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Vertical spread rate and intesification of dwarf mistletoe in western hemlock 1970

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THE VERTICAL SPREAD RATE AND INTENSIFICATION OF DWARF MISTLETOE IN WESTERN HEMLOCK by KENNETH STANLEY RICHARDSON B.S.F., U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e Department o f F o r e s t r y We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d . THE UNIVERSITY OF BRITISH COLUMBIA O c t o b e r , 1970 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 a v a i l a b l e 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 representative. It i s understood that copying or p u b l i c a t i o n 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 Forestry, The University of B r i t i s h Columbia, Vancouver 8, Canada. October 1st, 1970. X ABSTRACT The vertical rate of spread of dwarf mistletoe was studied in two actively growing, young hemlock stands. This was done by determining the height and age of successive oldest and highest female infections. The rate of spread was cal- culated by dividing the sum of the heights of advances by the total number of years lapse between successive advances. The mean vertical spread rate was 2.1 _+ 0.1 f t . / y r . in a relatively open stand and 1.0 _+ 0.1 f t . / y r . in a relatively dense stand. The mean rate of tree growth during the maximum growth phase in the open stand was 2.5 f t . / y r . and for the dense stand 1.5 f t . / y r . However, over the past 25 years, the growth rate of the trees in the open stand was 1.9 f t . / y r . and for the dense stand 1.1 f t . / y r . The number of new infections per year increased geometrically, doubling every four years in both the dense and open stands. However, the geometric increase levelled off six years ago in the open stand and fi v e years ago in i i the dense stand. During the maximum growth phase of hemlock in an open and dense stand, the most photosynthetically active upper portion of the crown remains free of mistletoe infection. Until the senescent phase i s reached, the trees can be expected to outgrow the mistletoe and intensification w i l l be restricted to the lower portions of the crowns. It i s tentatively concluded that provided there i s no overstory seed source and no disruption of the natural stand, such as thinning, dwarf mistletoe on hemlock w i l l not become serious u n t i l the rate of height growth of the trees f a l l s below the rate of vertical spread, i.e., not u n t i l after the presently accepted rotation age. XXI CONTENTS Page Abstract i Contents i i i Tables i v I l l u s t r a t i o n s v Acknowledgements v i Introduction 1 Li t e r a t u r e Review. 4 Methods 5 Ca l c u l a t i o n of the V e r t i c a l Rate of Spread 7 Ca l c u l a t i o n of the I n t e n s i f i c a t i o n Rate 11 Ca l c u l a t i o n of Tree Growth Rate 11 during the Linear Phase Results and Discussion 12 Heights of Advances and Number of Years Lapse 12 Rates of V e r t i c a l Spread, Tree Growth and 19 I n t e n s i f i c a t i o n Conclusions 26 Li t e r a t u r e Cited 28 TABLES Table Page I Sample tree basic measurements. 8 II Mean and range of heights of advance and 13 number of years lapse between successive advances and the i r 95% confidence l i m i t s . I l l Weighted mean v e r t i c a l spread rate of dwarf 20 mistletoe and mean growth rate of sample trees during the maximum growth phase and for the past 25 years. IV Average length of mistletoe-free crown. 21 V The regression analysis of the i n t e n s i f i c a t i o n 23 rate of dwarf mistletoe i n f e c t i o n s . V ILLUSTRATIONS F i g u r e Page 1 Example o f d e t e r m i n a t i o n o f h e i g h t s o f advance 9 and number o f y e a r s l a p s e . 2 Observed f r e q u e n c y d i s t r i b u t i o n f o r h e i g h t s o f 14 advances. 3 Frequency p o l y g o n o f the o b s e r v e d d i s t r i b u t i o n 15 of t h e number o f y e a r s l a p s e between t h e h e i g h t s of advance. 4 P l o t o f h e i g h t s o f advance o v e r c o r r e s p o n d i n g 17 number o f y e a r s l a p s e . 5 I n t e n s i f i c a t i o n o f dwarf m i s t l e t o e i n t h e 24 open (o) and dense (x) s t a n d s . ACKNOWLEDGEMENTS I would l i k e to express my indebtedness to Dr. B.J. van der Kamp fo r h i s considerable inter e s t and assistance throughout the study; to Dr. K. Graham and Dr. A. Kozak for th e i r review of the manuscript and subsequent suggestions. I would also l i k e to express thanks to my wife, Olga, for the typing of the f i n a l d r a f t . 1 THE VERTICAL SPREAD RATE AND INTENSIFICATION OF DWARF MISTLETOE IN WESTERN HEMLOCK I n t r o d u c t i o n Hemlock m i s t l e t o e ( A r c e u t h o b i u m campylopodum Engelm. forma t s u g e n s i s (Rosend.) G i l l ) , n a t i v e t h r o u g h o u t t h e c o a s t a l r a n g e o f w e s t e r n hemlock (Tsuga h e t e r o p h y l l a ( R a f . ) S a r g . ) , i s r e s p o n s i b l e f o r s e v e r e growth l o s s e s and low wood q u a l i t y i n h e a v i l y i n f e c t e d o v e r m a t u r e and mature s t a n d s ( B u c k l a n d and M a r p l e s , 1952; S m i t h , 1969; and Wellwood, 1956). Wellwood (1956) q u a n t i t a t i v e l y a s s e s s e d t h e d i f f e r e n c e s between t e n u n i n f e c t e d t o h e a v i l y i n f e c t e d o v e r - mature t r e e s a v e r a g i n g 210 y e a r s o l d w i t h a range o f 146 t o 256 y e a r s . He found t h a t s e v e r e l y i n f e c t e d t r e e s were s e r i o u s l y a f f e c t e d i n terms o f volume i n c r e m e n t , v i g o u r and 2 lower wood density. Smith (1969) recently studied the effects of dwarf mistletoe in a hemlock stand with an average age of 110 years. He found that the severely infected trees not only showed the greatest reduction in volume increment but also the greatest losses of height increment. It i s evident that i f maturing hemlock stands lightly to moderately infected with dwarf mistletoe remain unharvested they w i l l suffer increasing injury by the parasite. S i l v i c u l t u r a l procedures for the control of dwarf mistletoe in regenerating hemlock stands were discussed by Buckland and Marples (1952). They stated that since mistletoe occurred only in widely scattered areas in sub- sequent reproduction, the parasite may not constitute a major problem in future rotations i f such stands are managed on an even-aged basis. This conclusion was not substantiated quantitatively in the publication. Smith (1966), however, demonstrated the threat of a 35-ft. infected residual hemlock to regenerating hemlock stands. After trapping 1,962 seeds based on a 4% sampling intensity, he estimated that over 49,000 dwarf mistletoe seeds were dispersed. The dispersal area was 5,800 sq. f t . and the maximum distance of seed dispersal was 49.5 f t . 3 He c o n c l u d e d t h a t i t i s e s s e n t i a l t o remove a l l i n f e c t e d r e s i d u a l t r e e s d u r i n g or i m m e d i a t e l y a f t e r l o g g i n g i n o r d e r t o c o n t r o l m i s t l e t o e damage e f f e c t i v e l y s i n c e an i n f e c t e d 10 f t . t r e e a t t h e t i m e o f l o g g i n g can become a heavy seed p r o d u c e r w i t h i n 25 y e a r s . More q u a n t i t a t i v e d a t a on t h e e p i d e m i o l o g y o f dwarf m i s t l e t o e i n r e g e n e r a t i n g and m a t u r i n g hemlock s t a n d s a r e needed t o d e t e r m i n e the n e c e s s i t y o f c o n t r o l measures as c u t t i n g p r a c t i c e s s h i f t t o e a r l i e r r o t a t i o n s . Damage may be o f l i t t l e consequence i n i n f e c t e d s t a n d s managed on a s h o r t e r r o t a t i o n s i n c e the p a r a s i t e would be p r e v e n t e d f r o m i n t e n s i f y i n g t h r o u g h o u t t h e crown. I t was d e c i d e d , t h e r e f o r e , t o s t u d y an e p i d e m i o - l o g i c a l a s p e c t o f hemlock dwarf m i s t l e t o e i n immature hemlock, v i z . , t h e v e r t i c a l r a t e o f s p r e a d . I f t h e r a t e of t r e e h e i g h t g r o w t h d u r i n g t h e phase of maximum growth r a t e s i g n i f i c a n t l y exceeds t h e r a t e o f dwarf m i s t l e t o e v e r t i c a l s p r e a d then t h e r e w i l l be a s i g n i f i c a n t p r o p o r t i o n o f t h e most p h o t o s y n t h e t i c a l l y a c t i v e upper crown f r e e o f i n f e c t i o n s . In a d d i t i o n , t h e h i g h e s t p r o p o r t i o n o f i n f e c t i o n s w i l l be i n t h e l e s s p h o t o s y n t h e t i c a l l y a c t i v e lower crown. 4 L i t e r a t u r e Review Much o f t h e work on e p i d e m i o l o g y has been done on m i s t l e t o e s p e c i e s w h i c h i n f e c t p i n e s . M u i r (1968) found a l o g a r i t h m i c d e c r e a s e i n t h e number o f seed o f A. americanum N u t t . ex Engelm. w i t h i n c r e a s i n g d i s t a n c e f r o m t h e seed s o u r c e i n l o d g e p o l e p i n e ( P i n u s c o n t o r t a v a r . l a t i f o l i a E ngelm.). T h i s d i s t r i b u t i o n was i n c o n t r a s t t o t h e n e a r - n o r m a l d i s t r i b u t i o n he o b s e r v e d on t h e number o f i n f e c t i o n s f r o m an i n f e c t e d s i n g l e r e s i d u a l t r e e and i n f e c t e d s t a n d edges. T h i s was a t t r i b u t e d to t h e s m a l l t a r g e t a r e a o f t r e e s w i t h i n 10 f t . o f t h e i n f e c t e d t r e e s . Hawksworth (1961) f o u n d t h a t A. vaginatum ( W i l l d . ) P r e s l . forma cryptopodum (Engelm.) G i l l s p r e a d a p p r o x i m a t e l y 1.0 f t . / y r . i n even-aged ponderosa p i n e ( P i n u s ponderosa •Laws.) and t h a t A. amer icanum had a r a t e of spre a d i n young l o d g e p o l e p i n e o f 1.5 f t . / y r . i n open and 1.0 f t . / y r . i n c l o s e d s t a n d s . M u i r (1968) a l s o f o u n d a l o g a r i t h m i c i n c r e a s e i n t h e number o f i n f e c t i o n s o f A. americanum i n 30 y r . o l d s t a n d s o f l o d g e p o l e p i n e f r o m i n f e c t i o n age 10 t o 3 y e a r s . 5 The absence of i n f e c t i o n s less than three years o l d were at t r i b u t e d to the two year incubation period. Hawksworth (1969) determined the upward spread of a dwarf mistletoe by inoculating f i v e uninfected trees. He found that the mean upward rate of spread of A. occidentale Engelm. on Pinus sabiniana Dougl. was 2.3 f t . / y r . while growth rate of the trees averaged 2.0 f t . / y r . The work of Hawksworth, Muir and many others has provided a quantitative insight into the behaviour of several mistletoe species, the majority of which i n f e c t pines. These epidemiological aspects apply generally to western dwarf mistletoe but not to the mistletoe-hemlock complex because of obvious differences i n stand and crown c h a r a c t e r i s t i c s , e c o l o g i c a l conditions as well as d i f f e r i n g mistletoe species. Methods It was mentioned e a r l i e r that Hawksworth (1969) studied the v e r t i c a l spread of dwarf mistletoe by inoculating f i v e uninfected trees. The d i f f i c u l t y with t h i s method was that a period of 16 years was required. One of the main objectives of the present study was to develop a technique 6 to determine the vertical rate of spread in naturally infected, actively growing stands. Details of the develop- ment w i l l be discussed in the following sections. Trees were selected under the following restrictions: 1. Trees currently in their maximum growth phase (linear growth phase). 2. Trees having no lateral light source from openings created by logging. Additional light from these sources can stimulate abnormally high development of mistletoe plants and f r u i t s . (Baranyay, 1962). 3. Trees occupying a codominant or dominant position in the stand. Trees with an inferior position in the stand are subjected to seed bombardment from above which would result in an inaccurate estimate of upward spread. 4. Trees out of the range of infected over- mature residuals. Trees within the range of infected residuals are subjected to seed f a l l from above which would result in an inaccurate estimate of upward spread. 5. Areas having a relatively level terrain. 7 Two d i s t i n c t stand densities were recognized. Three trees i n a dense stand and 12 trees in an open stand met the above described s p e c i f i c a t i o n s . The dense stand consisted of two broadly d i s t i n c t height and age classes. The selected trees, which had been growing vigorously for an average of 47 years following suppression for an average of 44 years, occupied the dominant p o s i t i o n i n the stand. The understory was composed of 80 per cent hemlock and 20 per cent cedar a l l of which were 6 inches or less in diameter. The open stand was r e l a t i v e l y uniform and had been a c t i v e l y growing for an average of 42 years. Further information on the sample trees i s presented i n Table I. C a l c u l a t i o n of the V e r t i c a l Rate of Spread After the selected trees were f e l l e d , the heights of the apparent oldest female i n f e c t i o n s on each branch were recorded. The i n f e c t i o n s were l a b e l l e d and taken to the laboratory for age determination. The age of each i n f e c t i o n was found according to the technique outlined by Scharpf and Parmeter (1966). The tabulation of heights and ages of in f e c t i o n s for one of the sample trees i s given i n f i g u r e 1. The successive oldest and highest i n f e c t i o n s f o r each tree was determined (Figure 1). The d i f f e r e n c e between the heights and ages of these successive oldest highest i n f e c t i o n s provided i n d i v i d u a l heights of advance and the number of years lapse 8 Table I Sample tree basic measurements Dense Open no. trees 3 12 avg. ht. (ft.) 61. 3 (46.2-72.8) 85.2 (68.0-93.7) avg. dbh (in.) 10. 1 (8.8-12.2) 12.5 (9.3-15.4) avg. age (yrs.) 91 (86-97) 56 (38-78) avg. no. yrs. suppressed 44 (37-51) 14 (0-35) avg. no. yrs. actively growing 47 (45-49) 42 (35-48) avg. ht. when released from suppression (ft.) 9. 8 (7.5-12.0) 3.6 (0-10.0) avg. ht. growth from release (ft.) 51. 8 (39.7-62.8) 81.1 (66.6-91.8) Site index (Barnes, 1962) 80 160 inf ection no. height of inf ection age of infection (yrs) height of no. of yrs. advance (ft.) lapse oldest highest infection second oldest 5 highest infection 6 7 8 third fourth f i f t h 9 10 11 12 9.7 13.4 15.2 21.1 24.6 27.8 30.2 31.4 32.8 38.2 49.4 46.4 24 24 16 12 16 8 9 11 14 8 9 6 11.2 8.2 11.6 2.0 8 Figure 1. Example of determination of heights of advances and number of years lapse, 10 between each advance. I n d i v i d u a l r a t e s o f s p r e a d f o r each advance and t h e number o f y e a r s l a p s e between s u c c e s s i v e advances were c a l c u l a t e d . I t would be i n c o r r e c t s i m p l y t o sum t h e s e r a t e s and d i v i d e by t h e t o t a l number o f o b s e r v a t i o n s s i n c e t h i s w ould e x c l u d e t h e e f f e c t o f t h e v a r i a t i o n i n t h e number o f y e a r s l a p s e between each advance. A v e r t i c a l s p r e a d r a t e v a l u e i s p o s s i b l e f o r a ran g e o f y e a r s and h e i g h t s o f advance. F o r example a 11.2 f t . advance w i t h a l a p s e o f 8 y e a r s and a 7.0 f t . advance w i t h a l a p s e o f 5 y e a r s b o t h have a v e r t i c a l s p r e a d r a t e o f 1.4 f t . / y r . The r a t e o f v e r t i c a l s p r e a d and v a r i a n c e were c a l c u l a t e d a c c o r d i n g t o t h e f o l l o w i n g w e i g h t e d f o r m u l a e : * Y ' * y" S * y .• - I where: P = w e i g h t e d a v e r a g e r a t e o f v e r t i c a l s p r e a d h I /***,• c —• u n w e i g h t e d r a t e o f s p r e a d / t h i s h e i g h t o f an advance y;; number o f y e a r s l a p s e between an advance S s w e i g h t e d v a r i a n c e Each J2£ v a l u e has i t s own r e l a t i v e f r e q u e n c y w h i c h i s d e t e r m i n e d by i t s v a l u e . These d a t a were grouped i n t o a f r e q u e n c y d i s t r i b u t i o n w h i c h c l o s e l y a p p r o x i m a t e d a n e g a t i v e b i n o m i a l d i s t r i b u t i o n . Logarithmic and square-root transformations were applied which enabled a t-test comparison between the means of the dense and open stands. The rates of v e r t i c a l spread i n the infected portions of the upper and lower crown were also c a l c u l a t e d . The loss of branches with i n f e c t i o n s older and higher than the recorded oldest highest i n f e c t i o n s w i l l r e s u l t i n a greater height of advance, a 'lower number of years lapse, and hence an upward bias in the estimated mean v e r t i c a l rate of spread. These losses w i l l occur more often i n the lower crown due to branch suppression. C a l c u l a t i o n of the I n t e n s i f i c a t i o n Rate A l l of the i n f e c t i o n s of the trees i n the dense stand and a l l of the i n f e c t i o n s of nine of the trees i n the open stand were c o l l e c t e d to determine the rate of i n t e n s i f i - cation. The rate was estimated according to Muir's (1963) technique. He observed a logarithmic r e l a t i o n s h i p between the number of infec t i o n s and i n f e c t i o n age. The slope of the straight l i n e obtained by a transformation of the logarithmic r e l a t i o n s h i p was regarded as a meaningful parameter of increase of the mistletoe population. C a l c u l a t i o n of Tree Growth Rate during the Linear Phase 12 Each tree was bucked into f i v e - f o o t sections and ages were recorded at each section in order to f i n d the age when the maximum growth phase was i n i t i a t e d . The average growth rate of a tree during the li n e a r phase was determined by d i v i d i n g the difference i n height of the tree at the star t and end of the l i n e a r growth phase by the duration of the l inear phase i n years. In addition, the average height growth rate for the l a s t 25 years was cal c u l a t e d . Results and Discussion Heights of Advances and Number of Years Lapse (Table II) The average height of an advance was 6.0 f t . with a range of 0.4 to 25.6 f t . The observed frequency d i s t r i - bution (Figure 2) was, s t a t i s t i c a l l y , uniformly d i s t r i b u t e d up to 11.9 f t . The average number of years lapse between advances was 3.7 years with a range of 1 to 11 years. The observed frequency d i s t r i b u t i o n did not approximate any of the standard mathematically defined d i s t r i b u t i o n s (Figure 3). The r e s u l t s were given the following i n t e r p r e t a - t i o n . After the seeds have reached the host, they are subjected to the destructive e f f e c t s of wind, snow, r a i n , fungi and insects ( G i l l and Hawksworth, 1961; Roth, 1959 Table II Mean and range of heights of advance and number of years lapse between successive advances and their 95% confidence limits Average height Range Average number of Range years lapse Pop'n. estimate 6.0 +_ 0.8 0.4-25.6 3.7 +_ 0.6 1-11 Dense stand 4.1 + 1.6 0.7-9.0 4.0 + 1.5 1-11 Open stand 7.4 + 1.2 0.4-25.6 3.5 +_ 0.6 1- 9 w> w> L A i / i u*i in m H D ut u*t ut tf> DO L O u " > u - i m u " > o o o o o o o o o o o o O O O O O o o — r>a o tn r~. co cri o "~ « M r"i -» n-, t© •*** o o CN CN H E I G H T O F A D V A N C E ( F T . ) Figure 2. Observed frequency distribution for heights of advances. 15 20L N O . Y E A R S L A P S E F i g u r e 3 . F r e q u e n c y p o l y g o n of t h e o b s e r v e d d i s t r i b u t i o n o f t h e number o f y e a r s l a p s e between t h e h e i g h t s o f advance. 1 6 and W i c k e r , 1967). These f a c t o r s were r e s p o n s i b l e f o r removing o r k i l l i n g 90-96% o f m i s t l e t o e seeds on s e v e r a l c o n i f e r o u s s p e c i e s ( W i c k e r , 1967). In t h e p r e s e n t i n v e s t i g a t i o n , t h e maximum number of f r u i t s o b s e r v e d on t h e s h o o t s o f a s i n g l e i n f e c t i o n was a p p r o x i m a t e l y 300 w h i l e t h e ave r a g e i n f e c t i o n had 50-100 f r u i t s . The crown o f a t r e e , t h e r e f o r e , i s exposed to c r o s s - i n f e c t i o n f r o m a f i n i t e number o f seeds. F u r t h e r m o r e , t h e r e i s an u n i n f e c t e d p o r t i o n o f t h e crown above t h e i h i g h e s t e x i s t i n g f e m a l e i n f e c t i o n w h i c h i s exposed to seed f r o m one o r more i n f e c t i o n s a t one or more l e v e l s f r om one or more s u r r o u n d i n g t r e e s . The s u c c e s s o f a seed i n e s t a b l i s h i n g an i n f e c t i o n f r o m 0.4 f t . t o 25.6 f t . w i t h a l a p s e o f 1 t o a t l e a s t 11 y e a r s i s dependent on d e s t r u c t i v e f a c t o r s a c t i n g w i t h v a r i a t i o n i n s e v e r i t y f r o m year to y e a r . I t i s p o s s i b l e , t h e r e f o r e , t o have, a ra n g e o f advances f o r l a p s e s f r o m 1 t o a t l e a s t 11 y e a r s . T h i s was s u b s t a n t i a t e d by p l o t t i n g t h e o b s e r v e d h e i g h t s o f advances over t h e c o r r e s p o n d i n g number o f y e a r s l a p s e ( F i g u r e 4 ) . There was no r e l a t i o n s h i p between the v a r i a b l e s w h i c h was i n d i c a t e d s t a t i s t i c a l l y u s i n g t h e P e a r s o n product-moment c o e f f i c i e n t o f c o r r e l a t i o n f o r m u l a f o r ungrouped d a t a . The P v a l u e was -0.06. 25 1 7 1 6 1 5 I 4 1 3 1 2 — 1 1 - 1 0 | uj U z 9 < < u. 7 o 6 I— I 0 5 UJ 1 4 ' ' ' I I I I I I I I l ) 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 NO. YEARS LAPSE Figure 4. Plot of heights of advance over corresponding number of years lapse. 18 The o b s e r v e d e q u a l f r e q u e n c y d i s t r i b u t i o n f o r t h e h e i g h t s o f advances i n d i c a t e d t h a t p o s i t i o n s up t o 25.0 f t . on t h e u n i n f e c t e d p o r t i o n above t h e h i g h e s t e x i s t i n g f e m a l e i n f e c t i o n have a chance o f b e i n g i n f e c t e d . P o s i t i o n s up t o 12.0 f t . , however, have t h e g r e a t e s t and an e q u a l chance o f b e i n g i n f e c t e d . The ra n g e o f h e i g h t s o f advances o b s e r v e d a p p r o a c h t h e ran g e o f v a l u e s o b t a i n e d by Hawksworth (1961) i n h i s s t u d y on seed f l i g h t o f A. vaginaturn forma cryptopodum. He f o u n d t h a t t h e a v e r a g e v e r t i c a l d i s t a n c e a t t a i n e d by n a t u r a l l y d i s c h a r g e d seeds was 15.0 f t . f r o m an a v e r a g e a n g l e o f 45 d e g r e e s . The maximum a n g l e o f d i s c h a r g e was 67 d e g r e e s and v e r t i c a l d i s t a n c e 25 f t . R a t e s o f V e r t i c a l S p r e a d , T r e e Growth and I n t e n s i f i c a t i o n T a b l e I I I summarizes t h e i n v e s t i g a t i o n o f t h e v e r t i c a l r a t e o f s p r e a d o f dw a r f m i s t l e t o e and t h e growt h r a t e o f t h e sample t r e e s . I t was mentioned i n an e a r l i e r s e c t i o n t h a t l o s s o f i n f e c t i o n s f r o m b r a n c h s u p p r e s s i o n i n t h e l o w e r crown may c a u s e an upward b i a s i n t h e e s t i m a t e d v e r t i c a l r a t e o f s p r e a d . The mean r a t e i n t h e l o w e r crown was n o t s i g n i f i c a n t - l y d i f f e r e n t a t t h e 5% l e v e l f r o m t h e r a t e o b s e r v e d i n t h e upper crown. I t was o b s e r v e d t h a t many m i s t l e t o e i n f e c t e d b r a n c h e s p e r s i s t e d a t l e v e l s w e l l b e n e a t h t h e canopy. The 1 9 a v e r a g e h e i g h t t o t h e l o w e s t l i v i n g b r a n c h f o r t h e dense s t a n d was 17.8 f t . and f o r t h e open s t a n d 11.3 f t . T h i s p e r s i s t e n c e o f i n f e c t e d b r a n c h e s c o u l d a c c o u n t f o r t h e l a c k o f a s i g n i f i c a n t d i f f e r e n c e between t h e r a t e s o f v e r t i c a l s p r e a d i n t h e upper and lower p o r t i o n s o f t h e i n f e c t e d c rowns. The a v e r a g e r a t e s o f v e r t i c a l s p r e a d i n t h e open and dense s t a n d s were 2.1 and 1.0 f t . / y r . and t h e mean r a t e s o f t r e e h e i g h t growth d u r i n g t h e maximum growth phase f o r t h e open and dense s t a n d s were 2.5 and 1.5 f t . / y r . r e s p e c t i v e l y . However, over t h e p a s t 25 y e a r s t h e r a t e o f t r e e g r o w t h d e c r e a s e d t o an a v e r a g e o f 1.9 f t . / y r . i n t h e open s t a n d and 1.1 f t . / y r . i n t h e dense s t a n d . The d i f f e r - e nces between r a t e s o f v e r t i c a l s p r e a d and between t r e e h e i g h t g r o w t h i n t h e open and dense s t a n d s were s i g n i f i c a n t l y d i f f e r e n t a t t h e .05 p r o b a b i l i t y l e v e l . The combined v e r t i c a l r a t e o f s p r e a d i n b o t h s t a n d s was 1.6 f t . / y r . w h i l e t h a t o f t r e e g r o w t h r a t e was 2.1 f t . / y r . The combined av e r a g e t r e e growth r a t e f o r t h e p a s t 25 y e a r s was 1.6 f t . / y r . The h i g h e r r a t e o f s p r e a d i n t h e open s t a n d can be a t t r i b u t e d t o t h e open s t a n d c h a r a c t e r i s t i c w h i c h p r o v i d e s more l i g h t s t i m u l u s f o r shoot and f r u i t p r o d u c t i o n , seed g e r m i n a t i o n and l e s s o b s t r u c t i o n f o r e j e c t e d seeds. Table III Weighted mean vertical rate of spread of dwarf mistletoe and mean growth rate of sample trees during the maximum growth phase and for the past 25 years The 95% confidence limits are included. Pop'n estimate upper crown • lower crown Weighted mean ver t i ca1 spr ead rate (ft./yr.) 1.6 +_ 0.2 1.5 + 0.2 1.9 + 0.3 Mean height growth during the maximum growth phase (ft./yr.) 2.1 +_ 0.1 (1.4-3.'2) Mean height growth for the past 25 yrs. (ft./yr.) 1.6 _+ 0.3 (0.7-2.4) dense stand upper crown lower crown 1.0 + 0.1 0.9 +_ 0.5 1.1 + 0.4 1.5 +_ 0.2 (1.4-1.5) 1.1 + 0.7 (0.7-1.5) open stand upper crown lower crown 2.1 +_ 0.1 1.9 +_ 0.3 2.4 + 0.6 2.5 +_ 0.2 (2.1-3.2) 1.9 + 0.2 (1.5-2.4) to o 21 The differences between the tree growth rate during the linear phase and vertical rate of spread of dwarf mistle- toe could not be tested s t a t i s t i c a l l y since their means are from different populations. However, the end result of these differences when accumulated over tens of years, should mean that a significant proportion of the crown w i l l be either free of mistletoe infections or w i l l have a very low number of infections. The average length of female mistletoe-free crown above the highest recorded female infection for the dense stand was 25.7 f t . (range 20-31 ft.) or 51% of the average crown length of 50.0 f t . The open stand had an average of 27.6 f t . (range 20-40 ft.) or 41% of the average crown length of 66.5 f t . The length of mistletoe-free crown increased very l i t t l e over the last 15 years (TablelV). Table IV Average length of mistletoe-free crown 1955 1969 open 26.2 27.6 dense 24.5 25.7 I t was n o t e d i n t h e sample t r e e s t h a t t h e r e was dense f o l i a g e up t o a t l e a s t 5 f t . f r o m t h e t o p o f t h e crown. The l a c k o f i n c r e a s e o f dwarf m i s t l e t o e - f r e e crown l e n g t h i n d i c a t e d i n T a b l e IV was not due t o l a c k o f t a r g e t a r e a b u t r a t h e r t o t h e e q u a l i z a t i o n o f v e r t i c a l s p r e a d r a t e and t r e e growth r a t e ( T a b l e I V ) . W i t h t h e e x c e p t i o n o f a few male p l a n t s ( l e s s t h a n 4 on any one t r e e ) and l a t e n t i n f e c t i o n s , an a v e r a g e o f 67 and 60 i n f e c t i o n s per t r e e o c c u r r e d i n t h e dense and open s t a n d s , r e s p e c t i v e l y . These t r e e s would be c l a s s e d as l i g h t l y i n f e c t e d and w i l l have l o s t an i n s i g n i f i - c a n t p r o p o r t i o n o f volume t o t h e p a r a s i t e ( S m i t h , 1969). The r e g r e s s i o n a n a l y s i s o f t h e number o f dwarf m i s t l e t o e i n f e c t i o n s on age f o r i n f e c t i o n age tw e n t y t o s i x i n t h e open s t a n d and f r o m twenty t o f i v e i n t h e dense s t a n d i s summarized i n T a b l e V. The l i n e a r r e l a t i o n s h i p between t h e l o g a r i t h m o f t h e number o f i n f e c t i o n s and t h e i n f e c t i o n ages f o r b o t h t h e open and dense s t a n d s were s i g n i f i c a n t a t t h e 1% l e v e l . The h i g h c o e f f i c i e n t o f d e t e r m i n a t i o n v a l u e s f u r t h e r s u b s t a n t i a t e d t h i s s i g n i f i c a n c e i n t h a t f o r b o t h s t a n d s c l o s e t o 90% o f t h e v a r i a t i o n i n l o g a r i t h m o f t h e number o f i n f e c t i o n s was a s s o c i a t e d w i t h t h e i n f e c t i o n a g e s . There was no s i g n i f i c a n t d i f f e r e n c e a t t h e 5% l e v e l ( F - t e s t ) between t h e s l o p e s o f t h e r e g r e s s i o n l i n e s f o r t h e two s t a n d s i n d i c a t i n g no d i f f e r e n c e between t h e r a t e s o f i n t e n s i f i c a t i o n . T h i s a g r e e s w i t h M u i r ' s r e s u l t s i n h i s s t u d i e s o f A. americanum on l o d g e p o l e p i n e (1963 and 1968). T a b l e V The r e g r e s s i o n a n a l y s i s o f the i n t e n s i f i c a t i o n r a t e o f dwarf m i s t l e t o e i n f e c t i o n s open dense r e g r e s s i o n -0.0847145 -0.0715295 c o e f f i c i e n t i n t e r c e p t 2.15826 1.87958 F - t e s t o f 1.662 N.S. d i f f e r e n c e i n s l o p e s F - t e s t o f 5.708 * l e v e l s o f r e g r e s s i o n s N.S. Not s i g n i f i c a n t * S i g n i f i c a n t a t t h e .05 p r o b a b i l i t y l e v e l The d i f f e r e n c e between t h e l e v e l s o f t h e r e g r e s s i o n s was s i g n i f i c a n t a t t h e .05 p r o b a b i l i t y l e v e l . T h i s i n d i c a t e d t h a t s e p a r a t e e q u a t i o n s a r e needed t o r e p r e s e n t t h e i n t e n s i f i c a t i o n r a t e s i n b o t h s t a n d s . The p l o t t e d d a t a r e v e a l e d t h a t t h e number o f i n f e c t i o n s i n b o t h s t a n d s d o u b l e d e v e r y f o u r y e a r s ( F i g u r e F i g u r e 5. I n t e n s i f i c a t i o n o f dwarf m i s t l e t o e i n t h e open (o) and dense (x) s t a n d s . This increase appears to have levelled off six years ago in the open stand and five years ago in the dense stand. This may indicate that the parasite has passed i t s potential to increase logarithmically and that the number of new infections each year hence may remain more or less at a constant level. The loss of potential can be attributed to a closing canopy which results in lower light intensity in the lower portion of the infected crown. Low intensity of light r e s t r i c t s new infections and f r u i t production. If the stand were subsequently thinned or partially cut, a renewal of the geometric increase could be expected. The results of the investigation indicate that during the maximum growth phase of hemlock in an open and dense stand, the most photosynthetically active upper portion of the crowns i s free of mistletoe infections. Until the senescent phase is reached, the trees can be expected to outgrow the mistletoe, and intensification w i l l be restricted to the lower portions of the crowns. It was also shown that in the stands examined the mistletoe has lost i t s potential to increase logarithmically, and that the number of new infections may be expected to remain more or less constant. A study of the further intensification of the parasite and a comparison of volumes of uninfected and infected maturing stands from 50 to 80 years i s needed to determine the 26 significance of dwarf mistletoe losses. Conclusions The vertical rate of spread of dwarf mistletoe was studied in two actively growing, young hemlock stands. This was done by determining the height and age of successive oldest and highest female infections. The rate of spread was calculated by dividing the sum of the heights of advances by the total number of years lapse between successive advances. The mean vert i c a l spread rate in a relatively open stand was 2.1 +_ 0.1 f t . / y r . and in a relatively dense stand was 1.0 _+ 0.1 f t . / y r . The mean rate of tree growth during the maximum growth phase in the open stand was 2.5 f t . / y r . and for the dense stand 1.5 f t . / y r . However, over the past 25 years, the growth rate of the trees in the open stand was 1.9 ft . / y r . and for the dense stand 1.1 f t . / y r . The number of new infections per year increased geometrically, doubling every four years in both the dense and open stands. However, the geometric increase levelled 27 off six years ago in the open stand and five years ago in the dense stand. It i s tentatively concluded that provided there i s no overstory seed source and no disruption of the natural stand, such as thinning, dwarf mistletoe on hemlock w i l l not become serious u n t i l the rate of height growth of the trees f a l l s below the rate of vertical spread, i.e., not until after the presently accepted rotation age. 28 L i t e r a t u r e Cited Baranyay, J.A. 1962. Phenological observations on western hemlock dwarf mistletoe (Arceuthobium campylopodum G i l l forma tsugensis). Can. Dept. For., Bi-Mon. Prog. Rpt. 18(4): 3-4. Barnes, George H. 1962. Y i e l d of even-age stands of western hemlock. U.S. Dept. Agric. Tech. B u l l . 1273, 52 pp. Buckland, D.C. and E.G. Marples 0 1952. Management of western hemlock infected with dwarf mistletoe. B.C. Lumberman 36(5): 50, 51, 136, 138 and 140. G i l l , L.S. and F.G. Hawksworth. 1961. The mistletoes, a ^ l i t e r a t u r e review. U.S. Dept. Ag r i c . Tech. B u l l . 1242. 87 pp. Hawksworth, F.G. 1961. Dwarf mistletoe of ponderosa pine i n the South west. U.S. Dept. Agric. Tech. B u l l . 1246. 112 pp. Hawksworth, F.G. 1969. Rapid i n t e n s i f i c a t i o n and upward spread of dwarf mistletoe i n inoculated digger pines. Plant Disease Reporter 53(8): p. 615. Muir, J.A. 1963. A study of epidemics of lodgepole pine dwarf mistletoe i n Alberta. B.S.F. Thesis, Faculty of Forestry Univ. of B r i t i s h Columbia. 22 pp. Muir, J.A. 1968 Biology of dwarf mistletoe (Arceuthobium americanum) i n Alberta. Forest Research Laboratory, Calgary, Alberta, Internal Report A-15. Forestry Branch Dept. of F i s h e r i e s and Forestry. Sept. 1968. 20 pp. Roth, L.F. 1959. Natural emplacement of dwarf mistletoe seed on ponderosa pine. For. S c i . 5(4): 365-369. Scharpf, R.F. and Parmeter J.R. 1966. Determining the age of dwarf mistletoe i n f e c t i o n s on red f i r . U.S. For. Serv., Res. Note PSW 105, 5 p. P a c i f i c Southwest For. and Range Expt. Sta., Berkeley C a l i f . Smith, R.B. 1966. Hemlock and larch dwarf mistletoe seed ^ d i s p e r s a l . For. Chron. 42(4): 395-401. 2<r S m i t h , R.B. 1969. A s s e s s i n g dwarf m i s t l e t o e on w e s t e r n hemlock. F o r . S c i . 1 5 ( 3 ) : 277-85. Wellwood, R.W. 1956. Some e f f e c t s o f dwarf m i s t l e t o e on w e s t e r n hemlock. F o r . Chron. 3 2 ( 3 ) : 282-296. W i c k e r , E.F. 1967. Seed d e s t i n y as a k l e n d u s i c f a c t o r o f i n f e c t i o n and i t s impact upon p r o p a g a t i o n o f A r c e u t h o b i u m spp. P h y t o p a t h . 5 7 ( 1 1 ) : 1164-1168.

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