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A study of epidemics of lodgepole pine dwarf mistletoe in Alberta Muir, John Alexander 1966

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A STUDY OF EPIDEMICS OF LODGEPOLE PINE DWARF MISTLETOE IN ALBERTA by JOHN ALEXANDER MUIR B.S.F., U n i v e r s i t y of B r i t i s h Columbia, 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF FORESTRY i n the Department of F o r e s t r y We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1966 / / I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e 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 , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n T h e 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 V a n c o u v e r 8 , C a n a d a D e p a r t m e n t of ACKNOWLEDGMENT The data of t h i s study were c o l l e c t e d w i t h the ass i s t a n c e of Mr. E. A. Berndt while the w r i t e r was employed by the Canada Department of F o r e s t r y Regional Laboratory at Calgary, A l b e r t a . Considerable a s s i s t a n c e a l s o was given by Dr. A. Kozak, xfho c a l c u l a t e d the r e g r e s s i o n equations at the Computing Centre of the U n i v e r s i t y of B r i t i s h Columbia, and Drs. J . E. B i e r , P. G. Haddock, and 0. S z i k l a i who reviewed the manuscript. • ABSTRACT The increase of d i s c r e t e populations of dwarf m i s t l e t o e (Arceuthobium americanum Nutt.) p l a n t s or i n f e c t i o n s was st u d i e d i n 20 year o l d stands of lodgepole pine (Pinus  c o n t o r t a Dougl.). M i s t l e t o e i n f e c t i o n s were s e l e c t e d from i n f e c t e d t r e e s i n three areas 100 to 200 f t . i n diameter. The age of each i n f e c t i o n was determined by d i s s e c t i n g the i n f e c t e d host wood. I t was found t h a t the number of i n f e c t i o n s e s t a b l i s h e d annually had incre a s e d l o g a r i t h m i c a l l y during the preceding 8 to 10 years. Therefore the l o g a r i t h m i c r a t e s of increase i n the number of i n f e c t i o n s could be c a l c u l a t e d to describe the past epidemic growth of each m i s t l e t o e p o p u l a t i o n . The l o g a r i t h m i c r a t e s (equivalent to "compound" increase of 66 to 96 per cent per year) demonstrated r a p i d p o p u l a t i o n i n c r e a s e . Although the three m i s t l e t o e populations had developed i n apparently d i f f e r e n t environments, the po p u l a t i o n r a t e s of increase were not s t a t i s t i c a l l y d i f f e r e n t . However, f o r the study of e c o l o g i c a l aspects of dwarf m i s t l e -toe epidemics t h i s approach appears q u i t e promising. - i i i -TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES i v LIST OF ILLUSTRATIONS v INTRODUCTION 1 METHODS OF INVESTIGATION • 3 RESULTS OF THE INVESTIGATION 6 DISCUSSION OF RESULTS 13 LITERATURE CITED 17 - X V LIST OF TABLES Page Table 1. Measurements of i n f e c t e d t r e e s and dwarf m i s t l e t o e i n f e c t i o n s 6 i n three areas 2. Types of dwarf m i s t l e t o e i n f e c t i o n s 7 c o l l e c t e d from three areas of i n f e c t e d lodgepole pine 3- Ages of dwarf m i s t l e t o e i n f e c t i o n s 9 s e l e c t e d from three areas 4. Equation constants and s t a t i s t i c s f o r 12 the l i n e a r r e l a t i o n s h i p s between the number of i n f e c t i o n s expressed as n a t u r a l logarithms and i n f e c t i o n age (3 to 10 years) LIST OF ILLUSTRATIONS Age distribution of dwarf mistletoe infections selected from three areas - 1 -INTRODUCTION In many regions of western North America, dwarf m i s t l e t o e s (Arceuthobium spp.) have been considered very important agents of damage to f o r e s t t r e e s ( G i l l , 1935, 1957; K u i j t , 1955; Leaphart, 1959, 1963; Weir, 1916, 1918). However, c o n t r o l of the m i s t l e t o e s r a r e l y has been p r a c t i s e d i n f o r e s t management. Decisions to implement c o n t r o l have been hindered, p a r t i c u l a r l y i n Canada, by a s c a r c i t y of in f o r m a t i o n on the e f f e c t s of dwarf m i s t l e t o e p a r a s i t i s m , and l i t t l e i nventory data on the d i s t r i b u t i o n and i n t e n s i t y of dwarf m i s t l e t o e i n f e c t i o n . Although the dwarf m i s t l e t o e s appear q u i t e amenable to c o n t r o l ( K u i j t , 1955. p. 599), "...few, i f any of the methods of c o n t r o l have emerged from the h y p o t h e t i c a l or experimental stage" ( i b i d , p. 596 ). K u i j t ( i b i d , pp. 596 - 601) discusses some p o s s i b l e and attempted methods to c o n t r o l dwarf m i s t l e t o e s u s i n g p a r a s i t i c f u n g i , s e l e c t i v e h e r b i c i d e s , and s i l v i c u l t u r a l e r a d i c a t i o n . Recently, the use 01 a fungus p a r a s i t e (Septogloeum g i l l i i E l l i s ) of the m i s t l e t o e shoots (Mielke, 1959) and many h e r b i c i d e s (Quick, 1964) were found i n e f f e c t i v e . K u i j t ( i b i d , p. 602) a l s o discussed the r o l e of f o r e s t f i r e s i n l i m i t i n g the spread of dwarf m i s t l e t o e . - 2 -I t appears l i k e l y t h a t c o n t r o l treatments could be improved by more d e t a i l e d knowledge of the b i o l o g y of dwarf m i s t l e t o e s . Much of the a v a i l a b l e knowledge has been reviewed by G i l l ( 1 9 3 5 ) , G i l l and Hawksworth ( 1 9 6 1 ) , and K u i j t ( 1 9 5 5 , I960). Research on s e l e c t i v e h e r b i c i d e s could be f a c i l i t a t e d by the recent r e s u l t s of p h y s i o l o g i c a l s t u d i e s by Greenham and Leonard ( 1 9 6 5 ) , H u l l and Leonard ( 1 9 6 4 ), and McDowell ( 1 9 6 4 ) . For some dwarf m i s t l e t o e s , s i l v i c u l t u r a l c o n t r o l may be improved by recent observations on the minimum times necessary to complete the l i f e c y c l e s ( K u i j t , 1 9 6 1 ; Scharpf, 1 9 6 4 ; Smith, 1 9 6 5 ; Wagener, 1962). Since treatments are a p p l i e d to c o n t r o l l a r g e populations of dwarf m i s t l e t o e p l a n t s or i n f e c t i o n s , i t may be advantageous to study how d i f f e r e n t populations of i n -f e c t i o n s increase (Van der Plank, I 9 6 3 . p - l K The theory and methods to analyze some fungus epidemics, such as potato b l i g h t and wheat stem r u s t , have been developed r e c e n t l y by Van der Plank ( 1 9 6 3 , 1 9 6 5 ) . He demonstrated t h a t the r a t e of increase of such diseases could be used to measure the e f f e c t i v e n e s s of c o n t r o l treatments, such as r e s i s t a n t v a r i e t i e s of p l a n t s or f u n g i c i d e s . Such an approach, i f i t can be a p p l i e d s u c c e s s f u l l y to dwarf m i s t l e t o e epidemics, could be very u s e f u l i n developing more e f f e c t i v e c o n t r o l treatments. - 3 -Accordingly, a study of small populations of dwarf mistletoe (Arceuthobium americanum Nutt.) parasitizing lodge-pole pine (Pinus contorta Dougl.) in Alberta was undertaken. The primary objective of the project was to determine the increase in the number of infections of each population. A study of the spread of dwarf mistletoe from old, infected stands into adjacent 5 to 30 year-old stands (in Montana, Wyoming and Colorado) suggested that the establishment of dwarf mistletoe infections had occurred most frequently in certain years which were called "peak periods" of infection (Hawksworth and Graham, 1963)• Similar results have been obtained (R. F. Scharpf, 1965, personal communication) in California in a study of the spread of dwarf mistletoe from the infected overstorey to the healthy understorey trees in a partially logged stand. METHODS OF INVESTIGATION Although the Increase of dwarf mistletoe infections could be determined by observations repeated annually or less frequently, a quicker estimate of increase appeared possible. The age of individual, dwarf mistletoe infections can be determined by dissecting the infected host wood (Srivastava and - 4 -Esau, 1961; Hawksworth and Graham, 1963). Therefore, by randomly s e l e c t i n g i n f e c t i o n s and determining t h e i r ages, one can estimate the r e l a t i v e number of i n f e c t i o n s e s t a b l i s h -ed per year. Then one can c a l c u l a t e the r a t e of increase of the p o p u l a t i o n of i n f e c t i o n s (Van der Plank, 1963 p. 77) which can be used to compare popu l a t i o n s . Three s m a l l populations of dwarf m i s t l e t o e i n f e c -t i o n s were s e l e c t e d i n 22 to 25 year o l d stands of lodgepole pine. Young stands were sampled to reduce the l o s s of i n f e c t i o n s caused by the death of host t i s s u e . Dense and open stands were sampled to determine i f m i s t l e t o e i n f e c t i o n had v a r i e d s i g n i f i c a n t l y w i t h d i f f e r e n c e s i n l i g h t exposure or other f a c t o r s c o r r e l a t e d w i t h stand d e n s i t y . The abun-dance of a e r i a l shoots and seed production of dwarf m i s t l e t o e s g e n e r a l l y have been considered to be d i r e c t l y r e l a t e d to the degree of exposure to l i g h t (Wagener, 1961). One area (A) of i n f e c t e d t r e e s was found i n an open stand 20 miles south of the Kananaskis Experiment S t a t i o n ; the other two (B and C) were l o c a t e d i n denser stands 80 m i l e s south of the S t a t i o n and one-quarter mile apart. Each i n f e c t e d area had o r i g i n a t e d from one or more o l d e r , i n f e c t e d t r e e s : A and C each had one t r e e (now dead); B had 6 to 12. In each area stand d e n s i t y was estimated by c a l c u l a t i n g average t r e e crown width (Smith - 5 -and B a i l e y , 1964) and h e i g h t ( S m i t h e r s , 1956). From each a r e a about 450 i n f e c t i o n s were c o l l e c t e d u s i n g a square g r i d o f s a m p l i n g p o i n t s a t 10 f t . i n t e r v a l s . W i t h i n a 5 f t . r a d i u s o f each s a m p l i n g p o i n t randomly s e l e c t e d , a l l t he i n f e c t i o n s were c o l l e c t e d e i t h e r f r o m one i n f e c t e d t r e e c l o s e s t t o t h e p o i n t ( a r e a A) o r fr o m a l l i n f e c t e d t r e e s (B and C ) . A d d i t i o n a l o b s e r v a t i o n s , such as sex and number o f b e r r i e s on female p l a n t s , were made on each m i s t l e t o e i n f e c t i o n . I t was s u s p e c t e d t h a t a low p r o p o r t i o n o f male o r female i n f e c t i o n s i n a m i s t l e t o e p o p u l a t i o n c o u l d reduce the r a t e o f i n c r e a s e o f t h e number o f i n f e c t i o n s . U n p u b l i s h e d i n f o r m a t i o n , c o l l e c t e d by Dr. J . E. Nighswander, who was p r e v i o u s l y employed by t h e Canada Department o f F o r e s t r y a t C a l g a r y , had shown t h a t t h e r a t i o o f male t o female i n f e c t i o n s i n d i s c r e t e m i s t l e t o e p o p u l a t i o n s f r e q u e n t l y d e v i a t e d f r om an assumed 1:1 r a t i o . The age o f each i n f e c t i o n was e s t i m a t e d by c u t t i n g t r a n s v e r s e l y t h r o u g h t h e c e n t e r o f t h e i n f e c t i o n and by c o u n t i n g t h e number o f a n n u a l r i n g s i n w h i c h t h e deepest m i s t l e t o e s i n k e r was embedded ( S r i v a s t a v a and Esau, 1961). .The i d e n t i f i c a t i o n o f s i n k e r s was f a c i l i t a t e d by immersing each c r o s s s e c t i o n b r i e f l y i n methylene v i o l e t (Graham, 1964). No c o r r e c t i o n was added t o t h e e s t i m a t e d age f o r t h e i n t e r v a l between t h e c o n t a c t o f m i s t l e t o e seeds w i t h t h e h o s t t i s s u e s and the i n i t i a t i o n o f m i s t l e t o e s i n k e r s . The i n t e r v a l - 6 -between i n i t i a l i n f e c t i o n and s i n k e r i n i t i a t i o n was estimated, i n Colorado, at 2 years (Hawksworth and Graham, 1963). RESULTS OF THE INVESTIGATION Observations made i n the three areas sampled (Table 1) and d e t a i l e d observations on the m i s t l e t o e i n -f e c t i o n s (Table 2) i n d i c a t e d t h a t the m i s t l e t o e was growing v i g o r o u s l y . TABLE I. Measurements of i n f e c t e d t r e e s and dwarf m i s t l e t o e i n f e c t i o n s i n three areas. Area A B C Tree age (yrs.) 1. average (av.) 2. range 21.6 18-26 22.0 14-24 24-8 23-26 av. crown width ( f t . ) av. height ( f t . ) 5.0 18.0 2.5 12.1 2.9 10.6 No. t r e e s (and sampling p o i n t s ) 33(33) 202(9). 55(3) I n f e c t i o n s per t r e e 1. av. 2. range 14-3 1-118 2.2 1-12 8.7 1-83 Av. s i z e of I n f e c t i o n ( l e s s than 8 y r s . old) (sq. i n . ) 1.21 0.57 0.54 There was l i t t l e evidence of other f a c t o r s which might have i n f l u e n c e d m i s t l e t o e r e p r o d u c t i o n such as rodent damage to - 7 -mistletoe-infected bark, fungus disease of mistletoe shoots or other serious disease of the host trees. Also, very few dead, mistletoe infections were observed on dead trees or on dead branches of l i v i n g trees (Table 2 ) . These observations indicated that the number of mistletoe infections had not been reduced seriously by the death of host t i s s u e . Table 2. Types of dwarf mistletoe infe c t i o n s c o l l e c t e d from three areas of infected lodgepole pine.-Area A B C Type of i n f e c t i o n Number of infec t i o n s Dead 10 14 11 L i v i n g Male 246 183 174 Female with berries 110 173 42 with flowers only 100 85- 117 Sex indeterminable 1 5 12 148 Total 471 453 478 Sampling date May 26 July 4 Aug. 10 to to to June 25 21 11 An analysis of tree crown widths and heights from each area (Table 1) confirmed the subjective choice of areas - 8 -B and C as having s i g n i f i c a n t l y g r eater stand d e n s i t i e s than A. Area A al s o had a l a r g e r number of dwarf m i s t l e t o e i n -f e c t i o n s per t r e e and a s i g n i f i c a n t l y l a r g e r average s i z e of i n f e c t i o n . These d i f f e r e n c e s i n m i s t l e t o e incidence and growth might be r e l a t e d to the f a s t e r t r e e growth (Hawksworth, I 9 6 0 ) or presumably b e t t e r l i g h t c o n d i t i o n s (Baranyay, 1 9 6 2 ) i n area A. However, one species of dwarf m i s t l e t o e i n C a l i f o r n i a has been reported to grow b e t t e r under shaded co n d i t i o n s (Wagener, 1 9 6 1 ) . Other d i f f e r e n c e s between areas were observed i n the number of i n f e c t i o n s of v a r i o u s types (Table 2 ). The number of male and female i n f e c t i o n s i n each area deviated s i g n i f i c a n t l y from an assumed 1 : 1 r a t i o only i n area B which had s i g n i f i c a n t l y more female i n f e c t i o n s . The r a t i o of female i n f e c t i o n s w i t h b e r r i e s to female i n f e c t i o n s without .berries, however, was very d i f f e r e n t i n each area : i n A, 1 : 1 ; i n B, 2 : 1 ; and C, 1 : 3 • Such d i f f e r e n c e s may i n d i c a t e d i f f e r e n c e s i n the epidemic behavior of the m i s t l e t o e or, more l i k e l y , may r e f l e c t d i f f e r e n c e s i n the date of sampling (Table 2 ) . By August 1 0 , when area C was sampled, more young i n f e c t i o n s may have become v i s i b l e (as i n d i c a t e d by the l a r g e r number of "sex indeterminable" i n f e c t i o n s i n area C, Table 2) or seed discharge and the r e s u l t i n g disappearance - 9 -of b e r r i e s may have been i n i t i a t e d already, thus r e s u l t i n g i n the l a r g e r number of femals i n f e c t i o n s without b e r r i e s i n area C. A t o t a l of 1440 dwarf m i s t l e t o e i n f e c t i o n s were c o l l e c t e d i n the three areas, and the ages of 1402 i n f e c t i o n s were determined. The numbers of m i s t l e t o e i n f e c t i o n s per i n f e c t i o n age c l a s s f o r each area are summarized i n Table 3* TABLE 3 Ages of dwarf m i s t l e t o e i n f e c t i o n s s e l e c t e d from three areas. Area A. B C I n f e c t i o n Number of i n f e c t i o n s Age 2 122 92 173 3 171 161 130 4 77 109 69 5 45 48 47 6 30 14 22 7 14 4 17 8 8 5 14 9 3 4 3 10 2 3 11 3 12 2 13 1 2 14 2 15 3 16 1 17 18 1 T o t a l s 471 453 478 - 1 0 -In general, the numbers of infec t i o n s were largest i n the young age groups and decreased rapi d l y with increasing i n f e c t i o n age (Figure 1 ) . z Z ieo 160 140 120 100 8 0 6 0 4 0 2 0 + o x + + X O X O + 9 o AREA A X B + C O + $ + + * + + + 10 18 INFECTION AGE yrs. F i g . 1. Age d i s t r i b u t i o n of dwarf mistletoe infec t i o n s i n three areas of infected trees. The smaller number of 2 year as compared with 3 year old infect i o n s i n two areas, and the absence of 1 year in f e c t i o n s , were assumed to have been caused by the i n t e r v a l between the f a l l of mistletoe seed onto the host and the development of - 11 -symptoms of the i n f e c t i o n ; ; With the exception of one i n f e c t i o n , no i n f e c t i o n s o l d e r than 10 years were in c l u d e d i n the random samples from areas A and C. In area B, however, 1 to 3 i n f e c t i o n s per i n f e c t i o n age 10 to 16 years were found i n the sample. This may i n d i c a t e t h a t the m i s t l e t o e p o p u l a t i o n i n area 3 had expanded q u i t e s t e a d i l y f o r a while or that a l a r g e r number of o l d e r i n f e c t e d t r e e s i n B had produced a wider d i s t r i b u t i o n of o l d e r i n f e c t i o n s which increased t h e i r p r o b a b i l i t y of being s e l e c t e d . When the number of i n f e c t i o n s were p l o t t e d against i n f e c t i o n age ( F i g . 1 ) , a l o g a r i t h m i c r e l a t i o n s h i p between the two v a r i a b l e s was apparent (Freese, 1964, p. 126) . Therefore the l o g a r i t h m i c r e l a t i o n s h i p s could be transformed to s t r a i g h t l i n e ( l i n e a r ) r e l a t i o n s h i p s by p l o t t i n g the lo g a r i t h m of the number of i n f e c t i o n s . The slopes of the s t r a i g h t l i n e s are estimates of the l o g a r i t h m i c r a t e s of increase f o r the m i s t l e t o e p o p u l a t i o n s . The slopes we're" c a l c u l a t e d u s i n g s t a t i s t i c a l methods ( S t e e l and T o r r i e , I 9 6 0 , pp. 161-169) to evaluate how w e l l the transformed data was described u s i n g s t r a i g h t l i n e s . The l i n e a r equations were c a l c u l a t e d only f o r i n f e c t i o n ages 3 to 10 years since the number of i n f e c t i o n s l e s s than 3 and o l d e r than 10 years probably were a f f e c t e d by the m i s t l e t o e i n c u b a t i o n p e r i o d and the death of i n f e c t e d host t i s s u e . - 12 -The c a l c u l a t i o n s , summarized i n Table 4 , TABLE 4 Equation constants and s t a t i s t i c s f o r the l i n e a r r e l a t i o n s h i p s between the number of i n f e c t i o n s expressed as n a t u r a l logarithms and i n f e c t i o n age ( 3 to 1 0 years ). Equation constants I n t e r c e p t slope S t a t i s t i c s Area B Combined 7 . 2 5 5 6 . 7 4 1 6 . 3 4 9 6 . 7 8 1 - 0 . 6 7 3 - 0 . 6 0 4 - 0 . 5 0 6 - 0 . 5 9 4 l i n e a r c o r r e l a t i o n co-e f f i c i e n t F - t e s t of s i g n i f i c a n c e of i n d i v i d u a l slopes F - t e s t of s i g n i f i c a n c e of d i f f e r e n c e s between a l l slopes 0 . 9 8 6 0 . 9 5 6 0 . 9 8 2 O . 9 6 5 2 0 8 . 9 S 5 * " ' 63.795* 1 6 4 . 6 3 2 * * 3 0 2 . 0 7 4 * " 2 . 2 4 2 ns P r o b a b i l i t y of these F values o c c u r r i n g by chance i s l e s s than 0.01 ns P r o b a b i l i t y of t h i s F value o c c u r r i n g by chance i s gre a t e r than 0.05 demonstrated w i t h a high degree of confidence that the number of m i s t l e t o e i n f e c t i o n s had increased l o g a r i t h m i c a l l y . F u r ther-- 1 3 -more, a s t a t i s t i c a l t e s t (Freese, 1 9 6 4 . pp. 8 4 - 8 5 ) of the l o g a r i t h m i c r a t e s i n d i c a t e d t h a t the d i f f e r e n c e s between r a t e s were not s i g n i f i c a n t l y g r e a t e r than those which could be expected by chance v a r i a t i o n . Therefore i t was concluded th a t the r a t e of increase was the same i n a l l areas despite d i f f e r e n c e s between areas such as geographic l o c a t i o n and stand d e n s i t y . The r a t e s of increase were 0 . 5 1 to 0 . 6 7 per year f o r l o g a r i t h m i c increase or 6 6 to 9 6 per cent per year f o r "compound i n t e r e s t " i n c r e a s e . DISCUSSION OF RESULTS I t was demonstrated w i t h a high degree of con-fidence (with the s i g n i f i c a n t "F" values and l i n e a r cor-r e l a t i o n c o e f f i c i e n t s , Table 4 ) t h a t the number of dwarf m i s t l e t o e i n f e c t i o n s e s t a b l i s h e d per year had increased l o g a r i t h m i c a l l y i n the three populations of m i s t l e t o e sampled. These r e s u l t s suggested that s m a l l d i s c r e t e popula-t i o n s of dwarf m i s t l e t o e do not always increase i r r e g u l a r l y as suggested by Hawksworth and Graham ( 1 9 6 3 ) . In c o n t r a s t to t h e i r r e p o r t of more i n f e c t i o n s e s t a b l i s h e d i n c e r t a i n years ("peak periods") w i t h presumably b e t t e r c o n d i t i o n s f o r i n f e c t i o n , t h i s study has demonstrated a l o g a r i t h m i c increase of i n f e c t i o n s which i m p l i e s many successive years wi t h good c o n d i t i o n s f o r i n f e c t i o n . - 14 -"Peak p e r i o d s " of i n f e c t i o n have been found, however, during the i n i t i a l i n f e c t i o n of young stands (Hawksworth and Graham, 1963). These periods may be caused, I b e l i e v e , by i r r e g u l a r or infrequent p r o d u c t i o n of m i s t l e t o e seed on the o l d , i n f e c t e d t r e e s . This may e x p l a i n why m i s t l e t o e i n f e c t i o n s r a r e l y have been found i n stands 20 to 25 years o l d , adjacent to o l d e r , i n f e c t e d stands, i n c e n t r a l A l b e r t a (Dowding, 1929, p. 98) and i n the Highwood R i v e r v a l l e y (Near areas B and C) i n t h i s study. The data f o r area B (p. 11) al s o suggested that m i s t l e t o e populations may increase i r r e g u l a r l y at an e a r l y stage of the growth of the popu l a t i o n s . S h o r t l y t h e r e a f t e r , however, m i s t l e t o e popula-t i o n s may begin to increase r a p i d l y . The s c a r c i t y of i n f e c t i o n s on young t r e e s adjacent to o l d , i n f e c t e d stands was of considerable i n t e r e s t because at a distance of 100 f t . or more away from the apparently n o n - i n f e c t i v e stands i n the Highwood v a l l e y , many extensive areas of young i n f e c t e d t r e e s were found. These i n f e c t e d t r e e s were u s u a l l y found around one or more o l d t r e e s which were i n f e c t e d remnants of the previous stand. Weir (1916) and K u i j t (1955, P« 602) have speculated t h a t the i s o l a t e d , i n f e c t e d t r e e s may be more a c t i v e sources of new i n f e c t i o n s because of gre a t e r exposure to l i g h t . I t appears that the - 15 -d e s t r u c t i o n of i s o l a t e d , i n f e c t e d t r e e s should r e c e i v e high p r i o r i t y i n applying m i s t l e t o e c o n t r o l treatments, e s p e c i a l l y i n r e c e n t l y burned or logged areas. The most s i g n i f i c a n t r e s u l t of t h i s study was the s u c c e s s f u l use of a mathematical model to describe the progress of three dwarf m i s t l e t o e epidemics. This o b j e c t i v e approach should f a c i l i t a t e s t u d i e s of the i n f l u e n c e of environmental f a c t o r s on m i s t l e t o e epidemics. M u l t i p l e r e g r e s s i o n sampling could be used to i n v e s t i g a t e the e f f e c t of environmental f a c t o r s on the l o g a r i t h m i c r a t e of in c r e a s e . Some environmental f a c t o r s , such as stand d e n s i t y , were measured i n the present study, but t h e i r e f f e c t s were not s t u d i e d w i t h m u l t i p l e r e g r e s s i o n a n a l y s i s since only three populations were sampled. Nevertheless, the observed environmental d i f f e r e n c e s appeared unimportant s i n c e the r a t e s of increase were not s t a t i s t i c a l l y d i f f e r e n t (Table 4). This appears reasonable since approximately e q u a l - s i z e d areas of i n f e c t e d t r e e s were s e l e c t e d to be sampled. I t i s u n l i k e l y that a c t u a l d i f f e r e n c e s i n r a t e s of increase were concealed by c o l l e c t i n g an i n s u f f i c i e n t number of i n f e c t i o n s since very high c o r r e l a t i o n c o e f f i c i e n t s (0.96 to 0.99) were found. The three s i m i l a r r a t e s of increase and the d i f f e r e n t numbers of i s o l a t e d , i n f e c t e d t r e e s (6 to 12 i n B; 0 i n A and - 16 -C) suggested t h a t the o l d , i n f e c t e d t r e e s had c o n t r i b u t e d i n s i g n i f i c a n t l y - at l e a s t i n the recent past - to the increase of the m i s t l e t o e p o p u l a t i o n s . I t i s emphasized, however, that a l a r g e number of o l d , i n f e c t e d t r e e s may r e s u l t i n a l a r g e r number of d i s c r e t e epidemics or populations of dwarf m i s t l e t o e . Since some dwarf m i s t l e t o e epidemics can be des-c r i b e d by l o g a r i t h m i c curves, the epidemics could be s t u d i e d u s i n g Van der Plank*s (1963, 1965) approach. Many changes i n the b a s i c equations and many years of o b s e r v a t i o n would be necessary, however, to apply h i s comprehensive, mathematical models f o r p l a n t diseases, such as wheat stem r u s t , to dwarf m i s t l e t o e epidemics. The m u l t i p l e r e g r e s s i o n approach suggested i n t h i s study may provide i n f o r m a t i o n of value i n p r e d i c t i n g the development of dwarf m i s t l e t o e epidemics and the r e s u l t i n g damage to i n f e c t e d stands. - 17 -LITERATURE CITED Baranyay, J.A. 1962. Dowding, E.S. 1929. Freese, F. 1964. G i l l , L.S. 1935. . 1957-P h e n o l o g i c a l observations on western hemlock dwarf m i s t l e t o e (Arceuthobium  campylopodum G i l l forma t s u g e n s i s ) . Canada Department of F o r e s t r y B i -Monthly Progress Report 18(4):3-4. The v e g e t a t i o n of A l b e r t a . I l l The s a n d h i l l areas of c e n t r a l A l b e r t a w i t h p a r t i c u l a r reference to the ecology of Arceuthobium americanum Nutt. J o u r n a l of Ecology 17:82-105. Li n e a r r e g r e s s i o n models f o r f o r e s t research. U.S. Forest S e r v i c e Research Paper FPL 17. Madison, Wisconsin. 136pp. Arceuthobium i n the United S t a t e s . Connecticut Academy of A r t s and Sciences Transactions. 32:111-245. Dwarfmistletoe of lodgepole pine. U.S. Forest S e r v i c e . Forest Pest L e a f l e t 18. 7pp. and F. G. Hawksworth. 1961. The m i s t l e t o e s : a l i t e r a t u r e review. U. S. Forest S e r v i c e T e c h n i c a l B u l l e t i n 1242. 87pp. - 18 -Graham, S.O. 1964. D i f f e r e n t i a l s t a i n i n g of Arceuthobium t i s s u e s i n the host. (Abstr.) Phyto-pathology 54:1433. Greenham, C.G. and O.A. Leonard. 1965. The amino acid s of some m i s t l e t o e s and t h e i r hosts. American Journal of Botany 52:41-47-Hawksworth, F.G. I960. Growth r a t e of dwarf m i s t l e t o e i n -f e c t i o n s i n r e l a t i o n to the crown c l a s s of the host. U.S. Forest S e r v i c e , Rocky Mountain Forest and Range Experiment S t a t i o n . Research note 41* . and D. P. Graham 1963- Spread and i n t e n -s i f i c a t i o n of dwarf m i s t l e t o e i n lodgepole pine r e p r o d u c t i o n . J o u r n a l of F o r e s t r y 61:587-591-H u l l , R.J. and O.A. Leonard. 1964 P h y s i o l o g i c a l aspects of m i s t l e t o e s (Arceuthobium and Phoraden-dron). P l a n t Physiology 39:996-1017. K u i j t , J . 1955- Dwarf m i s t l e t o e s . B o t a n i c a l Review 21:569-627-.I960. Morphological aspects of p a r a s i t i s m i n the dwarf m i s t l e t o e s (Arceuthobium). U n i v e r s i t y of C a l i f o r n i a P u b l i c a t i o n s i n Botany 30:337-436. . 1961. Leaphart, CD.1959. . 1963. McDowell, L.L. 1964. M i e l k e , J.L. 1959. Parker, H.A. 1942. Quick, CR. 1964. - 19 -Observations on the l i f e c y c l e s i n Arceuthobium campylopodum. L e a f l e t s of Western Botany 9:133-134-This m i s t l e t o e gives the k i s s of death to t r e e s . Western Conservation J o u r n a l l 6 ( l ) : 4 4 - 4 7 . Dwarf m i s t l e t o e s : a s i l v i c u l t u r a l / challenge. J o u r n a l of F o r e s t r y 61: 4 0 - 4 6 . P h y s i o l o g i c a l r e l a t i o n s h i p s between dwarf m i s t l e t o e and ponderosa pine. Ph.D. Thesis Oregon State U n i v e r s i t y -U n i v e r s i t y M i c r o f i l m s . Ann Arbor, Michigan. ( D i s s e r t a t i o n A b s t r a c t s 25:53) I n f e c t i o n experiment w i t h Septogloeum  g i l l i i , a fungus p a r a s i t i c on dwarf m i s t l e t o e . J o u r n a l of F o r e s t r y 57: 925-926. C o n t r o l of M i s t l e t o e . Canada Department of Mines and Resources. Land, Parks and Forest Branch. S i l v i c u l t u r e L e a f l e t 15• 2pp. Experimental h e r b i c i d a l c o n t r o l of - 20 -dwarf m i s t l e t o e s on some C a l i f o r n i a c o n i f e r s . U. S. Forest S e r v i c e . P a c i f i c Southwest Forest and Range Experiment S t a t i o n . Research note 47 • 9pp. Scharpf, R.F. 1964. Epidemiology and p a r a s i t i s m of the dwarf m i s t l e t o e Arceuthobium cam-pylopodum Engelm. i n C a l i f o r n i a . Ph.D. Thesis.- U n i v e r s i t y of C a l i f o r n i a . U n i v e r s i t y M i c r o f i l m s . Ann Arbor, Michigan. ( D i s s e r t a t i o n A b s t r a c t s 24:3922) Smith, J.H.G. and G.R. B a i l e y . 1964. Influence of s t o c k i n g and stand d e n s i t y on crown widths of Douglas f i r and lodgepole pine. Commonwealth F o r e s t r y Review 43-' 243-246 Smith, R.B. 1965- Some r e s u l t s of a r t i f i c i a l i n o c u l a t i o n w i t h western hemlock dwarf m i s t l e t o e seeds. Canada Department of F o r e s t r y , Bi-Monthly Progress Report. 21(6) :3-4« Smithers, L.A. 1956. Assessment of s i t e p r o d u c t i v i t y i n dense lodgepole pine stands. Canada Department of Northern A f f a i r s and N a t i o n a l Resources. Forest Research - 21 -D i v i s i o n . T e c h n i c a l note no. 30- - 20pp. S r i v a s t a v a , L.M. and K. Esau. 1961. R e l a t i o n of dwarf m i s t l e t o e (Arceuthobium) to the xylem t i s s u e of c o n i f e r s . I . Anatomy of p a r a s i t e s i n k e r s and t h e i r connection w i t h host xylem. American J o u r n a l of Botany 48:159-167-S t e e l , R.G.D. and J.H. T o r r i e . I960. P r i n c i p l e s and pro-cedures of s t a t i s t i c s . McGraw-Hill Book Company, Inc. Toronto, Ontario. 481pp. Van der Plank, J.E. 1963. Pl a n t diseases: epidemics and c o n t r o l . Academic Press, Inc. New York, New York. 349 'pp. . 1965. Dynamics of epidemics of p l a n t disease. Science 147:120-124-Wagener, W.W. 1961. The i n f l u e n c e of l i g h t on the e s t a b l i s h -ment and growth of dwarf m i s t l e t o e on ponderosa and J e f f r e y pines. U. S. Forest S e r v i c e . P a c i f i c Southwest Forest and Range Experiment S t a t i o n . Research note 81. 5PP-. 1962 Dwarf m i s t l e t o e i n c u b a t i o n p e r i o d on ponderosa and J e f f r e y pines i n C a l i f o r n i a . Forest Science 8:16-20. - 22 -Weir, J.R. 1916. . M i s t l e t o e i n j u r y to c o n i f e r s i n the northwest. U. S. Department of A g r i c u l t u r e . B u l l e t i n 360. 36 pp. .1918. E f f e c t s of m i s t l e t o e on young c o n i f e r s . J o u r n a l of A g r i c u l t u r a l Research 12:715-718. 

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