l e a r c u t t i n g b l a c k spruce in e a s t e r n 87 T a b l e 3.6 Comparison of average a i r and f o r e s t f l o o r temperature between the o l d - g r o w t h f o r e s t and a 10 to 1 1 - y r - o l d c l e a r c u t (1971) between September 1982 and September 1983. Average Temperature (deg C) Time P e r i o d o l d -A i r * •growth FH l a y e r -10-yr A i r - o l d C l e a r c u t FH l a y e r * * S e p t . 8 - O c t . 1 4 11.0 11.7 Oct . 1 5 - Nov.8 6.3 8.9 7.4 4.5 Nov.9 - Dec.15 2.2 -- 3.1 --D e c . 1 6 - Jan 20 1 .7 1 .0 3.2 1 .0 J a n . 21- F e b . 20 3.0 . 2.2 4.5 1 .6 F e b . 21 - A p r i l 6 4.7 — 5.6 — A p r i l 7 - May 31 7.8 — 10.3 — June 1 - June 28 9.9 9.5 12.3 9.2 June 29 - J u l y 26 11.8 11.0 13.5 12.5 J u l y 27 - Sept . 7 14.2 11.7 15.2 15.9 Annual Average 7 2 * * * — 8.6 — — * - Average a i r temperature was moni tered u s i n g a hygrothermograph. * * - Average FH l a y e r temperature was e s t i m a t e d from 3 max-min thermometers in each of the above s tands . * * * - Average d i f f e r e n c e in a i r temperature was 1.4 deg C , which was s i g n i f i c a n t l y d i f f e r e n t from zero based on a p a i r e d t - t e s t at P=.05. 88 Canada. Whereas in my s t u d y , the d i f f e r e n c e i n average d a i l y t emperature i n the middle of the FH l a y e r between a 1 0 - y r - o l d and o l d - g r o w t h s tand was on ly about 2 deg C for the same time p e r i o d . Between-s tand d i f f e r e n c e s i n average temperature for the FH l a y e r might have been g r e a t e r i f the 3- or 5 - y r - o l d s tands would have been compared to the o l d - g r o w t h stand', c o n s i d e r i n g the -1 number o f s tems .ha in each s i t e . However, g i v e n a Q of 2 .0 , a 1 0 d i f f e r e n c e of between 10 to 20 deg C would be needed to account for t h r e e t imes h igher v a l u e s f o r c e l l u l o s e decompos i t i on a f t e r c l e a r c u t t i n g , un le s s d i f f e r e n c e s i n maximum d a i l y temperature and temperature t h r e s h o l d s are c o n s i d e r e d . Decomposers might a l s o respond to the change in temperature through changes i n numbers as w e l l as i n a c t i v i t y l e v e l s . S i m i l a r r e a s o n i n g i s needed to account f o r the l a r g e d i f f e r e n c e (P=.05) between N m i n e r a l i z a t i o n in the 3- (18.4) and 6 - y e a r - o l d c l e a r c u t s ( 1 5 . 3 ) , and the o l d --1 -1 growth s t a n d ( -2 .0 ppm.g .8 weeks ) d u r i n g the f a l l p e r i o d . A s i m i l a r p a t t e r n in both the l a b o r a t o r y and f i e l d of N m i n -e r a l i z a t i o n i n s o i l taken from 'bare zones' and 'young regrowth' areas a f t e r wave-form d i e b a c k i n mountain hemlock suggest the importance of s u b s t r a t e q u a l i t y as opposed to temperature in c o n t r o l l i n g n i t r o g e n m i n e r a l i z a t i o n a f t e r a d i s t u r b a n c e (Matson and Boone 1984). A f t e r c l e a r c u t t i n g l o b l o l l y p i n e s tands i n Nor th C a r o l i n a , V i t o u s e k and Matson (unpubl i shed) r e c o r d e d 2 to 4 f o l d i n c r e a s e s i n N m i n e r a l i z a t i o n , but on ly very minor i n c r e a s e s i n t e m p e r a t u r e . 3 . 4 . 2 . 3 . 2 M o i s t u r e content The method of f i e l d i n c u b a t i o n d i d not a l l o w for s i g n i f i c a n t 89 water vapour l o s s d u r i n g the e i g h t week p e r i o d of i n c u b a t i o n (Eno 1960). Thus , w i th respec t to m i n e r a l i z a t i o n r a t e s , o n l y i n i t i a l m o i s t u r e c o n t e n t s are of importance . On a s t a n d - b y - s t a n d b a s i s t h e r e were no s i g n i f i c a n t (P=.05) d i f f e r e n c e s in the i n i t i a l m o i s t u r e content in the summer i n c u b a t i o n s ( i . e . J u l y 1-3 to September 1-3) , but d i f f e r e n c e s i n f o r e s t f l o o r m o i s t u r e content between s tands had expressed themselves by August or September (Table 3 . 7 ) . In g e n e r a l , lowest m o i s t u r e content (MC) v a l u e s were r e c o r d e d in the 1 0 - y r - o l d c l e a r c u t s , i n t e r m e d i a t e MC's in the o l d growth and 2 6 - y r - o l d s tands and the h i g h e s t MC's in the 3- and 6-y r - o l d c l e a r c u t s d u r i n g the p e r i o d between J u l y and November of 1982. Carbon m i n e r a l i z a t i o n of s u r f a c e l i t t e r i s thought to be r e l a t i v e l y u n a f f e c t e d by moi s ture above about 100% by mass, a c c o r d i n g to Wiant (1967) and Piene and van C l e v e (1976) . In only a few cases—did MC f a l l below 100% by mass in the FH l a y e r s in the F l e e t R i v e r s tands d u r i n g the summer and f a l l of 1982. How-e v e r , the ' c r i t i c a l ' MC may not be the same for a l l the o r g a n i c l a y e r s or for m i n e r a l i z a t i o n of N or n i t r i f i c a t i o n , as compared w i t h l o s s of c a r b o n . A s i g n i f i c a n t c o r r e l a t i o n (r= 0 .93 , P=.01) was o b t a i n e d between average moi s ture content and average m i n e r a l i z e d N v a l u e s f o r the f a l l i n c u b a t i o n s (Table 3 . 5 ) . However, on a sample by sample b a s i s , the r - v a l u e dropped to 0.34 (Table 3 . 8 ) . E x -t r a c t e d ammonium and m i n e r a l i z e d ammonium were both s i g n i f i c a n t l y (P=.01) c o r r e l a t e d wi th moi s ture content (r=0.26 and 0.36) on a sample by sample b a s i s , as w e l l (Table 3 . 8 ) . I t appears tha t moi s ture content had v a r i a b l e e f f e c t s on n i t r i f i c a t i o n depending on the a b s o l u t e MC of the f o r e s t f l o o r , 90 T a b l e 3.7 M o i s t u r e content (% by mass) of the FH l a y e r of the f o r e s t f l o o r d u r i n g the summer and f a l l of 1982.** * S i t e / J u l y August September October C l e a r c u t o l d - g r o w t h 218 (46) 99 (27) 1 20 (61 ) 1 37 (33) 4 - y r - o l d 248 (81 ) 1 94 (83) 1 93 (70) 223 (63) 7 - y r - o l d 204 (69) 1 27 (64) 1 58 (55) 127 (48) 1 1 - y r - o l d 174 (62) 1 34 (37) 96 (30) 1 19 (54) 2 7 - y r - o l d 95 (21 ) 1 43 (46) * - Age of c l e a r c u t in 1982. * * - Means ( S . D . in b r a c k e t s ) are g r a v i m e t r i c d e t e r m i n a t i o n s of 5 FH samples per r e p l i c a t e , and 3 r e p l i c a t e s per age c l a s s c o l l e c t e d in the f i r s t week of each month. 91 which v a r i e d both between the summer and f a l l p e r i o d s , and the t ime s i n c e c l e a r c u t t i n g . For example, both e x t r a c t e d and m i n e r -a l i z e d n i t r a t e were p o s i t i v e l y c o r r e l a t e d wi th moi s ture content i n the o l d - g r o w t h , 26- and 1 0 - y r - o l d c l e a r c u t s i n the f a l l i n c u -b a t i o n s , where m o i s t u r e content had dropped to 120, 95 and 96% by mass. Whereas, o v e r a l l , and e s p e c i a l l y in the younger c l e a r c u t s d u r i n g the summer ( J u l y and August) p e r i o d , h i g h m o i s t u r e c o n -t e n t s were a s s o c i a t e d wi th lower n i t r a t e l e v e l s , p o s s i b l y due to d e n i t r i f i c a t i o n . A l though s e a s o n a l or temporal p a t t e r n s of N m i n e r a l i z a t i o n in a s s o c i a t i o n w i t h MC have been w e l l documented (eg. E l l e n b e r g (1977): in a r e n d z i n a under a 1imestone-beechwood f o r e s t over a two year p e r i o d or V i t o u s e k and Matson ( u n p u b l i s h e d ) : l o b l o l l y p i n e stands in N o r t h C a r o l i n a ) , m o i s t u r e c o n t e n t ' s i n f l u e n c e on N m i n e r a l i z a t i o n ' a f t e r c l e a r c u t t i n g has not always been i m p o r t a n t . The l a t t e r a u t h o r s d i d not observe s i g n i f i c a n t i n c r e a s e s in MC a f t e r c l e a r c u t t i n g i n Nor th C a r o l i n a , nor c o u l d they a t t r i b u t e the i n c r e a s e i n m i n e r a l i z e d n i t r a t e to i n c r e a s e d m o i s t u r e content a f t e r c l e a r c u t t i n g in I n d i a n a , a l t h o u g h they d i d demonstrate the i n f l u e n c e of MC in the l a b o r a t o r y (Matson and V i t o u s e k 1981). In c o n t r a s t , there appears to be a f a i r l y s t r o n g r e l a t i o n s h i p be-tween MC and m i n e r a l i z e d N in the f o r e s t f l o o r , based on the F a l l i n s i t u i n c u b a t i o n s for the chronosequence s i t e s in the F l e e t R i v e r study a r e a . The r e l a t i o n s h i p i s b e t t e r when s tand averages of MC and m i n e r a l i z e d - N are used , presumably because of h i g h w i t h i n - s t a n d v a r i a b i l i t y of many f a c t o r s . Based on s i m i l a r p a t -t e r n s of change in both FH and Ah h o r i z o n MC, and N m i n e r a l i z a -t i o n d u r i n g the summer months a f t e r c l e a r c u t t i n g Norway spruce 92 s tands in Germany, i t appears that s o i l MC was an important de terminant i n t h e i r study as w e l l (Glavac and Koenies (1978a) . 3 . 4 . 2 . 3 . 3 Ammonium and n i t r a t e l e v e l s T a b l e 3.8 summarizes the c o r r e l a t i o n c o e f f i c i e n t s between the i n i t i a l l e v e l s of e x t r a c t e d ammonium- or n i t r a t e - N and in  s i t u m i n e r a l i z e d ammonium-, n i t r a t e - , and (ammonium + n i t r a t e ) -N . M i n e r a l i z e d n i t r a t e was n e g a t i v e l y ( r=-0 .86 , P<.01) c o r r e l a t e d w i t h i n i t i a l n i t r a t e c o n c e n t r a t i o n s . I f i n i t i a l n i t r a t e l e v e l s were a good i n d i c a t i o n of the a c t i v i t y of n i t r i f y i n g b a c t e r i a , then a p o s i t i v e c o r r e l a t i o n would have been e x p e c t e d . I t i s p o s s i b l e tha t the n e g a t i v e c o r r e l a t i o n r e f l e c t s d e n i t r i f i c a t i o n w i t h i n the bags . S i e v i n g of the FH l a y e r m a t e r i a l w h i l e moist d i d tend to produce a 'mass ive ' s t r u c t u r e , which c o u l d have favoured d e n i t r i f i c a t i o n . Negat ive m i n e r a l i z a t i o n of n i t r a t e was more common i n the summer i n c u b a t i o n s , than in the f a l l i n c u b a t i o n s where i n i t i a l m o i s t u r e c o n t e n t s were c o n s i d e r a b l y l o w e r . M i n e r a l i z e d ammonium and m i n e r a l i z e d N were not s i g n i f i -c a n t l y (P=.05) c o r r e l a t e d w i t h i n i t i a l c o n c e n t r a t i o n s of ammonium or n i t r a t e . 3 . 4 . 2 . 3 . 4 " G a d g i l and G a d g i l e f f e c t " : i n d i r e c t ev idence The presence of t r e e cover i s thought to be important i n the r e g u l a t i o n of d e c o m p o s i t i o n , through the e f f e c t of t r e e r o o t s ( G a d g i l and G a d g i l 1975; 1978) and p o s s i b l y t h r o u g h f a l l (Gosz 1984) on decomposer o r g a n i s m s . U s i n g the three p l o t s i n the 3-(1978), 6- (1975) and 1 0 - y r - o l d (1971) c l e a r c u t s t o g e t h e r , r -v a l u e s of -0 .72 (P <.05) and -0 .84 (P<.01.) were o b t a i n e d between 93 T a b l e 3.8 C o r r e l a t i o n c o e f f i c i e n t (r ) between d i f f e r e n t parameters measured in the jm s i t u i n c u b a t i o n e x p e r i -ments of FH m a t e r i a l f or a l l s tands combined for the summer and f a l l of 1982.* V a r i a b l e 1. E x t r a c t e d N03-N** 1.00 2. E x t r a c t e d NH4-N - . 3 6 1.00 3. M i n e r a l i z e d N03-N .56 - . 3 0 1.00 4. M i n e r a l i z e d NH4-N - . 3 7 - . 9 0 - . 2 2 1.00 5. M i n e r a l i z e d N - .32 .88 - . 1 2 .99 1.00 6. M.C. (%) - . 3 7 .26 - . 2 2 .36 .34 1.00 7. I n i t i a l N03-N - . 0 6 .15 - . 8 6 .04 - . 0 5 .04 1.00 8. I n i t i a l NH4-N - .02 .35 - . 2 0 - . 1 0 - . 1 2 - . 1 7 .23 1.00 V a r i a b l e 1 2 3 4 5 6 7 8 * - r - c r i t i c a l (P=.05) =0.1771; (P=.01) =0.2315 * * - a l l v a r i a b l e s are in ppm, except m o i s t u r e content ( M . C . ) which i s in %. 94 f o r e s t f l o o r d e c l i n e between 1981 and 1983 and n i t r o g e n accumu-l a t e d in r e g e n e r a t i o n , and in both r e g e n e r a t i o n + u n d e r s t o r y v e g e t a t i o n , r e s p e c t i v e l y . The r e p l i c a t e s w i th the l e a s t d e c l i n e had the l e a s t amount of n i t r o g e n accumulated in e i t h e r r e g e n e r a -t i o n or u n d e r s t o r y biomass ( i . e . the r e p l i c a t e s i n the 1978 c l e a r c u t as w e l l as 1975-2) . The c o r r e l a t i o n p r o v i d e s i n d i r e c t ev idence s u p p o r t i n g the c o n c e p t ( s ) t h a t : 1. the absence of m y c o r r h i z a l r o o t s in the f o r e s t f l o o r promotes decompos i t i on and subsequent m i n e r a l i z a t i o n of n i t r o g e n a n d / o r 2. the absence of canopy wash c o n t a i n i n g i n h i b i t o r y c h e m i c a l s promotes decomposer a c t i v i t y . The d i f f e r e n -ces in temperature between the 3 - y r - o l d r e p l i c a t e s would appear to be s m a l l g i v e n the s m a l l d i f f e r e n c e s observed between the o l d growth and 1 0 - y r - o l d s i t e s for both average a i r and FH l a y e r t empera ture . M o i s t u r e content of the f o r e s t f l o o r in the 1978 r e p l i c a t e s d i d not vary s i g n i f i c a n t l y (P=.05) d u r i n g the months examined (Tab le 3 . 7 ) , a l t h o u g h the one wi th the g r e a t e s t amount of r e g e n e r a t i o n had a s l i g h t l y lower v a l u e for MC (203) than the o ther two (240 and 300%). F u r t h e r work i s necessary to assess the importance of t h r o u g h f a l l and a l l e l o p a t h i c e f f e c t s i n decom-p o s i t i o n and m i n e r a l i z a t i o n p r o c e s s e s , e s p e c i a l l y w i t h regards to the "assar t e f f e c t " . 3 . 4 . 3 I n d i c e s of n i t r o g e n a v a i l a b i l i t y T h i s s e c t i o n p r e s e n t s r e s u l t s on a l l the i n d i c e s used to a s ses s the a v a i l a b i l i t y of N for the chronosequence i n v e s t i g a t e d . These i n c l u d e s o i l water c h e m i s t r y , ion exchange r e s i n s , KC1 e x t r a c t i o n s in. the m i n e r a l s o i l , and a b i o a s s a y u s i n g f o l i a r N 95 and h e i g h t growth of a m a b i l i s f i r r e g e n e r a t i o n . Changes i n N a v a i l a b i l i t y r e f l e c t both changes in m i n e r a l i z a t i o n as w e l l as b i o l o g i c a l demand. In the case of i n d i c e s of s o i l N a v a i l a b i l i t y demand from the s o i l or (net change in s o i l N c a p i t a l c . f . M i l l e r 1984) w i l l i n f l u e n c e N a v a i l a b i l i t y more so than t o t a l demand ( i . e . t r e e uptake + r e t r a n s l o c a t i o n w i t h i n t r e e ) . Net change in s o i l N c a p i t a l i s e q u i v a l e n t to t h a t r e l e a s e d from t r e e s ( i . e . l i t t e r f a l l + canopy wash) + a tmospher ic input - t r e e u p t a k e ) . M i l l e r (1984) p r e s e n t e d ev idence tha t in P inus n i g r a v a r . m a r i --1 t ima p l a n t a t i o n s net change i n s o i l N c a p i t a l was 40 k g . N . h a -1 -1 -1 yr in a 1 0 - y r - o l d , as compared to on ly 13 k g . N . h a . y r i n a 4 0 - y r - o l d p l a n t a t i o n . The changes in a v a i l a b i l i t y of n i t r a t e as compared to ammo-nium and t o t a l - N w i l l vary in accordance to f a c t o r s which c o n t r o l n i t r i f i c a t i o n . Such f a c t o r s are d i s c u s s e d in g r e a t e r d e t a i l , e s p e c i a l l y w i t h r e f e r e n c e to l e a c h i n g l o s s e s i n Chapter 5. 3 . 4 . 3 . 1 S o i l water c h e m i s t r y T a b l e 3 .9 p r e s e n t s average c o n c e n t r a t i o n s (weighted by volume) of n i t r a t e - , ammonium-, o r g a n i c - N ( t o t a l - N - ( n i t r a t e + ammonium)) and t o t a l - N (TPN) ppm i n the f o r e s t f l o o r l e a c h a t e over a two year p e r i o d of o b s e r v a t i o n . As i n d i c a t e d i n T a b l e 3 . 9 , there were s l i g h t l y h i g h e r v a l u e s r e c o r d e d in the youngest c l e a r -c u t s and the o l d - g r o w t h s t a n d , but they were not s i g n i f i c a n t l y d i f f e r e n t (P=.05) from the o ther s t a n d s . T a b l e 3 .9 p r e s e n t s average weighted c o n c e n t r a t i o n s of n i -t r a t e , ammonium, o r g a n i c - N and t o t a l - N (TPN) ppm at 15 and 60 cm 96 T a b l e 3.9 Weighted average c o n c e n t r a t i o n s (ppm) of n i t r a t e , ammonium, organ1c-N and t o t a l - N (TPN) i n water c o l l e c t e d over a two year p e r i o d . Standard d e v i a t i o n s i n b r a c k e t s . S i t e * N i t r a t e - N Ammonium-N Organic-N *** T o t a l - N C l e a r c u t FF 15cm 60cm FF 15cm 60cm FF 15cm 60cm FF 15cm 60cm ** * * * * O.G. .04a -- .02b . 18a -- .04a 1. 13a -- .99a 1 ,42a -- 1 . 16a (.01) (.00) ( . 12) ( .02) ( .08) ( .09) ( 32) ( .08) ***** 1 -- -- . 19 -- -- .04 -- .76 -- -- 0 .99 3 .03a .01 . 13a . 11a .05 . 15a .81a . 90 ,74b .94a 1 .09 1 ,02ab ( 01) ( .04) ( .03) ( . 14) ( 08) ( . 18) (.11) ( . 3 3 ) 6 .03a .01 .02b .09a .02 .08a .68a . 73 .45c .81a . 76 . 55c ( .02) (.oo) ( .04) ( .04) ( . 12) ( 03) ( • 18) ( .02) 10 .02a .02 .01b . 10a .03 .04a .88a . 66 .70b .99a .71 . 74c ( .01) ( 02) ( .03) ( 00) ( 21) ( .08) ( .21) ( .06) 26 .02a .02b .06a .04a .81a -- . 74b .89a . 79bc ( 01) -- ( .00) ( .01) -- ( .01) ( . 17) -- ( 04) ( . 17) -- ( .04) * Age of s i t e In 1981 when l y s i m e t e r s were i n s t a l l e d , except the 1 - y r - o l d s i t e where age r e f e r s to age i n 1982 when they were i n s t a l l e d . ** O.G. = o l d - g r o w t h *** O r g a n i c - N = T o t a l - N (TPN) - ( N i t r a t e + Ammonium)-N **** Means f o l l o w e d by the same l e t t e r a r e n.s.d. at P=.05 ***** Only one r e p l i c a t e was sampled i n t h i s age c l a s s . in the m i n e r a l s o i l . S i g n i f i c a n t l y h i g h e r c o n c e n t r a t i o n s of n i -t r a t e were observed at 60 cm in young c l e a r c u t s 2-5 y e a r s a f t e r c l e a r c u t t i n g than a f t e r 7-12 years or more, and in o l d - g r o w t h s t a n d s . H i g h e s t c o n c e n t r a t i o n s of o r g a n i c - and t o t a l - N were r e -c o r d e d in the o l d - g r o w t h , and 3 - y r - o l d and o l d - g r o w t h s t a n d s , r e s p e c t i v e l y . The p a t t e r n was s i m i l a r for ammonium as for t o t a l -N , but no s i g n i f i c a n t (P=.05) d i f f e r e n c e s between s tands were observed for ammonium. Trends at 15 cm i n the youngest c l e a r c u t s were s i m i l a r to those at 60 cm. Al though the p a t t e r n s for t o t a l - N i n the f o r e s t f l o o r and m i n e r a l s o i l between s tands are s i m i l a r , only the p a t t e r n in the m i n e r a l s o i l i n d i c a t e d between-stand d i f f e r e n c e s (P=.05) . The h i g h e s t c o n c e n t r a t i o n s observed in the 3 - y r - o l d and o l d - g r o w t h s tands for t o t a l - N - a p p e a r to c o i n c i d e wi th the l e a s t demands for s o i l N as c a l c u l a t e d u s i n g M i l l e r ' s (1984) net change in s o i l N c a p i t a l e q u a t i o n . Annual net change i n s o i l N c a p i t a l were p o s i t i v e in both the o l d - g r o w t h (+9.4) and 3 - y r - o l d s tands -1 (+5.7 k g . N . h a ) , as compared to n e g a t i v e v a l u e s i n the o ther s t a n d s . However, the lowest c o n c e n t r a t i o n s for t o t a l - N which were r e c o r d e d i n the 6- and 1 0 - y r - o l d c l e a r c u t s , were not a s s o c i a t e d -1 w i t h the g r e a t e s t net change i n s o i l N c a p i t a l of - 42 .5 k g . N . h a recorded i n the 2 6 - y r - o l d ' s t a n d s . The l a t t e r va lue may be an o v e r e s t i m a t e of demand, as uptake in the 2 6 - y r - o l d s tands i s thought to be h i g h due to h i g h e s t i m a t e s for f o l i a r biomass (see T a b l e 7 . 4 ) . T h i s i s in c o n t r a s t to M i l l e r ' s f i n d i n g s , where he observed a much g r e a t e r demand f o r s o i l N in 10- as compared to 4 0 - y r - o l d p l a n t a t i o n s of P inus n i g r a v a r . m a r i t i m a . 98 3 . 4 . 3 . 2 Ion exchange r e s i n s -1 Tab le 3.10 p r o v i d e s q u a n t i t i e s (ueq .g of dry r e s i n ) of n i t r a t e , ammonium and ( n i t r a t e + ammonium) adsorbed by the an ion and c a t i o n exchange r e s i n bags i n the f o r e s t f l o o r and upper m i n e r a l s o i l . P a t t e r n s of a d s o r p t i o n w i t h time s i n c e c l e a r c u t t i n g are s i m i l a r for the f o r e s t f l o o r and m i n e r a l s o i l . The h i g h e s t amounts of r e s i n - b a g n i t r a t e were found in the m i n e r a l s o i l of the youngest c l e a r c u t s (1978 c l e a r c u t ; d u r i n g the f i f t h year a f t e r c l e a r c u t t i n g ) , which had v a l u e s more than 30 t imes ( s i g n i f i c a n t at P=.05) h i g h e r than the o l d - g r o w t h . T h i s had dropped to about 2.5 t imes the o l d - g r o w t h va lue by e i g h t years a f t e r c l e a r c u t t i n g . F o r e s t f l o o r v a l u e s were i n c r e a s e d f i v e t imes and twice ( s i g n i f i c a n t at P= .05) , r e s p e c t i v e l y , f o r the same c l e a r c u t s . T a b l e 3.11 i n d i c a t e s which groups of s tands were s i g -n i f i c a n t l y d i f f e r e n t u s i n g the K r u s k a l - W a l l i s s t a t i s t i c at P=.05. T h i s n o n - p a r a m e t r i c t e s t was used because of the l a r g e d i f f e r e n -ces in s t a n d a r d d e v i a t i o n s observed between s i t e s . B i n k l e y ' s (1982) s tudy of low, medium and h i g h e l e v a t i o n s tands on the e a s t e r n p a r t of Vancouver I s l a n d i n d i c a t e d l a r g e i n c r e a s e s ( i . e . 7-10 t imes) in ammonium adsorbed by r e s i n s p l a c e d in the f o r e s t f l o o r i n c l e a r c u t s as compared to v a l u e s f o r uncut stands at the two lower e l e v a t i o n s . No s i g n i f i c a n t i n c r e a s e s i n adsorbed ammonium were observed a f t e r c l e a r c u t t i n g i n the F l e e t R i v e r p l o t s , a l t h o u g h the i n c r e a s e s i n n i t r a t e are of a s i m i l a r magnitude to the ammonium i n c r e a s e s observed by B i n k l e y (1982). One of the major d i f f e r e n c e s between the two s t u d i e s was the s l a s h b u r n i n g treatment that f o l l o w e d c l e a r c u t t i n g i n the s i t e s 99 T a b l e 3.10 Q u a n t i t y of n i t r a t e and ammonium (ueq n i t r a t e - or ammonlum-N per gram of d r y r e s i n ) f i x e d by exchange r e s i n s 1n the f o r e s t f l o o r and at 30 cm In the m i n e r a l s o i l m the chronosequence s t a n d s . (Means w i t h s t a n d a r d d e v i a t i o n s i n 1n b r a c k e t s ) . * Stand Age Ion S o i l l a y e r o l d - g r o w t h 5 - y r - o l d 8 - y r - o l d 1 2 - y r - o l d 2 8 - y r - o l d N 1 t r a t e F o r e s t F1 oor O. . 58 (0.11) 2 .96 (1 .24) 0. .94 (0. 13) 0, .52 (0. 08) 0. 34 (0. .05) M i n e r a l s o l 1 0. 28 (0.04) 10. .05 (4 . 30) 0, ,76 (0. 14) 0. .41 (0. 07) 0. 44 (0. .07) Ammon1 urn F o r e s t F 1 oor O. .71 (0.23) 0. 65 (O. . 13) O. 75 (0. 23) 0. 72 (0. 12) 0. 59 (0. 16) M i n e r a l Soi 1 O. 54 (0.07) o. .44 (0. .05) 0. 60 (0. 12) 0. 65 (0. 10) 0. 35 (0. 06) M i n e r a l - N F o r e s t F l o o r 1 . 29 (0.21) 3. 61 (1 21) 1 . 69 (0. 30) 1 . 24 (0. 15) 0. 93 (0. 16) M i n e r a l Soi 1 0. 82 (0.10) 10. 50 (4. .50) 1 . 36 (0. 19) 1 . 06 (0. 16) 0. 79 (0. 10) * - r e f e r s t o the y e a r i n which samples were c o l l e c t e d a f t e r c l e a r c u t t i n g . T a b l e 3.11 I n t e r s t a n d comparison of q u a n t i t i e s of n i t r a t e and ammonium adsorbed by ion exchange r e s i n s . * Ion S o i l Layer S i g n i f i c a n t l y d i f f e r e n t groups ** N i t r a t e F o r e s t f l o o r 5- 8- o l d - g r o w t h 12- 2 8 - y r - o l d M i n e r a l s o i l 5- 8- 12- 2 8 - y r - o l d o l d - g r o w t h Ammonium F o r e s t f l o o r o l d - g r o w t h 12- 8- 5- 2 8 - y r - o l d M i n e r a l s o i l 12- o l d - g r o w t h 8- 5- 2 8 - y r - o l d M i n e r a l - N F o r e s t f l o o r 5- 8- o l d - g r o w t h 12- 2 8 - y r - o l d M i n e r a l s o i l 5- 8- 12- o l d - g r o w t h 2 8 - y r - o l d * - Groups are based on the K r u s k a l - W a l l i s s t a t i s t i c at P=.05. Large d i f f e r e n c e s i n s t a n d a r d d e v i a t i o n s between s i t e s made K-W t e s t more robus t than Duncans in d e t e c t i n g d i f f e r e n c e s . * * - r e f e r s to age i n 1983; sampl ing was done between Sept .82 and A p r i l 1983. 101 examined by B i n k l e y (1982) . In a d d i t i o n , the age and the e l e v a -t i o n do not e x a c t l y c o r r e s p o n d wi th those of the F l e e t R i v e r s t a n d s . F i n a l l y , there i s some i n d i c a t i o n that the mercury-t r e a t e d r e s i n s used by B i n k l e y may accumulate l e s s n i t r a t e than the u n t r e a t e d r e s i n s used i n the F l e e t R i v e r study ( B i n k l e y and Matson 1983). R e s u l t s s i m i l a r to those found by me suggests that mercury has no e f f e c t on n i t r a t e a d s o r p t i o n (David p e r s . comm.) V i t o u s e k and Matson ( u n p u b l i s h e d ) a l s o observed an i n c r e a s e i n ammonium and n i t r a t e on u n t r e a t e d ion exchange r e s i n s a f t e r c l e a r c u t t i n g l o b l o l l y p ine s tands i n North C a r o l i n a . They noted g r e a t e r v a r i a b i l i t y in amounts of ammonium ( r a t h e r than n i t r a t e ) adsorbed w i t h i n the same treatment as was observed i n my s t u d y . The i n c r e a s e d n i t r a t e a d s o r p t i o n by ion exchange r e s i n s in the m i n e r a l s o i l .of the youngest c l e a r c u t s r e f l e c t the e l e v a t e d m i n e r a l s o i l s o l u t i o n n i t r a t e c o n c e n t r a t i o n s of these p l o t s . However, the i n c r e a s e d a d s o r b t i o n of n i t r a t e in the f o r e s t f l o o r r e s i n bags in the 5- (1978) and 8 - y r - o l d (1975) c l e a r c u t s was not r e f l e c t e d in the p a t t e r n of n i t r a t e c o n c e n t r a t i o n s in f o r e s t f l o o r l y s i m e t e r water . The reason for t h i s i s unknown, but may be r e l a t e d to the movement through the f o r e s t f l o o r of water c o n t a i n i n g n i t r a t e , and the c o l l e c t i o n c h a r a c t e r i s t i c s of the r e s i n bags as compared w i t h that of the z e r o - t e n s i o n l y s i m e t e r s . The f o r e s t f l o o r l y s i -meters sample main ly the s a t u r a t e d flow and thus o n l y measure mass flow t r a n s f e r s of n i t r a t e . Ion exchange r e s i n s , on the o ther hand might sample both mass flow and d i f f u s i o n t r a n s f e r s , a l t h o u g h B i n k l e y (1984) sugges ted that l i t t l e of the n i t r a t e or ammonium appeared to be t r a n s p o r t e d by d i f f u s i o n i n s m a l l pots 1 02 c o n t a i n i n g s o i l at f i e l d c a p a c i t y . Given the above, both ion exchange and z e r o - t e n s i o n l y s i m e t e r s c o u l d be main ly c o l l e c t i n g n i t r a t e i n mass f low, but the n i t r a t e c o u l d be adsorbed by the r e s i n s p r i o r to the onset of a n a e r o b i c c o n d i t i o n s . There was no p r e v e n t a t i v e measure taken to ensure t h a t n i t r a t e c o l l e c t e d i n the z e r o - t e n s i o n l y s i m e t e r s was not d e n i t r i f i e d d u r i n g the p e r i o d of t ime between c o l l e c t i o n s . However, no such measure was taken for the m i n e r a l s o i l l y s i m e t e r water c o l l e c t i o n s and n i t r a t e c o n c e n t r a t i o n s f o l l o w e d the p a t t e r n of n i t r a t e a d s o r p t i o n e x h i -b i t e d by the ion exchange r e s i n s . Be fore s a t u r a t e d flow takes p l a c e , the s o i l has to be t h o r -oughly wet ted . T h i s c o u l d c r e a t e a n a e r o b i c m i c r o s i t e c o n d i t i o n s because of the r e l a t i v e l y low d i f f u s i o n r a t e of oxygen i n water ( i . e . 1/10,000 of the r a t e i n a i r ) , l e a d i n g to d e n i t r i f i c a t i o n i f n i t r a t e i s p r e s e n t . The presence of g r e a t e r n i t r i f i c a t i o n in the f o r e s t f l o o r in the youngest c l e a r c u t s i s i n d i c a t e d by both the in s i t u i n c u b a t i o n s and the i on exchange r e s i n bags . However, s i n c e the l y s i m e t e r s in the f o r e s t f l o o r c o l l e c t most ly d u r i n g t imes when the f o r e s t f l o o r i s s a t u r a t e d , they do not p r o v i d e a good i n d i c a t i o n of the n i t r i f i c a t i o n a c t i v i t y i n the f o r e s t f l o o r . Hence, d e n i t r i f i c a t i o n may account for the d i f f e r e n t p a t -t e r n i n n i t r a t e c o n c e n t r a t i o n s observed i n the f o r e s t f l o o r u s i n g the i on exchange r e s i n bags compared w i t h z e r o - t e n s i o n l y s i m e -t e r s . D e n i t r i f i c a t i o n w i l l be d i s c u s s e d f u r t h e r i n Chapter 6. The p a t t e r n of n i t r a t e a d s o r p t i o n by the ion exchange r e s i n p r o b a b l y r e f l e c t s to a g r e a t e r or l e s s e r degree the p a t t e r n of biomass a c c u m u l a t i o n (or net change in s o i l c a p i t a l ) on the 103 chronosequence s i t e s . I t i s p o s s i b l e tha t the i n c r e a s e in a d s o r p -t i o n of n i t r a t e by the ion exchange r e s i n bags in the 5- (1978) and the 8 - y r - o l d (1975) c l e a r c u t s both i n the f o r e s t f l o o r and m i n e r a l s o i l was due to reduced p l a n t u p t a k e . Net change in s o i l - 1 - 1 N c a p i t a l ( k g . N . h a . y r ) was c a l c u l a t e d to be +5.7, - 2 . 2 , - 4 . 6 , - 4 2 . 5 and +9.4 for the 3 - , 6- , 10-, 26- and o l d - g r o w t h s t a n d s , based on data from Tab le 7.4 i n Chapter 7. O b v i o u s l y net change i n s o i l N c a p i t a l does not f u l l y e x p l a i n the p a t t e r n of n i t r a t e a d s o r p t i o n by the ion exchange r e s i n s ( i . e . 30 and 2.0 t imes g r e a t e r in the 3 - y r - o l d as compared to the o l d - g r o w t h stands for the m i n e r a l s o i l and f o r e s t f l o o r , r e s p e c t i v e l y ) , o therwise n i t r a t e a d s o r p t i o n would have been much g r e a t e r in the o l d - g r o w t h s t a n d s . S i m i l a r l y , the i n c r e a s e in net change i n s o i l N c a p i t a l -1 -1 from -2 .2 to - 4 . 6 k g . N . h a . y r ( i . e g r e a t e r demand) does not e x p l a i n the 2 f o l d decrease in n i t r a t e adsorbed by r e s i n bags in the 10- as compared to the 6 - y r - o l d c l e a r c u t s . 3 . 4 . 3 . 3 K C l - e x t r a c t i o n s of N i n the upper m i n e r a l s o i l Tab le 3.12 p r o v i d e s average c o n c e n t r a t i o n s (ppm) and -1 amounts ( k g . N . h a ) of n i t r a t e - and ammonium-N i n the m i n e r a l s o i l based on midsummer KC1 e x t r a c t i o n s . As the e x t r a c t i o n s were c a r r i e d out o n l y once d u r i n g the s t u d y , the r e s u l t s are r e a l l y o n l y of va lue on a r e l a t i v e s c a l e . The p a t t e r n observed in m i d -summer may not be the same as in the wet ter p a r t of the y e a r . Averages are based on 3 r e p l i c a t e s per age (t ime s i n c e c l e a r c u t -t i n g ) wi th 8 d e t e r m i n a t i o n s be ing performed on composi te samples i n each r e p l i c a t e . Duncans MRT i n d i c a t e d between-stand d i f f e r e n -ces i n c o n c e n t r a t i o n s and amounts of n i t r a t e - N at P=.05. H i g h e s t • 1 04 T a b l e 3.12 Average midsummer K C l - e x t r a c t a b l e n i t r a t e - , ammonium-and m i n e r a l - N c o n c e n t r a t i o n s (ppm) and amounts in the upper m i n e r a l s o i l f o r 3 r e p l i c a t e s in each age c l a s s in the chronosequence . (Means and s t a n d a r d d e v i a t i o n s ) (25 samples were reduced to 8 compos i te samples from which each r e p l i c a t e mean was d e t e r m i n e d ) . * C o n c e n t r a t i o n (ppm) Amount ( k g . N . h a ) S i t e / C l e a r c u t N i t r a t e - N Ammonium-N N i t r a t e - N Ammonium-N M i n e r a l - N ** o l d - g r o w t h 1 1 . ,97 b 18. 1 2 b 7. ,90 c 12. , 1 0 a 20. ,0 a 1 . .67 7. 08 2. ,10 6. ,30 6. ,7 4 - y r - o l d 15. .80 a 16. 57 b 15. ,60 a 16. ,60 a 32. .2 a c l e a r c u t 0. .59 1 . 32 2. ,46 3. ,27 4. .6 7 - y r - o l d 10. .84 b 12. 06 b 1 1 . .20 be 12. .40 a 23. .6 a c l e a r c u t 1 . .33 3. 1 9 1 . ,96 4. .04 4, .9 1 1 - y r - o l d 10, .84 b 16. 80 b 10, .50 be 16, .00 a 26, . 5 a c l e a r c u t 1 , .13 4. 28 1 , . 1 5 2, .80 2, .7 2 7 - y r - o l d 16, .94 a 32. 90 a 12, .92 ab 25. .40 a 38, .3 a c l e a r c u t 0 .90 1 2 .8 0, .36 10, .30 8, .7 * - Age of c l e a r c u t i n 1982. * * - Means in columns f o l l o w e d by the same l e t t e r are not s i g n i f i c a n t l y d i f f e r e n t at P=.05, u s i n g Duncans MRT. 105 c o n c e n t r a t i o n s and amounts of n i t r a t e were observed in the 4-y e a r - o l d (1978) and 2 7 - y e a r - o l d (1955) s i t e s . The h i g h e r v a l u e s f o r the l a t t e r p a r a l l e l the p a t t e r n of h i g h e r t o t a l - N c o n c e n t r a -t i o n s (Table 4.1.0), which as d i s c u s s e d e a r l i e r c o u l d be due e i t h e r to f i n e root t u r n o v e r i n the upper m i n e r a l s o i l , or to p a r e n t m a t e r i a l i n f l u e n c e s on n i t r o g e n c a p i t a l . The h i g h e r c o n -c e n t r a t i o n s of n i t r a t e in the youngest c l e a r c u t agrees wi th a l l the o ther i n d i c a t o r s of n i t r o g e n a v a i l a b i l i t y . S i m i l a r l y , ammo-nium l e v e l s do not d i f f e r much between s tands a lon g the c h r o n o s e -quence , which agrees wi th o ther i n d i c e s of ammonium-N a v a i l a b i l -i t y . The h i g h e r v a l u e s for e x t r a c t e d n i t r a t e in the 2 6 - y r - o l d s tands may be due to the " i n a c t i v a t i o n " of subs tances i n h i b i t i n g n i t r i f i c a t i o n sometimes presen t i n t h r o u g h f a l l (Gosz 1984), which might o therwise l i m i t n i t r i f i c a t i o n i n s tands of t h i s age . N i t r a t e c o n c e n t r a t i o n s r e c o r d e d by a d s o r p t i o n on to exchange r e s i n s and by l y s i m e t e r s suggest r e l a t i v e l y l i t t l e n i t r i f i c a t i o n i n the 2 6 - y r - o l d s tands as compared to the o ther c l e a r c u t s . The s m a l l d i f f e r e n c e of 4 ppm (12 v s . 16) i n KCL e x t r a c t e d n i t r a t e (ppm) between the o l d - g r o w t h and 3 - y r - o l d c l e a r c u t (1978) i s much s m a l l e r than that observed by Popovic (1974) between an uncut spruce (.4 ppm) and an a d j a c e n t 1 - y r - o l d c l e a r c u t (2 ppm) i n Sweden. U n l i k e Popovic (1974), where an i n c r e a s e in e x t r a c t e d (ammonium + n i t r a t e ) - N observed (from 4 to 21 ppm), no s i g n i f i -cant (P=.05) p o s t - c l e a r c u t t i n g i n c r e a s e i n e x t r a c t e d (ammonium + n i t r a t e ) - N i n the m i n e r a l s o i l was found i n my s t u d y . The l a c k of d i f f e r e n c e s c o u l d be due to numerous r e a s o n s , i n c l u d i n g i m m o b i l i -z a t i o n by decomposers , d e n i t r i f i c a t i o n a f t e r n i t r i f i c a t i o n , a n d / o r g e n e r a l l y low n i t r i f i c a t i o n a c t i v i t y at the time of sam-106 p l i n g . 3 . 4 . 3 . 4 F o l i a r a n a l y s i s F o l i a r a n a l y s i s i s based on the idea tha t the c h e m i c a l com-p o s i t i o n of f o l i a g e r e f l e c t s both n u t r i e n t a v a i l a b i l i t y and b i o -l o g i c a l demand ( U l r i c h and H i l l s 1967). F o l i a g e has g e n e r a l l y been used because growth and n u t r i t i o n are a s s o c i a t e d wi th c h l o r -o p h y l l which can account for more than 50% of the N content of f o l i a g e (Powers 1984). F o l i a r a n a l y s e s tend to be used wi th r e f e r e n c e to a " c r i t i c a l va lue" or range of c o n c e n t r a t i o n s (mass b a s i s ) which d e f i n e the t r a n s i t i o n zone between d e f i c i e n c y and l u x u r y consumption when y i e l d and n u t r i e n t content are p l o t t e d . In my s tudy , I used the temporal p a t t e r n s of c o n c e n t r a t i o n s of f o l i a r N as an index of. t emporal p a t t e r n s of n i t r o g e n a v a i l a -b i l i t y . However, d i f f e r e n c e s in demand or uptake w i l l i n f l u e n c e N a v a i l a b i l i t y as w e l l , which w i l l v a r y somewhat w i t h time s i n c e c l e a r c u t t i n g . F o l i a r p e r c e n t - N , N - c o n t e n t (mg.N per 100 need-l e s ) , and N:P r a t i o of f o l i a g e of a m a b i l i s f i r r e g e n e r a t i o n w i l l be r e p o r t e d . For an in depth review of f o l i a r a n a l y s i s as an i n d i c a t o r of n i t r o g e n a v a i l a b i l i t y see Powers (1984) . 3 . 4 . 3 . 4 . 1 Percent N As o u t l i n e d in the Methods s e c t i o n , d i f f e r e n t whorls were sampled on the d i f f e r e n t s i t e s ( i . e . 2nd whorl on the 4 - y r - o l d s i t e , s i n c e i t a l l o w e d for sampl ing of both c u r r e n t and 1 - y r - o l d f o l i a g e , and i t r e p r e s e n t e d the f i r s t year where a pronounced i n c r e a s e i n h e i g h t growth was e v i d e n t ; the f i r s t and t h i r d whorls were sampled on the 7 - y r - o l d s i t e , w h i l e the the t h i r d whorl on 107 T a b l e 3.13 Percent n i t r o g e n , N:P r a t i o , and N content of c u r r e n t year a m a b i l i s f i r f o l i a g e c o l l e c t e d from d i f f e r -ent whorls (see below) i n the F a l l of 1982. Means and S . D . are g iven based on an average of three r e p l i c a t e s in the stands examined .* ** Age of c l e a r c u t N i t r o g e n (%) C u r r e n t y e a r s f o l i a g e N:P N-Content (mg.N per 100 needles ) *** 4 - y r - o l d 1 .22 a 0. 09 9. 1 1 a 0. 95 9.22 a 2. 37 (2nd)**** 7 - y r - o l d 0.94 b 0. 1 5 5. 1 3 b 1 . 09 7.08 ab 1 . 44 (1s t ) 7 - y r - o l d 0.78 c 0. 07 4. 93 b 0. 62 3.48 c 0. 33 (3rd) 1 1 - y r - o l d 0.83 c 0. 05 4. 89 b 0. 37 7.65 ab 1 . 44 (3rd) 2 7 - y r - o l d 0.93 b 0. 05 7. 25 ab 2. 00 4.79 b 0. 55 (upper crown) * - Between 10-15 t r e e s were sampled in each r e p l i c a t e . * * - Age of the c l e a r c u t in 1982. * * * - Means f o l l o w e d by the same l e t t e r are not s i g n i f i -c a n t l y d i f f e r e n t at P=.05 u s i n g Duncans MRT. * * * * _ r e f e r s to the whorl from which the f o l i a g e was sampled. (See t e x t for d e t a i l s ) 108 the 1 1 - y r - o l d s i t e and the upper t h i r d of the crown was sampled on the 2 7 - y r - o l d s i t e . N i t r o g e n c o n c e n t r a t i o n s in c u r r e n t f o l i a g e of a m a b i l i s f i r r e g e n e r a t i o n growing on the chronosequence s i t e s are g i v e n i n T a b l e 3 .13 . Percent N i n c u r r e n t f o l i a g e was h ighes t in the 4-y r - o l d (1978) (1.22%), f o l l o w e d by d e c l i n i n g c o n c e n t r a t i o n s of 0.94% (1st whorl) in the 7 - y r - o l d (1975), 0.83% in the 1 1 - y r - o l d (1971) and 0.78% (3rd whorl ) in the 7 - y r - o l d c l e a r c u t . C o n c e n t r a -t i o n s were s l i g h t l y h i g h e r in the 2 7 - y r - o l d s i t e s ( 0 . 9 3 ) , as compared to the 1 1 - y r - o l d c l e a r c u t s . T h i s c o u l d have been due to the e f f e c t of " r i c h e r " parent m a t e r i a l s of the 2 7 - y r - o l d s i t e s , as compared to the F l e e t R i v e r s i t e s (see Chapter 2 ) . Higher amounts of t o t a l - N i n the upper m i n e r a l s o i l were a l s o observed in the 27 - jyr -o ld s i t e s (Tab le 4 . 1 0 ) . The tempora l p a t t e r n f o r %N in c u r r e n t f o l i a g e i s c o n s i s t e n t w i th the p r e v i o u s e x p e r i m e n t a l ev idence of g r e a t e s t N a v a i l a b i l i t y in the 3-6 year p e r i o d a f t e r c l e a r c u t t i n g . I t does not n e c e s s a r i l y support the ev idence of g r e a t e r m i n e r a l i z a t i o n d u r i n g the 3-6 year p e r i o d because b i o l o -g i c a l demand w i l l i n f l u e n c e percent N in f o l i a g e . As i n d i c a t e d e a r l i e r , the demand for s o i l N, expressed as the net change in s o i l N c a p i t a l ( M i l l e r 1984), was e s t imated to be g r e a t e s t in the -1 -1 2 7 - y r - o l d s tands ( -42.5 k g . N . h a yr ) . However, %N in c u r r e n t need les taken from the upper t h i r d of the crown were s i g n i f i c a n t l y h igher than c u r r e n t needles taken from the t h i r d whorl of t r e e s in the 1 1 - y r - o l d s t a n d . As suggested above, the 2 7 - y r - o l d s tand c o u l d be e x p l o i t i n g a " r i c h e r " m i n e r a l s o i l than the 1 1 - y r - o l d s t a n d . A comparison of demand and %N l e v e l s between the d i f f e r e n t F l e e t R i v e r stands suggests t h a t %N l e v e l s are 109 r e f l e c t i n g net change in s o i l N c a p i t a l (Table 7.4) ( i . e . compare v a l u e s between 4 - y r - o l d , and 7- or 1 1 - y r - o l d s t a n d s ) . The average va lue of 1.22 for %N i n the 4 - y r 7 o l d (1978) s i t e compares f a v o u r a b l y wi th some of the h i g h e r v a l u e s r e p o r t e d i n the l i t e r a t u r e (Table 3.14)-; for example, v a l u e s of 1.05 to 1.85% N found by Beaton et a l . (1965) for 3 to 6 - y e a r - o l d s u b a l p i n e f i r (1.05-1,85%N) and 1.11 %N r e p o r t e d by B r i g g s ( u n p u b l . ) f or t h i n -ned and f e r t i l i z e d a m a b i l i s f i r (1.11%N). In c o n t r a s t , the . lower %N c o n c e n t r a t i o n s observed i n the 1 1 - y r - o l d c l e a r c u t ( .83) com-pare w i t h those of ' p o o r l y ' grown 3 0 - y r - o l d a m a b i l i s f i r ( .78) r e p o r t e d by Cameron ( u n p u b l . ) , but are h i g h e r than %N i n both ' w e l l ' ( .71) and ' p o o r l y ' (.55) grown a m a b i l i s f i r near C o u r t n e y , B . C . on the e a s t e r n p a r t of Vancouver I s l a n d (Husted 1982). 3 . 4 . 3 . 4 . 2 N i t r o g e n content N i t r o g e n content (mg.N.per 100 needles ) of c u r r e n t f o l i a g e f o l l o w s a s l i g h t l y d i f f e r e n t p a t t e r n than tha t of %N. A l t h o u g h the h i g h e s t N content i n c u r r e n t f o l i a g e was observed in t r e e s sampled i n the 4 - y r - o l d c l e a r c u t s ( 9 . 2 2 ) . Depending on the w h o r l used for c o m p a r i s o n , va lues were s i m i l a r in the 7- (7 .08) and 1 1 - y r - o l d (7.65) c l e a r c u t s , or d i s s i m i l a r in the 7 - (3 .48) and 11-y r - o l d (7 .65) c l e a r c u t s . A r e l a t i v e l y low v a l u e was r e c o r d e d i n the 2 7 - y r - o l d (4.79) c l e a r c u t s (Table 3 . 1 3 ) . T h i s i m p l i e s tha t needle biomass per 100 needles was r e l a t i v e l y g r e a t e r in the 4-and 1 1 - y r - o l d as compared to the 7 - y r - o l d s i t e , g i v e n the %N data p r e s e n t e d i n the l a s t s e c t i o n . The sharp d e c l i n e i n needle mass suggested by the %N and N content data in f i r s t and e s p e c i a l l y 110 T a b l e 3.14 F a l l f o l i a r n i t r o g e n c o n c e n t r a t i o n s (%) for Ab ie s spp. r e p o r t e d in the l i t e r a t u r e for c u r r e n t and 1 - y r - o l d n e e d l e s . C u r r e n t 1 - y e a r - o l d A b i e s a m a b i l i s Beaton et a l . (1965) 8 - y e a r - o l d 0 .99 0 .91 Husted (1982) 9 - y e a r - o l d w e l l grown 0 .71 0 .74 p o o r l y grown 0 .55 0 .58 G a l l a g h e r (1964) 'young' t r e e s 0 .92- 1 .22 0 . 81 - 0 .94 G e s s e l and K l o c k (1982) 2 3 - y e a r - o l d 0 .96 0 .78 Cameron ( u n p u b l . ) 3 0 - y e a r - o l d , t h i n n e d w e l l grown 1 .05 1 .04 p o o r l y grown 0 .78 0 .76 B r i g g s ( u n p u b l . ) t h i n n e d u n f e r t i l i z e d 0 .89 0 .93 f e r t i l i z e d 1 . 1 1 1 . 1 4 Schwab ( u n p u b l . ) dominant mature 1 . 20 - 1 .30 1 .05 - 1 . 1 5 G e s s e l and K l o c k (1982) o l d - g r o w t h 1 . 1 0 1 .02 A b i e s l a s i o c a r p a Beaton et a l . (1965) 3 - 6 - y e a r s - o l d 1 . 05 - 1 .85 0 .91- 1 .59 A b i e s g r a n d i s Lowenste in and P i t k i n (1971 ) f e r t i l i z e d 1 .29 u n f e r t i l i z e d 1 .18 A b i e s balsamea M o r r i s o n (1974) j u v e n i l e t r e e s 1 . 0 9 - 1 . 15 1 .02- 1 .07 Brazeau and B e r n i e r (1973) mature t r e e s 1 .34 1 .23 Lang et a l . (1982) 2 .48 2 .35* S p r u g e l and Bormann (1981) 1 . 3 5 - 2 . 1 Bruns (1973) 1 . 0 1 - 2 .09 Timmer and Stone (1978) -f e r t i l i z e d 1 . 7 5 - 2 .75 Czapowskyj (1980) 0 . 9 5 - 1 .40 Young and C a r p e n t e r (1967) 0 .95* a l l age need les 11 1 t h i r d whorl f o l i a g e on the 7 - y r - o l d s i t e , as w e l l as the more r a p i d d e c l i n e in h e i g h t growth a f t e r c l e a r c u t t i n g (next s e c t i o n ) suggests t h a t the 7 - y r - o l d (1975) s i t e may be s l i g h t l y "poorer" in terms of n u t r i e n t regime than the 4- and 1 1 - y r - o l d s i t e s . Because v a r i a t i o n in N content was h i g h e r than v a r i a t i o n i n %N i n c u r r e n t needles there were fewer s i g n i f i c a n t b e t w e e n - s i t e d i f f e r e n c e s for N content than for %N. G r e a t e s t v a r i a t i o n i n N content was observed in f o l i a g e sampled on the 4 - y r - o l d and the l e a s t on the 2 7 - y r - o l d s i t e s . T h i s might be expected c o n s i d e r i n g the r e l a t i v e l y g r e a t e r v a r i a t i o n in t r e e s i z e and response in the 4- v s . the 2 7 - y r - o l d s i t e . There are few p u b l i s h e d data for n i t r o g e n content i n amabi -l i s f i r r e g e n e r a t i o n a c r o s s a wide range of p o s t - c l e a r c u t t i n g s tand ages . Husted (1982) observed tha t f o l i a r N c o n t e n t in ' w e l l ' grown a m a b i l i s f i r r e g e n e r a t i o n was about double tha t of ' p o o r l y ' grown r e g e n e r a t i o n (Table 3 . 1 5 ) . N content of c u r r e n t f o l i a g e was about twice as h i g h i n the 4 - y r - o l d as compared to that in the 2 7 - y r - o l d s tand i n the F l e e t R i v e r s t u d y . B r i g g s ( u n p u b l . ) noted i n c r e a s e s of a s i m i l a r magnitude i n n i t r o g e n content in c u r r e n t and 1 - y e a r - o l d needles two years a f t e r f e r t i -l i z a t i o n w i t h n i t r o g e n i n 3 0 - y e a r - o l d t h i n n e d s tands of a m a b i l i s f i r . In comparison to o ther da ta i n T a b l e 3 .15 , N c o n t e n t s i n c u r r e n t f o l i a g e in the F l e e t R i v e r study are h i g h e r r a n g i n g from 7.65 to 9.22 in the 3- to 1 0 - y r - o l d c l e a r c u t s . These v a l u e s are s i m i l a r to c u r r e n t needle N-content of f e r t i l i z e d a m a b i l i s f i r r e p o r t e d by ( B r i g g s u n p u b l . ) . The lower N-content of c u r r e n t needles (4.8 mg.N per 100 need les ) sampled i n the 2 7 - y r - o l d c l e a r c u t s i s c l o s e to tha t observed by B r i g g s ( u n p u b l . ) f o r an 11 2 T a b l e 3.15 N i t r o g e n c o n t e n t in A b i e s s p p . f o l i a g e r e p o r t e d i n the l i t e r a t u r e and i n the F l e e t R i v e r area, (mg.N per 100 n e e d l e s ) . * C u r r e n t 1- 2 - y e a r - o l d A b i e s a m a b i l i s Husted (1982) 4th whor l w e l l grown 2.60 5.35 5.65 p o o r l y grown 1.39 2.54 2.77 B r i g g s (unpubl . ) upper crown f e r t i l i z e d 7.5 14.8 u n f e r t i l i z e d 4.2 6.5 A b i e s balsamea Brazeau and B e r n i e r (1973) upper crown mature t r e e s 3.42 5.44 A b i e s a m a b i l i s F l e e t R i v e r * * 4 - y r - o l d 9.22 7 - y r - o l d 7.08 3.48 1 1 - y r - o l d 7.65 2 7 - y r - o l d 4.79 * See T a b l e 3.14 f o r d e t a i l s of s tand age, e t c . . * * See T a b l e 3.13 f o r d e t a i l s of whorl number, e tc 1 13 u n f e r t i l i z e d 3 0 - y r - o l d s tand of a m a b i l i s f i r . T a b l e 3.15 suggests that 1 - y r - o l d need les tend to have h i g h e r N c o n t e n t s than c u r r e n t n e e d l e s , which i s p r o b a b l y due to h i g h e r f o l i a g e mass in 1 - y r - o l d n e e d l e s . 3 . 4 . 3 . 4 . 3 N:P r a t i o The temporal p a t t e r n for the N:P r a t i o in c u r r e n t f o l i a g e f o l l o w s more c l o s e l y tha t of %N than tha t of N c o n t e n t . The h i g h e s t r a t i o was found in the 4 - y r - o l d ( 9 . 1 1 ) , f o l l o w e d by lower r a t i o s of 5.13 and "4.89 i n the 7- and 1 1 - y r - o l d c l e a r c u t s , and a h i g h e r r a t i o of 7.25 i n the 2 7 - y r - o l d c l e a r c u t . The lowest %P c o n c e n t r a t i o n s ( .135 and .136) were found in f o l i a g e sampled in the 4- and 2 7 - y r - o l d c l e a r c u t s , r e s p e c t i v e l y , where r e l a t i v e l y h i g h %N c o n c e n t r a t i o n s were o b s e r v e d . T a b l e 3.16 p r e s e n t s N:P r a t i o s r e p o r t e d for a m a b i l i s f i r which range from a low of 3.1 (Husted 1982) to a h i g h of 6.2 (Beaton et a l . 1965) i n c u r r e n t f o l i a g e . N:P r a t i o s i n 1 - y e a r - o l d f o l i a g e are 16-50% h i g h e r than in c u r r e n t f o l i a g e . However, in most cases the v a l u e s r e c o r d e d are below the optimum of 6 .7 -12 .5 suggested by Inges tad (1967) for c o n i f e r o u s s p e c i e s . In c o n t r a s t , N:P r a t i o s observed i n my study were g r e a t e r than I n g e s t a d ' s lower l i m i t in both the 4- and 2 7 - y r - o l d c l e a r c u t s , but below the lower l i m i t i n the 7- and 1 1 - y r - o l d c l e a r c u t s . 3 . 4 . 3 . 5 He ight growth response F i g u r e s 3 .10-3 .12 i l l u s t r a t e the p a t t e r n of p o s t - c l e a r c u t t i n g h e i g h t growth u s i n g both a b s o l u t e and r e l a t i v e v a l u e s i n the 5-(1978) , 8-(1975) and 12- (1971) y r - o l d c l e a r c u t s . E x p r e s s i n g the 114 T a b l e 3.16 N:P r a t i o s in c o n i f e r o u s f o l i a g e r e p o r t e d in the l i t e r a t u r e and in the F l e e t R i v e r a r e a . C u r r e n t 1 - y e a r - o l d A b i e s a m a b i l i s Husted (1982) 9 - y e a r - o l d t r e e s w e l l grown 3. 9 5. 3 p o o r l y grown 3. 1 3. 6 Beaton et a l . (1965) j u v e n i l e t r e e s 6. 2 7. 5 Schwab (unpubl .) mature t r e e s 4. 8 7. 2 F l e e t R i v e r 4 - y r - o l d 9. 1 7 - y r - o l d 5. 0 1 1 - y r - o l d 4. 9 2 7 - y r - o l d 7. 2 C o n i f e r o u s spp. optimum Inges tad (1967) 6 . 7 - 1 2 . 5 115 data as r e l a t i v e he ight ( i . e . annual h e i g h t increment / t o t a l h e i g h t in 1983) reduced the v a r i a b i l i t y seen in the a b s o l u t e h e i g h t increment data that r e f l e c t s v a r i a t i o n in t r e e s i z e w i t h i n the p l o t s ( F i g u r e 3 . 1 0 ) . I f t o t a l h e i g h t in the year of increment was used as the denominator , r e l a t i v e h e i g h t growth would tend to decrease c o n t i n o u s l y , whereas u s i n g the h e i g h t r e l a t i v e to tha t in 1983 i n d i c a t e s an i n c r e a s e f o l l o w e d by a d e c l i n e , which more or l e s s f o l l o w s the p a t t e r n of a b s o l u t e h e i g h t i n c r e m e n t . F i g u r e 3.10 i n d i c a t e s that i t takes about 5 y e a r s a f t e r c l e a r c u t t i n g be fore annual h e i g h t growth approaches 50 cm. When h e i g h t growth a f t e r c l e a r c u t t i n g i s combined for the 5- and 12-y r - o l d c l e a r c u t s (age in 1983) ( F i g u r e 3 . 1 3 ) , i t p r o v i d e s a more complete p i c t u r e of a m a b i l i s f i r r e l e a s e h e i g h t growth for the f i r s t 12 y e a r s a f t e r c l e a r c u t t i n g . Both F i g u r e 3.12 (which shows, the p a t t e r n for r e g e n e r a t i o n on the 1 2 - y r - o l d c l e a r c u t ) and F i g u r e 3.13 (combined p a t t e r n for both the 5- and 1 2 - y r - o l d ) i n d i c a t e a d e c l i n e in h e i g h t growth about 9-10 y e a r s a f t e r c l e a r -c u t t i n g on these s i t e s . In c o n t r a s t , the h e i g h t growth response a f t e r c l e a r c u t t i n g on the 8 - y r - o l d s i t e (1975) tended to vary between the r e p l i c a t e p l o t s examined. P o s t - c l e a r c u t t i n g h e i g h t growth d e c l i n e d 7-8 y e a r s a f t e r c l e a r c u t t i n g on r e p l i c a t e #1 , which i s about 2 years e a r l i e r than i n the 1 2 - y r - o l d c l e a r c u t . However, p o s t - l o g g i n g r e g e n e r a t i o n which became e s t a b l i s h e d a f t e r c l e a r c u t t i n g on the 8 - y r - o l d #2 r e p l i c a t e , showed i n c r e a s i n g h e i g h t growth in that same p e r i o d . The 8 - y r - o l d #3 r e p l i c a t e e x h i b i t e d a p a t t e r n some-where i n between the o ther two. T a b l e 3.17 i n d i c a t e s c u r r e n t 1 16 ID CD i 1 i r 1 1 2 3 4 5 YEARS SINCE CLEARCUTTING o c6n o 1 1 1 1 1 1 0 l 2 3 A 5 YEARS SINCE CLEARCUTTING gure 3 . 1 0 P o s t - c l e a r c u t t i n g r e l a t i v e and a b s o l u t e h e i g h t i n c r e m e n t o f a m a b i l i s f i r r e g e n e r a t i o n on the 1 9 7 8 ( 5 - y r - o l d ) s i t e measured i n 1 9 8 3 (Means and S.D.). 1 1 7 YEARS SINCE CLEARCUTTING F i g u r e 3 . 1 1 P o s t - c l e a r c u t t i n g h e i g h t increment f o r a m a b i l i s f i r r e g e n e r a t i o n f o r the 1 9 7 5 ( 8 - y r - o l d ) s i t e (Means and S . D . ) . YEARS SINCE CLEARCUTTING F i g u r e 3.12 P o s t - c l e a r c u t t i n g r e l a t i v e h e i g h t i n c r e m e n t o f a m a b i l i s f i r r e g e n e r a t i o n f o r the 1971 ( 1 2 - y r - o l d ) s i t e (Means and S.D.). TliVE SINCE CLEARCUTTING (years) F i g u r e 3 . 1 3 P o s t - c l e a r c u t t i n g h e i g h t i n c r e m e n t o f a m a b i l i s f i r r e g e n e r a t i o n f o r the 1 9 7 8 ( 5 - y r - o l d ) s i t e and the 1 9 7 1 ( 1 2 - y r - o l d ) s i t e combined (Means and S.D.). T a b l e 3.17 A b s o l u t e and r e l a t i v e he ight increment in 1983 for a m a b i l i s f i r advance r e g e n e r a t i o n in the chronosequence examined. He ight Increment in 1983 (dm) * * * * * * C l e a r c u t n A b s o l u t e R e l a t i v e * * * * 5 - y r - o l d 32 4.77 a (1 .46) 0.514 a (0.142) 8 - y r - o l d - 1 15 1 .76 c (0 .96) 0.085 c (0.041 ) 8 - y r - o l d - 2 10 4.78 a (0 .44) 0.268 b (0.057) 8 - y r - o l d - 3 14 3.56 b (0 .34) 0. 197 b (0.046) 1 2 - y r - o l d 35 2.32 c (2 .00) 0.062 c (0.051) * - c l e a r c u t age in 1983. * * - n= the number of t r e e s sampled. * * * - R e l a t i v e h e i g h t increment=absolute h e i g h t increment d i v i d e d by t o t a l he ight in 1983. * * * * - Means f o l l o w e d by the same l e t t e r are not s i g n i f i c a n t -l y d i f f e r e n t at P=.05; ( S . D . in b r a c k e t s ) . 121 h e i g h t increment (both a b s o l u t e and r e l a t i v e ) in the r e p l i c a t e s examined. Duncans MRT i n d i c a t e d s i g n i f i c a n t d i f f e r e n c e s between r e p l i c a t e s at P=.05 for h e i g h t growth in 1983. Husted (1982) observed a p a t t e r n of d e c l i n i n g h e i g h t growth in a m a b i l i s f i r advance r e g e n e r a t i o n a f t e r c l e a r c u t t i n g near Courtney on Vancouver I s l a n d . Annual increment i n c r e a s e d to about 30 cm s i x y e a r s a f t e r l o g g i n g , and then d e c l i n e d to about 10 cm nine years a f t e r l o g g i n g i n ' p o o r l y ' grown t r e e s . In c o n t r a s t , ' w e l l ' grown t r e e s showed i n c r e a s i n g annua l h e i g h t increment of up to 50 cm e i g h t y e a r s a f t e r l o g g i n g . Husted (1982) was unable to e x p l a i n the p a t t e r n of v a r i a b l e h e i g h t growth i n a m a b i l i s f i r u s i n g age or h e i g h t at r e l e a s e or degree of aboveground c o m p e t i -t i o n in her study in 9 - y e a r - o l d c l e a r c u t s . S i m i l a r l y , Ivanco (1976) and G a l l a g h e r (1964) c o u l d not e x p l a i n v a r i a b l e h e i g h t growth of s u b a l p i n e f i r and a m a b i l i s f i r , r e s p e c t i v e l y , u s i n g s t o c k i n g d e n s i t y as an independent v a r i a b l e . D i f f e r e n c e s in r o o t -ing medium ( i . e . r o t t i n g wood versus m i n e r a l s o i l ) and thus in N a v a i l a b i l i t y between the ' p o o r l y ' and ' w e l l ' grown r e g e n e r a t i o n in H u s t e d ' s (1982) study was suggested as an e x p l a n a t i o n for the d i f f e r e n t p a t t e r n s of h e i g h t growth . Appendix 2c p r o v i d e s c o r r e l a t i o n c o e f f i c i e n t s between 1983 h e i g h t increment and f o l i a r N l e v e l s i n a m a b i l i s f i r r e g e n e r a t i o n , and Appendix 2d d i s c u s s e s c o r r e l a t i o n s between h e i g h t increment and o t h e r i n d i c e s of N a v a i l a b i l i t y . 3 . 4 . 3 . 6 F o l i a r N and o t h e r i n d i c e s of n i t r o g e n a v a i l a b i l i t y S o i l n i t r a t e i s c l e a r l y a b e t t e r i n d i c a t o r of n i t r o g e n a v a i l -a b i l i t y than s o i l ammonium on these s i t e s , a l t h o u g h most of the 122 v a r i a b l e s r e l a t i n g to ammonium have not been i n c l u d e d in T a b l e 3.18 s i n c e the r - v a l u e s tended to be much lower than for n i t r a t e . The r e l a t i v e l y low c o r r e l a t i o n s between i_n s i t u N m i n e r a l i z a t i o n and f o l i a r N was somewhat unexpected , c o n s i d e r i n g the r e l a t i v e l y good r e l a t i o n s h i p between m i n e r a l i z e d N from humus, and f o l i a r N and t r e e growth r e p o r t e d i n some s t u d i e s ( Z o t t l 1960). However, the l i t e r a t u r e i n d i c a t e s that c o n s i d e r a b l e v a r i a b i l i t y e x i s t s in the s t r e n g t h of the r e l a t i o n s h i p between m i n e r a l i z a b l e N and i n d i c e s of growth or p r o d u c t i v i t y (Powers 1984). There was c o n s i d e r a b l e v a r i a b i l i t y between r e p l i c a t e s r e s u l t i n g i n few s i g n i f i c a n t (P=.05) b e t w e e n - s i t e d i f f e r e n c e s in m i n e r a l i z a b l e N in my study (Table 3 . 5 ) . There was a c o n s i d e r a b l y s t r o n g e r c o r r e -l a t i o n between s i t e averages and f o l i a r p e r c e n t N than wi th r e p l i c a t e averages ( r= .94 , r - c r i t i c a l = . 9 5 at P=.05 and r - c r i t . =.87 at P= .10) . The sample s i z e of 5 in each r e p l i c a t e was c e r -t a i n l y not l a r g e enough. Powers (1984) suggests u s i n g s e v e r a l composi te samples to reduce w i t h i n - s i t e v a r i a b i l i t y , whi l e s t i l l o b t a i n i n g an i n d i c a t i o n of i t s magnitude . B e t w e e n - r e p l i c a t e d i f -f e r e n c e s in i m m o b i l i z a t i o n d u r i n g ir\ s i t u i n c u b a t i o n s of f o r e s t f l o o r in the younger c l e a r c u t s i s b e l i e v e d to have caused the poor c o r r e l a t i o n between both f o l i a r N and h e i g h t increment , and in s i t u N m i n e r a l i z a t i o n . A l t h o u g h a s i g n i f i c a n t c o r r e l a t i o n (P=.05) was observed b e t -ween %N i n the f o r e s t f l o o r and %N i n c u r r e n t needles the s t r e n g t h of the r e l a t i o n s h i p was c o n s i d e r a b l y weaker than for o ther i n d i c e s (Tab le 3 . 1 8 ) . ' Husted (1982) demonstrated that ' w e l l ' grown t r e e m i c r o - s i t e s w i t h i n the f o r e s t f l o o r had h i g h e r 1 23 T a b l e 3.18 C o r r e l a t i o n c o e f f i c i e n t s (r) of 1983 h e i g h t increment and % f o l i a r N i n c u r r e n t needles (1982) w i t h s e l e c t e d i n d i c e s of n i t r o g e n a v a i l a b i l i t y ( r e p l i c a t e averages were used u n l e s s o therwise n o t e d ) . He ight increment 1983 %Fol iar N A b s o l u t e R e l a t i v e * * * F o l i a r N (%)- c u r r e n t needles F o l i a r N content " " 1 - y e a r - o l d " C e l l u l o s e mass l o s s (%) F F - ion exchange r e s i n s -N03 MS- ion exchange r e s i n s -N03 F F - i n s i t u m i n e r a l i z a b l e N Summer F a l l F F - K C l - e x t r a c t a b l e N 0 3 - f a l l F F - m o i s t u r e content J u l y - 1 9 8 2 Sept-1982 MS- K C l - e x t r a c t a b l e N03 1982 Net change in FF N content between 1981 to 1983 F F - percent N in 1981 F F - N content i n 1981 MS- S o i l water N03 (ppm) 0.67 0.92* 0.87 0.99* 0.58* 0.53 0.92* 0.40 - 0 . 1 6 -0 .03 -0 .07 0.51 . 0.88* 0.78** 0.51 0.88* 0.65* **** -0 .12 -0 .08 0.30 ( .95) -0 .54 -0 .07 0.04 ( .94) 0.91* 0.86 0.61* 0.34 0.71 0.65* 0.37 0.75 0.74** 0.49 0.83 0.52 0.78 0.95* 0.83** 0.39 0.49 0.64* 0.79 0.96* 0.73** 0.56 0.90* 0.74** * - s i g n i f i c a n t at P= .05 , u s i n g c r i t i c a l r - v a l u e s . * * - s i g n i f i c a n t at P= . 0 1 , u s i n g c r i t i c a l r - v a l u e s . * * * - r e l a t i v e increment=abso lute d i v i d e d by t r e e h e i g h t . * * * * - r - v a l u e s based on c o r r e l a t i o n between averages of f o l i a r N and m i n e r a l i z e d - N for each of the 4- , 7 - , 11- and 2 7 - y r - o l d s tands (n=3 f o r summer i n c u b a t i o n s and n=4 for f a l l ) . R - c r i t . =.95 and 0.87 for P=.05 and .10 , r e s p e c -t i v e l y for n = 4) . FF - f o r e s t f l o o r ; MS - m i n e r a l s o i l 1 2 4 humus N than ' p o o r l y ' grown t r e e m i c r o s i t e s , a l t h o u g h no s i g n i f i -cant l i n e a r r e l a t i o n s h i p between e i t h e r humus N percent or C:N r a t i o and c u r r e n t f o l i a r n i t r o g e n percent was found . The problem wi th u s i n g humus N i s tha t much of the N may be bound (Coulson et a l . 1960; D a v i e s et a l . 1964) and not r e a d i l y a v a i l a b l e . Hence the reason f o r us ing m i n e r a l i z a b l e N as an index of n i t r o g e n a v a i l a b i l i t y . Other i n d i c e s which were s i g n i f i c a n t l y c o r r e l a t e d w i t h %N in f o l i a g e i n c l u d e d J u l y and September f o r e s t f l o o r m o i s t u r e c o n -t e n t . The h i g h e s t c o r r e l a t i o n c o e f f i c i e n t was observed between net change i n f o r e s t f l o o r n i t r o g e n c a p i t a l between 1981 and 1983 and percent n i t r o g e n in c u r r e n t f o l i a g e ( r=0 .83 ) , whi le the second h i g h e s t was that w i th n i t r a t e f i x e d by ion exchange r e s i n s (r = 0.78) . The r a t h e r poor c o r r e l a t i o n between percentage l o s s of c e l l u -l o s e and f o l i a r N or h e i g h t increment i n d i c a t e s tha t more than j u s t the r a t e of decompos i t ion of c e l l u l o s e i n f l u e n c e s n u t r i t i o n and h e i g h t growth of a m a b i l i s f i r r e g e n e r a t i o n . The c o n t i n u e d h i g h p o t e n t i a l for c e l l u l o s e decompos i t i on observed in the 12 -yr -o l d c l e a r c u t (1971) ( F i g u r e 3 . 8 ) , as compared to the o ther s i t e s i s not r e f l e c t e d in o ther i n d i c a t o r s of f o r e s t f l o o r d e c l i n e (eg. d e c l i n e of f o r e s t f l o o r mass for the chronosequence ( F i g u r e 3 . 8 ) ; f o r e s t f l o o r N ( F i g u r e 3 . 5 ) ; in s i t u N m i n e r a l i z a t i o n r a t e s (Table 3 . 5 ) ) . T h i s suggests t h a t a l t h o u g h the p o t e n t i a l for f o r e s t f l o o r l o s s s t i l l e x i s t s 10 to 12 y e a r s a f t e r c l e a r c u t t i n g , p r o b a b l y due to h i g h e r temperatures (or f l u c t u a t i o n s in) (Table 3 . 7 ) , a c t u a l l o s s of f o r e s t f l o o r mass does not occur because of o ther f a c t o r s . The r e - e s t a b l i s h m e n t of r o o t s and a s s o c i a t e d 1 25 m y c o r r h i z a e i n the f o r e s t f l o o r , and i n c r e a s i n g amounts of l i t -t e r f a l l p r o b a b l y are important in t h i s r e g a r d . However, the most important f a c t o r c o u l d be that a f t e r 10 -years , most of the e a s i l y decomposable m a t e r i a l ( i . e c e l l u l o s e ) has a l r e a d y decomposed, l e a v i n g the l e s s decomposable m a t e r i a l ( i . e l i g n i n ) b e h i n d . Sec-t i o n 3 . 4 . 2 . 1 d i s c u s s e d the idea tha t i t i s the more decomposable non-woody m a t e r i a l that undergoes decompos i t i on a f t e r c l e a r c u t -t i n g (See T a b l e 3 . 4 ) . 3 . 4 . 3 . 7 G r a p h i c a l a n a l y s i s of f o l i a r n i t r o g e n F i g u r e 3.14 i l l u s t r a t e s p i c t o r i a l l y the responses of a m a b i l i s f i r r e g e n e r a t i o n to the n u t r i e n t f l u s h u s i n g the g r a p h i c a l p r o c e -dure employed by Krause (1965), H e i n s d o r f (1967), and r e c e n t l y by Timmer and Stone (1978), and Weetman and F o u r n i e r (1982). The N c o n c e n t r a t i o n and content in c u r r e n t needles in the 2 7 - y r - o l d stands were used as s u b s t i t u t e s for i n i t i a l va lues p r i o r to c l e a r c u t t i n g . A m a b i l i s f i r r e g e n e r a t i o n on the 4 - y r - o l d (1978) c l e a r c u t s i l l u s t r a t e a ' p o s i t i v e ' response to n i t r o g e n f e r t i l i z a -t i o n , which in t h i s study i s the input of n i t r o g e n from the m o b i l i z a t i o n of the o l d - g r o w t h f o r e s t f l o o r . The ' p o s i t i v e ' response i s p r o b a b l y due to both l e s s e r b i o l o g i c a l demand and g r e a t e r m i n e r a l i z a t i o n r e s u l t i n g i n g r e a t e r N a v a i l a b i l i t y . I n t e r p r e t a t i o n of the p o s i t i o n of the 1 1 - y r - o l d (1971) c l e a r c u t s i n F i g u r e 3.14 d i f f e r s from past use of the g r a p h i c a l p r o c e d u r e . A c c o r d i n g to Timmer and Stone (1978) , the p o s i t i o n of the 1 1 - y r -o l d c l e a r c u t s in F i g u r e 3 .14 , i n d i c a t e a ' n o n - l i m i t i n g ' response to f e r t i l i z e r a d d i t i o n . However, g i v e n the ' p o s i t i v e ' response i n 126 cn d ' CD d" tv d" CD d . LEGEND O = 4-YR-OLD • = 7—YR—OLD A = 11-YR-OLD B = 27-YR-OLD O • O o / • • — I 1 1 1 1 1 40.0 60.0 80.0 100.0 120.0 140.0 0.0 20.0 NITROGEN CONTENT ( ug. needle" 1) F i g u r e 3 « l 4 G r a p h i c a l r e p r e s e n t a t i o n o f the response o f n a t u r a l r e g e n e r a t i o n o f a m a b i l i s f i r to the p o s t - c l e a r c u t t i n g " a s s a r e f f e c t " . the 4 - y r - o l d c l e a r c u t and the long time p e r i o d between the b e g i n -ning of the "assar t e f f e c t " and 11-years a f t e r c l e a r c u t t i n g , my i n t e r p r e t a t i o n i s that the p o s i t i o n of both the 1 1 - y r - o l d r e p l i -c a t e s as w e l l as the 7 - y r - o l d #1 r e p l i c a t e , i n d i c a t e tha t the p e r i o d of h i g h e r n i t r o g e n a v a i l a b i l i t y has ended. T h i s has r e s u l t e d i n lower n i t r o g e n c o n c e n t r a t i o n s due to inadequate N, and d e c l i n i n g needle mass, a l t h o u g h needle mass has not dropped to l e v e l s observed in the 2 7 - y r - o l d s t a n d s , presumably due to much l e s s e r demand i n the 11- as compared to the 2 7 - y r - o l d s t a n d . The p o s i t i o n of the 7 - y r - o l d # 2 r e p l i c a t e can be e x p l a i n e d by the r e l a t i v e l y l a r g e d e c l i n e i n f o r e s t f l o o r between 1981 and 1983 (Table 3 . 3 ) . 3.5 C o n c l u s i o n s Based on the accumulated ev idence for numerous i n d i c a t o r s of p o s t - c l e a r c u t t i n g n i t r o g e n a v a i l a b i l i t y for the chronosequence , the f o l l o w i n g c o n c l u s i o n s were made. H y p o t h e s i s 1, t h a t there s h o u l d be a s i g n i f i c a n t r e l e a s e of f o r e s t f l o o r n i t r o g e n and i t sh ou ld be r e f l e c t e d i n the growth response of advanced r e g e n e r a t i o n of a m a b i l i s f i r , was a c c e p t e d . There appeared to be a l a r g e m o b i l i z a t i o n of f o r e s t f l o o r N -1 amounting to a p p r o x i m a t e l y 950 k g . N . h a (which r e p r e s e n t s a 40% d e c l i n e ) , which i s a s s o c i a t e d w i t h a d e c l i n e i n f o r e s t f l o o r mass -1 of about 112 t . h a (35% d e c l i n e ) . These r e p r e s e n t r e l a t i v e l y s m a l l e r amounts (percent b a s i s ) than d e c l i n e s in f o r e s t f l o o r mass or N content observed in o t h e r comparable s t u d i e s . Changes in bu lk d e n s i t y and %N were more v a r i a b l e , a l t h o u g h there 1.28 appeared to be an e x p l a i n a b l e t empora l t r e n d i n the former , which i s p r o b a b l y r e l a t e d to s l a s h i n p u t s and mix ing of s o i l e i t h e r by y a r d i n g a n d / o r s o i l an imal a c t i v i t y . Rates of d e c o m p o s i t i o n , as measured u s i n g c e l l u l o s e s t r i p s , i n d i c a t e d l a r g e v a r i a t i o n w i t h i n c l e a r c u t s , but the t r e n d i n d i -c a t e d about a 3 t imes h i g h e r r a t e of d e c o m p o s i t i o n i n the 5-(1978), 8-(1975) and 1 2 - y r - o l d (1971) c l e a r c u t s , than in the o l d growth s t a n d s . The r e l a t i v e l y s m a l l d i f f e r e n c e i n average a i r temperature r e c o r d e d for the 1 0 - y r - o l d as compared to the o l d growth s tand was not great enough to account f o r the d e c l i n e in f o r e s t f l o o r a f t e r c l e a r c u t t i n g , or the h i g h e r v a l u e s f o r decom-p o s i t i o n of c e l l u l o s e . The e l e v a t e d l e v e l of c e l l u l o s e decomposi -t i o n i n the 1971 c l e a r c u t (12 y e a r s a f t e r c l e a r c u t t i n g ) , at a time when f o r e s t f l o o r mass l o s s appears to have s t a b i l i z e d , i s b e l i e v e d to be due to the c o n t i n u i n g e x i s t e n c e of more f a v o u r -ab le m i c r o c l i m a t i c c o n d i t i o n s ( temperature and m o i s t u r e ) ; how-e v e r , e x h a u s t i o n of such e a s i l y decomposed subs tances has p r o b a -b l y a l r e a d y taken p l a c e in the f o r e s t f l o o r by 12 y e a r s a f t e r c l e a r c u t t i n g , l e a v i n g o n l y the more r e c a l c i t r a n t components, n o t a b l y l i g n i n . The poor c o r r e l a t i o n between c e l l u l o s e decompos i -t i o n and f o l i a r N i s c o n s i s t e n t w i t h the above . I n d i c a t o r s of n i t r o g e n a v a i l a b i l i t y , i n c l u d i n g ij\ s i t u r a t e s of m i n e r a l i z a t i o n , s o i l water c o n c e n t r a t i o n s , and ion exchange r e s i n bags , documented h i g h e r l e v e l s of n i t r a t e i n the f o r e s t f l o o r a n d / o r m i n e r a l s o i l in the 3 to 5- and sometimes 6 to 8 - y r -o l d c l e a r c u t s , than i n the o t h e r s i t e s . A l t h o u g h t h e r e were s i m i l a r p o s t - c l e a r c u t t i n g t r e n d s for (ammonium + n i t r a t e ) , (eg. 1 29 m i n e r a l i z a b l e N i n the f a l l i n c u b a t i o n s ) , g r e a t e r w i t h i n - s i t e v a r i a t i o n u s u a l l y r e s u l t e d i n fewer s i g n i f i c a n t d i f f e r e n c e s be-tween s i t e s . N i t r o g e n a v a i l a b i l i t y , and e s p e c i a l l y n i t r a t e , appears to d e c l i n e by 8 - 1 0 . y e a r s a f t e r c l e a r c u t t i n g . M o i s t u r e content showed a good c o r r e l a t i o n wi th some in s i t u m i n e r a l i z a -t i o n r a t e s in the f a l l sugges t ing i t s importance in the m i n e r a l i -z a t i o n of f o r e s t f l o o r N a f t e r c l e a r c u t t i n g . Percent N and N content ( to a l e s s e r ex tent ) of c u r r e n t needles and 1983 he ight increment i n a m a b i l i s f i r r e g e n e r a t i o n r e f l e c t e d the p a t t e r n of N a v a i l a b i l i t y . F o l i a r N c o n t e n t s were h i g h l y c o r r e l a t e d with r e l a t i v e h e i g h t growth ( l e s s so w i th a b s o l u t e h e i g h t growth) , w i th the h i g h e s t v a l u e s be ing observed 5 y e a r s a f t e r c l e a r c u t t i n g . Other i n d i c e s of n i t r o g e n a v a i l a b i l i t y were a l s o c o r r e l a t e d with h e i g h t growth-. F o l i a r N l e v e l s tended to r e f l e c t the temporal p a t t e r n of n i t r o g e n a v a i l a b i l i t y sug-ges ted by o ther i n d i c a t o r s . Based on both the p a t t e r n of a m a b i l i s f i r he ight increment and i n d i c a t o r s of n i t r o g e n a v a i l a b i l i t y , the p e r i o d of h i g h e r N a v a i l a b i l i t y e x i s t s for up to 7 to 8 y e a r s a f t e r c l e a r c u t t i n g on these mesic CWHb s i t e s . 130 CHAPTER 4 S i n k s for F o r e s t F l o o r N A f t e r C l e a r c u t t i n g 4.1 I n t r o d u c t i o n The v a r i o u s p o s t - c l e a r c u t t i n g s i n k s for f o r e s t f l o o r n i t r o -gen that w i l l be d i s c u s s e d in t h i s chapter i n c l u d e : 1. p l a n t uptake , 2. i m m o b i l i z a t i o n in l a r g e s l a s h a n d / o r stumps (no exper -iments to e s t imate i m m o b i l i z a t i o n i n s l a s h or stumps were under-taken in my s t u d y , however) , and 3. m i n e r a l s o i l s t o r a g e . Losses of n i t r o g e n from the ecosystem in the form of s o l u t i o n f l u x and d e n i t r i f i c a t i o n w i l l be d i s c u s s e d i n Chapter 5 and 6, r e s p e c t i v e l y . An i n t e g r a t i o n of i n p u t s and output s and how they r e l a t e to the magnitude and t i m i n g of the "assar t e f f e c t " as des -c r i b e d in Chapter—3. w i l l be p r e s e n t e d in Chapter 7.. 4.2 L i t e r a t u r e Review 4.2 .1 N i t r o g e n A c c u m u l a t i o n in V e g e t a t i o n T o t a l uptake has been r e p o r t e d to range from 25-125 k g . N . h a -1 -1 . y r , i n p o l e s tage to mature s t a n d s , of which net uptake i s p r o b a b l y about 20% due to l i t t e r f a l l , f o l i a r l e a c h i n g and g r a z i n g l o s s e s (Keeney 1980). Net uptake as a percentage of t o t a l uptake may reach i t s h i g h e s t v a l u e s i n young p l a n t a t i o n s due to the s m a l l e r amounts of l i t t e r f a l l at t h i s s t a g e , but the g r e a t e s t t o t a l uptake i s u s u a l l y in p o l e s tage s t a n d s , at l e a s t i n temper-ate c l i m a t e s . M i l l e r (1984) has i n d i c a t e d tha t net change i n s o i l N c a p i t a l ( i . e . r e l e a s e from t r e e s + a tmospher ic input - t r e e 131 uptake (not i n c l u d i n g r e t r a n s l o c a t i o n w i t h i n t r e e ) was c o n s i d e r a b l y g r e a t e r i n a 1 0 - y r - o l d (-40) as compared to in a 40--1 -1 y r - o l d p l a n t a t i o n (-13 k g . N . h a . y r ) of P inus n i g r a v a r . m a r i -t ima in B r i t a i n . T h i s i m p l i e s a much g r e a t e r demand for or r e d u c -t i o n in s o i l N c a p i t a l in young p l a n t a t i o n s b e f o r e canopy c l o s u r e or the stage of maximum f o l i a g e b iomass . A f t e r canopy c l o s u r e much of t r e e uptake i s f u r n i s h e d by r e t r a n s l o c a t i o n and r e l e a s e of N through l i t t e r f a l l and t h r o u g h f a l l ( M i l l e r 1984). Comparison of e s t imates from v a r i o u s s t u d i e s of biomass and n u t r i e n t a c c u m u l a t i o n over time i s d i f f i c u l t due to d i f f e r e n c e s in s i t e q u a l i t y , c l i m a t i c f a c t o r s , i n i t i a l s t o c k i n g l e v e l s a f t e r c l e a r c u t t i n g , s p e c i e s mixes , management a c t i v i t i e s such as l o g -g i n g , precommerc ia l t h i n n i n g , e t c . Such comparisons do not p r o -v i d e a good b a s i s on which to a s se s s N.uptake and a c c u m u l a t i o n . . Cole (1981) r e p o r t e d a changing p a t t e r n of uptake wi th s tand age or development; percent uptake from the f o r e s t f l o o r i n c r e a s -ing from 55% in 10- to 2 0 - y e a r - o l d , to 100% i n 60- to 9 0 - y e a r - o l d s i t e c l a s s IV s tands of D o u g l a s - f i r , which c o r r e s p o n d s to a p a t t e r n of i n c r e a s i n g f o r e s t f l o o r o r g a n i c matter a c c u m u l a t i o n (Turner and Long 1975). In a d d i t i o n , p a t t e r n s of uptake would be expected to vary wi th s i t e q u a l i t y , s i n c e the "assar t e f f e c t " and the a v a i l a b i l i t y of n i t r o g e n in the f o r e s t f l o o r and m i n e r a l s o i l w i l l vary between good and poor s i t e s . On good s i t e s w i th a m u l l humus form, m i x i n g of o r g a n i c matter down i n t o the m i n e r a l h o r i -zons w i l l r e s u l t in c o n t i n u e d uptake from the m i n e r a l h o r i z o n s throughout s tand development . 132 4 .2 .1 .1 U n d e r s t o r y T a b l e 4.1 p r o v i d e s v a l u e s f o r u n d e r s t o r y biomass and n i t r o g e n c a p i t a l i n some west coas t f o r e s t s . Semi-mature to mature stands of a m a b i l i s f i r on the west c o a s t (Turner and S i n g e r 1976) appear to support l e s s biomass than comparable s tands of Doug las -f i r . For example, v a l u e s for u n d e r s t o r y biomass and N content in -1 stands of D o u g l a s - f i r range from 1.0 to 13.0 t . h a for biomass -1 and 6.0 to 66.6 k g . N . h a for N c o n t e n t as compared to v a l u e s for -1 t rue f i r s tands which range from 0.3 to 4.4 t . h a for biomass -1 and 5 to 14.9 k g . N . h a for N c o n t e n t . T h i s i s not unexpected c o n s i d e r i n g the r e l a t i v e l y g r e a t e r shade t o l e r a n c e of a m a b i l i s f i r than of D o u g l a s - f i r ( K r a j i n a et a l . 1982). In a l l these s t u d i e s , the p a t t e r n s are very s i m i l a r , w i th a g r e a t e r p r o p o r t i o n of the ecosystem biomass and n u t r i e n t content be ing c o n t a i n e d in the u n d e r s t o r y in the younger s tands (Swi tzer and Ne l son 1972; Turner 1975; Long 1976; MacLean and Wein 1977; and F e l l e r et a l • 1983). 4 . 2 . 1 . 2 O v e r s t o r y T a b l e 4.2 summarizes p u b l i s h e d v a l u e s of biomass and N content i n s tands of west c o a s t s p e c i e s and two Japanese Ab ie s spp. Biomass a c c u m u l a t i o n i n D o u g l a s - f i r s tands may d i f f e r from the more shade t o l e r a n t s p e c i e s , r e f l e c t i n g i t s s t a t u s as a p ioneer s p e c i e s in t h i s subzone . Western hemlock s tands in the CWH have been documented to accumulate g r e a t e r volumes of t imber over s i m i l a r t ime p e r i o d s and on s i m i l a r s i t e types than D o u g l a s - f i r . In a d d i t i o n , a m a b i l i s f i r appears to c a r r y a g r e a t e r amount of f o l i a g e than western hemlock , which c a r r i e s more than 133 T a b l e 4.1 N i t r o g e n a c c u m u l a t i o n 1n u n d e r s t o r y biomass f o r v a r i o u s stands 1n western North America. Dominant t r e e sp. Age ( y e a r s ) Locat1 on Blomass - 1 t.ha N1 t r o g e n -1 kg. ha Source A b i e s amabl11s/ Tsuga h e t e r o p h y l l a 175 WA 1 .8 14.9 Turner and Si n g e r 1976 A. amabl1 i s / A. l a s l o c a r p a CO 4 .2 5 . O Snel1 et a l . 1979 A. amabl11s/ T. h e t e r o p h y l l a 1 WA 0 .3 Long 1976 " " 3 WA 1 .2 II 1  n II 7 WA 1 . 7 M 1  " " 17 WA 1 .8 " " " " 25 WA 4 4 II 1  46 WA 0 .5 " " " " 250 WA 0 .9 1  1  " " 550 WA 0 .9' II 1  P. menzlesl1 3-good B C 4 .5 40.0 F e l l e r e t a l . 1983 * II II 3-poor B C 10 0 45.0 II II II II 6-good B C 3 5 35.0 •I n n II 5-poor B C 10 0 45.0 II II " " 11-good B C 3 5 35.0 " " n II 9-poor BC 13 0 65.0 Ii <i " " 19-good B C 1 0 10.0 II II II II 19-poor B C 5 5 30.0 II II II n 48-good B C 2 2 28 .0 II II II II 48-poor B C 6 0 35.0 II II 73-good B C 5 0 53.0 ii ir II •• 74-poor B C 4 5 40.0 II ii " " 15-20 B C 3 9 13.8 Webber et a l . 1977 " " 9 WA 5 3 46.2 Turner 1975 " " 22 WA 7 6 66.6 " " " " 30 WA 5 1 47.8 " " " " 36 WA 1 0 6.0 II II II n 42 WA 4 2 38 .6 " " " " 42 WA 3 4 28.6 " " II II 73 WA 2 8 17.8 n I. II it 95 WA 1 2 8.7 ti II * i n c l u d e s b o t h above and belowground biomass ( n i t r o g e n ) T a b l e 4.2 T o t a l aboveground t r e e and f o l i a r biomass and n i t r o g e n content i n v a r i o u s stands In w e s t e r n N o r t h America. Dominant t r e e sp. Age L o c a t i o n Biomass Ni t r o g e n Source Stand t F o l i a g e -1 ha Stand kg F o l I a g e - 1 ha A b i e s a m a b i l i s 3 & 9 WA 9 G r i e r and Lee 1982 " 23 WA 53 13 .6 193 H II " 130 OR 453 II II A. amabi11s/ T . m e r t e n s l a n a 180 WA 465 15 . 7 345 173 Turner and Si n g e r 1976 A . a m a b i l i s / C . n o o t a k a t e n s 1 s 180 WA 438 G r i e r and Lee 1982 A . a m a b i l i s 180 WA 445 21 7 G r i e r et a l . 1981 A . amabi11s/ T . m e r t e n s l a n a 420 BC 467 5 2 557 172 Krumlik 1979 A. l a s l o c a r p a / P . g l a u c a 110-130 BC * 296 Kimmins 1974 A. l a s l o c a r p a CO * 285 22 7 758 223 Snel1 et a l . 1979 A . p r o c e r a / P. menzles11 130 OR 880 Fuj imor1 et a l . 1976 II 310 WA 1687 , II .1 A . p r o c e r a 130 OR 614 G r i e r unpub.(Cole and Rapp A. mar 1ana Japan 129 Tsutsumi 1971 A . f l r m a 97-145 Japan 380 15 1 573 146 Ando ( C o l e and Rapp 1980 A. amabi1 I s / T . h e t e r o p h y l l a 25 WA 56 2 9 Long 1976 T . heterophy11 a/ A. p r o c e r a 4G WA 257 15 0 u II T . h e t e r o p h y l l a / P. menz e s i i / A. amabi1 i s 250 WA 1360 33 5 it II T . heterophy11 a/ A . a m a b I l l s 550 WA 1430 57 O it H T . heterophy11 a/ C . n o o t a k a t e n s 1 s 250 BC 56 3 8 70 27 Krumlik and Kimmins 1976 P. m e n z i e s i i 15-20 BC 65 9 4 185 101 Webber et a l . 1977 " 36 WA 205 9 1 320 102 Turner 1975 II 450 OR 521 8 9 188 75 G r i e r et a l . 1974 ti " 10-good BC * 15 2 8 72 27 F e l l e r e t a l . 1983 " " " 10-poor BC * <1 0 1 3 1 it .. II " 20-good BC * 200 15 3 457 140 H it " 20-poor BC * 18 2 7 54 23 II ti " " 48-good BC * 435 14 4 918 155 ti it M " 48-poor BC * 88 7 3 249 76 II tt ft " 74-good BC * 628 10 7 1 117 106 II it II " 74-poor BC * 85 7 8 283 107 II M above and belowground components combined. D o u g l a s - f i r , r e f l e c t i n g the r e l a t i v e shade t o l e r a n c e of these s p e c i e s . ( K r a j i n a et a l . 1982). Packee et a l . (1982) have r e -p o r t e d volumes in two a m a b i l i s f i r s tands at medium e l e v a t i o n s on southwestern Vancouver I s l a n d that are as much as 50% (1505 3 -1 m .ha ) g r e a t e r than those i n f u l l y s tocked n a t u r a l s tands of western hemlock. However, mature D o u g l a s - f i r on the h i g h e s t s i t e s -1 -1 have been r e p o r t e d to have volumes of 3000 m .ha (Lavender p e r s . comm.). L i t t l e i n f o r m a t i o n i s a v a i l a b l e on the v a r i a t i o n in biomass and e s p e c i a l l y N content in young s tands of A b i e s or A b i e s spp . m i x t u r e s through the f i r s t 100 y e a r s or so of s tand development . G r i e r and Lee (1982) have r e c o r d e d aboveground biomass va lues -1 r a n g i n g from about 9 t . h a in 3- and 9 - y r - o l d s tands to 53 -1 • -1 t . h a (and a N content of 193 k g . N . h a ) in a 2 3 - y r - c l d s tand in Washington , which r e p r e s e n t e d about 12% of the biomass va lues -1 of 445 to 465 t . h a o b s e r v e d i n a mature s tand n e a r b y . Long (1976) r e p o r t e d aboveground biomass accumula t ions in 25- and -1 4 6 - y r - o l d second growth s tands to be 56 and 257 t . h a ; f o l i a g e -1 biomass was e s t imated to be 3 and 15 t . h a , r e s p e c t i v e l y . Amabi-l i s f i r formed a somewhat minor component of these s tands (3 to 23%) r e l a t i v e to that of western hemlock. Tree biomass a c c u m u l a t i o n in D o u g l a s - f i r s tands between the ages of 10 and 74 years on Vancouver I s l a n d amounted to between -1 15 and 628, and 1 and 88 t . h a on 'good' and ' p o o r ' s i t e s , r e s p e c t i v e l y ( F e l l e r et a l . 1983). For s i m i l a r s i t e s , N content -1 ranged from 72 to 1117, and 3 to 283 k g . N . h a . The v a l u e s for biomass and N content f o r the 'good' s i t e were c o n s i d e r a b l y h i g h e r than most of those p r e s e n t e d in T a b l e 4 . 2 . However, 136 L o n g ' s (1976) e s t imates for s tand biomass for o l d - g r o w t h ( i . e > 250 years ) mixed c o n i f e r f o r e s t at medium e l e v a t i o n s in Washing-ton were a p p r o x i m a t e l y double those of the 48- and 7 4 - y e a r - o l d 'good' s i t e D o u g l a s - f i r s tands r e p o r t e d by F e l l e r et a l . (1983) . There appears to be l i t t l e p u b l i s h e d data on the r a t e of a c c u m u l a t i o n of biomass and v i r t u a l l y none of n u t r i e n t content in young r e g e n e r a t i n g s tands of a m a b i l i s f i r and western hemlock. 4 . 2 . 2 I m m o b i l i z a t i o n in s l a s h a n d / o r stumps. The extremely l a r g e stumps and a s s o c i a t e d root systems that are o f t en l e f t a f t e r c l e a r c u t t i n g o l d - g r o w t h stands can ac t as a s i n k for n i t r o g e n . Few s t u d i e s have i n v e s t i g a t e d t h i s source of i m m o b i l i z a t i o n . -1 I t was e s t i m a t e d that of 580 kg .ha of n i t r o g e n m i n e r a l i z e d i n 5 years a f t e r c l e a r c u t t i n g and a h e r b i c i d e a p p l i c a t i o n at -1 Hubbard Brook, 215 kg .ha was i m m o b i l i z e d most ly in d e c a y i n g wood, and that t h i s would subsequent ly be an important source of. n i t r o g e n for t r e e growth about 15 y e a r s a f t e r c l e a r c u t t i n g (Cov-i n g t o n 1981). However, i n s u f f i c i e n t da ta were p r e s e n t e d to t e s t t h i s h y p o t h e s i s . A recent study by Fahey (1983) i n lodgepo le p i n e stands -1 -1 i n d i c a t e d an annual i m m o b i l i z a t i o n r a t e of 1.2 k g . N . h a . y r d u r i n g the p e r i o d of 30-55 years a f t e r l o g g i n g . The author d i d not r e c o r d an i n c r e a s e in n i t r o g e n c o n t e n t i n downed t r e e bo l e s u n t i l w e l l a f t e r canopy c l o s u r e . Downed b o l e s suspended above the f o r e s t f l o o r showed a much slower d e c l i n e i n s p e c i f i c g r a v i t y and i n c r e a s e s i n N content than non-suspended b o l e s . I t was suggested 1 37 that i n c r e a s e s i n N content were due to t r a n s l o c a t i o n of N from the f o r e s t f l o o r ; however, no data were p r e s e n t e d to support t h i s h y p o t h e s i s . 1 5 Us ing N , V i t o u s e k and Matson (1985) found tha t between 60 and 80% of added N was i m m o b i l i z e d in m i c r o b i a l biomass and o r g a n i c matter i n the f o r e s t f l o o r and m i n e r a l s o i l d u r i n g the f i r s t s i x months a f t e r c l e a r c u t t i n g ; i n c o m p a r i s o n , on ly about 10% was i m m o b i l i z e d in U n d e r s t o r y v e g e t a t i o n and 2-5% in l o g g i n g s l a s h d u r i n g the same time p e r i o d . The p a t t e r n was s i m i l a r in both an u n d i s t u r b e d and s e v e r a l c u t o v e r s i t e s r e c e i v i n g d i f f e r e n t s i t e p r e p a r a t i o n s . Such experiments p r o v i d e i n f o r m a t i o n on s h o r t -term i m m o b i l i z a t i o n , but l i t t l e ev idence for l o n g - t e r m i m m o b i l i -z a t i o n i n l a r g e r m a t e r i a l s such as s l a s h and stumps. Exper iments i n S i t k a spruce p l a n t a t i o n s i n d i c a t e d that immo-b i l i z a t i o n of n i t r o g e n may occur i n stumps l e f t i n t a c t , a f t e r h a r v e s t i n g . An a b s o l u t e i n c r e a s e in n i t r o g e n content o c c u r r e d by the time 75-80% of the carbon had been l o s t , but the o r i g i n of the n i t r o g e n was not i d e n t i f i e d . I t c o u l d have come from s e v e r a l s o u r c e s , of which t r a n s l o c a t i o n from the f o r e s t f l o o r or m i n e r a l s o i l i s j u s t one (Heal et a l . 1982). Aber et a l . (1983) conducted a s h o r t - t e r m i m m o b i l i z a t i o n experiment u s i n g some t renched p l o t s i n e a s t e r n hardwood and p i n e s t a n d s . U s i n g the a d d i t i o n of l a r g e wood c h i p s to the s o i l in t renched p l o t s as an analogue to the presence of l a r g e woody root systems in the s o i l a f t e r c l e a r c u t t i n g , they e s t i m a t e d i m m o b i l i -- 1 - 1 z a t i o n to be a p p r o x i m a t e l y 10 kg .N h a . . y r in the p ine stands -1 -1 and 30 k g . N . h a . y r i n the hardwood s t a n d s . 138 Berg and S t a a f ' s (1981) f i n d i n g s suggest t h a t h i g h e r f e r -t i l i t y s i t e s shou l d have a g r e a t e r c a p a c i t y to i m m o b i l i z e n i t r o -gen and for longer p e r i o d s of t ime than lower f e r t i l i t y s i t e s due to the former s i t e ' s h i g h e r c r i t i c a l n i t r o g e n l e v e l b e f o r e r e -l e a s e o c c u r s . However, recent ev idence suggests tha t m i n e r a l i z a -t i o n may not be as s t r o n g l y c o n t r o l l e d by C:N r a t i o s as was once thought ( S o l l i n s et a l . 1984). The i m m o b i l i z a t i o n / m i n e r a l i z a t i o n p a t t e r n for stumps, and l a r g e r o o t s a f t e r c l e a r c u t t i n g r e q u i r e s f u r t h e r i n v e s t i g a t i o n , e s p e c i a l l y w i t h r e g a r d to the l o n g - t e r m b e n e f i t s of p l a n t i n g a d j a c e n t to stumps. 4 . 2 . 3 M i n e r a l s o i l s torage 4 . 2 . 3 . 1 T h e o r e t i c a l b a s i s for a c c u m u l a t i o n in m i n e r a l s o i l The m i n e r a l s o i l r e p r e s e n t s a l o g i c a l medium for s torage of n i t r o g e n compounds moving through the s o i l in s o l u t i o n or suspen-s i o n . Three d i f f e r e n t ' types of s t o r a g e ' w i l l be o u t l i n e d i n r e l a t i o n to the "assart e f f e c t " a f t e r c l e a r c u t t i n g : 1. exchangea-b l e ammonium, 2. f i x e d ammonium and , 3. a d s o r p t i o n of o r g a n i c - N compounds by c l a y s . N i t r a t e i s b e l i e v e d to be r e a d i l y l e a c h e d out of the s o i l p r o f i l e in humid temperate f o r e s t s and so w i l l not be d i s c u s s e d i n terms of m i n e r a l s o i l s t o r a g e . C a t i o n s are a t t r a c t e d to n e g a t i v e l y charged c o l l o i d s , where-upon they are h e l d by e l e c t r o s t a t i c charges on the s u r f a c e of the c o l l o i d . P r e f e r e n c e of c l a y s for e q u i v a l e n t amounts of monovalent or d i v a l e n t c a t i o n s can be expres sed by the l y o t r o p i c s e r i e s ( A l > Ca > Mg > K > Na) (Tan 1982; Nommik and V a h t r a s 1982). 139 The combinat ion of c a t i o n s and exchange s u r f a c e s a v a i l a b l e w i l l determine the p r o p o r t i o n of ions i n s o l u t i o n y_s. those on exchange s i t e s . For example, humic a c i d s may not adsorb a p p r e c i a -b l e q u a n t i t i e s of ammonium, when the r a t i o of ammonium to c a l c i u m i s low in s o l u t i o n , u n t i l muscovi te exchange p o s i t i o n s are n e a r l y s a t u r a t e d (Black 1968). F i x e d c a t i o n s r e f e r to adsorbed c a t i o n s h e l d on so t i g h t l y that they are not r e c o v e r e d by exchange r e a c t i o n s (Tan 1982). F i x a t i o n of potass ium and ammonium c a t i o n s o c c u r s by entrapment of ions in the i n t e r - m i c e l l a r r e g i o n s of the c l a y m i n e r a l s . These r e g i o n s are capab le of a c c e p t i n g p o t a s s i u m or ammonium when c l a y l a t t i c e s are expanded, whi le c o n t r a c t i o n l eads to entrapment of ions between c l a y l a y e r s . They are then c o n s i d e r e d to be non-exchangeable or f i x e d .(van der M a r e l 1 959) . S o i l m i n e r a l s tha t have been r e p o r t e d to c o n t r i b u t e to f i x a t i o n i n c l u d e m i c a s , i l l i t e s , m o n t m o r i l l o n i t e s and v e r m i c u l i t e s (Tan 1982). Doram and Evans ' (1983) study i n d i c a t e d that v e r m i c u l i t e was more important than i l l i t e in ammonium f i x a t i o n i n those southwestern O n t a r i o s o i l s examined. However, Sowden et a l . (1978) r e p o r t e d t h a t more than j u s t the v e r m i c u l i t e content was c o n t r o l l i n g the ammonium f i x i n g c a p a c i t y in some e a s t e r n Canadian s o i l s . C o n t i n u e d a p p l i -c a t i o n of ammonium or potass ium f e r t i l i z e r s u s u a l l y reduces sub-sequent f i x a t i o n ( T i s d a l e and Ne l son 1975). I t i s not known how r e v e r s i b l e the proces s i s ; however, added f e r t i l i z e r - N tends to be r e c o v e r e d by subsequent crops a f t e r temporary f i x a t i o n (Nommik and V a h t r a s 1982). A d s o r p t i o n of o r g a n i c compounds by s o i l c o l l o i d s has r e c e n t l y r e c e i v e d g r e a t e r a t t e n t i o n , e s p e c i a l l y w i th r e s p e c t to 1 40 p e s t i c i d e s and h e r b i c i d e s . Past s t u d i e s have focused on the r o l e of o r g a n i c complexes i n the process of s o i l f o r m a t i o n e s p e c i a l l y p o d z o l i z a t i o n ( B r u c k e r t et a l . 1970). C o n s i d e r i n g the c h a r a c t e r of the s o i l s in the F l e e t R i v e r study a r e a , i t i s probab le that c o n s i d e r a b l e amounts of o r g a n i c - N are t r a n s p o r t e d to the m i n e r a l s o i l be fore and a f t e r c l e a r c u t t i n g . V a r i o u s types of bonds have been i m p l i c a t e d in a d s o r p t i o n , i n c l u d i n g van der Waals , hydrogen , e l e c t r o s t a t i c and metal che -l a t e type complexes ( B a i l e y and White 1970). The nature of the bond v a r i e s w i th the na ture of the o r g a n i c compound ( i . e . o r g a n i c c a t i o n v s . a n i o n , v s . an uncharged o r g a n i c compound). For a complete account of the p o s s i b i l i t i e s see Tan (1982) . A l though no agreement has been reached i n the l i t e r a t u r e as to the exact nature of the a d s o r p t i o n i s o t h e r m , a d s o r p t i o n of o r g a n i c com-pounds such as humic a c i d s (Doram and Evans 1983) has been r e -p o r t e d to f o l l o w the Langmuir type of e q u a t i o n , which i n d i c a t e s a d s o r p t i o n has some s o r t of f i n i t e l i m i t (Tan 1982). M o l e c u l a r s i z e i s thought to be an important determinant of r a t e s of a d s o r p t i o n and has been summarized by B a i l e y and White (1970) as f o l l o w s : 1. a d s o r p t i o n of n o n e l e c t r o l y t e s by nonpolar ads or be nt s i n c r e a s e s as m o l e c u l a r s i z e i n c r e a s e s 2. van der Waals f o r c e s of a d s o r p t i o n i n c r e a s e wi th i n c r e a s i n g m o l e c u l a r s i z e 3. a d s o r p t i o n decreases w i t h i n c r e a s i n g m o l e c u l a r s i z e due to s t e r i c h i n d r a n c e . Tan (1976) r e p o r t e d that o r g a n i c compounds of molecu-l a r weights between 1500 and 10,000 are p r e f e r e n t i a l l y adsorbed over o t h e r s . 141 4 . 2 . 3 . 2 Methods for e s t i m a t i n g changes in n i t r o g e n r e s e r v e s Exchangeable ammonium and n i t r a t e can be r e a d i l y e s t i m a t e d by e x t r a c t i n g the s o i l wi th KC1 (Bremner and Mulvaney 1982). A l -though t h e r e i s s t i l l some c o n t r o v e r s y as to the r e c o v e r y of f i x e d ammonium from some s o i l s u s i n g the r e g u l a r K j e l d a h l method, many of the s o i l s examined to date have shown s i m i l i a r r e c o v e r i e s of f i x e d ammonium when e i t h e r the r e g u l a r K j e l d a h l or the H F -K j e l d a h l methods have been u t i l i z e d (Bremner and Mulvaney 1982). Both the K j e l d a h l and the H F - K j e l d a h l methods e s t imate ammonium and o r g a n i c - N in the s o i l of i n t e r e s t , p l u s some p o r t i o n of the n i t r a t e , but q u a n t i t a t i v e recovery of the l a t t e r i s not u s u a l . Hence, i t i s not p o s s i b l e to s imply add n i t r a t e e s t i m a t e d by e x t r a c t i o n to the K j e l d a h l n i t r o g e n to e s t imate t o t a l - N (Bremner and Mulvaney 1982). Sco t t et a l . (1960) found that K j e l d a h l d i g e s t i o n , H r e s i n and Na r e s i n methods i n d i c a t e d s i m i l a r + amounts of f i x e d ammonium. On the o ther hand, both K r e s i n and 1N KC1 s o l u t i o n methods r e s u l t e d i n underes t imates of f i x e d ammonium. 4 . 2 . 3 . 3 E s t i m a t e s of n i t r o g e n c a p i t a l i n the m i n e r a l s o i l -1 T a b l e 4.3 i n d i c a t e s the range of n i t r o g e n c a p i t a l s ( k g . N . h a ) in f o r e s t s o i l s of v a r y i n g s i t e q u a l i t i e s and o v e r s t o r y v e g e t a t i o n . V a l u e s for Ab ie s a m a b i l i s s tands range from 1,980--1 15,855 k g . N . h a , a l t h o u g h not a l l i n v e s t i g a t o r s q u a n t i f i e d the amount of N in the complete s o i l p r o f i l e . S i m i l a r l y , N c a p i t a l s -1 ( k g . N . h a ) in low e l e v a t i o n f o r e s t s o i l s i n western Washington 142 show a wide range from 6,334-21,758 a c c o r d i n g to Wooldr idge (1961) and from 3,365-23,262 a c c o r d i n g to G e s s e l et a l . (1973). V a r i a t i o n in percent n i t r o g e n , the c o a r s e fragment c o n t e n t , bu lk d e n s i t y , and depth w i l l r e s u l t i n l a r g e d i f f e r e n c e s in n i t r o g e n c a p i t a l between s t a n d s . F a c t o r s which tend to i n f l u e n c e the above four v a r i a b l e s , namely s l o p e , s l ope p o s i t i o n , a s p e c t , parent m a t e r i a l , e t c . , w i l l i n f l u e n c e the s o i l n i t r o g e n c a p i t a l on a s i t e (Jenny et a l . 1949). I n v e s t i g a t i o n s of changes i n s i t e N c a p i t a l f o l l o w i n g c l e a r -c u t t i n g can be obscured by the g e n e r a l l y l a r g e w i t h i n - s t a n d or s i t e v a r i a b i l i t y ( F e l l e r 1980; C o u r t i n et a l . 1983). In a d d i t i o n , the use of a chronosequence to i n f e r changes i n t ime i s f u r t h e r c o m p l i c a t e d by b e t w e e n - s i t e v a r i a b i l i t y . Both MacLean and Wein ( 1978a, 1978b) and F e l l e r et a l . (19-83) f a i l e d to observe c o n s i s -tent t rends wi th s tand age w i t h r e s p e c t to m i n e r a l s o i l n u t r i e n t c o n t e n t . The former a t t r i b u t e d the l a c k of a c o n s i s t e n t p a t t e r n to d i f f e r e n c e s i n f i r e h i s t o r y amongst the jack p i n e s tands examined. The l a t t e r suggested that e i t h e r s l a s h b u r n i n g , f i r e h i s t o r y , or b e t w e e n - s i t e v a r i a b i l i t y c o u l d be r e s p o n s i b l e for the l a c k of c o n s i s t e n t t r e n d s w i t h s tand age in a chronosequence of second-growth D o u g l a s - f i r s tands on Vancouver I s l a n d ( F e l l e r et a l . 1983). Chronosequences in areas i n f r e q u e n t l y d i s t u r b e d by f i r e might be expec ted to be l e s s v a r i a b l e . Few s t u d i e s have examined changes i n n i t r o g e n c a p i t a l of the m i n e r a l s o i l a f t e r c l e a r c u t t i n g . P r e l i m i n a r y exper iments of Ugo-l i n i (1982) to t e s t the s t a b i l i t y of the i l l u v i a l c h a r a c t e r of the Bf h o r i z o n , i n d i c a t e d tha t i t i s not g r e a t l y a f f e c t e d by c l e a r c u t t i n g . Based on s o i l s o l u t i o n c o n c e n t r a t i o n s of i r o n and 143 T a b l e 4.3 M i n e r a l s o i l n i t r o g e n c o n t e n t f o r v a r i o u s stands i n western North Amer i c a . Dominant t r e e sp. Age/ L o c a t i o n Depth N i t r o g e n Source - 1 s i t e cm Kg.N.ha A b i e s a m a b i l i s o l d - q r o w t h BC 0-30 3976 Krumlik et a l . 1982 H II 180 WA 0-26 3555 Turner and Sin g e r n n 180 WA 0-60 15855 1976 A. amabi11s/ Tsuga h e t e r o p h y l l a OR s o i 1 1980 G r i e r unpub.(Cole II II OR II II 2320 and Rapp 1980) A b i e s g r a n d l s GO WA 3-30 2127 Gess e l and Klock 1982 A b i e s l a s l o c a r p a 110-350 BC s o i 1 4451 Kimmins 1974 Pseudotsuqa menzles11 OR s o i 1 5682 F r a n k l i n e t a l . 1968 H II 36 WA 0-60 2809 C o l e et a l . 1967 M " 42 WA 0-60 2790 G r i e r and C o l e 1972 n II 3-good BC 0-100 4970 F e l l e r et a l . 1983 n II 3-poor BC 0-30 1530 II II it n 5-good BC 0-100 3710 n II it n 6-poor BC 0-40 2270 n II II M 9-good BC o - i o o 5490 II n II II 11-poor BC 0-40 4440 II II II II 19-good BC 0- 100 4390 II II II II 19-poor BC 0-30 2890 II II n it 48-good * BC 0-100 13090 a II i i •• 48-poor BC 0-30 2250 •1 II II II 74-good BC 0-100 5710 it n n n 74-poor BC 0-20 3990 II II Tsuga h e t e r o p h y l l a / A b i e s amabi1 i s SM-M** BC 0-45 3950 Lewis 1976 T. h e t e r o p h y l 1 a/ Th u j a p i i c a t a SH-H BC 0-45 4526 II II T. h e t e r o p h y l 1 a/ A. amabi11s M-SH BC 0-45 3990 •1 !• it II H BC 0-45 10600 II II II H H BC 0-45 5150 t l II low e l e v a t i o n WA 0-23- 6334- WooldMdge 1961 c o n l f e r s 0-135 21758 t i it •I n WA 0-28 3365- S t e i n b r e n n e r unpub. 0-165 23262 (G e s s e l et a l . 1973) * 4 8 - y r - o l d good s i t e had some r e s i d u a l a l d e r on the s i t e . ** - SM = submesic, M = mesic, SH = s u b h y g r i c , H = h y g r i c ; s u b j e c t i v e l y d e t e r m i n e d m o i s t u r e regime based on the u n d e r s t o r y v e g e t a t i o n r e c o r d e d f o r the s i t e s . t o t a l o r g a n i c carbon ( T O O i n the s o i l p r o f i l e of h i g h e l e v a t i o n a m a b i l i s f i r s tands in Washington , i t appears tha t the podzo l i - za -t i o n p r o c e s s i s very much o p e r a t i v e in c l e a r c u t s . In f a c t , max i -mum i r o n c o n c e n t r a t i o n s were about 3 t imes g r e a t e r in a recent c l e a r c u t than i n the nearby mature s tand ( U g o l i n i 1982). T h i s sugges ts that the proces s of p o d z o l i z a t i o n might be the major mechanism of o r g a n i c - N t r a n s l o c a t i o n from the f o r e s t f l o o r and upper m i n e r a l s o i l to the lower s o i l p r o f i l e . Based on the low i r o n and TOC l e v e l s in s o i l s o l u t i o n in the C h o r i z o n , U g o l i n i (1982) c o n c l u d e d that most of the o r g a n i c - N had p r e c i p i t a t e d out i n the B h o r i z o n . 4.3 Methods 4 .3 .1 U n d e r s t o r y a c c u m u l a t i o n U n d e r s t o r y biomass was s e p a r a t e d i n t o 6 c a t e g o r i e s which i n c l u d e d the f o l l o w i n g : 1. mosses ( R h y t i d i a d e l p h u s l o r e u s , H y l o - comium s p l e n d e n s , R h y t i d i o p s i s robus ta and P l a g i o t h e c i u m u n d u l a - tum), 2. Ep i lob ium, a n g u s t i f o l i u m ( f i r e w e e d ) , 3. s t a n d i n g dead f i r e w e e d , 4. V a c c i n i u m spp. (which i n c l u d e d , main ly V . a la skaense H o w e l l . , w i th l e s s e r amounts of V . o v a l i f o l i u m Smith in Rees and V . p a r v i f o l i u m Smith in R e e s ) , 5. G a u l t h e r i a s h a l l o n P u r s h . ( s a l a l ) and 6. o t h e r . The ' o t h e r ' ca t egory i n c l u d e d s e v e r a l u n d e r s t o r y s p e c i e s of minor abundance. Each of the above 6 c a t e -g o r i e s was oven d r i e d at 70 deg C for 48 h o u r s , weighed to the n e a r e s t gram and subsamples ground to pass a 2 mm mesh s i e v e in a W i l e y m i l l . Percent n i t r o g e n was determined on .1 g samples u s i n g a K j e l d a h l procedure (Twine and and W i l l i a m s 1972), f o l l o w e d by c o l o r i m e t r i c a n a l y s i s on a T e c h n i c o n A u t o a n a l y z e r . 145 Belowground biomass was based on aboveground/belowground dry mass r a t i o s for s i m i l a r - a g e d s tands on Vancouver I s l a n d taken from ( F e l l e r et a l . 1983). Root samples excavated d u r i n g the i n v e n t o r y of the f o r e s t f l o o r were used to o b t a i n root n i t r o g e n c o n c e n t r a t i o n s , from which root n i t r o g e n content c o u l d be c a l c u -l a t e d . 4 .3 .2 O v e r s t o r y accumula t ion Four 5 m r a d i u s c i r c u l a r p l o t s were randomly l o c a t e d in each of the 15 chronosequence s t a n d s . A l l t r e e s g r e a t e r than 2.5 cm in d iameter at the base of the stem (bsd) were t a l l i e d for bsd in the 3 - , 6- , and 1 0 - y r - o l d c l e a r c u t s ; d iameter at b r e a s t h e i g h t (dbh) was t a l l i e d for a l l . i n d i v i d u a l s g r e a t e r than 2.5 cm dbh i n the 2 6 - y r - o l d and o l d - g r o w t h s t a n d s . I n d i v i d u a l s l e s s than 2.5 cm bsd in the young c l e a r c u t s , or l e s s than 2.5 cm dbh in the o l d e r s tands were t a l l i e d i n the i n v e n t o r y of u n d e r s t o r y v e g e t a t i o n , s i n c e t h e i r s m a l l s i z e and l a r g e numbers made i t more p r a c t i c a l to sample them in the 1 X 1 m u n d e r s t o r y sample p l o t s . The c h o i c e of a d i f f e r e n t lower l i m i t in the young v e r s u s o l d s tands was done for p r a c t i c a l r e a s o n s . The o l d - g r o w t h biomass e q u a t i o n s of G h o l t z et a l . (1979) were based on dbh measurements, whi l e the r e g r e s s i o n s used i n the young c l e a r c u t s were based on bsd measurements. R e g r e s s i o n equat ions for aboveground biomass for a m a b i l i s f i r and western hemlock were deve loped by h a r v e s t i n g i n d i v i d u a l t r e e s r e p r e s e n t i n g the range of bsd or dbh (cm) o b s e r v e d . A t o t a l of 7 a m a b i l i s f i r and 7 western hemlock were h a r v e s t e d , r a n g i n g 146 i n s i z e from 1.0 cm bsd to 22.0 cm dbh . Separate equat ions were deve loped for each of the s p e c i e s ( a m a b i l i s f i r and western hemlock) . A l t h o u g h the sample s i z e i s s m a l l , the use of these e q u a t i o n s seemed p r e f e r a b l e to us ing e q u a t i o n s from the l i t e r a -t u r e (see s e c t i o n 4 . 4 . 1 . 2 . 1 ) . T h i s w i l l be d i s c u s s e d f u r t h e r in the r e s u l t s and d i s c u s s i o n s e c t i o n . Biomass components f o r t r e e s sampled in the o l d - g r o w t h stands were p r e d i c t e d from dbh u s i n g r e g r e s s i o n e q u a t i o n s of G h o l t z et a l . (1979) . F r e s h mass of i n d i v i d u a l components ( i . e . f o l i a g e , branches and stem biomass) were o b t a i n e d in the f i e l d . Sys temat ic subsam-p l e s of f o l i a g e and branches w i t h i n the crown, and d i s c s from the b o l e , were taken back to the l a b o r a t o r y to be weighed f r e s h and a f t e r d r y i n g at 70 deg C (to c o n v e r t f r e s h mass to dry mass) . E n t i r e s m a l l e r t r e e s were taken back to the l a b o r a t o r y and oven d r i e d and weighed. Need le s were removed from branches a f t e r d r y i n g to determine the p r o p o r t i o n of f o l i a g e to branch b iomass . Subsamples were ground i n a Wi l ey m i l l to pass a 2 mm mesh s i e v e . Average f o l i a r n i t r o g e n c o n c e n t r a t i o n s for the crown as a whole f o r a m a b i l i s f i r , wes tern hemlock, and western redcedar were o b t a i n e d by sampl ing t r e e crowns i n r e p l i c a t e 3 of the o l d - g r o w t h s tand a f t e r the s tand was c l e a r c u t i n the summer of 1983. Concen-t r a t i o n s for b r a n c h e s , b a r k , and wood tend to be l e s s v a r i a b l e between s t a n d s , and so v a l u e s for these components were taken from K r u m l i k (1979) f o r a m a b i l i s f i r and western hemlock, and were o b t a i n e d from F e l l e r ( p e r s . comm.) for western r e d c e d a r . For a l l the samples c o l l e c t e d , percent N was de termined u s i n g a K j e l d a h l procedure (Twine and W i l l i a m s 1972) and a T e c h n i c o n A u t o a n a l y z e r . 147 S e e d l i n g s l e s s than 2.5 cm at bsd and 2.5 cm at dbh in the young c l e a r c u t s and o l d e r s t a n d s , r e s p e c t i v e l y , were t a l l i e d d u r i n g the i n v e n t o r y of u n d e r s t o r y v e g e t a t i o n u s i n g 1 x 1 m sampl ing f rames . Biomass of these t r e e s down to 1.0 cm bsd was e s t i m a t e d u s i n g the r e g r e s s i o n s . An average c o n c e n t r a t i o n for a l l components combined was used to e s t imate n i t r o g e n content s i n c e the biomass was smal l in r e l a t i o n to stems g r e a t e r than 2.5 cm bsd in the young c l e a r c u t s , and to stems g r e a t e r than 2.5 cm at dbh in the 2 6 - y r - o l d c l e a r c u t and o l d - g r o w t h s t a n d s . 4 . 3 . 3 Storage of n i t r o g e n compounds in the upper m i n e r a l s o i l M i n e r a l s o i l was excavated down to a depth of 30 cm at 25 randomly d e t e r m i n e d l o c a t i o n s w i t h i n each of the 15 s tands in the summer of 1982. T o t a l p i t volume was determined in the f i e l d u s i n g a g r a d u a t e d c y l i n d e r , water , and p l a s t i c bags to l i n e the p i t . The volume of l a r g e r c o a r s e fragments was e s t i m a t e d in the f i e l d by water d i s p l a c e m e n t , and l a t e r c o n v e r t e d to a mass b a s i s -3 u s i n g a d e n s i t y of 2.65 g.cm ( H i l l e l 1980). Except for the above , and a few subsamples (see be low) , t o t a l mass of each f r a c t i o n was measured to the neares t g a f t e r d r y i n g at 105 deg C f o r 48 h r s . Bulk d e n s i t y was c a l c u l a t e d as t o t a l mass d i v i d e d by t o t a l volume. S o i l samples were s i e v e d i n t o 3 c a t e g o r i e s : 1. l e s s than 2 mm 2. 2-12 mm, and 3. g r e a t e r than 12 mm, a f t e r which the l e s s than 2 mm f r a c t i o n was randomly b u l k e d i n t o 8 s e t s to be used to determine t o t a l K j e l d a h l N (TKN). The 2-12 mm and > 12 mm f r a c -t i o n s were l a t e r combined. In each of the 15 s t a n d s , e i g h t compo-1 48 s i t e samples were a n a l y z e d for t o t a l K j e l d a h l N, wh i l e 5 of these samples were e x t r a c t e d wi th 1N KC1 and shaken for 1 hour to determine e x t r a c t a b l e n i t r a t e and ammonium (Bremner 1965). Due to the long time i n t e r v a l (3 months) between c o l l e c t i o n and l a b o r a t o r y a n a l y s i s , oven d r i e d samples were used . However, a s m a l l number of a i r d r i e d subsamples (< 10% m o i s t u r e content ) were e x t r a c t e d for purposes of c o m p a r i s o n . There was very l i t t l e d i f f e r e n c e in c o n c e n t r a t i o n s of ammonium and n i t r a t e between 5 a i r d r i e d and 5 oven d r i e d o l d - g r o w t h m i n e r a l s o i l samples . A l l ana lyse s were conducted on a T e c h n i c o n A u t o a n a l y z e r u s i n g e i t h e r the indophenol b lue method for TKN and e x t r a c t a b l e ammonium or h y d r a z i n e and h y d r a t e d c u p r i c s u l f a t e r e d u c t i o n of n i t r a t e to n i t r i t e f o l l o w e d by the G r e i s s - I l o s v a y procedure to determine n i t r a t e (Keeney and Nelson 1982). S o l u t i o n s were s t o r e d frozen l e s s than one week p r i o r to a n a l y s i s . Amounts of TKN, ammonium and n i t r a t e were c a l c u l a t e d as f o l l o w s : -1 TKN (kg .ha )= [%TKN/100] x [mass of < 2mm f r a c t i o n ] x -3 5 [ B . D . ( g . c m )] x [Depth (cm)] x 10 4.4 R e s u l t s and D i s c u s s i o n S i n k s f o r m o b i l i z e d f o r e s t f l o o r n i t r o g e n which were exa-mined i n c l u d e d : 1. u n d e r s t o r y a c c u m u l a t i o n , 2. o v e r s t o r y accumu-l a t i o n and 3. m i n e r a l s o i l s t o r a g e . 149 4.4 .1 Biomass a c c u m u l a t i o n 4 . 4 . 1 . 1 U n d e r s t o r y and s e e d l i n g biomass As e x p l a i n e d i n the methods s e c t i o n , u n d e r s t o r y v e g e t a t i o n was c a t e g o r i z e d i n t o 6 c l a s s e s : 1. mosses, 2. l i v e f i r eweed , 3. s t a n d i n g dead f i r e w e e d , 4. V a c c i n i u m spp. 5. s a l a l and 6. o t h e r . A f t e r a d e c l i n e in t o t a l u n d e r s t o r y biomass (from 1019 to 576 -1 kg .ha ) d u r i n g the f i r s t 3 y e a r s a f t e r c l e a r c u t t i n g , f ireweed -1 emerged as the dominant u n d e r s t o r y p l a n t by 6 years (860 kg.ha ) and m a i n t a i n e d i t s dominance at l e a s t u n t i l 10 years (820 kg . -1 ha (See F i g . 4 . 1 ) . Because l i v e f i reweed biomass had s t a r t e d to d e c l i n e , and was exceeded by s t a n d i n g dead biomass by 10 y e a r s , f i r eweed biomass p r o b a b l y peaked between 6 and 10 years a f t e r c l e a r c u t t i n g . Given the a s s o c i a t i o n of f i reweed wi th n i t r o g e n a v a i l a b i l i t y ( e s p e c i a l l y n i t r a t e ; E l l e n b e r g 1978), the p a t t e r n of f i reweed biomass suggests that n i t r o g e n a v a i l a b i l i t y had s t a r t e d to d e c l i n e by 10 y e a r s a f t e r c l e a r c u t t i n g . The biomass of Vacc in ium spp . deve loped more s l o w l y , r e a c h i n g a maximum somewhere between 10 and 26 y e a r s a f t e r c l e a r c u t t i n g . A f t e r a s m a l l d e c l i n e i n biomass a f t e r c l e a r c u t t i n g , moss ap-peared to be r e c o v e r i n g i n the 1 0 - y e a r - o l d c l e a r c u t s . Being a r a t h e r s m a l l component, s a l a l showed s m a l l f l u c t u a t i o n s in b i o -mass over the s tands examined, as d i d the ' o t h e r ' c a t e g o r y . T o t a l u n d e r s t o r y biomass v a l u e s appear to be s i m i l a r to those -1 r e p o r t e d in o ther s t u d i e s . L o n g ' s (1976) e s t i m a t e of 0.9 t . h a of u n d e r s t o r y biomass in o l d - g r o w t h mixed c o n i f e r f o r e s t i s c l o s e -1 to my e s t imate of 1.0 t . h a . However, u n d e r s t o r y biomass accumu-l a t i o n in the c l e a r c u t s examined by Long (1976) was c o n s i d e r a b l y 150 F i g u r e 4 . 1 Biomass o f i m p o r t a n t u n d e r s t o r y s p e c i e s (kg.ha ) f o r the chronosequence o f stands examined. -1 -1 lower ( i . e . 1.7 t . h a @ 7 years and 1.8 t . h a @ 17 y e a r s , -1 -1 v s . 4.3 t . h a @ 6 years and 5.2 t . h a @ 10 y e a r s ) , sug-g e s t i n g t h a t : 1. the F l e e t R i v e r c u t o v e r s underwent g r e a t e r d i s t u r b a n c e d u r i n g l o g g i n g ; t h i s e x p l a n a t i o n i s suppor ted by the absence of f i reweed i n the c l e a r c u t s examined by Long (1976) . In a d d i t i o n to the above, d i f f e r e n c e s in u n d e r s t o r y biomass c o u l d have been due t o : 1. d i f f e r e n c e s in the amount of advance or p o s t - l o g g i n g r e g e n e r a t i o n a n d / o r 2. d i f f e r e n t s i t e t y p e s . The u n d e r s t o r y biomass in the 2 5 - y r - o l d s tand observed by Long (1976) was of the same magnitude as tha t measured in the 2 6 - y r - o l d -1 s tands in my study ( i . e . 4.4 v s . 5.2 t . h a ) . The p a t t e r n of a c c u m u l a t i o n of n i t r o g e n in u n d e r s t o r y v e g e t a -t i o n f o l l o w s that of i t s biomass (Table 4 . 4 ) . Duncans MRT (P=.05) i d e n t i f i e d two groups w i t h i n which the e lements were not s i g n i f i -c a n t l y d i f f e r e n t : 1. o l d - g r o w t h and 3 - y r - o l d c l e a r c u t s and , 2. 6 - , 10- and 2 6 - y r - o l d c l e a r c u t s . Above and belowground biomass and n i t r o g e n content f o l l o w the same p a t t e r n except t h a t there i s p r o p o r t i o n a t e l y more belowground biomass and n i t r o g e n in V a c c i - nium spp . than i n s p e c i e s such as f i r e w e e d . Thus , r e l a t i v e to the o t h e r s i t e s , the t o t a l (above and belowground) biomass and n i t r o -gen c o n t e n t are p r o p o r t i o n a t e l y g r e a t e r in the 2 6 - y r - o l d c l e a r -c u t s . In comparison to s l a s h b u r n e d second-growth D o u g l a s - f i r p l a n t a t i o n s on "good" s i t e s on Vancouver I s l a n d ( F e l l e r et a l . 1983), where the n i t r o g e n content i n u n d e r s t o r y v e g e t a t i o n was -1 e s t i m a t e d to be 40 k g . N . h a a f t e r 3 y e a r s , 35 a f t e r 6 and 11 -1 y e a r s , and on ly 10 k g . N . h a a f t e r 19 y e a r s , the r a t e of under-1 52 T a b l e 4.4 N i t r o g e n c o n t e n t 1n the u n d e r s t o r y biomass f o r the chronosequence examined i n 1981. -1 * N i t r o g e n c o n t e n t (kg.N.ha ) S i t e Aboveground Belowground ** T o t a l Age * * * ** * * o l d-growth 7. ,45 a (3 .06) 1 .6 9 .0 a 3 5 ,92 a (2 .44) 1 .8 7 .7 a 6 32 . , 10 b (2 . 20) 6 . 3 38 . 4 b 10 40. , 13 b (11 .07) 1 1 .0 51 . 1 b 26 32. ,04 b (12 34) 18 .8 50 .8 b * s i t e age i n 1981 ** - belowground biomass e s t i m a t e d from above/belowground r a t i o s t h a t were e s t a b l i s h e d by e x c a v a t i o n . *** - means f o l l o w e d by the same l e t t e r a r e not s i g n i f i c a n t l y d i f f e r e n t a t P=.05 u s i n g Duncans MRT. **** - s t a n d a r d d e v i a t i o n 1n b r a c k e t s s t o r y development and subsequent d e c l i n e appears to be much slower in the F l e e t R i v e r s i t e s . T h i s i s p r o b a b l y due to a com-b i n a t i o n of t h r e e f a c t o r s : 1. the d i f f e r e n c e in the subzones ( i . e . dry v s . wet subzone) , 2. the e f f e c t of s l a s h b u r n i n g , and 3. the d i f f e r e n c e in s i t e q u a l i t y , a l l of which would tend to hasten s tand development i n the case of the D o u g l a s - f i r p l a n t a -t i o n s . The amount of n i t r o g e n accumulated i n p i n c h e r r y biomass 4-6 years a f t e r c l e a r c u t t i n g e a s t e r n hardwood f o r e s t s at Hubbard -1 Brook was e s t i m a t e d to be 146 to 192 k g . N . h a (Marks 1974). T h i s i s about 4-5 t imes g r e a t e r than the u n d e r s t o r y n i t r o g e n content observed at a s i m i l a r time a f t e r c l e a r c u t t i n g on the F l e e t R i v e r s i t e s . -1 T a b l e 4.5 i n d i c a t e s the number of s tems.ha , the biomass and the n i t r o g e n content i n t r e e s e e d l i n g stems l e s s than 2.5 cm at bsd in the 3 - , 6- and 1 0 - y r - o l d c l e a r c u t s , and t r e e s e e d l i n g stems l e s s than 2.5 cm at dbh in the o l d e r s t a n d s . N content in -1 s e e d l i n g biomass ranged from 0.2 to 5.4 k g . N . h a a c r o s s the range of s tand ages examined. Only i n the 3 - y r - o l d c l e a r c u t s do s e e d l i n g s r e p r e s e n t a s i g n i f i c a n t c o n t r i b u t i o n to t r e e biomass a c c u m u l a t i o n ( i . e . g r e a t e r than 5%), where the n i t r o g e n they c o n t a i n e d was 13% of the n i t r o g e n accumulated i n stems g r e a t e r than 2.5 cm b s d . 4 . 4 . 1 . 2 O v e r s t o r y a c c u m u l a t i o n 4 . 4 . 1 . 2 . 1 Development of r e g r e s s i o n e q u a t i o n s The u n c r i t i c a l a p p l i c a t i o n of p u b l i s h e d r e g r e s s i o n s for e s t i m a t i n g biomass can l e a d to s i g n i f i c a n t o v e r - or u n d e r - e s t i m a -1 54 T a b l e 4.5 Average d e n s i t y , biomass and N c o n t e n t of t r e e s w i t h stems < 2.5cm b a s a l stem d i a m e t e r or dbh f o r the chrono-sequence of s t a n d s examined. S i t e Age Dens 1ty B i omass 1 •1 stems.ha kg. ha N i t r o g e n Content -1 kg.N.ha o l d - g r o w t h 12667 * * * (2517) 1024 (223) 5.4 (1.2) 3 - y r - o l d 3667 (3786) 245 (52) 1 . 3 (0.2) 6 - y r - o l d 1333 (577) 33 (30) 0.2 (0.2) 1 0 - y r - o l d 5000 ( 1732) 266 (326) 2.6 (1.7) 2 6 - y r - o l d 3667 ( 1528) 317 (363) 1 . 7 ( 1 9 ) * - s i t e age In 1981 ** - stems < and < 2.5 2.5 cm cm a t BSD f o r the 1978. DBH i n o t h e r s . 1975, and 1971 c l e a r c u t s *** - S t a n d a r d d e v i a t i o n s based on average of t h r e e r e p l i c a t e s p e r age c l a s s . t i o n ( G r i e r and M i l n e 1980). U s i n g biomass r e g r e s s i o n s p u b l i s h e d i n G h o l t z et a l . (1979) and G r i e r and M i l n e (1980), e s t imates of f o l i a r biomass were p r e d i c t e d f o r h a r v e s t e d a m a b i l i s f i r between a p p r o x i m a t e l y 1 and 22 cm i n d i a m e t e r . Biomass r e g r e s s i o n s r e -p o r t e d in G h o l t z et a l . (1979) were based on l a r g e t r e e s in o l d -growth s t a n d s , ' whereas r e g r e s s i o n s p u b l i s h e d in G r i e r and M i l n e (1980) were d e r i v e d in a 2 3 - y e a r - o l d s tand for stems between 0.5 and 6.0 cm at b a s a l stem d i a m e t e r . A comparison of e s t imates of f o l i a r biomass from these r e g r e s s i o n s w i th the a c t u a l measured f o l i a r biomass of some h a r v e s t e d a m a b i l i s f i r in the young c l e a r c u t s arid 2 6 - y e a r - o l d s tands (Appendix 3 ) , l e d to the f o l l o w i n g o b s e r v a t i o n s : 1. the e q u a t i o n d e r i v e d from t r e e s i n o l d - g r o w t h stands (Ghol tz et a l . (1979)) , tended to u n d e r - e s t i m a t e f o l i a r biomass by 200-500% for a l l the t r e e s h a r v e s t e d ; t h i s might be expected c o n s i d e r i n g the range of d iameters used in d e v e l o p i n g the e q u a t i o n , 2. the equa-t i o n i n G r i e r and M i l n e (1980), which was based on stems < 6.0 cm, d e s p i t e the s t a n d ' s age of 23 y e a r s , u n d e r - e s t i m a t e d by 200% the measured v a l u e s of f o l i a r biomass of the s m a l l t r e e s (< 10.7 cm), but s l i g h t l y o v e r - e s t i m a t e d f o l i a r biomass of the 17-22 cm diameter t r e e s h a r v