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Salal understory removal effects on the soil water regime and tree transpiration rates in a Douglas-fir.. 1985

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SALAL UNDERSTORY REMOVAL EFFECTS ON THE SOIL WATER REGIME AND TREE TRANSPIRATION RATES IN A DOUGLAS-FIR FOREST by FRANCIS MAURICE KELLIHER A. A.S., Paul Smith's College, 1975 B. S., State University of New York, College of Environmental Science and Forestry, 1977 M.S., Oklahoma State University, 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 S o i l Science We accept this thesis as conforming"to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June 1985 0 Francis Maurice K e l l i h e r , 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of $o ) / S<̂> ê|<C€l_ The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 »E-6 (3/81) - i i - ABSTRACT Sal a l (Gaultheria shallon Pursh.) understory i n a 800 tree/ha 31-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand was cut and removed from around one of each of four pairs of adjacent trees, the root zones of which were is o l a t e d using p l a s t i c sheeting buried to bedrock. The differences i n the courses of the average root zone s o i l water content (9) during the growing season were small (maximum difference = 0.03 m m ) because t o t a l evapotranspiration was only s l i g h t l y higher where s a l a l was present than where i t had been removed. Porometer and lysimeter measurements on selected days indicated that s a l a l t r a n s p i r a t i o n was 0.5-1 mm d _ 1 greater than forest f l o o r evaporation i n cut subplots and that Douglas-fir t r a n s p i r a t i o n was 0.2-0.5 mm d - 1 higher where s a l a l had been removed. The s l i g h t increase i n 6 where s a l a l had been removed corresponded to s i g n i f i c a n t l y higher s o i l water po t e n t i a l and Douglas-fir pre-dawn twig xylem water p o t e n t i a l at low values of 6, owing to the steepness of the water retention curve for the gravelly sandy loam s o i l . This resulted i n s i g n i f i c a n t l y greater tree diameter growth where s a l a l had been removed than where i t remained. Shuttleworth's development of the Penman-Monteith equation for multilayer, p a r t i a l l y wet forest canopies was modified for use i n the hypostomatous canopies of Douglas-fir and s a l a l . This evapotranspiration theory was combined with standard hourly mlcrometeorological measurements, transfer resistance functions and canopy and root zone water balance equations to provide calculations of forest evapotranspiration (E) over - i i i - extended growing season periods. There was generally good agreement between calculated values of E and values determined using Bowen ratio/energy balance, water balance and porometer measurements. The s l i g h t l y higher values of 9 r e s u l t i n g from understory removal corresponded to s i g n i f i c a n t l y higher tree t r a n s p i r a t i o n rates calculated over early (June) and late (August) growing season periods. Most of the difference i n calculated tree t r a n s p i r a t i o n occurred during the f i n a l one-half of these periods when at low values of 6 s l i g h t l y lower 9 corresponded to s i g n i f i c a n t l y lower T s where s a l a l remained, leading to a reduction i n Douglas-fir t r a n s p i r a t i o n due to stomatal closure. The increase i n calculated tree t r a n s p i r a t i o n as a resu l t of understory removal was greatest where understory leaf area index was highest and trees were la r g e s t . - i v - TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES ix NOTATION x i v ACKNOWLEDGEMENTS xxi INTRODUCTION 1 CHAPTER 1 - EFFECTS OF SALAL UNDERSTORY REMOVAL ON THE SOIL WATER REGIME AND GROWTH OF DOUGLAS-FIR TREES 3 1. INTRODUCTION 4 2. METHODS 5 1. Site Description 5 2. Experimental Design 6 3. Measurements 7 1. Growing Season Weather Observations 7 2. Root Zone S o i l Water Content and Evapotranspiration Rate 8 3. S o i l Water Po t e n t i a l and Tree Pre-dawn Twig Xylem Water P o t e n t i a l 10 4. Forest Floor Evaporation Rate 12 5. Transpiration of Understory and Trees 13 6. Tree Diameter Growth 14 3. RESULTS AND DISCUSSION 15 1. Growing Season Weather Observations 15 2. Root Zone S o i l Water Content 18 - v - Page 3. S o i l Water P o t e n t i a l and Tree Pre-dawn Twig Xylem Water P o t e n t i a l 26 4. Evapotranspiration Rates Calculated from the S o i l Water Balance . . . . . . . . 26 5. P a r t i t i o n i n g Evapotranspiration Between Douglas-fir and S a l a l Transpiration and Forest Floor Evaporation 34 6. Tree Diameter Growth 36 4. CONCLUSIONS 36 5. REFERENCES 40 CHAPTER 2 - APPLICATION OF AN EVAPOTRANSPIRATION MODEL TO ESTIMATING SALAL UNDERSTORY REMOVAL EFFECTS IN A DOUGLAS-FIR FOREST 43 1. INTRODUCTION 44 2. THEORY 45 3. METHODS 47 1. Site and Experimental Design 47 2. Micrometeorological Measurements 48 3. Canopy Resistance Functions 49 4. Root Zone Water Balance Equation 54 5. Testing the Evapotranspiration and Root Zone Water Balance Equations . . . . . 54 4. RESULTS AND DISCUSSION 56 1. Measured and Calculated Courses of E 56 2. Forest Floor Evaporation After S a l a l Removal . . . . 56 3. Measured and Calculated Courses of 6 62 1. Using Equations (1) to (4) 62 2. E f f e c t of Assuming r ^ i = 0 i n Equations (1) to (4) 62 - v i - Page 4. P a r t i t i o n i n g of Evapotranspiration i n Cut and Uncut Subplots 68 5. CONCLUSIONS 72 6. REFERENCES 73 CONCLUSIONS 76 APPENDIX I - DERIVATION OF EQUATION (1) IN CHAPTER 2 81 APPENDIX II - DERIVATION OF THE EQUATION FOR r c SHOWING DEPENDENCE ON THE FRACTION OF WET LEAF AREA . . . . 85 APPENDIX III - RELATIONSHIP BETWEEN STOMATAL RESISTANCES OF AMPHISTOMATOUS AND HYPOSTOMATOUS LEAVES THAT RESULTS IN EQUAL TRANSPIRATION RATES 91 APPENDIX IV - DERIVATION OF EQUATION (5) IN CHAPTER 2 94 APPENDIX V - ELECTRICAL ANALOGS OF THE LATENT AND SENSIBLE HEAT FLUXES IN UNCUT AND CUT SUBPLOTS 96 APPENDIX VI - UNDERSTORY REMOVAL EFFECTS ON THE BELOW-TREE- CANOPY RADIATION REGIME 100 APPENDIX VII - MEASUREMENTS OF r s i n DOUGLAS-FIR AND SALAL . . . . 118 APPENDIX VIII - SOIL WATER RETENTION CURVE 121 - v i i - LIST OF TABLES Table Page 1.1 Measured average depth (mm) to bedrock (± one standard deviation) i n the eight subplots. Also shown are the four plot average values (± one standard d e v i a t i o n ) . . . . 19 1.2 Depth (mm) to base of the neutron probe access tubes placed In the eight subplots 20 1.3 Minimum measured values of average root zone water 3 3 content (m m ) i n the eight subplots 25 1.4 Douglas-fir pre-dawn t o t a l twig xylem water p o t e n t i a l (MPa) i n the eight subplots 30 1.5 Average evapotranspiration rates (mm d ) In the eight subplots in August 1981 as calculated from -(A0/At)£ + P assuming drainage was n e g l i g i b l e . On August 24, 9 mm of r a i n f e l l while during the* other three periods there was no r a i n . For July 30-August 19 and August 19-27, standard deviations were t y p i c a l l y 0.5 and 0.3 mm d - 1 r e s p e c t i v e l y . 31 1.6 Average evapotranspiration rates (mm d - 1 ) i n the eight subplots in May and June 1982 as calculated from -(A6/At)c + P assuming drainage was n e g l i g i b l e . During the period A p r i l 19—May 27, 12 mm o f r a i n f e l l while during the other three periods there was no r a i n . Standard deviations were t y p i c a l l y 0.3 mm d - 1 . 32 1.7 Measured rates (mm d - 1 ) of Douglas-fir and s a l a l t r a n s p i r a t i o n ( E j ) , forest f l o o r evaporation (E Q) and t o t a l evapotranspiration (E) i n plot 2 using porometry and equations (2) and (3) and small weighing lysimeters. The root mean square errors for Douglas-fir and s a l a l E j were t y p i c a l l y 0.1-0.2 mm d - 1 . These errors were determined by d i f f e r e n t i a t i o n of (2) and (3). For (2), a 30% error was assumed for a s and r D and a 20% error for e* - e a while the standard deviation of the r s measurements was used as the error i n r s . For (3), a 30% error was assumed for A t£ and a 10% error for D while the standard deviation of the r s measurements was used as the error i n r s . For E Q , standard deviations were t y p i c a l l y 0.1 m r n d - . • 35 1.8 Diameter ( i n c l u d i n g bark), at the 1.37 m height, of the four pairs of adjacent trees using a tape with a 1 mm ( i n diameter) r e s o l u t i o n 38 - v i i i - Table Page 2 1 1 1.9 Bas a l area increment (mm t r e e - year ) f o r the four p a i r s of adjacent trees c a l c u l a t e d u s i n g Table 1.8 39 2.1 Values of the e m p i r i c a l constants X, u, v, £ and o i n the stomatal r e s i s t a n c e ( r s ) c h a r a c t e r i s t i c s f u n c t i o n r s (| m"1 ) = exp[X - u ( ^ s + v) + U + o ( t s + v))D ] where ¥ s i s average root zone s o i l water p o t e n t i a l (MPa) and D i s vapour pressure d e f i c i t (kPa). . . 50 2.2 D a i l y t o t a l net r a d i a t i o n f l u x density above the f o r e s t ( R n a ) (MJ m - 2 d - 1 ) and d a i l y measured and c a l c u l a t e d values of e v a p o t r a n s p i r a t i o n r a t e (E) (mm d - 1 ) f o l l o w i n g i n i t i a l i z a t i o n of c a l c u l a t i o n s on August 20, 1982 when measured 6 was 0.16 m3 m~3 (¥ s = -0.3 MPa) 57 2.3 Average values of the minimum measured and c a l c u l a t e d average root zone water content (m m ) on the same day i n the cut (C) and uncut (U) subplots 65 2.4 Average c a l c u l a t e d values (mm) of t o t a l e v a p o t r a n s p i r a t i o n ( E ) , t r a n s p i r a t i o n (E^,), evaporation of i n t e r c e p t e d water (E|) and f o r e s t f l o o r evaporation ( E Q ) i n the cut (C) and uncut (U) subplots f o r the periods J u l y 24-Septeraber 3, 1981 and May 27-July 1, 1982. 69 AVI . l Sky view f a c t o r s (S.V.F.) determined from photographs taken with a f i s h eye lens i n each of the four p l o t s and along the path traversed by the tram 112 AVI.2 D a i l y t o t a l values of the r a t i o of below to above tree canopy net r a d i a t i o n f l u x density (Rnb/ Rna) a n <* s o l a r i r r a d i a n c e (K+b/K+a) f o r the cut and uncut po r t i o n s of the tram's path for seven days i n 1982. Also shown i s the s o l a r i r r a d i a n c e and net r a d i a t i o n f l u x density above the f o r e s t (K+ a and R n a ) f o r the same days. 113 - i x - LIST OF FIGURES Figure Page 1.1 Courses of f i v e day average daily values of r a i n f a l l rate (P), maximum and minimum forest canopy a i r temperature ( J a ± r ) and solar irradiance above the forest (K+ a) for the experimental s i t e from A p r i l 1 - October 31, 1981. Measurements of P and T a-j_ r are from the Campbell River airport (13 km north of the s i t e ) from A p r i l 1 - May 14 and October 15-31. For the same periods, K+ a measurements are from Nanaimo Departure Bay (130 km south of the s i t e ) 16 1.2 Same as for F i g . 1.1. except for 1982 and A p r i l 1 - May 19 and September 15 - October 31 17 1.3 Courses of average root zone s o i l water content (9) i n the cut (0) and uncut (0) subplots of plot 1 from May 27 - October 22, 1981 and from A p r i l 29 - October 6, 1982. Also shown i s the d a i l y r a i n f a l l rate (P). Root zone depth was 731 mm. 21 1.4 Same as for F i g . 1.3 except for plot 2 and 667 mm 22 1.5 Same as for F i g . 1.3 except for plot 3 and 618 mm 23 1.6 Same as for F i g . 1.3 except for plot 4 and 613 mm 24 1.7 P r o f i l e s of s o i l water content (9) on selected days i n 1982 i n the cut (top half of graph) and uncut (bottom half of graph) subplots of plot 2. 27 1.8 Courses of average root zone s o i l water p o t e n t i a l ( ¥ 3 ) i n the cut (0) and uncut (0) subplots of plot 2 from July 24 - September 1, 1981 28 1.9 Same as for F i g . 1.8 except for June 9 - 25, 1982 29 1.10 Courses of Douglas-fir basal area i n the cut (0) and uncut (0) subplots of plot 2 from June 4 - September 23, 1981 and A p r i l 29 - September 14, 1982 37 _2.1 Relationship between d a i l y throughfall (above the s a l a l ) and r a i n f a l l rates at the experimental s i t e i n 1978. A l i n e of unit slope along the upper l i m i t of the data i s also shown. The negative intercept of this l i n e (0.6 mm d - 1 ) gives the maximum water storage of the Douglas-fir subcanopy following Rutter et_ al_. (1971) 53 Figure - x - Page 2.2 Courses of net r a d i a t i o n flux density and vapour pressure d e f i c i t above the forest ( R n a ( ) and D ( - - - ) ) and measured ( ) and calculated (- - -) forest evapotranspiration rate (E) (with understory) on August 25, 1982, a clear day when average root zone s o i l water po t e n t i a l (Yg) was about -0.7 MPa. Errors i n measured E were approximately 0.02-0.04 mm h - 1 (Spittlehouse and Black 1980). Root mean square errors in calculated E were 0.04-0.06 mm h - 1 as determined by d i f f e r e n t i a t i o n of (1) applied to two layers and s o i l . A 10% error was assumed for Dj_, a 20% error for ( R n i - G) and a 30% error for the transfer resistances ( r s i , r b i , and r a i ) , L E 0 and Rno 58 2.3 Relationship between forest f l o o r d i f f u s i v e resistance ( r c o ) and average root zone s o i l water content (9) i n the cut subplot of plot 2 for ten days i n July and August 1981. For 6 less than 0.185, r c o (s m _ 1) = -83000 9 + 16100 (R 2 = 0.96) as shown by the s o l i d l i n e . For 9 greater than.0.185, r c o was 800 s m - 1, on average, as shown by the dashed l i n e . 59 2.4 Courses of net radiation flux density and vapour pressure d e f i c i t above the forest f l o o r ( R n o ( ) and D Q (- - -)) and measured ( ) and calculated (- - -) forest f l o o r evaporation rate (E Q) i n the cut subplot of plot 2 on July 24-25, 1981, two clear days when average root zone s o i l water p o t e n t i a l (^g) was about -0.05 MPa. Standard deviations for measured E n values l l were t y p i c a l l y 0.004 mm h at night and 0.015 mm h during the daytime. Root mean square errors for calculated E 0 values were s i m i l a r . These errors were determined by d i f f e r e n t i a t i o n of (1) with 6 Q = 0. A 20% error was assumed for (R no ~ G) and r c o > a 10% error for DQ and a 30% error for r^o 61 2.5 Courses of measured (symbols) and calculated ( l i n e s ) average root zone s o i l water content (9) and s o i l water p o t e n t i a l (Vs) i n the cut ( A and - - -) and uncut (A and ) subplots of plot 2 for the period July 24 - September 3, 1981. Also shown i s the d a i l y r a i n f a l l rate (P). 63 2.6 Same as for F i g . 2.5 except for May 27 - July 1, 1982 and 0 and 64 - x i - Figure Page 2.7 Courses of measured (symbols) and calculated ( l i n e s ) average root zone s o i l water content (6) in the cut (0) and uncut (•) subplots of plot 2 for the period July 30 - August 18, 1981. Calculated values with and without the aerodynamic and boundary-layer transfer resistances for the Douglas-fir and s a l a l subcanopies are shown by the s o l i d and dashed l i n e s , r e s p e c t i v e l y . Error bars are one standard deviation. 67 2.8 Courses of calculated tree t r a n s p i r a t i o n rates i n the cut (- - -) and uncut ( ) subplots of plot 2 for the period July 24 - September 3, 1981. Also shown i s the d a i l y r a i n f a l l rate (P) 70 2.9 Same as for F i g . 2.8 except for May 27 - July 1, 1982. . . . 71 C.l Relationship between s a l a l leaf area index and Douglas-fir stand basal area i n 31-36 year old stands close to and in c l u d i n g the one at the experimental s i t e . The curve indicates s a l a l leaf area index = 288 (Douglas-fir stand basal area (m2 h a - 1 ) ) " 1 * 5 7 (R 2 • 0.89) 79 C.2 Relationship between the average area of s a l a l leaves and Douglas-fir stand basal area i n 31-36 year old stands close to and in c l u d i n g the one at the experimental s i t e . The l i n e indicates average area of s a l a l leaves (mm2 l e a f - 1 ) = 0.41 (Douglas-fir stand basal area (m2 h a - 1 ) ) + 16 (R 2 = 0.41). 80 A I . l E l e c t r i c a l analog depicting the transfer of latent and sensible heat fluxes for a single canopy layer i (LE^ and H^) where T|^ i s the " e f f e c t i v e " surface temperature of the layer ( i . e . wet and dry portions) and other symbols have been previously defined. The depiction shows the i d e n t i c a l l e a f , single source l i m i t of the Penman-Monteith equation described by Shuttleworth (1979) 83 AV.l E l e c t r i c a l analog depicting the transfer of latent and sensible heat fluxes i n an uncut subplot 98 AV.2 Same as for F i g . AV.l except for cut subplot. 99 - x i i - Figure Page AVI.l Construction of a hemisphere of radius r 0 over a small area of forest f l o o r (dAj). The angle <J> between the normal to dA^ and the stra i g h t l i n e connecting dAj and a small area on the hemisphere (dA 2) i s TT/4 radians. Using the azimuthal angle a and the derived geometric r e l a t i o n s h i p s shown i n this f i g u r e , dA 2 = r 0 s i n <f> da r Q d<|>. The p a r t i a l obscuration of dA 2 indicates the forest canopy. . . . . . . . 105 AVI.2 Grid used for analyses of f i s h eye lens photographs of radius r Q . The 0-r o/3 annulus of the grid was divided into nine equal size sectors while the r 0/3 - 2r 0/3 and 2r 0/3 - r D annul! were each divided into eighteen equal size sectors. 108 AVI.3 F i s h eye lens photograph taken at about the 300 mm height from the 1981 loc a t i o n of the net radiometer (below the tree canopy) i n the cut subplot of plot 2 on November 23, 1983. The forest floor-sky view factor for th i s l o c a t i o n was 0.28 109 AVI.4 Courses of solar irradiance (top l i n e ) and net radi a t i o n f l u x density above the forest (K+ a and R n a ) for the period July 31 - September 1, 1981. Also shown are the courses of the r a t i o of below to above tree canopy solar irradiance (K+ b/K+ a) for the cut subplot of plot 2 and the r a t i o of below to above tree canopy net ra d i a t i o n flux density (R nb^na) ^ o r t n e c u t (~ ~ ~) and uncut (——) subplots of plot 2 during the same period • • • 111 AVI.5 Courses of net ra d i a t i o n flux density above the forest ( R n a ) and the r a t i o of below to above tree canopy net ra d i a t i o n flux density (R nb/ Rna) ^ o r t n e c u t (~ ~ ~) and uncut ( ) portions of the tram path on July 30, 1982 115 AVI.6 Same as for AVI.5 except for August 18, 1982 116 AVI.7 Same as for AVI.5 except for September 1, 1982 and Rnb/Rna f ° r t n e c u t portion of the tram path measured using a single stationary net radiometer ( • ) 117 - x l i i - Figure AVII.1 Page Relationship between stomatal resistance ( r s ) and vapour pressure d e f i c i t (D) i n Douglas-fir and s a l a l adjacent to the four plots at the experimental s i t e f o r June-August 1980 (0) and 1981 (0) when average root zone s o i l water p o t e n t i a l (Vs) was greater than -0.3 MPa and photon f l u x density was greater than 2.5 and 1.0 m mol m - 2 s - 1 for Douglas-fir and s a l a l r e s p ectively. Curves show c h a r a c t e r i s t i c r s values for fg = -0.01 (lower curve) and -0.3 MPa (upper curve) (see Table 2.1) 120 AVIII.l Relationship between average root zone s o i l water p o t e n t i a l ( f s ) and water content (9) i n plot 2. The s o i l water retention curve shown i s Y s (MPa) = -0.005 ( 9 / 0 . 3 ) - 6 - 5 . 123 - x i v - NOTATION A t £ tree leaf area on a one-sided basis (m2 t r e e " 1 ) ki area on the forest f l o o r (m ) A2 area on a hemisphere constructed over a small area on the forest f l o o r (dAj) (m2) 2 1 1 BAI basal area increment (mm t r e e " period ) C canopy water storage (mm) D vapour pressure d e f i c i t (kPa) vapour pressure d e f i c i t just above layer 1 (kPa) D vapour pressure d e f i c i t just above layer i-1 (and within layer i ) (kPa) D 0 vapour pressure d e f i c i t within the canopy ( i . e . at the e f f e c t i v e source height) (kPa) E evapotranspiration rate above a forest c o n s i s t i n g of a number of layers (eg. trees, understory and s o i l ) (mm p e r i o d - 1 ) E^ evapotranspiration rate above layer i (mm p e r i o d - 1 ) Ej evapotranspiration rate from layer i (I.e. E^ - E^_^) where I i s 2 for trees, 1 for understory and 0 for forest f l o o r (mm p e r i o d - 1 ) E| rate of evaporation of intercepted water from tree or understory subcanopy (mm p e r i o d - 1 ) rate of evaporation of intercepted water from layer i (mm p e r i o d - 1 ) E,j, t r a n s p i r a t i o n rate (mm p e r i o d - 1 ) Ê , t r a n s p i r a t i o n rate from tree or understory subcanopy (mm p e r i o d - 1 ) F root zone drainage rate (mm p e r i o d - 1 ) G s o i l heat f l u x density (W m- ) H sensible heat flux density (W m- ) o Hi ' sensible heat flux density above layer i (W m- ) I radiance (W m - 2) - XV - K+ a solar Irradiance above the forest (W m ) K+jj solar irradiance below the tree canopy (W m ) Rr D solar irradiance below the tree subcanopy which i s r e f l e c t e d from the understory subcanopy or forest f l o o r surface (W m - 2) L latent heat of vaporization of water (J k g - 1 ) LE latent heat flux density (W m - 2) LEf latent heat flux density above layer i (W m- ) M forest canopy heat storage rate (W m ) Ojj porosity of the dry forest f l o o r (and s o i l ) surface layer (dimensionles s) P r a i n f a l l rate (mm p e r i o d - 1 ) Q subcanopy drainage rate (mm (15 min.) - 1) 2 R c o e f f i c i e n t of determination i n a l i n e a r regression analysis (dimensionless) 2 Rn net r a d i a t i o n flux density (W m~ ) Rria net r a d i a t i o n flux density above the forest (W m~2) 2 R nb net r a d i a t i o n flux density below the tree canopy (W m- ) 2 Rni net r a d i a t i o n flux density above layer 1 (W m- ) S maximum water storage of a subcanopy (mm) T a £ r a i r temperature (°C) T£ a i r temperature just above layer i (°C) T s i surface temperature of layer I (°C) Tijfi surface temperature of the dry portion of layer i (°C) T | i " e f f e c t i v e " surface temperature of layer i inc l u d i n g wet and dry portions (°C) Tgi surface temperature of the wet portion of layer i (°C) V 1 2 f r a c t i o n of view by area 1 that i s occupied by area 2 (dimensionless) - x v i - V f c forest floor-canopy view factor (dimensionless) V f s forest floor-sky view factor (dimensionless) Wi f r a c t i o n of leaf area In layer 1 that i s completely wet (dimensionless) 2 2 a l e a f area index on a one-sided basis (m m~ ) 2 2 a± l e a f area index on a one-sided basis for layer i (m m ) a s s a l a l leaf area index on a one-sided basis (m m ) c 30 second count when neutron probe i s i n s o i l (s) c p s p e c i f i c heat of moist a i r (J k g - 1 "C - 1) c s 30 second count when neutron probe i s withdrawn into the s h i e l d ( i . e . standard count) (s) d day d^ leaf diameter (mm) e a vapour pressure of the a i r (kPa) e± vapour pressure of the a i r above layer I (kPa) e* saturated vapour pressure of the stomatal c a v i t i e s (kPa) e * ( T s i ) saturated vapour pressure of the a i r just above layer I at the surface temperature of layer I (kPa) f(<|>,a) f r a c t i o n of a small area on a hemisphere constructed over a small area of forest f l o o r which i s not obscured by the forest canopy (dimensionless) f(<))) average non-obscuration f r a c t i o n for a small annulus on a hemisphere constructed over a small area of forest f l o o r (dimensionless) f ' ( r i ) average non-obscuration f r a c t i o n for the annulus r ^ , of width A r j , on the projection of a hemisphere constructed over a small area of forest f l o o r (eg. f i s h eye photograph) (dimensionless) thickness of the dry forest f l o o r (and s o i l ) surface layer (m) - x v i i - p free throughfall c o e f f i c i e n t (dimensionless) q^ 2 flux emitted by area 1 toward area 2 (W) r distance from the center of a projection of a hemisphere constructed over a small area of forest f l o o r to the projection of an object on the hemisphere (m) r^ annulus on a projection of a hemisphere constructed over a small area of forest f l o o r (dimensionless) r 0 radius of a hemisphere constructed over a small area of forest f l o o r (m) r a i eddy d i f f u s i v e resistance above layer 1 assuming the s i m i l a r i t y of eddy d i f f u s i v e resistances to sensible heat and water vapour transfer (s m ) r eddy d i f f u s i v e resistance to sensible heat transfer above layer am . — 1 \ i (s m ) r eddy d i f f u s i v e resistance to water vapour transfer above layer a v l J / — 1 \ 1 (s m ) r t o t a l aerodynamic resistance for layer i assuming the s i m i l a r i t y of t o t a l aerodynamic resistances to sensible heat and water vapour transfer (s m - 1) r.T„ t o t a l aerodynamic resistance to sensible heat transfer for layer AHI , _ i N i (s m ) r t o t a l aerodynamic resistance to water vapour transfer for layer A V 1 i (s m"1) r b boundary-layer resistance assuming the s i m i l a r i t y of boundary- layer resistances to sensible heat and water vapour transfer (s m"1) r D i boundary-layer resistance for layer i assuming the s i m i l a r i t y of boundary-layer resistances to sensible heat and water vapour transfer (s m - 1) r c i canopy resistance for layer i (s m - 1) r u boundary-layer resistance to sensible heat transfer (s m - 1) H r t o t a l boundary-layer resistance of a leaf to sensible heat transfer on a one-sided leaf area basis divided by the leaf area index on a one-sided basis (s m~ ) - x v i i i - "HBOT Hi HTOP HTOT boundary-layer resistance to sensible heat transfer on the bottom side of a leaf (s m"1) boundary-layer resistance to sensible heat transfer on a one-sided leaf area basis for layer i (s m - 1) boundary-layer resistance to sensible heat transfer of a leaf on the top side (s m - 1) t o t a l boundary-layer resistance of a leaf to sensible heat transfer on a one-sided leaf area basis (s m - 1) '•sa rsBOT r s h r s i rsTOP Va VBOT V i Vsa VTOP VTOT stomatal resistance on a one-sided leaf area basis (s m - 1) stomatal resistance on a one-sided leaf area basis for an amphistomatous leaf (s m~*) stomatal resistance on the bottom side of a leaf (s m - 1) stomatal resistance on a one-sided leaf area basis for a hypostomatous leaf (s nf 1 ) stomatal resistance on a one-sided leaf area basis for layer 1 (s m"1) stomatal resistance on the top of a leaf (s m"1) boundary-layer resistance to water vapour transfer on one side of a leaf assuming the boundary-layer resistance to water vapour transfer i s i d e n t i c a l on the top and bottom sides of the leaf (s m-1) t o t a l boundary-layer resistance of a leaf to water vapour transfer on a one-sided leaf area basis divided by the leaf area index on a one-sided basis (s m""1 ) boundary-layer resistance to water vapour transfer on a bottom side of a leaf (s m"1) boundary-layer resistance to water vapour transfer on a one-sided leaf area basis for layer I (s m"1) t o t a l resistance to water vapour transfer of a leaf on a one-sided leaf area basis divided by the leaf area index on a one-sided basis (s m - 1) boundary-layer resistance to water vapour transfer on the top side of a leaf (s m - 1) t o t a l resistance of a leaf to water vapour transfer on a one-sided leaf area basis (s m"1) - JCLX - rVTOT t o t a l boundary-layer resistance of a leaf to water vapour transfer on a one-sided leaf area basis (s m"1) s slope of the saturated vapour pressure curve (kPa ° C - 1 ) t time period (15 min., 1 hour or 1 day) u wind speed (m s - 1 ) u i 5 m wind speed at the 15 m height ( i . e . 1 m above the forest) (m s- 1) flt " t o r t u o s i t y " factor for the dry forest f l o o r (and s o i l ) surface layer (dimensionless) a azimuthal angle (radians) 8 Bowen r a t i o (dimensionless) Y psychrometric constant (kPa "C - 1) 6 £ term i n (1) of chapter 2 to account for the net ra d i a t i o n and latent heat flux densities below layer i i n determining the evapotranspiration rate from layer 1 (kPa) 6| term i n (AI.6) to account for the net r a d i a t i o n and latent heat flux densities below layer 1 i n determining the evapotranspiration rate above layer I (kPa) ei emittance of area 1 (W m~ ) X, root zone depth (mm) n r a t i o of the radius of a hemisphere constructed over a small area of forest f l o o r to the constant width of an annuli on a projection of the hemisphere (dimensionless) 3 3 6 volumetric s o i l water content (m m- ) 3 3 6 average volumetric root zone water content (m m~ ) 6<j volumetric water content of the dry forest f l o o r (and s o i l ) surface layer (m3 m~3) K$ d i f f u s i v i t y for water vapour of the dry forest f l o o r (and s o i l ) surface layer (m2 s _ 1 ) A,u,v,£,o constants for the emperical stomatal resistance c h a r a c t e r i s t i c s functions (dimensionless) q p density of moist a i r (kg m~ ) - XX - 4> angle between the normal to a small area on the forest f l o o r (dAi) and the str a i g h t l i n e connecting dA^ and a small area (dA 2) on a hemisphere constructed over dA^ (radians) ? s s o i l water p o t e n t i a l (MPa) t o t a l twig xylem water p o t e n t i a l (MPa) u s o l i d angle (steradians) - xxi - ACKNOWLEDGEMENTS I am most gr a t e f u l to my family L e s l i e , Anna and Alex for t h e i r l o v i n g support and encouragement. I wish to thank the f a c u l t y and s t a f f of the Department of S o i l Science for t h e i r help. In p a r t i c u l a r , the contribution of my supervisor, Dr. Andy Black, i s greatly appreciated. I could not have completed my f i e l d work without the friendship and assistance of Ralph Adams, Alan Barr, Doug Beames, Ron Emerson and Dave P r i c e . Fran Dixon and the s t a f f of the Oyster River Farm and John Harwijne and Dave McBride of Crown Forest Industries Ltd. also provided e s s e n t i a l support i n the f i e l d . Bob Stathers gave me necessary help i n w r i t i n g the computer programs for Chapter 2 and Appendix VI. Dave Spittlehouse generously supplied his neutron probe data analysis program as well as valuable s o i l physics, throughfall and r a i n f a l l data. My stipend was provided by contracts from the B.C. M i n i s t r y of Forests, a U.B.C. Graduate Fellowship and a contract from Crown Forest Industries Ltd . Funds for the research project were provided by contracts from the B.C M i n i s t r y of Forests and a grant from the Natural Science and Engineering Research Council. I've saved a s p e c i a l thanks for L i l a Harter who provided the typing wizardry. - 1 - INTRODUCTION Where forest s o i l s have low water storage capacity and r a i n f a l l rates during the growing season are low, i t i s common s i l v i c u l t u r a l practice to reduce or eliminate competing vegetation when planting valuable conifer seedlings. In pine stands of the southern United States, this practice i s often extended u n t i l tree canopy closure by c o n t r o l l e d burning of the f o r e s t s . However, r e l a t i v e l y few studies have dealt with the e f f e c t s of understory vegetation on tree water use and growth. Competition for s o i l water by s a l a l understory i n young Douglas-fir stands on dry s i t e s on the eastern coast of Vancouver Island i s considered to be a major reason for the often observed poor tree growth response to thinning. This problem led to the conduction of a two year s a l a l understory removal experiment i n a 31-year-old, thinned Douglas-fir stand near Courtenay, B r i t i s h Columbia. To minimize the e f f e c t s of s o i l and tree v a r i a b i l i t y i n the stand, four plots 7 m i n diameter were selected so that each contained two very s i m i l a r trees not more than four metres apart. Each plot was divided into two semicircular subplots, each containing one of the two s i m i l a r trees. A l l understory was cut close to the ground and removed from one subplot of each plot at the s t a r t of the experiment. The root zone of each of the eight trees was i s o l a t e d using p l a s t i c sheeting buried to bedrock. The r e s u l t s of this study form the basis of the two chapters comprising this t h e s i s , which has been written i n paper format. - 2 - In Chapter 1, the eff e c t s of understory removal on root zone water content, s o i l water p o t e n t i a l , pre-dawn twig xylem water p o t e n t i a l and tree diameter growth are reported. A simple root zone water balance analysis i s used to compare the evapotranspiration rates of adjacent subplots with and without understory present. Measurements of forest f l o o r evaporation and stomatal resistance of understory and trees on selected days are used to estimate the amount of add i t i o n a l water which understory removal provided to the trees during summer drying periods. In Chapter 2, Shuttleworth's development of the Penman-Monteith evapotranspiration equation for multilayer, p a r t i a l l y wet forests i s modified for use i n hypostomatous canopies. This evapotranspiration theory i s combined with standard hourly micrometeorological measurements, transfer resistance functions and canopy and root zone water balance equations to give c a l c u l a t i o n s of forest evapotranspiration over extended periods during the growing season. Calculations are tested using Bowen ratio/energy balance, water balance, porometer and lysimeter measurements. The cal c u l a t i o n s are then applied to explain the ef f e c t s of s a l a l understory removal on Douglas-fir t r a n s p i r a t i o n rates during the growing season. In Appendices I-IV, derivations of the equations from Shuttleworth's evapotranspiration theory as modified for use i n hypostomatous canopies are given. Appendix V shows e l e c t r i c a l analogs of the latent and sensible heat fluxes i n subplots with and without understory present. Appendix VI reports -the measurements of the below tree canopy r a d i a t i o n regime and sky view factors are derived from fish-eye lens photographs. Appendix VII reports stomatal resistance measurements made i n 1980 and 1981 adjacent to the four plots at the experimental s i t e . Appendix VIII reports the s o i l water retention curve measurements. - 3 - CHAPTER 1 EFFECTS OF SALAL UNDERSTORY REMOVAL ON THE SOIL WATER REGIME AND GROWTH OF DOUGLAS-FIR TREES - 4 - CHAPTER 1 EFFECTS OF SALAL UNDERSTORY REMOVAL ON THE SOIL WATER REGIME AND GROWTH OF DOUGLAS-FIR TREES 1. INTRODUCTION Competition for s o i l water by s a l a l (Gaultherla shallon Pursh.) understory i n young Douglas-fir (Pseudotsuga menzlesii (Mirb.) Franco) stands on dry s i t e s on the eastern coast of Vancouver Island i s considered to be a major reason for the observed poor tree growth response to thinning (Black et_ a l . 1980). Following thinning, s a l a l understory i n a Douglas-fir stand near Courtenay, B r i t i s h Columbia used up to 50% of the t o t a l s o i l water over a 30-day summer drying period (Black et_ a_l. 1980). In the U.K., Roberts et_ a l . (1980) found that, during warm dry periods, bracken (Pteridium aquilimum L.) understory accounted for more than 50% of the evapotranspiration i n a 50-year-old Scots pine (Pinus s y l v e s t r i s L.) stand. In the U.S., workers have found s i g n i f i c a n t increases in tree growth of l o b l o l l y (Pinus taeda L.) and shortleaf (Pinus echinata M i l l . ) pine (Grano 1970; Balmer et a l . 1978) and ponderosa pine (Pinus ponderosa Laws.) (Oliver 1979) r e s u l t i n g from understory removal. Zahner (1958), also working In the U.S., found summertime root zone water storage was up to 50 mm greater (0.04 m3 m - 3 greater) i n upland l o b l o l l y and shortleaf pine stands with understory removed than with i t present. He concluded that the understory competed s i g n i f i c a n t l y for s o i l water. Where forest s o i l s have low water storage capacity and r a i n f a l l rates during the growing season are low, s i l v i c u l t u r a l - 5 - pr a c t i c e s that reduce or eliminate understory vegetation can result i n more e f f i c i e n t tree water use and growth (Barrett and Youngberg 1965; Black and Spittlehouse 1981). The objective of this chapter i s to report the e f f e c t s of s a l a l understory removal i n a Douglas-fir stand on ( i ) root zone water content, ( i i ) s o i l water p o t e n t i a l and pre-dawn twig xylem water p o t e n t i a l , and ( i i i ) tree diameter growth over two successive growing seasons. A root zone water balance analysis w i l l be used to compare the evapotranspiration rates of plots with and without understory present. Measurements of forest f l o o r evaporation and stomatal resistance of understory and trees on selected days w i l l be used to determine how much add i t i o n a l water s a l a l understory removal provides to the trees during summer drying periods. 2. METHODS 1. S i t e Description The f i e l d experiment was conducted i n a thinned Douglas-fir stand approximately 27 km northwest of Courtenay on the eastern coast of Vancouver Island (49° 50 ' N , 125° 14'W, 150 m above sea l e v e l ) . The trees were planted as 2-0 stock of unknown provenance i n 1952 ( J . Harwijne, Crown Forest Industries Ltd., personal communication) and thinned from about 1500 to about 800 trees h a - 1 i n 1974. The stand was f e r t i l i z e d once (February 28, 1981), with urea applied a e r i a l l y at a rate of 200 kg (nitrogen) h a - 1 . At the end of the 1982 growing season, stand (excluding understory) basal area was 16 m2 h a - 1 and average tree height was 14 m. Average tree leaf area index on a one-sided leaf area basis, was about 6 i n 1982 as calculated from dbh (diameter, including bark, at the 1.37 m height (breast height)) measurements and a r e l a t i o n s h i p between dbh and tree leaf area index for - 6 - t h i s s i t e (Spittlehouse 1981). S a l a l leaf area Index on a one-sided leaf area basis, obtained from measurements made on eight 1 m plots, was about 3 i n 1982. The s o i l , an Orthic Humo F e r r i c Podzol (Anonymous 1978), i s a gravelly sandy loam of variable depth (0.3 - 1.0 m) and volumetric coarse fragment (>2 mm diameter) content (0.10 - 0.45) over sandstone bedrock. The s i t e i s on a s l i g h t slope of less than 10%, with a northeast aspect. Further d e s c r i p t i o n of the s i t e can be found i n Nnyamah and Black (1977) and Spittlehouse and Black (1981). 2. Experimental Design Four plots, each consisting of two subplots (one with understory to be removed and the other with i t to be l e f t i n place), were selected i n May 1981. To minimize the e f f e c t s of s o i l and tree v a r i a b i l i t y i n the stand, plots 7 m i n diameter were selected so that each one contained two very s i m i l a r trees not more than four metres apart. Each plot was divided into semicircular subplots, each containing one of the two s i m i l a r trees, by drawing a l i n e b i s e c t i n g at right angles the l i n e j oining the pair of trees. Each subplot contained two or three other trees. A l l understory was cut close to the s o i l surface using a gas-powered, rotary brush saw and removed from one subplot of each plot on May 21, 1981. In order to prevent possible l a t e r a l flow of water from a cut to an uncut subplot and water extraction from a cut subplot by the trees and s a l a l i n an uncut subplot, a 300 mm wide trench was excavated to bedrock between each pair of trees along the 7 m bisect l i n e . A sheet of 0.15 mm thick polyethylene p l a s t i c was placed on both sides of the trench and the trench r e f i l l e d with s o i l . A - 7 - layer of p l a s t i c was placed on the top of the trench to prevent r a i n from entering the s o i l between the sheets of p l a s t i c . The root zones of a l l eight experimental trees were is o l a t e d during the f i r s t two weeks of May 1982 by completing the trench (and p l a s t i c b a r r i e r ) around each tree. 3. Measurements 1. Growing Season Weather Observations In 1981 and 1982, standard forest micrometeorological measurements were made from May 15 u n t i l October 14, and May 20 u n t i l September 14, r e s p e c t i v e l y . Solar irradiance was measured with a pyranometer (Kipp and Zonen, D e l f t , Holland) located on the top of a 15 m t a l l open l a t t i c e t riangular tower adjacent to the four p l o t s . A tipping bucket r a i n gauge (Sierra Misco, Inc., Berkeley, C a l i f o r n i a , U.S.A.), with a 200 mm diameter o r i f i c e , was located on a horizontal bar extending 3 m from the tower at the 11 m height. A r e l a t i v e humidity sensor (Phys-Chemical Research Corporation, New York, NY, U.S.A.) coupled with a thermistor (Fenwal E l e c t r o n i c s Corporation, Framingham, Massachusetts, U.S.A.) for measurement of a i r temperature was located at the 6 m height i n a small Stevenson screen (450 mm by 300 mm by 300 mm) attached to the tower. The r e l a t i v e humidity sensor and thermistor were frequently checked in the f i e l d using an Assmann psychrometer. Windspeed was measured at the 15 m height using a s e n s i t i v e Casella (C.F. Casella and Company, Ltd., London, England) cup anemometer. Based on 38 days of windspeed measurements (one hour averages) at the 1 m height i n July and August 1980, the windspeed at this height was 13% of that at the 15 m height. Data were e l e c t r o n i c a l l y logged as one hour averages or - 8 - t o t a l s using a Campbell S c i e n t i f i c model CR-21 data logger, stored on audio cassette tape and l a t t e r transferred to a microcomputer for processing. 2. Root Zone S o i l Water Content and Evapotranspiration Rate The average root zone depth of each subplot was determined by dr i v i n g a 12 mm diameter s t e e l rod v e r t i c a l l y into the s o i l u n t i l i t struck sandstone bedrock. There were about 20 samples per subplot ( i . e . within the area enclosed by the p l a s t i c b a r r i e r ) taken i n a grid pattern. Roots were observed at a l l depths i n the s o i l on faces of the trenches excavated around each of the eight subplots. During the t h i r d week of May 1981, thin wall aluminum neutron moisture probe access tubes (48 mm inside diameter) were i n s t a l l e d i n the s o i l i n a l l of the subplots. An access hole was obtained by dri v i n g a s t e e l pipe (38 mm inside diameter, 51 mm outside diameter) v e r t i c a l l y into the s o i l with a sledge hammer. The pipe was c a r e f u l l y withdrawn from the hole, at about 100 ram i n t e r v a l s , and the s o i l was removed from inside the pipe. This procedure continued u n t i l bedrock was reached. Three aluminum tubes were placed i n each subplot of plots 1, 2 and 4, while four aluminum tubes were placed i n each subplot of plot 3, for a t o t a l of 26 tubes. A Campbell P a c i f i c Nuclear hydroprobe (model CPN 503, Sacker S c i e n t i f i c Company, Edmonton, Alberta) was used to measure changes i n s o i l water content (0) using the neutron moderation technique. The neutron probe was lowered into each aluminum tube at 150 mm i n t e r v a l s beginning at a depth of 150 mm and continuing to bedrock. One 30 second reading was taken at each depth. In 1981 and 1982, measurements of 6 were made from May 27 u n t i l - 9 - October 22, and A p r i l 29 u n t i l October 6 respectively, i n one to two (and sometimes three week) i n t e r v a l s . During 1981 and 1982, the neutron probe was c a l i b r a t e d i n the f i e l d using s o i l samples obtained with a heavy gauge open bucket-type auger (60 mm inside diameter) within 1.5 m of an aluminum tube, used for c a l i b r a t i o n , near the four p l o t s . S o i l samples were obtained at 100 ram i n t e r v a l s from the s o i l surface down to bedrock and placed i n aluminum s o i l moisture sampling cans. The cans were l a t e r weighed, dried at 105°C for 48 hours, reweighed and 6 was calculated using a bulk density and coarse fragment content obtained from an adjacent s o i l p i t . C a l i b r a t i o n of the neutron probe was conducted i n 1981 for values of 8 ranging from 0.10 to 0.51 m3 m~3. A l i n e a r equation, 6 = 0.224 ( c / c s ) - 0.009 (sample size = 44, R =0.85) where c i s the neutron count per 30 seconds when the probe i s in the s o i l and c s i s the neutron count per 30 seconds when the probe i s withdrawn into the shield ( i . e . the standard count), was found to describe the c a l i b r a t i o n data for a l l depths quite w e l l . An Independent c a l i b r a t i o n of the same neutron probe using the same procedure during the same measurment period and for si m i l a r s o i l s at Mesachie Lake, B.C., by Giles (1983) resulted i n a very s i m i l a r equation (6 = 0.23 (c/ c s ) - 0.021). The same neutron probe was used i n 1982 and c a l i b r a t i o n was then conducted for values of 6 ranging from 0.09 to 0.46 m m~3. Since the 1982 c a l i b r a t i o n data lay along the 1981 c a l i b r a t i o n l i n e , the 1981 c a l i b r a t i o n l i n e was used for both 1981 and 1982. - 10 - Root zone water content and r a i n f a l l data were used to calculate average evapotranspiration rates (E) (mm d - 1 ) i n cut and uncut subplots using the water balance equation written as E = -(A6/At)c + P (1) where A8/At i s the average rate of change i n the average root zone water content (9) over a 1-2 week period, £ i s the root zone depth and P i s the average r a i n f a l l rate over the same period. Calculations were done for periods 2 or more days af t e r r a i n f a l l and since the s o i l was a sandy loam, small decreases i n 9 corresponded to large decreases In unsaturated hydraulic conductivity so that root zone drainage could be neglected (Black and Spittlehouse, 1981). 3. S o i l Water P o t e n t i a l and Tree Pre-dawn Twig Xylem Water P o t e n t i a l On June 18, 1981 and June 8, 1982, thermocouple psychrometers (model PCT-55 measured using a model HR-33T dew-point microvoltimeter, Wescor Inc., Logan, Utah, U.S.A.) were I n s t a l l e d i n each subplot of plot 2. The thermocouple psychrometers were ca l i b r a t e d i n the laboratory just p r i o r to i n s t a l l a t i o n , using s a l t solutions of known osmotic p o t e n t i a l (Nnyamah and Black 1977). A 150 mm by 300 mm hole was excavated to bedrock about 1 m from each of the two experimental trees i n plot 2. Access holes were made by d r i v i n g a 9 mm diameter s t e e l rod about 150 mm h o r i z o n t a l l y into the s o i l on opposite sides of the large hole. Careful i n s e r t i o n of the thermocouple psychrometer to the end of the access hole assured that the porous ceramic cup was completely embedded i n s o i l . The remainder of the access hole was - 11 - then r e f i l l e d with stone-free s o i l which was l i g h t l y compacted. Most of the wire from the thermocouple psychrometer was wound about the perimeter of the large hole at the depth of i n s t a l l a t i o n . Only a length of the wire about 300 mm greater than the depth of i n s t a l l a t i o n remained to be led to the s o i l surface. This was done to minimize heat conduction down the wire to the thermocouple junction. Pairs of psychrometers were i n s t a l l e d at about 150 mm depth i n t e r v a l s beginning at a depth of 150 mm and continuing to bedrock. Following i n s t a l l a t i o n , the large hole was r e f i l l e d with the same s o i l (including stones) removed during excavation. The psychrometers were read i n dew-point and psychrometer mode after being P e l t i e r cooled for 5 seconds. On June 18, 1981, tensiometers fabricated following van Bavel et^ a l . (1968) were also i n s t a l l e d i n each subplot of Plot 2. The porous ceramic cups ( S o i l Moisture Equipment Corp., Santa Barbara, C a l i f o r n i a , U.S.A.) had an a i r - e n t r y value of 0.1 MPa, were 10 mm outside diameter and 25 mm long with 5 mm sealed i n a length of 13 mm outside diameter clear a c r y l i c tube. An access hole was obtained by d r i v i n g a 12 mm diameter s t e e l rod v e r t i c a l l y into the s o i l to the desired depth. The tensiometer was inserted into the hole u n t i l the porous ceramic cup was completely embedded i n the s o i l at the bottom of the hole. Tensiometers were i n s t a l l e d at about 150 mm i n t e r v a l s beginning at a depth of 150 mm and continuing to bedrock. Tensiometers were not i n s t a l l e d i n 1982. In 1981 and 1982, measurements of s o i l water p o t e n t i a l (T" s) were made every two to seven days from July 4 u n t i l September 3, and June 9-25, respectively. - 12 - The pre-dawn t o t a l twig xylem water p o t e n t i a l (^Ttx) °f t n e eight measurement trees was measured using a pressure chamber, using the procedure described by K e l l i h e r et_ al_. (1984). In 1981, measurements were made on August 12 and 20 and October 23. In 1982, measurements were made on June 9, 17, 23 and 30. 4. Forest Floor Evaporation Rate Forest f l o o r evaporation rate was determined in 1981 using small weighing lysimeters. The lysimeters, 150 mm i n diameter and 120 mm deep, were made by removing the top and bottom portions of a p l a s t i c jar (0.5 mm t h i c k ) . S o i l for the lysimeter was excavated from the cut subplot and trimmed so that the p l a s t i c tube could be snugly f i t t e d over the undisturbed s o i l core. The top of the cylinder was temporarily covered and the core was placed on i t s side so that a p l a s t i c bottom could be taped i n place. The lysimeter was then placed i n the p l a s t i c sleeve of a hole excavated on the cut subplot, with the top fl u s h with the s o i l surface. The sleeve prevented s o i l from adhering to the outside of the lysimeter which would have caused an increase i n weight. Every one to two hours the lysimeter was c a r e f u l l y l i f t e d out and car r i e d to a nearby e l e c t r i c balance to determine weight l o s s . The resolution of the balance was equivalent to 0.001 mm depth of water. Four lysimeters were located i n the cut subplot of plot 2, and one was situated beneath the s a l a l i n the uncut subplot of plot 2. Lysimeter s o i l cores were replaced every 1 to 2 days (Boast and Robertson 1982). S o i l -cores were generally obtained near the p l a s t i c sleeves, but on August 11 and .18, s o i l for the cut subplot lysimeters was obtained from cut subplots of plot 1 and 4 as well as plot 2. - 13 - 5. Transpiration of Understory and Trees The t r a n s p i r a t i o n rate (ET-) of the understory (kg m - 2 d - 1 or mm d _ 1 ) was estimated using E T = a s ( p c p / L Y ) ( e * - e a ) / ( r s + r b ) (2) where a s i s the s a l a l leaf area index, p i s the density of moist a i r , Cp i s the s p e c i f i c heat of moist a i r , L i s the latent heat of vaporization of water, y i s the psychrometric constant, e* i s the saturated vapour pressure of the stomatal c a v i t i e s , e a i s the vapour pressure of the a i r , r s i s the average stomatal resistance and r b i s the average boundary layer resistance of the leaves. The value of r b was determined using the re l a t i o n s h i p between r b and windspeed found for a r t i f i c i a l s a l a l leaves by Spittlehouse and Black (1982) and an estimated shelter factor of 2 (Thorn 1971). This resulted i n values of r b of about 250 s m - 1. The vapour pressure was measured hourly using an Assmann psychrometer immediately above the s a l a l canopy. Stomatal resistance measurements of s a l a l i n the uncut subplot of plot 2 were made each hour from early morning u n t i l late afternoon on f i v e days i n 1981 from July 24 u n t i l Aug. 20, as well as Oct. 23. Measurements of abaxial r s were made on at least ten leaves. A representative sample of leaves i n both sun and shade was obtained each hour. A ve n t i l a t e d d i f f u s i o n - 14 - porometer described by Turner et^ a l . (1969) and modified by Tan and Black (1978) was used. S a l a l leaf temperature was measured i n the shade and sun using a Barnes PRT-10 i n f r a r e d thermometer (Barnes Engineering Company, Stamford, Connecticut, U.S.A.). Hourly estimates of the f r a c t i o n of the subplot that was s u n l i t were used with the above measurements to c a l c u l a t e s a l a l E^. The t r a n s p i r a t i o n rate of the experimental trees (kg t r e e - 1 s - 1 ) was estimated using a s i m p l i f i e d version of (2) i n which r D * 0 so that e* - e a » D, the vapour pressure d e f i c i t of the Douglas-fir canopy (Tan et_ a l . 1978). This can be written as E T - A t £ ( p c p / L Y ) D/r s (3) where A t£ i s the projected leaf area of the tree and r s i s the average stomatal resistance of the tree. Conversion of Ef from a tree to a stand (mm d - 1 ) basis used a stocking density of 800 trees h a - 1 . Stomatal resistance measurements were made on the experimental trees of the cut and uncut subplots of plot 2 each hour from dawn u n t i l late afternoon on Aug. 12, Aug. 20, and Oct. 23, 1981 and June 9, 17, 23 and 30, 1982. Measurements were made using the v e n t i l a t e d porometer on at least two samples of four needles taken at the mid-crown l e v e l (approximately 7 m height) of each tr e e . 6. Tree Diameter Growth The dbh of the eight experimental trees was measured using a diameter tape (with a r e s o l u t i o n of 1 mm change i n diameter) on June 4 and December 10, 1981, October 6, 1982 and November 28, 1983. Dendrometer bands, - 15 - f a b r i c a t e d following Liming (1957), were i n s t a l l e d at the 1.37 m height on the experimental trees of plot 2 on July 3, 1981 and A p r i l 30, 1982 ( a d d i t i o n a l dendrometer bands). The bands were made of 25 mm wide by 0.125 mm thick s t a i n l e s s s t e e l with extension springs made of 0.625 mm diameter s t a i n l e s s s t e e l wire into a 9 mm outside diameter by 63 mm long c o i l . Our verniers provided by Dr. H. Brix of the Canadian Forestry Service were pressure s e n s i t i v e , s i l k screened aluminum labels with a r e s o l u t i o n of 0.1 mm (change i n diameter). In 1981 and 1982, measurements of dbh using dendrometer bands were made from July 3 u n t i l December 10, and A p r i l 30 u n t i l October 6 respectively, i n one to two (and sometimes three to four) week i n t e r v a l s . 3. RESULTS AND DISCUSSION 1. Growing Season Weather Observations The 1981 growing season was characterized by exceptionally high r a i n f a l l ( t o t a l r a i n f a l l from A p r i l through October was 760 mm ( F i g . 1.1)). The only extended dry period i n 1981 occurred between July 30 and August 24 when d a i l y maximum a i r temperature exceeded 25°C for f i f t e e n consecutive days (August 5-19) and only two cloudy days (da i l y t o t a l solar irradiance < 12 MJ m - i d a y - 1 ) occurred (July 30 and August 3). By contrast, t o t a l r a i n f a l l for the 1982 growing season was 540 mm, of which one-half f e l l during October ( F i g . 1.2). An extended dry period occurred i n 1982 between May 19 and June 26 when only 3 mm of r a i n f e l l , on June 2 (the only cloudy day during this period), and d a i l y maximum a i r temperature exceeded 25°C for ten consecutive days (June 15-24). - 16 - i i i l i i A P R WAY J U M J U L A U G S E P OCT 1 9 6 1 Figure 1.1 Courses of f i v e day average d a i l y values of r a i n f a l l rate (P), maximum and minimum forest canopy a i r temperature (T aj_ r) and solar irradiance above the forest (K+ a) for the experimental s i t e from A p r i l 1 - October 31, 1981. Measurements of P and Ta±T are from the Campbell River a i r p o r t (13 km north of the s i t e ) from A p r i l 1 - May 14 and October 15-31. For the same periods, K+ a measurements are from Nanaimo Departure Bay (130 km south of the s i t e ) . - 17 - 32 24 t- _ 16 M A X I M U M n. 20 § n. o - M I N I M U M _ 24 I- ' E Jl Jl N LTMl ' I I I 1 1 A P R M A Y J U N J U L A U G S E P O C T 1 9 8 2 Figure 1.2 Same as for F i g . 1.1. except for 1982 and A p r i l 1 - May 19 and September 15 - October 31. - 18 - 2. Root Zone S o i l Water Content Average depth to bedrock for adjacent subplots was not s i g n i f i c a n t l y d i f f e r e n t (Table 1.1). However, i t was found that coarse fragments impeded penetration of the s t e e l rod to bedrock i n a 1 m s o i l p i t excavated to bedrock. Based on the re l a t i o n s h i p between s o i l depths obtained by the rod penetration and excavation methods, root zone depth for each plot was taken as the average of the adjacent subplot values plus one standard deviation based on a l l samples taken i n the plot (Table 1.1). The neutron probe measurement made when the probe was at the bottom of the access tube (Table 1.2) was applied to the s o i l layer between the bottom of the access tube and the plot root zone depth. There was l i t t l e difference i n the course of 9 i n adjacent subplots i n the 1981 and 1982 growing seasons ( F i g s . 1.3-1.6). Except for plot 1 i n 1981, minimum values of 9 were s l i g h t l y higher i n the cut (0.15) than i n the uncut (0.14) subplots (Figs. 1.3-1.6 and Table 1.3). Zahner (1958) and Barrett and Youngberg (1965) observed much higher minimum values of 9 following understory removal from pine stands on s i l t loam, and pumice s o i l s , r e s p e c t i v e l y . For a sandy s o i l , Roberts et a l . (1982) found that minimum values of 9 were s l i g h t l y higher i n a pine stand without understory than i n an adjacent pine stand with bracken understory. Following considerable winter r a i n f a l l , which resulted i n a uniformly wetted root zone, s o i l water content p r o f i l e s during the 1982 springtime drying period were used to examine the ef f e c t of s a l a l removal on water depletion patterns. As the drying period progressed, the s o i l p r o f i l e dried uniformly with depth i n adjacent subplots i n d i c a t i n g equal depletion from - 19 - Table 1.1 Measured average depth (mm) to bedrock (± one standard deviation) i n the eight subplots. Also shown are the four plot average values (± one standard deviation). Subplot 1 2 3 4 Uncut 538±127 512+123 443±109 515±136 Cut 608±181 530±169 524±147 434+121 Plot 573±158 521±146 484±134 474±139 - 20 - Table 1.2 Depth (mm) to base of the neutron probe access tubes placed i n the eight subplots. Plot Uncut Cut 1 650 650 650 400 480 500 2 450 480 490 500 350 350 3 560 640 650 620 650 500 500 4 350 350 350 350 400 430 - 21 - T 1 1 i r M A Y JUN JUL AUG SEP OCT Figure 1.3 Courses of average root zone s o i l water content (0) i n the cut (0) and uncut (•) subplots of plot 1 from May 27 - October 22, 1981 and from A p r i l 29 - October 6, 1982. Also shown i s the d a i l y r a i n f a l l rate (P). Root zone depth was 731 mm. - 22 - M A Y JUN JUL AUG SEP OCT Figure 1.4 Same as for F i g . 1.3 except for plot 2 and 667 mm. - 2 3 . - MAY JUN JUL AUG SEP OCT Figure 1.5 Same as for F i g . 1.3 except for plot 3 and 618 mm. - 24 - MAY JUN JUL AUG S E P OCT Figure 1.6 Same as for F i g . 1.3 except for plot 4 and 613 mm. - 25 - Table 1.3 Minimum measured values of average root zone water content (m m - 3) i n the eight subplots. Aug. 27, 1981 Jun. 25, 1982 Plot Uncut Cut Uncut Cut 1 0.14 0.13 0.14 0.15 2 0.13 0.15 0.12 0.14 3 0.14 0.17 0.14 0.17 4 0.16 0.17 0.16 0.16 Ave. 0.14 0.16 0.14 0.16 - 26 - the entire root zone ( F i g . 1.7). Over the entire drying period s i m i l a r quantities of water were lo s t from the top portion of the root zone (0.08 3 3 and 0.10 m m~ for the cut and uncut subplot, respectively) as from the bottom portion (corresponding values were 0.09 and 0.08 m3 m - 3). 3. S o i l Water P o t e n t i a l and Tree Pre-Dawn Twig Xylem Water P o t e n t i a l During drying periods in 1981 and 1982, T s was s i g n i f i c a n t l y higher as a result of s a l a l removal in plot 2 ( F i g s . 1.8 and 1.9). In 1981, minimum values of were -0.9 and -1.2 MPa i n the cut and uncut subplots, r e s p e c t i v e l y , while i n the 1982 the corresponding values were -1.0 and -1.5 MPa. As this gravelly sandy loam s o i l dried, small differences i n 8 corresponded to large difference i n f s owing to the steepness of the ( f i e l d determined) water retention curve described using an equation of the form proposed by Campbell (1974) ( ¥ 3 (MPa) = -0.005 ( 8/0. 3 ) - 6 ' 5 ) . Douglas-fir pre-dawn ^xtx w a s 0.1-0.2 MPa higher, during drying periods, as a result of s a l a l removal (Table 1.4) except for plot 4 i n 1982. K e l l i h e r et a l . (1984) showed that Douglas-fir pre-dawn r'Ttx w a s generally s i m i l a r to Y s for < -0.4 MPa. For Vs >-0.4 MPa, Douglas-fir pre-dawn fxtx w a s about 0.4-0.5 MPa less than f s (eg. August 12 and October 23, 1981 and June 30, 1982). 4. Evapotranspiration Rates Calculated from the S o i l Water Balance When s o i l moisture was adequate (July 30 - August 13, 1981 and A p r i l 29 - May 27, 1982), E (Tables 1.5 and 1.6) was s i m i l a r to those values determined by water balance analysis (Black et_ al_. 1980) and weighing lysimeter (Fritschen et a l . 1977) i n s i m i l a r well-watered Douglas-fir f o r e s t s . For most of the 1981 and 1982 drying periods, E was generally s l i g h t l y higher i n the uncut subplots (Tables 1.5 and 1.6). This result and - 27 - 0.10 0 200 ~ 400 h E - 600 0 9 (m 3m' 3) 0.14 0.18 200 400 h 600 TTT T 0.22 - ~ l 0.26 JUN 25l i IJUNII JUN \8<-\—\ iMAY 27 APR 29 1 r I . i ! ! ! L . _L . n L ! i! JUN 25| _ JUN !8<-j r-i IJUN i i :MAY 27 APR 29 _ J Figure 1.7 P r o f i l e s of s o i l water content (6) on selected days i n 1982 i n the cut (top half of graph) and uncut (bottom half of graph) subplots of plot 2. Figure 1.8 Courses of average root zone s o i l water p o t e n t i a l (T" s) i n the cut (0) and uncut (•) subplots of plot 2 from July 24 September 1, 1981. Figure 1.9 Same as for F i g . 1.8 except for June 9 - 2 5 , 1982. - 30 - Table 1.4 Douglas-fir pre-dawn t o t a l twig xylem water p o t e n t i a l (MPa) i n the eight subplots. 1981 Plot 1 2 3 4 Ave. August 12 Uncut Cut -0.5 -0.4 August 20 Uncut Cut -0.5 -0.4 -1.3 -0.9 -1.1 -0. 9 -1.0 -1.1 -0.8 -0.6 -0.7 -0.8 October 23 Uncut Cut -0.6 -0.6 -0.6 -0.6 -0.7 -0.6 -0.6 -0.6 -0.6 -0.6 1982 Plot 1 2 3 4 Ave. June 9 June 17 Uncut Cut -0.8 -1.0 -0.8 -0.8 -0.8 -0.7 -0.7 -0.7 -0.9 -0.8 Uncut Cut -1.0 -1.3 -1.0 -1.2 -1.1 -0.7 -0.8 -0.8 -1.4 -0.9 June 23 Uncut Cut June 30 -1.3 -0.9 -1.6 -1.1 -1.3 -1.0 -1.2 -1.6 -1.4 -1.2 Uncut Cut -0.4 -0.8 -0.4 -0.5 -0.5 -0.6 -0.6 -0.5 -0.8 -0.6 - 31 - Table 1.5 Average evapotranspiration rates ( • d" ) i n the eight subplots i n August 1981 as calculated from -(A9/At)£ + P assuming drainage was n e g l i g i b l e . On August 24, 9 mm of ra i n f e l l while during the other three periods there was no r a i n . For July 30-August 19 and August 19-27, standard deviations were t y p i c a l l y 0.5 and 0.3 mm d~ respectively. July 30-Aug. 6 Aug. 6--13 Plot Uncut Cut Uncut Cut 1 2.7 3.3 2.6 2.1 2 1.9 1.6 2.8 1.3 3 1.8 1.6 1.9 2.1 4 2.2 1.3 1.2 2.0 Ave. 2.2 2.0 2.1 1.9 Aug. 13--19 Aug. 19--27 Plot Uncut Cut Uncut Cut 1 1.1 0.9 1.2 1.6 2 1.9 0.8 1.7 1.7 3 0.9 1.3 1.3 1.8 4 0.7 1.5 1.1 1.6 Ave. 1.2 1.1 1.3 1.7 - 32 - Table 1.6 Average evapotranspiration rates (mm d ) i n the eight subplots i n May and June 1982 as calculated from -(A6/At)£ + P assuming drainage was n e g l i g i b l e . During the period A p r i l 19-May 27, 12 mm of ra i n f e l l while during the other three periods there was no r a i n . Standard deviations were t y p i c a l l y 0.3 mm d - 1 . Apr. 29-May 27 May 27-June 11 Plot Uncut Cut Uncut Cut 1 1.8 2.1 1.0 0.9 2 1.5 1.5 1.6 1.0 3 1.4 1.4 0.8 0.9 4 1.5 1.3 0.7 0.8* Ave. 1.6 1.6 1.0 0.9 June 11--18 June 18--25 Plot Uncut Cut Uncut Cut 1 0.8 0. 7 0.9 1.1 2 0.7 0.9 1.1 0.9 3 0.4 0.5 0.7 0.6 4 0.5 0.8* 0.7 0.9 Ave • 0.6 0.7 0. 8 0.9 *period was May 27 - June 18 - 33 - the higher values of Vs and tree pre-dawn Yftx o n c u t subplots during these periods suggest that much of the s o i l water which would have been used by s a l a l was immediately taken up by the tree i n the cut subplot. This would not explain why E in the cut subplot of plots 3 and 4 during August 6-27, 1981 exceeded that i n the adjacent uncut subplot. A possible explanation i s as follows. Tree t r a n s p i r a t i o n rate i s equal to the tree rooting area ( i . e . the e f f e c t i v e ground area occupied by the tree's roots) m u l t i p l i e d by the rate of change i n root zone water storage i n this area due to water extraction by the tree ((A8/At)£). Now i f one of two trees with the same t r a n s p i r a t i o n rate has a 10% smaller rooting area than the other, then the former w i l l have 10% larger values of (A0/At)c than the l a t t e r . This suggests that i n 1981 the tree i n the cut subplot of plots 3 and 4 had a smaller rooting area than the tree i n the adjacent uncut subplot. The fact that this was not the case i n 1982 suggests that the completion of the i s o l a t i o n trench confined adjacent trees to an equal rooting area. For the period August 6-19, 1981, E in the uncut subplot of plot 2 was almost twice that i n the cut subplot. This suggests an i n i t i a l saving of s o i l water i n the cut subplot for use l a t e r i n the drying period. During most of the 1982 drying period, E was much lower than during the 1981 drying period when si m i l a r values of 0 existed. The 1982 drying period occurred during the emergence of new leaves for both Douglas-fir and s a l a l so that E j was estimated to be about 20% less than l a t e r i n the growing season (eg. August) for the same s o i l moisture and meteorological conditions. For most of the 1982 drying period, vapour pressure d e f i c i t s were higher than during the 1981 drying period. This suggests that r s of both Douglas-fir and s a l a l were higher i n 1982 than i n 1981. It would - 34 - appear that the increase i n r s was great enough that i t offset the increase i n e*-e i n (2) and D i n (3) so that E-p i n 1982 was less than i n 1981. 5. P a r t i t i o n i n g Evapotranspiration Between Douglas-fir and S a l a l T r a n s p i r a t i o n and Forest Floor Evaporation Comparison of d a i l y t o t a l values of forest f l o o r evaporation and s a l a l t r a n s p i r a t i o n for 5 days i n the 1981 drying period i n plot 2 indicated that the former was about 0.5-1 mm d _ 1 less than the l a t t e r (Table 1.7). On August 12 and 20, 1981, Douglas-fir E^ was about 0.2 mm d - 1 higher i n the cut than the uncut subplot of 2. Similar r e s u l t s were obtained using the heat pulse technique described by Cohen et_ a l . (1985) i n the same plot on August 3 and 4, 1981 suggesting that these differences i n Douglas-fir E-p were not a result of error i n r s measurements (Leverenz et_ al_. 1982). On October 23, 1981, when the root zone was recharged with water, Douglas-fir Ey; was very s i m i l a r i n the adjacent subplots i n d i c a t i n g that e a r l i e r differences were due to differences In s o i l water stress between the two subplots. Daily t o t a l values of E on August 12 and 20, 1981, i n Table 1.7 are s i m i l a r to values for plot 2 i n Table 1.5 In the corresponding periods. This suggests that the marked difference i n water balance estimates of E between adjacent subplots i n plot 2 (Table 1.5) was not a r e s u l t of error i n 6 measurements and that, for some trees (eg. plot 2 and, to a lesser extent, plot 1), s a l a l removal led to a reduction i n E during part of the 1981 drying period. In 1982, Douglas-fir E T was lower than i n 1981 since tree leaf area was lower as indicated e a r l i e r , early i n the springtime of 1982 and vapour pressure d e f i c i t s were much higher i n 1982 ( p a r t i c u l a r l y on June 17, 1982). - 35 - Table 1.7 Measured rates (mm d ) of Douglas-fir and s a l a l t r a n s p i r a t i o n ( E T ) , forest f l o o r evaporation ( E 0 ) and t o t a l evapotranspiration ( E ) i n plot 2 using porometry and equations (2) and (3) and small weighing lysimeters. The root mean square errors for Douglas-fir and s a l a l E ^ were t y p i c a l l y 0.1-0.2 mm d~* . These errors were determined by d i f f e r e n t i a t i o n of (2) and (3) . For (2), a 30% error was assumed for a s and r b and a 20% error for e* - e while the standard deviation of the SL a measurements was used as the error i n r s . For (3), a 30% error was assumed for A t £ and a 10% error i n D while the standard deviation of the r s measurements was used as the error i n . r s . For E Q , standard deviations were t y p i c a l l y 0.1 mm d - 1 . Subplot Uncut Cut E T • E D E E T Eo E F i r S a l a l F i r 1981 Jul y 24 1.5 0.6 July 31 1.5 0.5 Aug • 4 0.7 0.2 Aug. 12 1.1 1.6 0.1 2.8 1.4 0.3 1.7 Aug. 20 0.6 0.8 0.1 1.5 0.8 0.1 0.9 Oct. 23 1.7 0.9 1.8 1982 June 9 0.3 0.8 June 17 0.4 0.4 June 23 0.5 0.7 June 30 0.4 — _ 0.5 - 36 - Douglas-fir Ej was 0.2-0.5 mm d higher as a r e s u l t of s a l a l removal i n plot 2 during the 1982 drying period (except for June 17). Following 26 mm of r a i n from June 26-28, 1982, Douglas-fir was si m i l a r i n the adjacent subplots of plot 2 on June 30. 6 . Tree Diameter Growth There was close agreement between basal area increments (BAI) (mm t r e e - 1 y e a r - 1 ) calculated from dendrometer band ( F i g . 1.10) and diameter tape measurements (Tables 1.8 and 1.9). For three years following s a l a l removal, BAI was higher i n cut subplots except for plot 4. Increase i n BAI as a res u l t of s a l a l removal was greatest i n 1982 when the spring drying period l i m i t e d tree growth where s a l a l was present ( F i g . 1.10). This shows the important e f f e c t of the higher values of H'g on tree growth i n the cut subplots during May and June 1982 when most of the BAI occurred. During the 1982 drying period, tree pre-dawn fxtx w a s a°out 0.2-0.4 MPa higher i n the uncut than the cut subplot of plot 4 (Table 1.4). Brix (1972) and Lassoie (1979) found that bole expansion of field-grown Douglas-fir trees ceased when was about -0.5 MPa. Values of ¥ s were less than -0.5 MPa i n plot 2 during the t h i r d week of August 1981 when BAI ceased. In 1982, BAI i n plot 2 ceased (temporarily) during June when fg was again less than -0.5 MPa. However, during this period, BAI continued for an a d d i t i o n a l week in the cut subplot of plot 2. 4 . CONCLUSIONS S a l a l understory removal resulted i n a s l i g h t increase i n 9 during growing season drying periods. This was a r e s u l t of E being only s l i g h t l y higher where s a l a l was present than where i t had been removed. Because - 37 - i 1 r MAY JUN JUL AUG SEP Figure 1.10 Courses of Douglas-fir basal area i n the cut (0) and uncut (•) subplots of plot 2 from June 4 - September 23, 1981 and A p r i l 29 - September 14, 1982. - 38 - Table 1.8 Diameter (including bark), at the 1.37 m height, of the four pairs of adjacent trees using a tape with a 1 mm (i n diaraete r e s o l u t i o n . Subplot June 4, 1981 Dec. 10, 1981 Oct. 6, 1982 Nov. 28, 1 Uncut 161 166 172 1 Cut 183 187 198 2 Uncut 179 182 185 188 2 Cut 180 185 192 198 3 Uncut 120 122 127 132 3 Cut 123 128 132 137 4 Uncut 112 117 123 130 4 Cut 102 105 109 116 - 39 - Table 1.9 Basal area increment (mm tre e " year ) for the four pairs of adjacent trees calculated using Table 1.8. Plot 1981 1982 1983 Uncut Cut Uncut Cut Uncut Cut 1 1280 1160 1590 3330 2 850 1430 860 2070 880 1840 3 380 990 980 820 1020 1060 4 900 490 1130 670 1390 1240 Ave. 850 1020 1140 1720 1100 1380 - 40 - s a l a l t r a n s p i r a t i o n was 0.5-1 mm d~ greater than forest f l o o r evaporation i n cut subplots, Douglas-fir t r a n s p i r a t i o n was higher by 30 and 58%, on average, i n 1981 and 1982 as a r e s u l t of s a l a l removal. The s l i g h t increase i n 9 corresponded to s i g n i f i c a n t l y higher values of T " s and Douglas-fir pre-dawn H 'T / tx owing to the steepness of the s o i l water retention curve for t h i s gravelly sandy loam s o i l at low values of 9. Douglas-fir BAI was increased s i g n i f i c a n t l y following s a l a l removal as a r e s u l t of increased i n cut subplots during growing season drying periods. The r e s u l t s of t h i s study suggest that i n stands where consumption of water by understory i s a growth l i m i t i n g f actor, understory control w i l l r e s u l t i n more tree water use and growth. 5. REFERENCES Anonymous. 1978. The Canadian system of s o i l c l a s s i f i c a t i o n . Can. S o i l Survey Com., Subcom. on S o i l C l a s s i f i c a t i o n , Can. Dept. of A g r i c . Publ. 1646, Supply and Services Can., Ottawa, Can. B a r r e t t , J.W. and C.T. Youngberg. 1965. E f f e c t of tree spacing and understory vegetation on water use i n a pumice s o i l . S o i l S c i . Soc. Am. Proc. 29: 472-475. Balmer, W.E., K.A. Utz and O.G. Langdon. 1978. F i n a n c i a l returns from c u l t u r a l work i n natural l o b l o l l y pine stands. South. J . Appl. For. 2: 111-117. Black, T.A., C S . Tan and J.U. Nnyamah. 1980. Transpiration rate of Douglas-fir trees i n thinned and unthinned stands. Can. J . S o i l S c i . 60: 625-631. Black, T.A. and D.L. Spittlehouse. 1981. Modeling the water balance for watershed management, pp. 117-129. In: D.M. Baumgartner (ed.) Proc. I n t e r i o r West Watershed Mgt. A p r i l R-10, 1980. Spokane, WA, USA. Boast, CW. and T.M. Robertson. 1982. A "mlcro-lysimeter" method for determining evaporation from bare s o i l : Description and laboratory evaluation. S o i l S c i . Soc. Am. J . 46: 689-696. - 41 - Brix, H. 1972. Nitrogen f e r t i l i z a t i o n and water e f f e c t s on photosynthesis and earlywood-latewood production i n Douglas-fir. Can. J . For. Res. 2: 467-478. Campbell, G.S. 1974. A simple method of determining unsaturated conductivity from moisture retention data. S o i l S c i . 117: 311-314. Cohen, Y., F.M. K e l l i h e r and T.A. Black. 1985. Determination of sap flow i n Douglas-fir trees using the heat pulse technique. Can. J . For. Res. 15: 422-428. Fritschen, L.J., J. Hsia and P. Doraiswamy. 1977. Evapotranspiration of a Douglas-fir determined with a weighing lysimeter. Water Resour. Res. 13: 145-148. G i l e s , D.G. 1983. S o i l water regimes on a forested watershed. M.Sc Thesis, Univ. of B.C., Vancouver, B.C. Grano, C.X. 1970. Small hardwoods reduce growth of pine overstory. USDA Forest Serv. South. Forest Exp. Sta. Res. Pap. S0-55. K e l l i h e r , F.M., T.A. Black and A.G. Barr. 1984. Estimation of twig xylem water p o t e n t i a l i n young Douglas-fir trees. Can. J. For. Res. 14: 481-487. Lassoie, J.P. 1979. Stem dimensional f l u c t u a t i o n s i n Douglas-fir of d i f f e r e n t crown classes. For. S c i . 25: 132-144. Leverenz, J . , J.D. Deans, E.D. Ford, P.G. J a r v i s , R. Milne and D. Whitehead. 1982. Systematic s p a t i a l v a r i a t i o n of stomatal conductance i n a Sitka spruce plantation. J . Appl. E c o l . 19: 835-851. Liming, F.G. 1957. Homemade dendrometers. J . For. 55: 575-577. O l i v e r , W.W. 1979. Ea r l y response of ponderosa pine to spacing and brush: observations on a 12-year old plantation. USDA Forest Serv. Pac. Southwest For. and Range Exp. Sta. Res. Note PSW-341. Nnyamah, J.U. and T.A. Black. 1977. F i e l d performance of the dewpoint hygrometer i n studies of s o i l - r o o t water r e l a t i o n s . Can. J . S o i l S c i . 57: 437-444. Roberts, J . , C.F. Pymar, J.S. Wallace and R.M. Pitman. 1980. Seasonal changes i n leaf area, stomatal conductance and t r a n s p i r a t i o n from bracken below a forest canopy. J . Appl. E c o l . 17: 409-422. Roberts, J. , R.M. Pitman and J.S. Wallace. 1982. A comparison of evaporation from stands of Scots pine and corsican pine i n Thetford Chase, East Anglia. J . Appl. E c o l . 19: 859-872. Spittlehouse, D.L. 1981. Measuring and modelling evapotranspiration from Douglas-fir stands. Ph.D. Thesis, Univ. of B.C., Vancouver, B.C. - 42 - Spittlehouse, D.L. and T.A. Black. 1981. A growing season water balance model applied to two Douglas-fir stands. Water Resour. Res. 17: 1651-1656. Spittlehouse, D.L. and T.A. Black. 1982. A growing season water balance model used to p a r t i t i o n water use between trees and understory. pp. 195-214 In: P r o c Can. Hydrol. Symp. 82, Hydrol processes i n f o r . areas. June 14-15, 1982, Fredericton, N.B. Tan, C S . and T.A. Black. 1978. Evaluation of a ve n t i l a t e d d i f f u s i o n porometer for the measurement of stomatal d i f f u s i o n resistance of Douglas-fir needles. Arch. Met. Geoph. B i o k l . , Ser. B. 26: 257-273. Tan, C.S., T.A. Black and J.U. Nnyamah. 1978. A simple d i f f u s i o n model of tr a n s p i r a t i o n applied to a thinned Douglas-fir stand. E c o l . 59: 1221-1229. Thorn, A.S. 1971. Momentum absorption by vegetation. Quart. J. R. Met. Soc. 97: 414-428. Turner, N.C, F . C C Pederson and W.H. Wright. 1969. An aspirated d i f f u s i o n porometer for f i e l d use. Conn. Agr. Sta. Special S o i l s B u l l . 29. van Bavel, C.H.M., G.B. S t i r k and K.J. Brust. 1968. Hydraulic properties of a clay loam s o i l and the f i e l d measurement of water uptake by roots: I. Interpretation of water content and pressure p r o f i l e s . S o i l S c i . Soc. Am. Proc. 32: 310-317. Zahner, R. 1958. Hardwood understory depletes s o i l water i n pine stands. For. S c i . 4: 178-184. - 43 - CHAPTER 2 APPLICATION OF AN EVAPOTRANSPIRATION MODEL TO ESTIMATING SALAL UNDERSTORY REMOVAL EFFECTS IN A DOUGLAS-FIR FOREST - 44 - CHAPTER 2 APPLICATION OF AN EVAPOTRANSPIRATION MODEL TO ESTIMATING SALAL UNDERSTORY REMOVAL EFFECTS IN A DOUGLAS-FIR FOREST 1. ISTRODPCTION The Penman-Monteith equation (Monteith 1965) has provided useful insight into the physical and ph y s i o l o g i c a l factors a f f e c t i n g forest evapotranspiration (Stewart and Thorn 1973; Tan and Black 1976). A further development of the equation for multilayer, p a r t i a l l y wet forest canopies has provided a p r a c t i c a l one-dimensional model (Shuttleworth 1978 and 1979) despite the sim p l i f y i n g assumptions regarding within canopy turbulent transfer (Jarvis et a l . 1976; Raupach and Thorn 1981; Finnegan 1985). With standard hourly micrometeorological measurements and stomatal resistance c h a r a c t e r i s t i c s , the model can be combined with a root zone water balance model (e.g. Spittlehouse and Black 1982) to provide estimates of forest evapotranspiration over extended periods. This chapter ( i ) describes the evapotranspiration model as modified for use i n hypostomatous canopies, ( i i ) tests the model using energy and water balance measurements and ( i i i ) uses the model to explain the e f f e c t s of s a l a l (Gaultheria shallon Pursh.) understory removal on tree t r a n s p i r a t i o n rates i n a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) f o r e s t . - 45 - 2 . THEORY Using Shuttleworth's (1979) evapotranspiration theory and assuming the s i m i l a r i t y of sensible heat and water vapour aerodynamic trans f e r r e s i s t a n c e s , i t can be shown that the water vapour f l u x density from layer I ( E ' i ) with l e a f area (one side) Index (a^) within a multilayer forest canopy of hypostomatous leaves can be expressed as (see Appendix I) E , _ s(Rni - C) + p c p ( D j - 6 j ) / r A 1 1 " L[s + y d + r c i / r M ) ] where E'^ i s the difference between the water vapour f l u x density above and below the layer ( i . e . - Ei-l)» Rni a n d D i a r e t n e n e t r a d i a t i o n f l u x density and vapour pressure d e f i c i t above layer i r e s p e c t i v e l y , G i s the s o i l heat f l u x density, 5 i " [ s ( R n i - l ~ G ) ( r b i / ( 2 a i ) ) + (s + Y ) L E i - i r a i ] / ( p c p ) , (2) r ^ i i s the t o t a l aerodynamic resistance (Thorn 1972) given by * A i " r a i + r b i / ( 2 a i ) (3) and rci i s the canopy resistance of layer i expressed as (see Appendix II) r c i Wi 1 - Wi -1 (1 + s / Y ) r b l / ( 2 a i ) r s i / a i + (2 + s / Y ) r b l / ( 2 a i ) _ - (1 + s / Y ) r b l / ( 2 a i ) (4) - 46 - where r a ^ i s the eddy d i f f u s i v e resistance above layer i , r D i and r s j are the boundary-layer and stomatal resistances of one side of the leaves In layer 1 r e s p e c t i v e l y and i s the f r a c t i o n of leaf area i n layer i that i s completely wet. The remaining symbols are as conventionally defined (see Notation). Stomatal resistances of hypostomatous leaves of Douglas-fir and s a l a l on a one-sided basis have been rel a t e d to l i g h t , leaf and s o i l water p o t e n t i a l and D (Tan et_ a l . 1977 and 1978). Equation (1) i s recognized as the Penman-Monteith equation with an a d d i t i o n a l term which accounts for the net r a d i a t i o n , latent and sensible heat flux densities below layer 1. Black et a l . (1970) used (1) as an evapotranspiration model for a dry snap bean canopy where r D i « r s i was assumed so that 6^ was equal to the s o i l evaporation rate m u l t i p l i e d by r a i ( s + Y)/(PCp). I f TA± i s assumed to be zero, then (1) reduces to the Penman-Monteith equation applied to the layer ( i . e . using the vapour pressure d e f i c i t and the av a i l a b l e energy flux density ( R n i - ^ n i - i ) within the l a y e r ) . Equation (4) gives the canopy resistance of a p a r t i a l l y wet layer assuming that the i n d i v i d u a l leaves are either completely wet or completely dry so that the wet and dry leaves have d i f f e r e n t temperatures. Equation (4) d i f f e r s s l i g h t l y from Shuttleworth's (1978) equation (32) because his derivation was for amphistoraatous leaves. When Ŵ  = 0, r c i = r s i / a i + rbi/(2a-L) compared to r c i = rsi/(2a±) for an amphistoraatous leaf canopy as i n (24) of Shuttleworth (1978) (see Appendix I I I ) . As expected when Wj_ = 1, TC± = 0 i n both hypostomatous and amphistomatous cases. Making use of Shuttleworth's (1978) theory, i t can be shown that the - 47 - rate of evaporation of intercepted water from layer i (E'n) i s given by (see Appendix IV) E ' i W i [ ( 2 + s / Y ) r b l + 2 r s i ] E ' l l - (5) (1 + s / Y ) r b l + W i ( r b i + 2 r s l ) The vapour flux density above layer 1 (E^) i s given by summing (1) from 0 to i , so that for a canopy of n layers the evapotranspiration rate i s n E = I E ' i (6) i=o where E Q = E ' 0 i s the evaporation rate from the forest f l o o r with 6 Q = 0 and r c o being the forest f l o o r d i f f u s i v e r e s i stance. In t h i s study, the forest canopy was divided into two layers (n = 2), where the Douglas-fir subcanopy was designated as layer 2 and the s a l a l subcanopy as layer 1 (see Appendix V). 3. METHODS 1. S i t e and Experlaental Design The s i t e and experimental design are described in d e t a i l i n Chapter 1. B r i e f l y , the s i t e was a thinned 31-year-old Douglas-fir stand with s a l a l underst ory ov6v sbout 700 mm o f gravelly sandy loam s o i l . The experimental design consisted of four c i r c u l a r plots about 7 m i n diameter, each containing two subplots, one with s a l a l understory present and the other with i t cut and removed. Each subplot contained one tree about 14 m t a l l - 48 - and 160 mm i n diameter (at the 1.37 m height) which was si m i l a r i n size to the one i n the adjacent subplot. The root zones of a l l eight trees were i s o l a t e d by a trench and p l a s t i c b a r r i e r . 2 . Mlcroneteorological Measurements Standard hourly micrometeorological measurements of solar irradiance, a i r temperature ( T a j [ r ) , vapour pressure d e f i c i t , wind speed (u) and p r e c i p i t a t i o n (P) above the forest are described i n Chapter 1. In 1981, R n above the forest ( R n a ) was measured using a Swissteco S-l net radiometer. In 1981, ̂  below the tree canopy ( R n o ) was measured i n one plot using one net radiometer above the s a l a l canopy and another above the for e s t f l o o r surface, where s a l a l had been removed. In 1982, was estimated, p r i o r to August, from the solar irradiance measurements following Gates (1980) and Spittlehouse and Black (1981). During August 1982, R ^ was measured using an S-l net radiometer. In the same month, R ^ was measured using an S-l net radiometer mounted on a tram t r a v e l l i n g at the 1 m height along a 10 m path where s a l a l along one half of the path had been removed. The tram t r a v e l l e d at 0.5 m min" 1 and automatically reversed when i t reached each end. The net radiometer output voltage was measured every 10 seconds (see Appendix VI). P r i o r to August 1982, R ^ was estimated to be approximately 0.16 R n a and 0.14 R ^ for uncut and cut subplots, r e s p e c t i v e l y (see Appendix VI). In 1981 and 1982, R ^ below the s a l a l canopy was estimated using = ((s + y ) / s ) L E 0 + G where L E Q was 2 W m - 2 (measured using small weighing lysimeters (150 mm diameter by 120 mm deep) (see Chapter 1)). The s o i l heat flux density i n each subplot of one plot at the 50 mm depth was measured using three s o i l heat f l u x plates (100 mm long - 49 - x 25 mm wide x 3 mm t h i c k ) , s i m i l a r to those described by Fuchs and Tanner (1968), connected i n series and corrected for the rate of change of heat storage i n the upper 50 mm of s o i l . In 1982 ( p r i o r to August), G was estimated to be 0.02 R n a and 0.03 R n a for uncut and cut subplots, r e s p e c t i v e l y . In 1981 and 1982, T a ± T  and D were estimated below the tree canopy using hourly average values at the 6 m height with the relationships T a ^ r (°C) (0.5 m height) = 0.93T a i r (°C)(6 m height) + 1.2 and D (kPa) (0.5 m height) = 0.89 D(kPa) (6 m height) - 0.03 (based on 33 hourly average measurements of T a ^ r and D at both heights on July 24 and 25, 1981). Hourly Assmann psychrometer measurements confirmed the v a l i d i t y of these re l a t i o n s h i p s on several days i n August 1981 and June 1982. 3. Canopy Resistance Functions Stomatal resistances of Douglas-fir and s a l a l were estimated using average root zone s o i l water p o t e n t i a l (^g) and D for the layer following i n t e r p o l a t i o n of the r g c h a r a c t e r i s t i c functions i n Tan £t_ a l . (1978) (Table 2.1)( see Appendix VII). When R n a was negative ( i . e . 1900-0700 hours PST), r s of both species was set to 10 5 s m*~ 1 . Boundary-layer resistances were estimated using a function for a r t i f i c i a l leaves i n Spittlehouse and Black (1982) and a shelter factor of two (Landsberg and Powell 1973; Ja r v i s et_ a l . 1976) ( r b (s m _ 1) = 2 x 184 ( d ^ / u ) 0 , 5 ) where djj, i s leaf diameter (1 mm for Douglas-fir and 60 mm for s a l a l ) and u i s the windspeed near the leaf (0.5 u 1 5 m for Douglas-fir and 0.1 u i 5 m for s a l a l where u l 5 m i s the windspeed at the 15 m height ( i . e . 1 m above the f o r e s t ) ) . In 1982 ( p r i o r to August), u 1 5 m was estimated to - 50 - Table 2.1 Values of the empirical constants A, u, v, £ and o i n the stomatal resistance ( r 8 ) c h a r a c t e r i s t i c s function r s (s m"1) = exp[X - y ( Y s + v) + (C + o(-f 8 + v ) ) D 2 ] where i s i s average root zone s o i l water p o t e n t i a l (MPa) and D i s vapour pressure d e f i c i t (kPa). Species *s X y V 5 o S a l a l >-0.10 0.40 7.0 0.01 0.19 0.10 S a l a l >-0.35 to <-0.10 1.05 1.5 0.10 0.20 0.10 S a l a l >-0.95 to <-0.35 1.40 1.0 0.35 0.24 0.02 S a l a l >-1.10 to <-0.95 1.95 1.8 0.95 0.27 0.02 S a l a l <-1.10 2.20 0.5 1.10 0.27 0.02 Douglas-fir >-0.10 1.10 5.0 0.01 0.37 0.20 Douglas-fir >-0.35 to <-0.10 1.60 0.6 0.10 0.38 0.10 Douglas-fir >-0.95 to <-0.35 1*80 1.4 0.35 0.40 0.20 Douglas-fir >-1.10 to <-0.95 2.60 1.5 0.95 0.54 0.10 Douglas-fir <-1.10 2.85 0.5 1.10 0.56 0.10 - 51 - be 3 m s - 1 for 1100-2000 h PST and 1.5 m s ~ 1 for the rest of the day. Div i d i n g these values by 2a^ gave mean boundary-layer resistances s i m i l a r to those estimated following the relationships given by Garratt and Hicks (1973). The eddy d i f f u s i v e resistance above the Douglas-fir subcanopy was estimated assuming a logarithmic wind p r o f i l e (Jarvis et_ a l . 1976) with a zero plane displacement height of 8.5 m (Szeicz et a l . 1969) and a roughness length of 1.5 m ( S t a n h i l l 1969), while that above the s a l a l subcanopy was estimated assuming an exponential eddy d i f f u s i v i t y p r o f i l e from the top of the trees to the s a l a l subcanopy (Thorn 1975) with an attenuation c o e f f i c i e n t of 2. This value was based on the r a t i o of the above to below tree canopy windspeeds being about 0.13. Daytime eddy d i f f u s i v e resistances above the s a l a l subcanopy were calculated to be about 40 s m - 1 compared to 20 s m~1 for the 600 trees h a - 1 Thetford (Shuttleworth 1979; Roberts et^ al_. 1980) and the 400 trees h a - 1 Jadraas (Lindroth 1984) f o r e s t s . For the forest f l o o r , values of r a£ estimated for the s a l a l subcanopy were used. Forest f l o o r d i f f u s i v e resistances for ten days i n July and August 1981 were determined by choosing a value which resulted i n agreement between d a i l y t o t a l values of measured and calculated E Q on each day. These values were l i n e a r l y related to average root zone water content for calcu l a t i o n s of forest f l o o r d i f f u s i v e resistances on other days. For each Douglas-fir tree, leaf area was estimated from the diameter at the 1.37 m height using a function i n Spittlehouse (1981). Ground area occupied by the tree was estimated using a tree l o c a t i o n map and the "polygon of occupancy" (Santantonio et_ al_. 1977). Tree leaf area divided by the polygon of occupancy was taken as the value of a for Douglas-fir i n each subplot. For s a l a l , a was estimated from 1 ra2 sample measurements made in each of the cut subplots on May 21, 1981. - 52 - The leaf wetness variable for each subcanopy was estimated using the r a t i o of the subcanopy water storage (C) to the maximum water storage of the subcanopy (S). The value of S for the Douglas-fir subcanopy was determined by p l o t t i n g 24 hour throughfall (above the s a l a l ) against the corresponding r a i n f a l l using data of Spittlehouse (1981) for 1978 ( F i g . 2.1). Rutter et_ a l . (1971) showed that the negative intercept of the l i n e of unit slope along the upper l i m i t of the throughfall data gives the value of S. This was found to be 0.6 mm d"*1. Since a of the Douglas-fir i n 1978 was 5, the average depth of water on the leaves was 0.12 mm. This was very close to the depth of water a f t e r drainage on a f o l i a t e d Douglas-fir branch sprayed i n the laboratory (Spittlehouse and Black 1982). The value of S for the s a l a l subcanopy was approximated by multiplying s a l a l a by 0.12. The value of C was calculated for each time j using the following water balance equation for each subcanopy C i = C 1 - l + [<1-P> P -« " E i j " Q j ! At (7) where At i s 15 minutes and Qj i s the drainage of Intercepted water for the i n t e r v a l between j-1 and j . The free throughfall c o e f f i c i e n t p was c a l c u l a t e d by taking the average of the r a t i o of 24 hour throughfall (above s a l a l ) to r a i n f a l l for 5 days from Spittlehouse (1981) where P < S (Rutter et a l . 1971). I t was found to be 0.6. Drainage was assumed to be zero u n t i l Cj exceeded S. At t h i s point drainage was calculated as the amount by which (1-p) P j - E ' j j exceeded the remaining water storage capacity of the leaves (S - C j ) . - 53 - I 1 I I I L 0 4 8 RAINFALL (mm d"1) Figure 2.1 Relationship between d a i l y throughfall (above the s a l a l ) and r a i n f a l l rates at the experimental s i t e i n 1978. A l i n e of unit slope along the upper l i m i t of the data i s also shown. The negative intercept of t h i s l i n e (0.6 mm d - 1 ) gives the maximum water storage of the Douglas-fir subcanopy following Rutter et a l . (1971). - 54 - 4. Root Zone Water Balance Equation The course of average root zone volumetric water content (8) during two summer periods was calculated using the following water balance equation applied to the stand and i t s s i n g l e - l a y e r root zone 9k = 9 k - l + CPk " E k " * k ) A t / r (8) where P^, and F k are the rates of r a i n f a l l , evapotranspiration and root zone drainage respectively, at time k, At i s the time i n t e r v a l between k and k-1 (one hour except for when W-̂  > 0 and then At i s 15 minutes) and S i s root zone depth. Evapotranspiration rates were calculated using (1) through (7) with the canopy divided into two layers (tree and understory subcanopies) and s o i l . Drainage from the root zone was calculated as a function of 8 (F (mm d - 1 ) = 100 (8/0.3) 1 7) (Spittlehouse and Black 1981). During most of the summer, drainage was a small term i n the root zone water balance equation so that 8 was larg e l y determined by r a i n f a l l and evapotranspiration. 5. Testing the Evapotranspiraton and Root Zone Water Balance Equations During August 1981 and June 1982, 8 was measured i n one to two week i n t e r v a l s using the neutron moderation technique with a probe being lowered into access tubes placed i n each subplot. Thermocouple psychrometers and tensiometers were used to measure Y s every two to seven days i n one of the p l o t s . Measured values of 8 and Y s were compared during the two summer - 55 - periods with calculated values obtained using (8) and a f i e l d determined s o i l water retention c h a r a c t e r i s t i c (fg (MPa) = -0.005 (9/0.3)~ 6* 5) (see Appendix VI I I ) . Forest evapotranspiration was measured on four days i n August 1982 using the Bowen ratio/energy balance technique. Half-hourly measurements of the Bowen r a t i o ($) were made using a D.C. powered rotating psychrometric apparatus described by Spittlehouse and Black (1980). The apparatus was located at the top of a 15 m t a l l tower adjacent to the four p l o t s , with the v e r t i c a l separation between the two psychrometers being 3 m. The lower psychrometer was about 1 m above the tops of the trees. Forest evapotranspiration was calculated using E = ( R n a - G - M)/(L(1 + 6)) (9) where M i s the rate of canopy heat storage estimated following Stewart and Thorn (1973). This measurement of E was considered to include tree and understory t r a n s p i r a t i o n and s o i l evaporation since the area where understory had been removed was small and 20 m from the tower i n a d i r e c t i o n at right angles to the p r e v a i l i n g wind d i r e c t i o n . These measurements of E were compared to calculated values obtained using ( l ) - ( 6 ) (Ŵ  = 0) applied to a two layer canopy (trees and understory) plus s o i l evaporation. Stomatal resistance measurements were made on the two trees and s a l a l understory i n one plot using a ve n t i l a t e d d i f f u s i o n porometer described by Tan et a l . (1978). Hourly measurements were made from sunrise u n t i l l a t e afternoon on August 12 and 20, 1981 and June 9, 17, 23 and 30, 1982 (trees - 56 - o n l y ) . These measurements were used to test the r s c h a r a c t e r i s t i c functions (Table 2.1) and the rates of tree and understory t r a n s p i r a t i o n calculated using (1). 4. RESULTS AND DISCUSSION 1 . Measured and Calculated Daytime Courses of E There was generally good agreement between d a i l y values of E measured during the 4-day test period using the energy balance/Bowen r a t i o and values calculated for the stand with understory present using (1) to (8) (Table 2.2) Agreement was not as good when comparing the daytime courses of measured and calculated E ( F i g . 2.2). However, both measured and calculated E was highest for the period prior to noon on August 25, 1982. Calculated Douglas-fir r s increased markedly af t e r 1400 h (> 6000 s m - 1) owing to the high values of D and s a l a l E was highest for the period 1100-1400 h (« 0.1 mm h - 1 ) . Measurement error accounts for some of the disagreement since Bowen r a t i o s were high (> 2) on August 24-26 and wet and dry bulb gradients small on August 27 (Spittlehouse and Black 1980). 2. Forest Floor Evaporation After S a l a l Removal For 9 less than 0.185, forest f l o o r d i f f u s i v e resistance was l i n e a r l y related to 9 ( r c o (s m - 1) = -83000 9 + 16100, R 2 = 0.96) ( F i g . 2.3). On three days when 9 > 0.185, r c o was 700 s m~1 (9 = 0.189) and 900 s m - 1 (9 = 0.200 and 0.203). This meant that E 0 was l i m i t e d by a dry surface layer whose thickness (£(j) can be related to r c o using (Denmead 1984; Novak and Black 1985) - 57 - Table 2.2 Daily t o t a l net r a d i a t i o n flux density above the forest ( R n a ) (MJ m~2 d - 1 ) and d a i l y measured and calculated values of evapotranspiration rate (E) (mm d - 1 ) following i n i t i a l i z a t i o n of c a l c u l a t i o n s on August 20, 1982 when measured 8 was 0.16 m3 m - 3 (Y s = -0.3 MPa). Date (August, 1982) 24 25 26 27 Rna 11.5 11.6 12.6 1.4 Measured E 1.8 2.2 1.8 0.2 Modelled E 1.9 2.0 2.0 0.3 - 58 - 6 0 0 C\J E 4 0 0 - g 2 0 0 CM ' E LU 6 12 hour (PST) Figure 2.2 Courses of net r a d i a t i o n f l u x density and vapour pressure d e f i c i t above the forest ( R n a ( ) and D (- - - )) and measured ( ) and calculated (- - -) forest evapotranspiration rate (E) (with understory) on August 25, 1982, a clear day when average root zone s o i l water p o t e n t i a l CPg) was about -0.7 MPa. Errors i n measured E were approximately 0.02-0.04 mm h - 1 (Spittlehouse and Black 1980). Root mean square errors i n calculated E were 0.04-0.06 mm h - 1 as determined by d i f f e r e n t i a t i o n of (1) applied to two layers and s o i l . A 10% error was assumed for T>i, a 20% error for ( R ^ - G) and a 30% error for the transfer resistances ( r s i , r ^ and r a ^ ) , L E Q and Rno* 59 - Figure 2.3 Relationship between forest f l o o r d i f f u s i v e resistance ( r c o ) and average root zone s o i l water content ( 9 ) in the cut subplot of plot 2 for ten days i n July and August 1981. For 0 less than 0.185, r c o (s n f 1 ) = -83000 9 + 16100 (R 2 = 0.96) as shown by the s o l i d l i n e . For 9 greater than 0.185, r c o was 800 s nf" , on average, as shown by the dashed l i n e . *d " r c o n t (°d - ed> <d (11) where ftt i s a " t o r t u o s i t y " factor (0.66), 0<j and 6<j are the porosity and volumetric water content of the dry layer and Is the d i f f u s i v i t y for water vapour (25.7 X 10~ 6 m2 s - 1 at 20°C). For the forest f l o o r , 0 d and were taken from Plamondon (1972) as 0.88 ( i . e . bulk density = 150 kg m~3, organic matter density = 1300 kg m~3 (Van Wijk and De Vries 1963)) and 0.20 ( i . e . matric p o t e n t i a l = -1.5 MPa). Using these values and r c o = 700 s m-1 i n (11) r e s u l t s i n £<} = 8 mm. F i e l d observations support this c a l c u l a t i o n . At about midday on the day following an evening i r r i g a t i o n equivalent to 100 mm of r a i n , the forest f l o o r surface of a cut area ( i . e . s a l a l cut and removed) adjacent to the four plots was observed to be dry. The 8 mm value of SL^ for r c 0 = 700 s m"1 suggests that the top 8 mm of f o r e s t f l o o r consisted of r a p i d l y draining l i t t e r ( i . e . undecomposed leaves and twigs) while the bottom 2-12 mm was humified with some water storage capacity. The s o i l surface was dry u n t i l r c o was 850 to 1500 s m-1 (9 = 0.184 to 0.176). For 8 = 0.125 ( f s = -1.5 MPa), £<j was 68 mm so that the top 48-58 mm of s o i l was dry. For July 24-25, 1981, two consecutive clear days when V s  was about -0.05 MPa, r c o = 900 s m-1 provided agreement between calculated and measured d a i l y values of E Q (0.6 mm d - 1 ) . Hourly values of calculated E Q generally agreed with lysimeter measurements i n the cut subplot of plot 2 ( F i g . 2.4). Aerodynamic resistances varied from 10-50 s m-1 and were highest for the period 0200-0600 on July 25. The high v a r i a b i l i t y i n the - 61 - hour (PST) Figure 2.4 Courses of net ra d i a t i o n flux density and vapour pressure d e f i c i t above the forest f l o o r ( R n o ( ) and D Q (- - -)) and measured ( ) and calculated (- - -) forest f l o o r evaporation rate ( E Q) i n the cut subplot of plot 2 on July 24-25, 1981, two clear days when average root zone s o i l water p o t e n t i a l (fg) was about -0.05 MPa. Standard deviations for measured E D values were t y p i c a l l y 0.004 mm h - 1 at night and 0.015 mm h _ 1 during the daytime. Root mean square errors for calculated E Q values were s i m i l a r . These errors were determined by d i f f e r e n t i a t i o n of (1) with 6 Q = 0. A 20% error was assumed for ( R n 0 - G) and r c o , a 10% error for D Q and a 30% error for r A o . - 62 - measured values of R ^ Q and E Q was due to sun fle c k s which were generally common to the net radiometer and lysimeters. About 16% of the d a i l y measured E Q occurred at night (2200-0600 h, F i g . 2.4) while for the c a l c u l a t i o n s the value was 10%. 3 . Measured and Calculated Courses of 6 1. Using Equations (1) to (4) Calculations of courses of 6 using the complete evapotranspiration theory and water balance equations were made for the eight subplots for the periods July 24-September 3, 1981 and May 27-July 1, 1982. There was good agreement between measured and calculated courses of 9 i n cut and uncut subplots ( F i g s . 2.5 and 2.6 and Table 2.3). In p a r t i c u l a r , there was excellent agreement i n the measured and calculated differences between 9 i n paired subplots. S a l a l understory removal resulted i n s l i g h t l y higher values of 9 and s i g n i f i c a n t l y higher values of ¥ s (see Chapter 1). Owing to the steepness of the retention curve for this gravelly sandy loam s o i l at low values of 9, a small decrease In 9 corresponded to a s i g n i f i c a n t decrease i n V s  ( F i g s . 2.5 and 2.6). 2. E f f e c t of Assuming r ^ i = 0 i n Equations (1) to (4) Considerable s i m p l i f i c a t i o n of the evapotranspiration theory i s achieved when Ŵ  = 0 by using the l i m i t r ^ i •*• 0 i n (1) - (4) so that E| = P c p D i / r c i where r c£ = rg^/a^. Working i n the same stand as i n this study, Tan et a l . (1978) obtained good agreement between energy balance/Bowen r a t i o evapotranspiration measurements and values calculated using the above procedure i n 1975 following heavy thinning of the stand. - 63 - Figure 2.5 Courses of measured (symbols) and calculated ( l i n e s ) average root zone s o i l water content (9) and s o i l water p o t e n t i a l (V  s ) i n the cut (A and - - -) and uncut (A and ) subplots of plot 2 for the period July 24 - September 3, 1981. Also shown i s the d a i l y r a i n f a l l rate (P). Figure 2.6 Same as for F i g . 2.5 except for May 27 - July 1, 1982 and 0 and t . - 65 - Table 2.3 Average values of the minimum measured and calculated average root zone water content (m3 m~3) on the same day i n the cut (C) and uncut (U) subplots. August 27, 1981 June 25 , 1982 Measured Calculated Measured Calculated 1U 0.14 0.14 0.14 0.12 1C 0.13 0.14 0.15 0.14 2U 0.13 0.13 0.12 0.12 2C 0.15 0.14 0.14 0.14 3U 0.14 0.13 0.14 0.12 3C 0.17 0.15 0.17 0.14 4U 0.16 0.14 0.16 0.13 AC 0.17 0.16 0.16 0.15 U 0.14 0.13 0.14 0.12 c 0.16 0.15 0.16 0.14 - 66 - Use of th i s procedure i n the cut subplot of plot 2 ( s a l a l understory cut and removed) for the r a i n l e s s period July 30-August 18, 1981 resulted i n only s l i g h t l y higher calculated 6 values than those obtained when r ^ i was not assumed to be n e g l i g i b l e ( F i g . 2.7). This i s to be expected since r ^ i i s much smaller than r s ^ for Douglas-fir so that r s ^ / a j i s a good approximation of the Douglas-fir canopy resistance (Tan et a l . 1978). The reason for the small difference between calculated 6 values (with r ^ j included) i n F i g . 2.7 and those i n F i g . 2.5 i s due to the difference i n the s t a r t i n g dates for c a l c u l a t i o n s i n the two f i g u r e s . For s a l a l , r ^ i B r s ^ so that neglecting r^£ for s a l a l and Douglas-fir subcanopies i n (1) - (4) resulted i n lower calculated 8 values i n the uncut subplot of plot 2 for the same r a i n l e s s period ( F i g . 2.7). When r^i for both subcanopies were included, calculated tree t r a n s p i r a t i o n i n the uncut subplot for th i s 20 day period was 29 mm. The corresponding value was 24 mm when r ^ j for both subcanopies were neglected. The diff e r e n c e between these two values resulted from more s a l a l understory t r a n s p i r a t i o n being calculated using the l a t t e r procedure. Since s a l a l stomatal resistance c h a r a c t e r i s t i c s and leaf area index remained r e l a t i v e l y constant from 1975-1981, i t appears that the reduction i n s a l a l t r a n s p i r a t i o n from 1975-1981 resulted from forest canopy closure leading to a reduction i n the r a t i o of below to above tree canopy wind speed and an increase i n rA± f ° r t n e s a l a l subcanopy. - 67 - Figure 2.7 Courses of measured (symbols) and calculated ( l i n e s ) average root zone s o i l water content (0) in the cut (0) and uncut (•) subplots of plot 2 for the period July 30 - August 18, 1981. Calculated values with and without the aerodynamic and boundary-layer transfer resistances for the Douglas-fir and s a l a l subcanopies are shown by the s o l i d and dashed l i n e s , r e s p e c t i v e l y . Error bars are one standard deviation. - 68 - 4. P a r t i t i o n i n g o f E v a p o t r a n s p i r a t i o n i n C u t a n d U n c u t S u b p l o t s Table 2.4 gives the t o t a l c a l c u lated values of evapotranspiration, t r a n s p i r a t i o n and int e r c e p t i o n of Douglas-fir and s a l a l and forest f l o o r evaporation for cut and uncut subplots for the 1981 and 1982 periods discussed i n Section 3.1. Calculated t o t a l values of E for the uncut subplots were s l i g h t l y larger than those for the cut subplots i n both years. Throughout these periods, calculated values of E of the uncut subplots were also s l i g h t l y higher than i n the cut subplots. This was also found using water balance measurements i n Chapter 1 except for plot 2 for August 6-19, 1981 when the measured average values of E were 2.4 and 1.1 mm d - 1 for the uncut and cut subplots r e s p e c t i v e l y compared to calculated average values of 1.7 and 1.5 mm d - 1 r e s p e c t i v e l y . Calculated tree t r a n s p i r a t i o n rates for the f i r s t 19 days of the 1981 period and 11 days of the 1982 period were s l i g h t l y higher where s a l a l had been removed but were considerably higher during the rest of the respective periods (Figs. 2.8 and 2.9). On August 12, 1981, calculated tree t r a n s p i r a t i o n rates were 1.5 and 1.1 mm d - 1 i n the cut and uncut subplots of plot 2 (leaf area index was 5 for both trees) r e s p e c t i v e l y . The corresponding values on August 20 were 1.1 and 0.5 mm d - 1 . Using (1) and r s measurements made at about the midcrown height of the trees In plot 2, tree t r a n s p i r a t i o n rates were estimated to be 1.4 and 1.1 mm d - 1 on August 12 and 0.8 and 0.6 on August 20, 1981. There was not as good agreement between calculated and estimated tree t r a n s p i r a t i o n rates i n plot 2 for June 9, 17 and 23, 1982; however the differences between the cut and uncut subplots were i n good agreement (see Table 1.7 i n Chapter 1). - 69 - Table 2.4 Average calculated values (mm) of t o t a l evapotranspiration (E), tr a n s p i r a t i o n (E^,), evaporation of Intercepted water (E') and forest f l o o r evaporation (E Q) In the cut (C) and uncut (U) subplots for the periods July 24-September 3, 1981 and May 27-July 1, 1982. E T E i E q 1981 F i r S a l a l F i r S a l a l 1U 92 47 25 15 2 3 1C 84 57 — 15 - 12 2U 88 39 28 15 3 3 2C 79 51 — 15 - 13 3U 83 32 31 14 3 3 3C 84 53 — 14 - 17 4U 84 38 27 14 2 3 4C 75 43 — 14 - 18 U 88 39 28 15 3 3 C 81 51 — 15 — 15 1982 1U 61 23 17 17 1 3 1C 64 39 — 17 - 8 2U 66 23 21 17 2 3 2C 58 33 — 17 - 8 3U 60 17 21 17 2 3 3C 60 32 — 17 - 11 4U 64 22 21 ' 17 1 3 4C 55 26 — 17 — 12 U 63 21 20 17 2 3 C 59 32 — 17 - 10 - 70 - 1981 Figure 2.8 Courses of calculated tree t r a n s p i r a t i o n rates i n the cut (_ _ _) a n d uncut ( ) subplots of plot 2 for the period July 24 - September 3, 1981. Also shown i s the d a i l y r a i n f a l l rate (P). - 71 - JUN2 JUN 10 JUN 18 JUN 26 1982 Figure 2.9 Same as for F i g . 2.8 except for May 27 - July 1, 1982. - 72 - During the 1981 and 1982 periods, s a l a l removal resulted In an average increase i n tree t r a n s p i r a t i o n rate of 31 and 52% r e s p e c t i v e l y i n the four p l o t s . These r e s u l t s agree with the conclusion i n Chapter 1 that s a l a l removal resulted i n an average increase i n tree t r a n s p i r a t i o n rate of 30 and 58% i n 1981 and 1982 based on r s measurements. Calculations indicate that the increase i n tree t r a n s p i r a t i o n rate was greatest i n plot 3 where s a l a l leaf area index (3 and 2.A i n 1981 and 1982 r e s p e c t i v e l y ) was highest and was least i n plot A where s a l a l leaf area index was lowest (2.1 and 1.7 i n 1981 and 1982). Although s a l a l leaf area index values i n plot 1 were s i m i l a r to those i n plot A, the larger tree In plot 1 (Douglas-fir leaf area index was 6 and 3.1 i n plots 1 and A In 1981 (less by 10% i n 1982)) responded more to s a l a l removal than the small tree i n plot A. T o t a l c a l c u l a t e d values of s a l a l t r a n s p i r a t i o n (plus forest f l o o r evaporation below the s a l a l ) were about twice those of forest f l o o r evaporation a f t e r s a l a l removal i n the 1981 and 1982 periods. The above difference l a r g e l y accounts for the Increased tree t r a n s p i r a t i o n following s a l a l removal since Douglas-fir i n t e r c e p t i o n i n adjacent subplots was i d e n t i c a l and s a l a l i n t e r c e p t i o n was a small term i n the water balance. 5. COHCLUSIOHS Shuttleworth's evapotranspiration theory with canopy and root zone water balance models proved to be p r a c t i c a l i n c a l c u l a t i n g the courses of 6, f g and tree t r a n s p i r a t i o n during extended periods i n the growing season. The d i f f i c u l t y i n using the theory i s i n estimating the transfer resistances - 73 - ( rsi> rbi> r a i a n a* forest f l o o r d i f f u s i v e resistance) although r s ^ i s often a v a i l a b l e from p h y s i o l o g i c a l studies. Simplifying the evapotranspiration theory when W-̂  = 0 by using the l i m i t r ^ i •*• 0 i n (1) - (6) resulted in more understory t r a n s p i r a t i o n being calculated because of the r e l a t i v e l y closed forest canopy and an underestimation of the courses of 9, and tree t r a n s p i r a t i o n i n uncut subplots. This s i m p l i f i c a t i o n resulted i n l i t t l e change in cut subplots since r^j. i s much smaller than r s i f ° r Douglas-fir. Calculations showed that the s l i g h t l y higher values of 9 as a result of understory removal corresponded to s i g n i f i c a n t l y higher tree t r a n s p i r a t i o n rates. During early (June 1982) and late (August 1981) growing season drying periods, most of difference i n tree t r a n s p i r a t i o n occurred during the l a t t e r one-half of the period owing to the steepness of the s o i l water retention curve for low values of 9 and stomatal closure by Douglas-fir where s a l a l remained. Increase i n tree t r a n s p i r a t i o n as a r e s u l t of understory removal was greatest where understory leaf area index was highest and trees were largest. 6. REFERENCES Black, T.A., C.B. Tanner and W.R. Gardner. 1970. Evapotranspiration from a snap bean crop. Agron. J. 62: 66-69. Denmead, O.T. 1984. Plant p h y s i o l o g i c a l methods for studying evapotranspiration: Problems of t e l l i n g the forest from the trees. Agric. Wat. Mgt. 8: 167-189. Finnegan, J . J . 1985. Turbulent transport In f l e x i b l e plant canopies, pp. 443-480. In: B.A. Hutchison and B.B. Hicks (eds.) The Forest-Atmosphere Interaction. D. Reidel Publ. Co., Boston. Fuchs, M. and C.B. Tanner. 1968. C a l i b r a t i o n and f i e l d test of heat f l u x p l a t e s . S o i l S c i . S o c Am. P r o c 32: 326-328. - 74 - Garratt, J.R. and B.B. Hicks. 1973. Momentum, heat and water vapour transfer to and from natural and a r t i f i c i a l surfaces. Quart. J. R. Met. Soc. 99: 680-687. Gates, D.M. 1980. Biophysical Ecology. Springer-Verlag, New York. J a r v i s , P.G., G.B. James and J. J . Landsberg. 1976. Coniferous f o r e s t . pp. 171-240. In: J.L. Monteith (ed.) Vegetation and the Atmosphere. Vo l . 2, Case Studies, Academic Press, New York. Landsberg, J . J . and D.B.B. Powell. 1973. Surface exchange c h a r a c t e r i s t i c s of leaves subject to mutual interference. Agric. Met. 12: 169-184. Lindroth, A. 1984. Seasonal v a r i a t i o n i n pine forest evaporation and canopy conductance. Ph.D. thesis, Univ. of Uppsala, Uppsala, Sweden. Monteith, J.L. 1965. Evaporation and environment. Symp. S o c Exp. B i o l . XIX: 205-234. Novak, M.D. and T.A. Black. 1985. Theoretical determination of the surface energy balance and thermal regime of bare s o i l s . Boundary-Layer Met. (i n press). Plamondon, A.P. 1972. Hydrologic properties and water balance of the forest f l o o r of a Canadian west coast watershed. Ph.D. thesi s , Univ. of B.C., Vancouver, B.C. Raupach, M.R. and A.S. Thorn. 1981. Turbulence i n and above plant canopies. Ann. Rev. of F l u i d Mech. 13: 97-129. Roberts, J. C.F. Pymar, J.S. Wallace and R.M. Pitman. 1980. Seasonal changes i n leaf area, stomatal conductance and tr a n s p i r a t i o n from bracken below a forest canopy. J . Appl E c o l . 17: 409-422. Rutter, A.J., K.A. Kershaw, P.C. Robins and A.J. Morton. 1971. A pre d i c t i v e model of r a i n f a l l i n t e rception i n fores t s , 1. Derivation of the model from observations i n a plantation of Corsican pine. A g r i c . Met. 9: 367-384. Santantonio, D., R.K. Herman and W.S. Overton. 1977. Root biomass studies i n forest ecosystems. Pedobiol. 17: 1-31. Shuttleworth, W.J. 1978. A s i m p l i f i e d one-dimensional t h e o r e t i c a l d e s c r i p t i o n of the vegetation-atmosphere i n t e r a c t i o n . Boundary-Layer Met. 14: 3-27. Shuttleworth, W.J. 1979. Below-canopy fluxes i n a s i m p l i f i e d one-dimensional t h e o r e t i c a l description of the vegetation-atmosphere i n t e r a c t i o n . Boundary-Layer Met. 17: 315-331. Spittlehouse, D.L. 1981. Measuring and modelling forest evapotranspiration. Ph.D. thesis, Univ. of B.C., Vancouver, B.C. - 75 - Spittlehouse, D.L. and T.A. Black. 1980. Evaluation of the Bowen ratio/energy balance method for determining forest evapotranspiration. Atmos-Ocean 18: 98-116. Spittlehouse, D.L. and T.A. Black. 1981. A growing season water balance model applied to two Douglas-fir stands. Water Resour. Res. 17: 1651-1656. Spittlehouse, D.L. and T.A. Black. 1982. A growing season water balance model used to p a r t i t i o n water use between trees and understory. pp. 195-214. In: Proc. Can. Hydrol. Symp. 82, Hydrol. processes i n f o r . areas. June 14-15, 1982, Fredericton, N.B. S t a n h i l l , G. 1969. A simple instrument for the f i e l d measurement of turbulent d i f f u s i o n f l u x . J . Appl. Met. 8: 509-513. Stewart, J.B. and A.S. Thorn. 1973. Energy budgets in a pine f o r e s t . Quart. J. R. Met. Soc. 99: 154-170. Szeicz, G., G. Endrodi and S. Tajchman. 1969. Aerodynamic and surface factors i n evaporation. Water Resour. Res. 5: 380-394. Tan, C S . and T.A. Black. 1976. Factors a f f e c t i n g the canopy resistance of a Douglas-fir f o r e s t . Boundary-Layer Met. 10: 475-488. Tan, C.S., T.A. Black and J.U. Nnyamah. 1977. C h a r a c t e r i s t i c s of stomatal d i f f u s i o n resistance i n a Douglas-fir forest exposed to s o i l water d e f i c i t s . Can. J. For. Res. 7: 595-604. Tan, C.S., T.A. Black and J.U. Nnyamah. 1978. A simple d i f f u s i o n model of t r a n s p i r a t i o n applied to a thinned Douglas-fir stand- E c o l . 59: 1221-1229. Thom, A.S. 1972. Momentum, mass and heat exchange of vegetation. Quart. J . R. Met. Soc. 98: 124-134. Thom, A.S. 1975. Momentum, mass and heat exchange of plant communities. pp. 57-109. In: J.L. Monteith (ed.) Vegetation and atmosphere. Vol.1, P r i n c i p l e s , Academic Press, New York. Van Wijk, W.R. and D.A. De V r i e s . 1963. Periodic temperature v a r i a t i o n , pp. 102-143. In: W.R. Van Wijk (ed.) Physics of Plant Environment. North Holland Publishing Co., Amsterdam. - 76 - CONCLUSIONS S a l a l understory removal resulted i n l i t t l e change i n the growing season course of 8 because E was only s l i g h t l y higher where s a l a l was present than where i t had been removed. The s l i g h t increase i n 8 where s a l a l had been removed corresponded to s i g n i f i c a n t l y higher V s  at low values of 8 owing to the steepness of the s o i l water retention curve. This resulted i n s i g n i f i c a n t l y greater tree water use and diameter growth where s a l a l had been removed than where i t remained. Shuttleworth's evapotranspiration theory when modified for use i n hypostomatous canopies and combined with canopy and root zone water balance equations proved to be p r a c t i c a l i n c a l c u l a t i n g the courses of 0, y s  and tree t r a n s p i r a t i o n over extended growing season periods. The d i f f i c u l t y i n using the theory i s i n estimating the transfer resistances ( r s j , r D l , r a i > and forest f l o o r d i f f u s i v e r e s i s t a n c e ) . S i m p l i f y i n g the theory when = 0 by using the l i m i t r ^ •*• 0 resulted In an overestimate of understory t r a n s p i r a t i o n and an underestimate of 8, f s and tree t r a n s p i r a t i o n i n uncut subplots. This s i m p l i f i c a t i o n r e s u l t e d i n l i t t l e change In the ca l c u l a t i o n s for cut subplots. Calculations showed that most of the s a l a l removal e f f e c t to increase tree t r a n s p i r a t i o n occurred during the f i n a l one-half of drying periods owing to the steepness of the retention curve at low values of 8 leading to stomatal closure and a reduction i n tree t r a n s p i r a t i o n where s a l a l remained. Increases i n tree t r a n s p i r a t i o n rates as a r e s u l t of s a l a l removal were calculated to be greatest where s a l a l leaf area Index was highest and trees were l a r g e s t . - 77 - The evapotranspiration theory was developed for extensive homogeneous surfaces so that i t s use i n the cut subplots Included employment of below-tree-canopy values of D and T a j r l a r g e l y determined by the presence of s a l a l i n the surrounding f o r e s t . Values of below-tree-canopy D and ^ a i r would be expected to be higher following extensive s a l a l removal; however, i t i s d i f f i c u l t to estimate the magnitude of the increase. McNaughton and J a r v i s (1983) show that the D above a conifer forest canopy i s l i k e l y to be well coupled to the outer mixed portion of the planetary boundary layer. Consequently, they argue that a 50% reduction i n forest leaf area index would not r e s u l t In an Increase In above-forest D and therefore would r e s u l t i n a s i g n i f i c a n t reduction i n forest E. Since D below the tree canopy i n t h i s study was well correlated to that above ( i n agreement with the r e s u l t s reported by Stewart (1984) for Thetford f o r e s t ) , i t i s probable that only a s l i g h t increase i n below-tree-canopy D would result from extensive s a l a l removal. The mechanism for this below to above-tree-canopy D coupling may be a t t r i b u t e d to the gust penetration phenomenon observed i n the U r i a r r a forest by Bradley et a l . (1985). Recent experimental evidence from 30 m by 40 m plots In a s i m i l a r Douglas-fir forest about 130 km south of the experimental s i t e also showed l i t t l e change i n the growing season course of 8 and that E was only s l i g h t l y higher where s a l a l was present than where i t had been removed (Black et a l . 1985). Operational removal of s a l a l understory would l i k e l y require the use of herbicides as regrowth could be considerable. At the end of the f i r s t and second year following s a l a l understory removal i n a 50 m area adjacent to the four p l o t s , s a l a l leaf area Index was 1.0 and 1.6 ( o r i g i n a l l y , i t was 3.7). A l t e r n a t i v e l y , Black and Spittlehouse (1981) suggested that, for dry s i t e s with s a l a l understory, the r a t i o of Douglas-fir to stand t r a n s p i r a t i o n - 78 - ( i n c l u d i n g s a l a l understory) may be increased by increasing stocking density. In young Douglas-fir stands close to and including the one used i n this study, increased stand basal area corresponded to decreased s a l a l understory leaf area index ( F i g . C.l) and increased s a l a l leaf s i z e ( F i g . C.2). The former r e s u l t would d i r e c t l y decrease the amount of s a l a l t r a n s p i r a t i o n while the l a t t e r , when combined with the l i k e l y decreased below-tree-canopy wind speeds i n the denser tree stands, would tend to increase s a l a l r ^ i and thus decrease s a l a l t r a n s p i r a t i o n . The r e s u l t s of t h i s study indicate that on dry, salal-dominated s i t e s where s a l a l control i s not f e a s i b l e , tree water use and wood produciton may be jeopardized i n low density stands. REFERENCES Black, T.A. and D.L. Spittlehouse. 1981. Modeling the water balance for watershed management, pp. 117-129. In: D.M. Baumgartner (ed.) Proc. syrap. i n t e r i o r west watershed mgt. Apr. 8-10, 1980, Spokane, WA, U.S.A. Black, T.A., D.T. P r i c e , P.M. Osberg and D.G. G i l e s . 1985. E f f e c t of reduction of s a l a l competition on evapotranspiration and growth of early stage Douglas-fir plantations. Contract Research Report to Research Branch, B r i t i s h Columbia Min. of For., V i c t o r i a , B.C. Bradley, E.F., O.T. Denraead and G.W. T h u r t e l l . 1985. Measurements of turbulence and heat and moisture transport i n a forest canopy. Quart. J. Roy. Met. S o c ( i n preparation). McNaughton, K.G. and P.G. J a r v i s . 1982. pp. 1-47. In: T.T. Kozlowski (ed.). Water D e f i c i t s and Plant Growth, Vol. VII. Academic Press, New York. Stewart, J.B. 1984. Measurement and p r e d i c t i o n of evaporation from forested and a g r i c u l t u r a l catchments. A g r i c Water Mgt. 8: 1-28. - 79 - CM i _ CVJ < LU < < _IUJ CO < 0 1 1 1 1 • 1 1 ^•^ 1 1 1 1 1 i 12 20 28 36 D O U G L A S - F I R S T A N D B A S A L A R E A ( m 2 h a 1 ) Figure C l Relationship between s a l a l leaf area index and Douglas-fir stand basal area In 31-36 year old stands close to and in c l u d i n g the one at the experimental s i t e . The curve indicates s a l a l leaf area index = 288 (Douglas-fir stand basal area (m2 h a - 1 ) ) - 1 ' 5 7 (R 2 = 0.89). - 80 - D W E 3 2 E Li_ O co LU < LU LU < cr > < cr LU < < < LU CO > 16 0 1 i i i i i — • • • — • — i i i i • 2 0 2 8 3 6 D O U G L A S - F I R S T A N D B A S A L A R E A ( m 2 h a H ) Figure C.2 Relationship between the average area of s a l a l leaves and Douglas-fir stand basal area i n 31-36 year old stands close to and i n c l u d i n g the one at the experimental s i t e . The l i n e indicates average area of s a l a l leaves (mm2 l e a f - ) = 0.41 (Douglas-fir stand basal area (m2 h a - 1 ) ) + 16 (R 2 = 0.41). - 81 - APPENDIX I DERIVATION OF EQUATION (1) IN CHAPTER 2 - 82 - Appendix I DERIVATION OF EQUATION (1) IH CHAPTER 2 The derivation i s e s s e n t i a l l y the same as that given i n Shuttleworth (1979) (p. 321). Applying Ohm's Law to the e l e c t r i c a l analog of the model for canopy layer 1 shown i n F i g . A I . l , the relationships between fluxes and resistances can be written as T i " V l " ^ a H i ^ p ( A i a > T i - 1 " T s i = " ( % " H i _ 1 ) ( r H 1 / 2 a 1 ) / p c p (AI.2) * ± " e*(T| ±) = - ( L E i r a V 1 Y / p c p ) -(LE^-LE^_^) ( r ^ + ( r V i / 2 a ± ) ) y/pc p (AI.3) where T|^ , the e f f e c t i v e temperature of the leaves i n layer i , can be calculated by sub s t i t u t i n g ( A I I . l ) , (All.20) (see Appendix II) and the sensible heat transfer equation into the energy balance equation for the layer. Using the Penman transformation, we have e i - e ^ T f i ) - D t + 8 ( T i - Ti.x) + s C T ^ - T ^ ) (AI.4) - 83 - e : A r aVi AW—'VW *<T8'> si r H i A W aHi L E i- l H i - l Figure A I . l E l e c t r i c a l analog depicting the transfer of latent and sensible heat fluxes for a single canopy layer i (LEj and H^) where i s the " e f f e c t i v e " surface temperature of the layer ( i . e . wet and dry portions) and other symbols have been previously defined. The depiction shows the i d e n t i c a l l e a f , single source l i m i t of the Penman-Monteith equation described by Shuttleworth (1979). - 84 - where, since - i s not large, the same value of s i s used for both temperature d i f f e r e n c e s . The energy balance equation for a l l layers 1 to i (neglecting energy storage) i s R n l - G = Hi + L E i (AI.5) Su b s t i t u t i n g ( A I . l ) , (AI.2) and (AI.3) into (AI.4), using (AI.5) and d i v i d i n g by L, we have s ( R n i - G) + p c p ( D i + ep/TAm EJ = (AI.6) Us + Y ( r A V i / r A H i ) ( l + ( r c i / r m ) ) ) where 6 i = [ L E i - i t ( r c i + ( r v i / 2 a i ) ) Y + ( r H i / 2 a i ) s ] - s C * ^ " G ) ( r H i / 2 a i ) ] / p c p , (AI.7) rAHi = r H i / 2 a i + r a H i and r A V i = r V i / 2 a i + r a V i ' Equation (AI.6) i s the same as (9) i n Shuttleworth (1979) . Subtracting E j _ i from (AI.6) and assuming s i m i l a r i t y ( i . e . r ^ i • r y i and r a H i = t a y i ) gives (1). - 85 - APPENDIX II DERIVATION OF THE EQUATION FOR r c SHOWING DEPENDENCE ON THE FRACTION OF WET LEAF AREA - 86 - APPENDIX II DERIVATION OF THE EQUATION FOR r c SHOWING DEPENDENCE OH THE FRACTION OF WET LEAF AREA The Penman-Monteith equation for a canopy i s LE = s(R n - G) + p c p D 0 / r H a S + Y r V a / r H a ( 1 + ( r c / r V a ) ) ( A I I . l ) This i s the combination equation for the latent heat flux from an extended isothermal f l a t leaf with one side having boundary layer resistances r ^ a and r y a to sensible heat and vapour transfer respectively and a canopy or surface resistance r c to vapour t r a n s f e r . If r n a = r y a _ s ( R n - G) + pCpD 0/r H a LE — • s + y ( l + (rc/rua)) ( A l l . 2 ) The latent heat f l u x on a ground area basis from a canopy with a projected leaf area index (a) and a f r a c t i o n of the leaf area wet (W) can be written as LE = Wa + (1-W) a s(Rn - G)/a + p c p D o / r H T 0 T s + YrVTOT/rHTOT s(R n - G)/a + p c p D o / r H T 0 T s + ^VTOT^HTOT ( A l l . 3 ) - 87 - T h e f i r s t b r a c k e t t e d t e r m o f ( A l l . 3 ) i s t h e e v a p o r a t i o n r a t e f r o m t h e a v e r a g e w e t l e a f o n a p r o j e c t e d l e a f a r e a b a s i s , t h e s e c o n d b r a c k e t t e d t e r m is t h e t r a n s p i r a t i o n r a t e f r o m t h e a v e r a g e d r y l e a f o n a p r o j e c t e d l e a f a r e a b a s i s . T h e r e s i s t a n c e s a r e d e f i n e d a s r H T O T _ 1 * ^ T O P " 1 + ^ O T " 1 " 2 / r H « ( A I I > 4 ) r ' ~* - r , T n , ~ l + r „ . - 1 = 2 / r „ , ( A l l . 5 ) VTOT VTOP VBOT V v ' r V T O T _ 1 " ( r V T O P + r s T O P r l + ( r V B O T + SBOT 5 " 1 ' ( A l l . 6 ) w h e r e T O P , BOT a n d TOT r e f e r t o t o p , b o t t o m a n d t o t a l r e s p e c t i v e l y . E q u a t i o n ( A l l . 3 ) c a n b e r e w r i t t e n a s L E = W s ( R n - G ) + p C p D 0 / ( r H T o x / a ) 8 + Y r V T O T / r H T O T + ( 1 - W ) s ( R n - G ) + P C p D Q / ( r H T 0 T / a ) 6 + Y r V T O T ^ r H T O T ( A l l . 7 ) I n o r d e r t h a t ( A l l . 3 ) c a n b e r e w r i t t e n i n t h e f o r m o f ( A I I . l ) b y m a k i n g r c a f u n c t i o n o f W, we r e q u i r e r H a " r H T O T / a " r H / ( 2 a ) ( A l l . 8 ) - 88 - and W 1-W S + Y r V a / r H a ( 1 + < r c / r V a » S + ^ v T O T / r H T O T S + ^ V T O T / r H T O T ( A l l . 9 ) Solving for r c we have W 1-W ( s / y ) r H a + r ' V T 0 T / a ( s / y ) ^ + r ^ -1 - ( s / y ) r H a - r V a (All.10) where rVsa = rVTOT/ 3 (All.11) In order that r c = 0, when W = 1, r v a must be rVa = rVT0T / a = r V / ( 2 a ) (All.12) S u b s t i t u t i n g (All.12) into (All.10) we have r = c W 1-W (s/y)r + r (s/y)r + r Ha Va Ha Vsa -1 - (s/y)r - r (All.13) Ha Va with the resistances given by ( A l l . 8 ) , (All.11) and ( A l l . 1 2 ) . For amphistomatous leaves, ryxoT i s given by ( A l l . 6 ) , but i f rĝ op = rsB0T = r s a n d rVT0P = rVBOT = rV» s o t h a t - 89 - then rVTOT = ( t V + r s ) / 2 (All.14) rVsa = ( r V + r s > / ( 2 a ) (All.15) For hypostomatous leaves with, say, r STrjp = 0 0 and r SBQT = rs> then from ( A l l . 6 ) so that rVTOT = TV + r s (All.16) rVsa = ( r V + r s ) / a ( A I I * 1 7 ) When W = 1, r c = 0 for both leaf types. When W = 0, r c for the amphistomatous leaf i s r c = r s/2a (All.18) while for the hypostomatous leaf i t i s r c = r 8 / a + r v / 2 a (All.19) which i s the sum of the average stomatal resistance and average boundary layer resistance of the canopy. Equation (All.19) i s correct since without the boundary layer resistance term, halving r s of the hypostomatous leaf would r e s u l t i n the same t r a n s p i r a t i o n as that of an amphistomatous leaf - 90 - with the same i n i t i a l r g (see Appendix I I I ) . In the hypostomatous leaf a l l vapour has to go through a single ry (one side) rather than two rv's i n p a r a l l e l as In the amphistomatous case. In summary, r c , as defined by ( A I I . l ) , for the hypostomatous Douglas-fir canopy of t h i s study can be written as follows (after s u b s t i t u t i n g ( A l l . 8 ) , (All.12) and (All.17) into (All.10)) W + 1 - w -1 r c (s/Y)r H/(2a) + r y / ( 2 a ) ( s / y ) r H / ( 2 a ) + ( r y + r g ) / a - ( s / Y ) r H / ( 2 a ) - r y / ( 2 a ) (All.20) where r ^ , ry and r s are the one-sided resistances of the l e a f . Note we have made the reasonable assumption that the bottom and top boundary layer resistances are equal i n ( A l l . 4 ) and ( A l l . 5 ) . - 91 - APPENDIX III RELATIONSHIP BETWEEN STOMATAL RESISTANCES OF AMPHISTOMATOUS AND HYPOSTOMATOUS LEAVES THAT RESULTS IN EQUAL TRANSPIRATION RATES - 92 - APPENDIX III RELATIONSHIP BETWEEN STOMATAL RESISTANCES OF AMPHISTOMATOUS AND HYPOSTOMATOUS LEAVES THAT RESULTS IN EQUAL TRANSPIRATION RATES This appendix derives the expression for the stomatal resistance of an amphistomatous leaf i n terms of that of a hypostomatous leaf which would re s u l t i n equal t r a n s p i r a t i o n rates for the two leaves. Following Monteith (1973), LE from an isothermal leaf can be estimated from the Penman-Monteith equation written as where n = 1 or 2 for amphistomatous (rsxop = rsBOT = r s ) o r hypostomatous leaves, r e s p e c t i v e l y . Assuming ry = r ^ , ( A I I I . l ) can be written for amphistomatous leaves as LE = s(R n - G) + pc pD/(r H/2) ( A I I I . l ) s + n y r v / r H ( l + r g / r v ) LE = s(R n - G) + pc D/(r /2) n p rl (AIII.2) s + Y d + (r /2)/(r„/2)) and for hypostomatous leaves as LE = s(R - G) + pc D/(r /2) n p n (AIII.3) s + Y d + ( r g h + r R / 2 ) ) ( r H / 2 ) - 93 - where r s a and r s n are the stomatal resistances of amphistomatous and hypostomatous leaves r e s p e c t i v e l y . Comparing (AIII.2) and (AIII.3), we see that for t r a n s p i r a t i o n rates for the two leaves to be equal r 12 = r , + r n/2 sa sh H or r = 2r + r u . sa sh H If the apmhistomatous leaf equation i s used to calculate LE from a hypostomatous l e a f , the stomatal resistance of t h i s "equivalent" amphistomatous leaf must be calculated using (AIII.5). REFERENCES Monteith, J.L. 1973. P r i n c i p l e s of environmental physics. American E l s e v i e r Publ. Co., Inc., New York. (AIII.4) (AIII.5) - 94 - APPENDIX IV DERIVATION OF EQUATION (5) IN CHAPTER 2 - 95 - APPENDIX IV DERIVATION OF EQUATION (5) IN CHAPTER 2 The water vapour flux density from layer i (Ep with leaf area (one side) index (a^) within a multilayer forest canopy of hypostomatous leaves is given by s ( R n i - R n i_!) + pc p D i _ 1 / ( r H i / 2 a i ) E' = (AIV.l) L(s + Y ( r v l / r H i ) ( l + ^ / ( r ^ a ^ ) where D^_i i s the vapour pressure d e f i c i t within the layer ( i . e . at the e f f e c t i v e "source" height). This can be obtained by se t t i n g r a ^ = 0 (e.g. r a H i = r a vi =0) i n (1), (2) and (3) of Chapter 2. The rate of evaporation of intercepted water from layer 1 (E-J^) i s obtained by multip l y i n g the f r a c t i o n of leaf area (one side) that i s completely wet (Wj.) by (AIV.l) with r c i = 0 which gives W i t s ( R n i - Rni-l) + P^p D ^ / ( r H i / 2 a i ) ] E l = (AIV.2) L(s + Y ( r v i / r H i ) ) D i v i d i n g (AIV.2) by (AIV.l), assuming s i m i l a r i t y ( i . e . r H i = r V i = r b i ) , making use of (4) and rearranging gives (5). - 96 - APPENDIX V ELECTRICAL ANALOGS OF THE LATENT AND SENSIBLE HEAT FLUXES IN UNCUT AND CUT SUBPLOTS - 97 - APPENDIX V ELECTRICAL ANALOGS OF THE LATENT AND SENSIBLE HEAT FLUXES IH UNCUT AND CUT SUBPLOTS The purpose of t h i s appendix i s to present e l e c t r i c a l analogs depicting the transfer of latent and sensible heat fluxes for uncut and cut subplots (F i g s . AV.l and AV.2). The forest canopy was divided into two layers (n = 2) where the Douglas-fir subcanopy was designated as layer 2 and the s a l a l subcanopy as layer 1 (the forest f l o o r was layer 0). Individual leaves were considered either completely wet or completely dry so that wet and dry leaves had d i f f e r e n t surface temperatures ( T ^ and T|̂ ). For cut subplots, forest f l o o r aerodynamic resistances were estimated by r a v i and rafli« - 98 - LE H r o V 2 a V i r V a 2 r s 2 / a 2 -A/W—WV—e (Ts a 2) * IT* \ e (Ts2) ' V o 2 ViV - ' V o l r s l / Q l -AM,—m— e* (Tj,) r V a l * w -AM- e (Ts,) ' s 2 - l s 2 ' s i ' s i ' H o 2 r H o 2 ' H a l -AM- r H a l -AMr r o H 2 r o H I LE, Figure AV.l E l e c t r i c a l analog depicting the transfer of latent and sensible heat fluxes i n an uncut subplot. - 9 9 - L E roV2 W r s 2 / Q 2 - , -WWVW-e*(T s d 2) rVo2 -̂ vw '(T.S) 's2 •s2 'Ho2 - / V W - rHo2 - / W v - roH2 'oVI roHI LE, H, Figure AV.2 Same as for F i g . AV.l except for cut subplot. - 100 - APPENDIX VI UNDERSTORY REMOVAL EFFECTS ON THE BELOW-TREE-CANOPY RADIATION REGIME - 101 - APPENDIX VI UNDERSTORY REMOVAL EFFECTS ON THE BELOW-TREE-CANOPY RADIATION REGIME 1. INTRODUCTION The r a d i a t i o n balance below the tree canopy i s Important i n determining the rate of s o i l evaporation and the t r a n s p i r a t i o n rate of understory vegetation. In th i s appendix, below-tree-canopy r a d i a t i o n data c o l l e c t e d as part of the s a l a l understory removal experiment from July 24-September 1, 1981 and July 30-September 1, 1982 are presented. Sky view factors determined from photographs taken with a f i s h eye lens are also presented. 2. METHODS 1. Radiation Measurements Net r a d i a t i o n and solar Irradiance above the forest ( R n a and K+ a) were measured using a Swissteco S-l net radiometer and Kipp and Zonen pyranometer located on top of a 15 m t a l l tower. In 1981, hourly average values of net r a d i a t i o n below the tree canopy (R nb) were measured i n one of the four plots (plot 2) using one net radiometer above the s a l a l canopy and another above the forest f l o o r surface, where s a l a l had been removed. Measurements of solar irradiance below the tree canopy (K+ b) were also made i n 1981 with a pyranometer located next to the net radiometer over the forest f l o o r surface. - 102 - In 1982, and K+jj were measured using a net radiometer and pyranometer mounted on a tram t r a v e l l i n g at a rate of about 0.5 m min - 1 at the 1 m height along a 10 m path where s a l a l along a portion of the path had been removed. Measurements of Rn a, Riib> K+a a n ^ ^*b were made every 2 seconds and 10 second average values were recorded using a Campbell S c i e n t i f i c CR-21 data logger. A control c i r c u i t for the tram provided a p o s i t i v e or negative voltage i n d i c a t i n g the d i r e c t i o n of t r a v e l by the tram. This voltage was also recorded every 10 seconds using the data logger. A negative voltage indicated that the tram was traversing to the west ( i . e . to the cut ( s a l a l cut and removed) portion of the path) while a p o s i t i v e voltage Indicated the reverse ( i . e . to the uncut portion of the path). A change i n the sign of the voltage indicated that the tram had Just passed one end of the path and had reversed i t s d i r e c t i o n toward the other end. Using the rate and d i r e c t i o n of t r a v e l and the time when the tram passed the end of the path, periods were designated when the tram passed over the cut and uncut portions of the path. This was the period when the tram was located between a i m buffer east (uncut) or west (cut) of the cut/uncut border and the end of the path. The tram t r a v e l l e d over the cut or uncut portion of the path twice during the period, going to and from the end of the path. The period was about 13 minutes for the cut portion (distance from west end of path to cut/uncut border was A.2 m) and about 19 minutes for the uncut portion (distance from east end of path to the cut/uncut border was 5.8 m). - 103 - On September 1, 1982, the tram was run i n conjunction with a net radiometer next to the tram path i n the cut portion. A l l r a d i a t i o n measurements on t h i s day were made every two seconds and recorded by the data logger as 10 second average values. 2. Determination of Sky View Factors The view factor V 1 2 Is defined as follows: q 1 2 = V 1 2 A l E l (AVI.l) where q 1 2 Is the flux emitted by area Ax toward area A 2, and ej i s the emittance of A^ and V 1 2 i s the f r a c t i o n of A ^ that i s intercepted by A 2 or the f r a c t i o n of the view by fi^ that i s occupied by A 2. The general expression for V j 2 can be obtained as follows: d q i 2 =» li dA± cos <}> doo (AVI.2) where <f> i s the angle between the normal to dAj and the straight l i n e connecting dA.^ and dA 2, dto i s the small s o l i d angle given by dA 2 divided by the square of the distance between dA^ and dA 2, and 1^ i s the radiance from A x which i s related to el by (Reifsnyder 1967): I x = ex/n (AVI.3) so that dqj.2 = (EiAOdAi cos $ dm (AVI.4) - 104 - Integrating over and A 2 we have q 1 2 - (e^ir) J / dAj cos (J) du 1 A2 (AVI.5) Assuming A^ i s small, (AVI.6) From (AVI.l) we see that v12 = / A cos <j> doi/ir (AVI.7) A2 as given in (6) of Reifsnyder (1967). A hemisphere of radius r Q can be constructed over a small area on the forest f l o o r (dAj ) ( F i g . AVI.l) to give the forest floor-sky view factor ( V f s ) TT/2 ,2̂  o v f s = / / c o s • f U » a ) r o s i n * d a ro d"t'/( i r ro) (AVI.8) <j)=0 a=0 where a i s the azimuthal angle, f(<{>,a) i s the f r a c t i o n of dA2 not obscured by the forest canopy and dto i s replaced by d A 2 / r D . Taking f (<J>) to be an average non-obscuration f r a c t i o n for an entire annulus (0 < a < 2TT), (AVI.8) can be rewritten as: - 105 - Figure AVI.l Construction of a hemisphere of radius r D over a small area of forest f l o o r (dA^). The angle $ between the normal to dAx and the s t r a i g h t l i n e connecting dAĵ  and a small area on the hemisphere (dA 2) i s T T / A radians. Using the azimuthal angle a and the derived geometric r e l a t i o n s h i p s shown in t h i s f i g u r e , dA2 = r Q s i n <|> da r 0 d<j>. The p a r t i a l obscuration of dA2 i n d i c a t e s the forest canopy. - 106 - v f s = 2 / 1 T / 2 f (•) c o s • 8 i n • d<t> (AVI.9 ) An equiangular fish-eye lens projection r e s u l t s i n an image of an object at an angle 4> from the normal being located at a distance from the centre of the p rojection (r) defined by r = 2<J> r 0 / 7 r (AVI. 10) which indicates the p r o p o r t i o n a l i t y between r a d i a l distance on the projection (photograph) and <|>. Consequently, d<J> = [ i r / ( 2 r 0 ) ] d r . (AVI.11) Expressing (AVI.9) as a sum of n annuli (Steyn 1980) and using (AVI.10) and the f i n i t e difference form of (AVI.11) gives v f s " (*/*<>> ^ f ' ( r i ) cos ( i r / 2 ( r 1 / r 0 ) ) s i n ( T t / 2 ( r i / r 0 ) ) hr± (AVI.12) where f ' ( r ^ ) (« f(<t>i)) i s the average non-obscuration f r a c t i o n for the annulus rj_, of width Arj_, on the projection (eg. photograph). Using constant width annuli Ar^ = A r 0 , (AVI.12) becomes - 107 - V f s - (n/n) I f ' ( r i ) cos ( T T / 2 ( T ± / T Q ) ) s i n ( T r / 2 ( r i / r 0 ) ) (AVI.13) 1=1 where r\ - r 0 / A r 0 . The forest floor-canopy view factor ( V f c ) i s equal to 1 - V f s . Photographs taken using an equiangular fish-eye lens were divided into three annuli 0-r 0/3, r Q / 3 - 2 / 3 r 0 and 2 / 3 r 0 - r 0 which correspond to the 0-30°, 30-60° and 60-90° zenith angles r e s p e c t i v e l y . The midpoint of each range was taken as r for the annulus ( i . e . r 0 / 6 , r D/2 and 5 r 0 / 6 ) . The 0-r o/3 annulus was divided into nine equal size sectors while the r 0 / 3 - 2 / 3 r 0 and 2 / 3 r 0 - r 0 annuli were each divided into eighteen equal size sectors. Using a clear p l a s t i c sheet with the annuli and sectors marked on i t ( F i g . AVI.2) over the photograph (eg. F i g . AVI.3), the f r a c t i o n of sky v i s i b l e i n each sector of the photograph was estimated by eye to 0.05 and the average value of a l l sectors i n each annulus was taken as f (rj_). The forest f l o o r sky view factor was determined using (AVI.13). Ten black and white photographs were taken with a f i s h eye lens (using Kodak PX-135 panchromatic f i l m , 125 ASA) i n the stand on November 23, 1983. Photographs were taken from about a 300 mm lens height at the locations of the 1981 R n b measurements in plot 2 ( i . e . two photographs), the centres of the three other plots and from about the 1 m height along the tram path ( f i v e photographs). Pr i n t s with a 78 mm diameter c i r c u l a r f i e l d were used i n the analysis ( F i g . AVI.3). - 108 - Figure AVI.2 Grid used for analyses of f i s h eye lens photographs of radius r Q . The 0-r o/3 annulus of the grid was divided into nine equal size sectors while the r 0 / 3 - 2r Q/3 and 2r Q/3 - r Q annuli were each divided into eighteen equal size sectors. log - Figure AVI.3 F i s h eye lens photograph taken at about the 300 mm height from the 1981 location of the net radiometer (below the tree canopy) i n the cut subplot of plot 2 on November 23, 1983. The forest floor-sky view factor for t h i s l o c a t i o n was 0.28. - 110 - 3. RESULTS In August 1981, d a l l y t o t a l R n D In plot 2 was generally s i m i l a r for the s a l a l canopy (uncut subplot) and forest f l o o r (cut subplot) except near the end of the month when i t was very cloudy ( F i g . AVI.A). The r a t i o R n D/R na w a s f a i r l y constant during t h i s period with the average values being 0.15 and 0.13 for the s a l a l canopy and forest f l o o r r e s p e c t i v e l y . The sky view factor for the net radiometer over the s a l a l canopy was s l i g h t l y greater than that for the net radiometer over the forest f l o o r (Table A V I . l ) . For the forest f l o o r , there was l i t t l e difference between values of Rnb/ Rna and K+b/K+a ( F i g . AVI.A). In 1982, d a i l y t o t a l values of R n D for the uncut portion of the tram path were usually s l i g h t l y higher than for the cut portion (Table AVI.2). The sky view factor and values of K+D/K+a for the uncut portion of the tram path were also higher than for the cut portion (Tables AVI.l and AVI.2). On two clear days (August 7 and 18), R nb/ Rna a n d K + b / K + a were s i g n i f i c a n t l y higher i n the uncut portion. Average values of the r a t i o K+D/K4-D (i«e« below-tree-canopy albedo) for August 10-19 were 0.28 and 0.19 for the s a l a l canopy and forest f l o o r r e s p e c t i v e l y . There was a downward trend i n the values of R nb/ Rna a n d K+i,/K4-a from July 30-Septeraber 1. The zenith angle of the sun at noon decreased from 31° on July 30 to 42° on September 1 ( L i s t 1971). However, the average values of Rnb/Rna for the uncut and cut portions of the tram path (0.16 and 0.1A, res p e c t i v e l y ) were very s i m i l a r to those values obtained i n 1981 using stationary net radiometers. - I l l - AUG 6 AUG 14 AUG 22 AUG 30 1981 Figure AVI.4 Courses of solar irradiance (top l i n e ) and net r a d i a t i o n flux density above the forest (K+ a and R n a ) for the period July 31 - September 1, 1981. Also shown are the courses of the r a t i o of below to above tree canopy solar irradiance (K+b/K4-a) for the cut subplot of plot 2 and the r a t i o of below to above tree canopy net r a d i a t i o n flux density ( R n b / R n a ) for the cut ( ) and uncut ( ) subplots of plot 2 during the same period. - 112 - Table AVI. 1. Sky view factors (S.V.F.) determined from photographs taken with a f i s h eye lens i n each of the four plots and along the path traversed by the tram. LOCATION S.V.F. Centre of plot 1 0.22 Centre of plot 3 0.24 Centre of plot 4 0.26 Net radiometer i n subplot 2 uncut (1981) 0.30 Net radiometer in subplot 2 cut (1981) 0.28 West end of tram path 0.25 1 m west of tram path's cut/uncut border 0.21 1 m east of tram path's cut/uncut border 0.19 East end of tram path 0.25 Net radiometer adjacent to tram path i n the cut portion on September 1, 1982 0.23 - 113 - Table AVI.2. Daily t o t a l values of the r a t i o of below to above tree canopy net r a d i a t i o n f l u x density (R nb/Rna) a n a < solar irradiance (K+ D/K+ a) ^ o r t n e c u t a n c * u n c u t portions of the tram's path for seven days i n 1982. Also shown i s the solar irradiance and net r a d i a t i o n f l u x density above the forest (K+ a and Rn a) for the same days. 1982 Rnb / Rna K+b/K+a K+ a Rna Uncut Cut Uncut Cut (MJ m" J u l . 30 0.21 0.19 0.17 0.16 8.6 5.7 Aug. 4 0.18 0.17 0.14 0.13 15.8 8.9 7 0.18 0.10 0.17 0.11 26.2 15.4 17 0.15 0.15 0.13 0.12 19.5 10.5 18 0.14 0.10 0.12 0.09 24.4 13.5 25 0.14 0.13 0.11 0.11 21.5 11.6 Sept. 1 0.13 0.14 0.09 0.09 17.6 11.4 - 114 - During a cloudy day, July 30, 1982, Rnb/^na w a s generally constant ( F i g . AVI.5) while on a clear day, August 18, 1982, Rnb/Rna was quite v a r i a b l e ( F i g . AVI.6). There was no apparent pattern In the v a r i a t i o n of R nb/R na o n t h i s c l e a r day, r e f l e c t i n g the complex canopy geometry of the f o r e s t . On September 1, 1982, values of Rnb/R na were s i m i l a r for most of the day for the tram net radiometer along the cut portion and an adjacent stationary net radiometer i n the cut portion of the tram path ( F i g . AVI.7). The d a i l y t o t a l values of Rnb/ Rna o n t h i s day were 0.14 and 0.12 for the tram and stationary net radiometers. 4. CONCLUSIONS Measurements of d a i l y t o t a l Rnb were s i m i l a r using tram and adjacent stationary net radiometers. The sky view factor was s i m i l a r for a l l four p l o t s and the tram path. In the absence of R ^ measurements or a major below-tree-canopy net r a d i a t i o n modelling e f f o r t , the best estimates of R nb/R n a for uncut and cut areas i n the forest would be 0.16 and 0.14, r e s p e c t i v e l y . 5. REFERENCES L i s t , R.J. 1971. Smithsonian meteorological tables (6th ed.). Smithsonian Miscellaneous C o l l e c t i o n s V o l . 114. Smithsonian I n s t i t u t i o n Press, Washington, D.C. Reifsnyder, W.E. 1967. Radiation geometry i n the measurement and i n t e r p r e t a t i o n of r a d i a t i o n balance. Agr. Meteorol. 4: 255-265. Steyn, D.G. 1980. The c a l c u l a t i o n of view factors from fisheye-lens photographs. Atmos.-Ocean 18: 254-258. - 115 - 4 0 0 8 12 hour ( P S T ) Figure AVI.5 Courses of net ra d i a t i o n flux density above the forest ( R n a ) and the r a t i o of below to above tree canopy net r a d i a t i o n flux density (R nb/ Rna) f o r t h e c u t (~ ~ ~) a n d uncut ( ) portions of the tram path on July 30, 1982. - 116 - 10 12 hour (PST) 14 Figure AVI.6 Same as for AVI.5 except for August 18, 1982 - 117 - KP 600 'E 400 h ° 200 o c en or 0 0.3 h 0.2 h 0.1 h 0 10 12 14 h o u r (PST) Figure AVI.7 Same as for AVI.5 except for September 1, 1982 and R n b/R n a for the cut portion of the tram path measured using a single stationary net radiometer (• • • ) • - 118 - APPENDIX VII MEASUREMENTS OF r s IN DOUGLAS-FIR AND SALAL - 119 - APPENDIX VII MEASUREMENTS OF r„ IN DOUGLAS-FIR AND SALAL The purpose of this appendix i s to report r s measurements made, following the procedure of K e l l i h e r et_ a l . (1984), i n 1980 and 1981 adjacent to the four plots at the experimental s i t e . In 1980, measurements were made on June 18, 19 and 30, July 17, 18 and 22 and August 7, 8 and 20. In 1981, measurements were made on July 3 and August 5. On August 20, 1980, Vs was -0.3 MPa and for other days (1980 and 1981) Y s > -0.3 MPa. S a l a l r s was far less responsive to increasing D than Douglas-fir r s i n agreement with the r e s u l t s of Tan e_t al_. (1978) for r s measurements made at the same s i t e i n 1975 ( F i g . A V I I . l ) . However, Douglas-fir r s in 1980 and 1981 increased somewhat more in response to increasing D than in 1975 (the value of £ i n Table 2.1 was 0.27 i n 1975 (Tan et a l . (1978), equation (7a)) and 0.38 in 1980 and 1981). The r s c h a r a c t e r i s t i c functions for T"s of -0.01 and -0.30 MPa (lower and upper curves i n F i g . AVII.l) describe the range of the 1980 and 1981 r s measurements f a i r l y well although there was considerable v a r i a t i o n i n r s for Douglas-fir when D was about 1-1.5 kPa. REFERENCES -Kelliher, F.M., T.A. Black and A.G. Barr. 1984. Estimation of twig xylem water p o t e n t i a l i n young Douglas-fir trees. Can. J. For. Res. 14: 481-487. Tan, C.S., T.A. Black and J.U. Nnyamah. 1978. A simple d i f f u s i o n model of t r a n s p i r a t i o n applied to a thinned Douglas-fir stand. E c o l . 59: 1221-1229. - 120 - o1 1 1 1 1 1 1 I i ' ' ' 0 0.8 1.6 2.4 D (kPo) Figure AVII.l Relationship between stomatal resistance ( r s ) and vapour pressure d e f i c i t (D) i n Douglas-fir and s a l a l adjacent to the four plots at the experimental s i t e for June-August 1980 (0) and 1981 (•) when average root zone s o i l water p o t e n t i a l (fg) was greater than -0.3 MPa and photon flux density was greater than 2.5 and 1.0 m mol m~2 s - 1 for Douglas-fir and s a l a l r e s p e c t i v e l y . Curves show c h a r a c t e r i s t i c r s values for f g = -0.01 (lower curve) and -0.3 MPa (upper curve) (see Table 2.1). - 121 - APPENDIX VIII SOIL WATER RETENTION CURVE - 122 - APPENDIX VIII SOIL WATER RETENTION CURVE The purpose of this appendix i s to report 9 and Y " s measurements used to determine the s o i l water retention curve at the experimental s i t e . Measurements were made i n plot 2 as described i n Chapter 1. An equation of the form proposed by Campbell (1974) was f i t t e d to average root zone values of 9 and Vs Q t a (MPa) = -0.005 (9/0.3)~6-5) ( F i g . A V I I I . l ) . REFERENCES Campbell, G. 1974. A simple method of determining unsaturated conductivity from moisture retention data. S o i l S c i . 117: 311-314. - 123 - Figure AVTII.l Relationship between average root zone s o i l water p o t e n t i a l (^g) and water content (9) i n plot 2. The s o i l water retention curve shown i s f s (MPa) = -0.005 ( 9 / 0 . 3 ) - 6 * 5 .

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