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Climate and outbreaks of the forest tent caterpillar in Ontario Daniel, Colin John 1990

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CLIMATE AND OUTBREAKS OF THE FOREST TENT CATERPILLAR IN ONTARIO by COLIN JOHN DANIEL B.A.Sc, The University of Waterloo, 1985 A thesis submitted i n p a r t i a l f u l f i l l m e n t of the requirements for the degree of MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s as conforming to the required standard THE UNIVERSITY OF June, © Colin John BRITISH COLUMBIA 1990 Daniel, 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of ^tO^^ The University of British Columbia Vancouver, Canada DE-6 (2/88) ABSTRACT A review of the current understanding of f o r e s t tent c a t e r p i l l a r (Malacosoma disstria Hbn.) p o p u l a t i o n dynamics i n Ontario suggests that two c l i m a t i c f a c t o r s , the temperature at the time of l a r v a l feeding and the minimum temperature through the winter, p l a y important r o l e s i n determining outbreaks. Comparing the p a t t e r n of d e f o l i a t i o n t o s i m i l a r l y s c a l e d temperature records over 41 years i n Ontario shows no r e l a t i o n s h i p between the year t o year dynamics of outbreaks and e i t h e r the temperature through the l a r v a l feeding p e r i o d or the minimum o v e r w i n t e r i n g temperature. A long-term a n a l y s i s suggests t h a t outbreaks are l e s s severe i n those regions w i t h low o v e r w i n t e r i n g temperatures and a patchy d i s t r i b u t i o n of host. This l a t t e r f i n d i n g , combined w i t h an a n a l y s i s of the synchrony and spread of d e f o l i a t i o n , suggests t h a t a d u l t d i s p e r s a l may pl a y an important r o l e i n shaping the dynamics of outbreaks. i i TABLE OF CONTENTS LIST OF TABLES V LIST OF FIGURES v i ACKNOWLEDGEMENTS X 1. INTRODUCTION 1 2. CURRENT UNDERSTANDING 5 L i f e H i s t o r y 5 H i s t o r y of Outbreaks 6 Host Preference 7 Impact upon Hosts 8 Egg M o r t a l i t y 8 L a r v a l M o r t a l i t y 9 Pupal M o r t a l i t y 12 Adult D i s p e r s a l . • 13 Summary 14 3. PATTERN OF OUTBREAKS 16 D e f o l i a t i o n Maps 16 Maps and Insect Abundance 22 P a t t e r n of D e f o l i a t i o n 25 Summary 32 i i i TABLE OF CONTENTS (continued) 4. CLIMATE AND FOREST COMPOSITION 33 L a r v a l Feeding Temperature 34 Overwintering Temperature 5 6 Forest Composition and Long-Term Climate 63 Summary 7 8 5. DISCUSSION 79 L a r v a l Feeding Temperature 7 9 Overwintering Temperature 8 8 Host A v a i l a b i l i t y 91 Long-Term Climate 92 Other F a c t o r s 95 6. CONCLUSIONS AND RECOMMENDATIONS 105 7. LITERATURE CITED 107 APPENDIX A. Successive year overlays of d e f o l i a t i o n maps 113 APPENDIX B. Feeding degree-days and d e f o l i a t i o n f o r 16 s t a t i o n s 124 APPENDIX C. Minimum overwintering temperature and d e f o l i a t i o n f o r 16 s t a t i o n s 129 APPENDIX D. D e t a i l s of the data a n a l y s i s 134 i v LIST OF TABLES I . S t u d i e s p r o v i d i n g e v i d e n c e o f t h e r e l a t i o n s h i p between f o r e s t t e n t c a t e r p i l l a r o u t b r e a k s and t h e t e m p e r a t u r e t h r o u g h t h e e a r l y l a r v a l f e e d i n g p e r i o d 35 I I . A c t u a l h a t c h d a t e s and t h o s e p r e d i c t e d by t h e M i n n e s o t a model f o r v a r i o u s t i m e s and l o c a t i o n s i n O n t a r i o 40 D-I. P r o p o r t i o n o f a r e a d e f o l i a t e d each y e a r , 1948-88, f o r t h e 20 c l i m a t e s t a t i o n s 138 D - I I . The l o c a t i o n o f t h e A t m o s p h e r i c Environment S e r v i c e c l i m a t e s t a t i o n s used i n t h e a n a l y s i s . . 140 D - I I I . P r e d i c t e d h a t c h date each y e a r , 1948-88, f o r t h e 20 c l i m a t e s t a t i o n s 141 D-IV. F e e d i n g degree-days each y e a r , 1948-88, f o r t h e 20 c l i m a t e s t a t i o n s 143 D-V. Minimum o v e r w i n t e r i n g t e m p e r a t u r e each y e a r , 1948-88, f o r t h e 20 c l i m a t e s t a t i o n s 145 v L I S T OF FIGURES 1. Annual maps of the area w i t h i n which moderate t o severe d e f o l i a t i o n has occurred i n Ontario, 1948-1988 18 2. T o t a l area w i t h i n which moderate t o severe d e f o l i a t i o n has occurred each year i n Ontario, 1948-88 26 3. Composite maps showing the t o t a l extent of the moderate t o severe d e f o l i a t i o n f o r each of the four province-wide outbreaks from 1948-88 27 4. The number of outbreaks t h a t have occurred throughout the province from 1948-88 28 5. The l o c a t i o n of the c e l l s d e f i n i n g the 4 outbreak regions at each of 3 s c a l e s 30 6. The p r o p o r t i o n of land area d e f o l i a t e d w i t h i n each of the 4 outbreak regions, at 3 d i f f e r e n t s c a l e s . . 31 7. The l o c a t i o n of the 20 cli m a t e s t a t i o n s 36 8. The p r e d i c t e d hatch date each year f o r 4 of the 20 cli m a t e s t a t i o n s 41 9. The mean hatch date, 1948-88, f o r each of the 20 cli m a t e s t a t i o n s -. . 42 10. The mean number of feeding degree-days, 1948-88,^ f o r each of the 20 cli m a t e s t a t i o n s 45 11. The feeding degree-days each year f o r 4 of the 20 cli m a t e s t a t i o n s 46 v i L I S T OF FIGURES (continued) 12. The feeding degree-days and the proportion of area d e f o l i a t e d each year for 4 of the 20 climate stations 48 13. Relationship between the change i n percentage of de f o l i a t e d area and the feeding degree-days, for a l l stations and years with a non-zero change i n d e f o l i a t i o n 50 14. The e f f e c t of varying the c e l l size on the re l a t i o n s h i p between the change i n percentage of de f o l i a t e d area and the feeding degree-days, for a l l stations and years with a non-zero change i n d e f o l i a t i o n 51 15. The e f f e c t of changing the c l i m a t i c measure on the rel a t i o n s h i p between the change i n percentage of def o l i a t e d area and the feeding degree-days, for a l l stations and years with a non-zero change i n d e f o l i a t i o n 53 16. The d i s t r i b u t i o n of 4-year averages of feeding degree-days p r i o r to i n i t i a l increase years and i n i t i a l decrease years 55 17. The mean of the minimum overwintering temperature, 1948-88, for each of the 20 climate stations . . . . 58 18. The minimum overwintering temperature and the proportion of area defoliated each year for 4 of the 20 climate stations 59 v i i L I S T OF FIGURES (continued) 19. R e l a t i o n s h i p between the change i n percentage of d e f o l i a t e d area and the minimum ov e r w i n t e r i n g temperature, f o r a l l s t a t i o n s and years w i t h a non-zero change i n d e f o l i a t i o n 61 20. The e f f e c t of v a r y i n g the c e l l s i z e on the r e l a t i o n s h i p between the change i n percentage of d e f o l i a t e d area and the minimum o v e r w i n t e r i n g temperature, f o r a l l s t a t i o n s and years w i t h a non-zero change i n d e f o l i a t i o n 62 21. C e l l system f o r the f o r e s t inventory i n Ontario . . 65 22. The year i n which the stand i n f o r m a t i o n was recorded f o r each c e l l i n the f o r e s t inventory 66 23. P r o p o r t i o n of each c e l l ' s area c l a s s i f i e d as non-forested 68 24. P r o p o r t i o n of each c e l l ' s f o r e s t e d area t h a t i s c l a s s i f i e d as e i t h e r mixedwood or hardwood . . . . 69 25. Breakdown of the deciduous component i n each c e l l by genus 71 26. The long-term patterns of d e f o l i a t i o n and c l i m a t e , 1948-88, f o r each of the 20 c l i m a t e s t a t i o n s . . . . 76 27. R e l a t i o n s h i p between the number of years of d e f o l i a t i o n and the long-term c l i m a t e , 1948-88, f o r the 20 c l i m a t e s t a t i o n s 77 28. The feeding degree-days and the p r o p o r t i o n of area d e f o l i a t e d each year f o r Sioux Lookout 81 v i i i LIST OF FIGURES (continued) 29. The d i s t r i b u t i o n of maximum feeding degree-day values i n a p e r i o d 2-4 years p r i o r t o i n i t i a l i n c r e a s e years and i n i t i a l decrease years 84 30. The p r o b a b i l i t y of having an above average feeding-degree day value as a f u n c t i o n of the p e r i o d of successive years considered 85 31. The minimum ove r w i n t e r i n g temperature and the p r o p o r t i o n of area d e f o l i a t e d each year f o r Fort Frances 90 32. The feeding degree-days each year f o r p a i r s of neighbouring and d i s t a n t s t a t i o n s 97 33. The s p a t i a l correlograms f o r the annual values of feeding degree-days and minimum ov e r w i n t e r i n g temperature 100 i x ACKNOWLEDGEMENTS I would l i k e t o thank Gord Howse o f t h e F o r e s t I n s e c t and D i s e a s e Survey (FIDS) f o r h i s s u p p o r t and h o s p i t a l i t y , and f o r p r o v i d i n g me w i t h t h e d e f o l i a t i o n and h a t c h d a t a . Mike A p p l e j o h n , Dave C o n s t a b l e , Wayne Ingram, Bob S a j a n and A l K e i z e r showed me t h e f i e l d s i d e o f t h e Survey, f o r w h i c h I am g r a t e f u l . I would a l s o l i k e t o thank t h e FIDS Technology Development Group a t t h e Petawawa N a t i o n a l F o r e s t r y I n s t i t u t e f o r t h e i r s u p p o r t and h o s p i t a l i t y : Mike Power f o r p r o v i d i n g me w i t h a c c e s s t o t h e i r computer f a c i l i t i e s ; L a r r y O ' Brien and C o l i n B r e t h o u r f o r t h e i r u n e n d i n g h e l p w i t h my l o n g d i s t a n c e computing. The t e m p e r a t u r e d a t a was p r o v i d e d by Mike Webb o f t h e A t m o s p h e r i c Environment S e r v i c e , w h i l e John Osborne o f t h e O n t a r i o M i n i s t r y o f N a t u r a l R e s o u r c e s a l l o w e d me t o use t h e f o r e s t i n v e n t o r y . My s t u d i e s a t U.B.C. were s u p p o r t e d by a N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l p o s t g r a d u a t e s c h o l a r s h i p . I would l i k e t o thank Tad U l r i c h , Tim Webb and Don Ludwig f o r t h e i r d i s c u s s i o n s . J o s h Korman and B r i a n K l i n k e n b e r g c o n t r i b u t e d b o t h t h r o u g h d i s c u s s i o n s and by r e a d i n g my t h e s i s d r a f t . My many s u p e r v i s o r s , Buzz H o l l i n g , C a r l W a l t e r s and Judy Myers, each c o n t r i b u t e d i n t h e i r own way t o my e c o l o g i c a l u p b r i n g i n g . F i n a l l y , thank you C h e r y l f o r y o u r p a t i e n c e and s u p p o r t . x CHAPTER 1 INTRODUCTION Four b l i n d men are l e d i n t o a cou r t y a r d t o experience an elephant f o r the f i r s t time. The f i r s t grasps the trunk and declares t h a t elephants are f i r e hoses. The second touches an ear and maintains t h a t elephants are rugs. The t h i r d walks i n t o i t s s i d e and b e l i e v e s t h a t elephants are a k i n d of w a l l . The f o u r t h f e e l s a l e g and decides t h a t elephants are p i l l a r s . Parable, from O ' N e i l l et al. (1986, p.3) Many s t u d i e s of f o r e s t i n s e c t p o p u l a t i o n dynamics can be c l a s s i f i e d as " l o c a l " ; i . e . smaller i n both temporal and s p a t i a l s c a l e than many of the pat t e r n s whose u n d e r l y i n g mechanisms they are t r y i n g t o uncover. Consider, f o r example, s t u d i e s of the po p u l a t i o n dynamics of the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria Hbn. Using p l o t s of a few hectares monitored over 3 to 5 years, v a r i o u s c l i m a t i c f a c t o r s have been a s s o c i a t e d w i t h year-to-year changes i n m o r t a l i t y (Hodson, 1941; W i t t e r et al., 1975). These st u d i e s have provided some i n s i g h t i n t o the p o p u l a t i o n dynamics that occur over an area the s i z e of a stand and over a time span of a few years. Now suppose that we are i n t e r e s t e d i n e x p l a i n i n g the year-to-year changes i n abundance, but over an area t h a t spans s e v e r a l thousand square k i l o m e t r e s . We have now expanded our s p a t i a l s c a l e w e l l beyond t h a t of the l o c a l s t u d i e s . While c l i m a t i c f a c t o r s may be capable of indu c i n g changes i n m o r t a l i t y i n a p a r t i c u l a r l o c a t i o n or year, i t i s - 1 -not c l e a r whether they are s i g n i f i c a n t i n determining the changes i n abundance that have occurred over t h i s l a r g e r area. For example, s e v e r a l l a r g e r s c a l e s t u d i e s have noted t h a t f o r e s t t e n t c a t e r p i l l a r outbreaks seem to spread over time from l o c a l areas of i n f e s t a t i o n (Brown, 1938; S i p p e l l , 1962; Hodson, 1977). This suggests t h a t d i s p e r s a l may be important i n determining the s p a t i a l p r o g r e s s i o n of outbreaks. The r o l e of d i s p e r s a l d i d not become evident, however, u n t i l one looked beyond separate p l o t s and examined s e v e r a l s u c c e s s i v e r e g i o n a l d e f o l i a t i o n maps. Having expanded our s p a t i a l s c a l e from stand t o region, we can a l s o expand the temporal s c a l e . Instead of l o o k i n g at the year-to-year changes i n abundance, consider the dynamics over three or four outbreaks. Do we d i s c o v e r any f a c t o r s at t h i s s c a l e t h a t went unnoticed at previous ones? S i p p e l l (1962), f o r example, looked at three successive f o r e s t t e n t c a t e r p i l l a r outbreaks i n Ontario and n o t i c e d t h a t d e f o l i a t i o n had never occurred i n some p a r t s of the pr o v i n c e . However, no attempt was made to e x p l a i n t h i s p a t t e r n . Are the dynamics at t h i s s c a l e i n f l u e n c e d by the f o r e s t composition across the province? Or p o s s i b l y climate? Another example of the importance of s c a l e i n f o r e s t entomology i n v o l v e s eastern spruce budworm, Choristoneura fumiferana (Clem.), research. While many s t u d i e s , most notably the Green R i v e r p r o j e c t (Morris, 1963), have s t u d i e d the p o p u l a t i o n dynamics l o c a l l y , other work has examined the -2-pattern of outbreaks at larger scales. B l a i s (1968) uncovered outbreak patterns i n tree rings across eastern North America over the l a s t two centuries, and suggested that recent outbreaks have been increasing i n frequency, extent and sev e r i t y . This change i n the pattern of outbreaks has since been associated with changes i n forest composition, through harvesting, f i r e protection and refo r e s t a t i o n (Blais, 1983). Other research has re l a t e d large scale outbreak patterns, using d e f o l i a t i o n maps for 1938-1980 from across a l l of eastern North America, to s i m i l a r l y scaled vegetative and c l i m a t i c information (Hardy, 1984; Hardy et al., 1986). The results of these analyses further emphasize the pot e n t i a l large scale importance of climate and forest composition. It appears that carrying out analyses at more than one scale should be enlightening. However, for most forest insects r e l a t i v e l y few studies have been done at larger s p a t i a l and temporal scales, presumably due to the d i f f i c u l t y i n f i n d i n g and analysing large scaled information. One system for which such data e x i s t i s the forest tent c a t e r p i l l a r i n Ontario. These data, when combined with other l o c a l studies, o f f e r an opportunity to explore the dynamics of a system over larger s p a t i a l and temporal scales, and form the basis for t h i s t h e s i s . The objective of t h i s thesis i s to assess the contribution of large scaled information to our -3-understanding of the po p u l a t i o n dynamics of the f o r e s t tent c a t e r p i l l a r i n Ontario. To provide a b a s e l i n e of understanding from which I can judge the added c o n t r i b u t i o n of t h i s i n f o r m a t i o n , I review the l i f e h i s t o r y and the current thoughts on p o p u l a t i o n dynamics of the f o r e s t t e n t c a t e r p i l l a r i n Chapter 2. I then present 41 years of province-wide maps of f o r e s t t e n t c a t e r p i l l a r d e f o l i a t i o n i n Chapter 3. The maps provide only a coarse p i c t u r e of f o r e s t t e n t c a t e r p i l l a r abundance, and t h i s chapter begins w i t h a d i s c u s s i o n of the u n c e r t a i n t y i n the map data and the i m p l i c a t i o n s f o r my a n a l y s i s . These data are then used t o explore how the dynamics of outbreaks vary as a f u n c t i o n of the s p a t i a l and temporal s c a l e over which they are measured. In Chapter 4 I present the r e s u l t s of my analyses u s i n g the d e f o l i a t i o n maps, i n conjunction w i t h s i m i l a r l y s c a l e d c l i m a t i c and v e g e t a t i v e i n f o r m a t i o n , t o explore some of the c l i m a t i c f a c t o r s that have been p r e v i o u s l y a s s o c i a t e d w i t h l o c a l changes i n abundance. F i n a l l y , i n Chapter 5, I discuss the f i n d i n g s of Chapters 3 and 4 i n l i g h t of the ideas presented i n the l i t e r a t u r e review. - 4 -CHAPTER 2 CURRENT UNDERSTANDING A number of s t u d i e s of the f o r e s t t e n t c a t e r p i l l a r have been c a r r i e d out i n and around Ontario over the l a s t f i f t y years. From these s t u d i e s a consensus has been formed regarding the p o p u l a t i o n dynamics. The f o l l o w i n g chapter reviews t h i s c urrent understanding. L i f e H i s t o r y The range of the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria Hiibner, extends from P.E.I, t o B r i t i s h Columbia and from James Bay t o L o u i s i a n a (Stehr and Cook, 1968) . In Ontario, egg l a y i n g by the female moth u s u a l l y occurs i n e a r l y t o mid-July. Female moths l a y a s i n g l e egg mass c o n t a i n i n g approximately 200 eggs. The egg mass i s l a i d as a band on the small twigs of v a r i o u s deciduous t r e e species, u s u a l l y i n the upper crown (Witter, 1979). Embryonic development begins immediately and continues f o r about three weeks u n t i l the pharate l a r v a e are f u l l y formed w i t h i n the egg. Regardless of the temperature, the pharate l a r v a e then undergo an o b l i g a t o r y 3 month diapause (Hodson and Weinman, 1945). The post-diapause pharate l a r v a e remain dormant through the w i n t e r , w i t h egg hatch c o i n c i d i n g roughly w i t h the budburst of the host t r e e s i n May. -5-The l a r v a l p e r i o d u s u a l l y l a s t s about 6 weeks, w i t h 95% of the feeding o c c u r r i n g i n the f o u r t h and f i f t h i n s t a r s (Hodson, 1941). Through the e a r l y i n s t a r s the c a t e r p i l l a r s are gregarious, feeding together on a s i n g l e t r e e . By the l a t e f o u r t h and f i f t h i n s t a r s they become more independent i n t h e i r feeding patterns and, given a shortage i n the food supply, w i l l wander between t r e e s i n search of s u i t a b l e f o l i a g e . The l a t e i n s t a r l a r v a e w i l l feed upon most of the deciduous t r e e s n a t i v e to c e n t r a l and northern Ontario. By mid- t o late-June the f i f t h i n s t a r l a r v a e form cocoons i n r o l l e d leaves of any remaining v e g e t a t i o n . They remain as pupae f o r about 2 weeks, emerging as a d u l t moths i n e a r l y t o mid-July. The a d u l t f l y i n g p e r i o d l a s t s f o r 1-2 weeks, w i t h the moths becoming a c t i v e i n l a t e afternoon and e a r l y evening and c o n t i n u i n g t h e i r a c t i v i t y throughout the n i g h t . History of Outbreaks Records from 18 67 to 1982 suggest t h a t there have been 12 outbreaks i n Ontario during t h i s p e r i o d , w i t h the time between the onset of each outbreak ranging from 6 to 14 years ( S i p p e l l , 1962; Annual Reports of the Forest Insect and Disease Survey f o r 1948-1982). S i p p e l l (1962) described an outbreak at a t y p i c a l l o c a t i o n i n Ontario as c o n s i s t i n g of 2-3 years of i n c r e a s i n g p o p u l a t i o n s , 1-2 years of high -6-p o p u l a t i o n s and 1 year of d e c l i n e . Brown (1938), S i p p e l l (1962) and Hodson (1977) have a l l found t h a t outbreaks g e n e r a l l y spread south and east over time, corresponding t o the d i r e c t i o n of p r e v a i l i n g winds. Host Preference The p r e f e r r e d host species of the f o r e s t t e n t c a t e r p i l l a r v a r i e s across i t s range. In the northern and western p a r t s of Ontario, which are p r i m a r i l y w i t h i n the Borea l F o r e s t , t r e m b l i n g aspen (Populus tremuloides) i s the p r e f e r r e d choice of egg l a y i n g moths ( S i p p e l l , 1957). This i s a l s o t r u e of the P r a i r i e provinces ( H i l d a h l and Campbell, 1975) and northern Minnesota (Hodson, 1941). As one moves f u r t h e r south, i n t o a region w i t h mixed c o n i f e r o u s and deciduous t r e e s , the preference f o r egg l a y i n g s h i f t s t o a l s o i n c l u d e sugar maple, Acer saccharum, and red oak, Quercus rubrus (Forest Insect and Disease Survey, pers. comm.). Other t r e e species on which eggs have been observed to hatch and subsequently feed i n c l u d e ( S i p p e l l , 1957; Connola et al., 1957): - w i l l o w (Salix species) ; - l a r g e t o o t h aspen (Populus grandidentata); - p i n cherry (Frunus pennsylvanica); -black cherry (Prunus serotina); -apple (Malus s p e c i e s ) . -7-While eggs and young la r v a e are confined p r i m a r i l y to the aforementioned species, wandering l a t e i n s t a r l a r v a e w i l l feed upon v i r t u a l l y a l l deciduous t r e e s n a t i v e t o Ontario. L i t t l e i s known about the f a c t o r s t h a t i n f l u e n c e the s e l e c t i o n of host species by the egg l a y i n g moths, although Hodson (1941) and S i p p e l l (1957) have both suggested t h a t the t i m i n g of budbreak may be an important f a c t o r i n determining e a r l y l a r v a l m o r t a l i t y . Impact upon Hosts Two s t u d i e s have assessed the host m o r t a l i t y and growth l o s s caused by f o r e s t t e n t c a t e r p i l l a r d e f o l i a t i o n (Duncan and Hodson, 1958; H i l d a h l and Reeks, 1960) . Both found t h a t s e v e r a l years of d e f o l i a t i o n caused some growth l o s s , w i t h growth r e t u r n i n g t o p r e - d e f o l i a t i o n l e v e l s i n the second year f o l l o w i n g an outbreak's c o l l a p s e . Tree m o r t a l i t y was g e n e r a l l y considered n e g l i g i b l e . Egg M o r t a l i t y Most s t u d i e s have found egg m o r t a l i t y t o be r e l a t i v e l y low through the course of an outbreak. M o r t a l i t y due t o egg p a r a s i t i s m has been found to range from 0 t o 10% (Hodson, 1941; Connola et al., 1957; Ives, 1971; W i t t e r and Kulman, 1972). One c l i m a t i c f a c t o r t h a t has been l i n k e d t o higher egg -8-m o r t a l i t y i s extremely low winter temperatures. A study of the seasonal v a r i a t i o n i n g l y c e r o l content of eggs found that they could be cooled t o -41°C i n January without f r e e z i n g (Hanec, 1966). A subsequent study i n northern Minnesota found egg m o r t a l i t y , from f a c t o r s other than p a r a s i t i s m and i n f e r t i l i t y , t o vary from 0 t o 65% over a 9 year p e r i o d (Witter et a l . , 1975). Egg m o r t a l i t y f o r the 5 years f o r which the temperature d i d not drop below -4 0°C ranged from 0 to 10%, while i n the 4 years where temperatures dropped below t h i s t h r e s h o l d m o r t a l i t y ranged from 10 t o 65%. Hodson (1941), however, has reported the presence of l a r v a e i n areas which experienced temperatures as low as -47°C the previous w i n t e r . Other experiments examining the pre-hatch e f f e c t s of temperature have shown th a t eggs kept below 5°C w i l l never hatch (Hodson and Weinman, 1945), and eggs can s u r v i v e exposure to c o l d temperatures (as low as -20°C) a few days before hatching (Wetzel et a l . , 1973; Raske, 1975). L a r v a l M o r t a l i t y C l i m a t i c f a c t o r s Laboratory experiments have shown th a t l a r v a e are able to s u r v i v e temperatures as low as -12°C immediately a f t e r hatch (Ives, 1971). As prolonged extreme temperatures such -9-as these are r a r e l y encountered i n the f i e l d through the l a r v a l stage, they are not thought t o be d i r e c t l y r e s p o n s i b l e f o r much l a r v a l m o r t a l i t y (Raske, 1975). S t a r v a t i o n There are s e v e r a l r e p o r t s of s p r i n g storms d e s t r o y i n g host f o l i a g e and subsequently causing e a r l y i n s t a r l a r v a l m o r t a l i t y through s t a r v a t i o n (Hodson, 1941; B l a i s et al., 1955; Connola et al., 1957). Two of these r e p o r t s a l s o i n c l u d e a mention of areas adjacent t o those whose f o l i a g e was destroyed where s t a r v a t i o n d i d not occur (Hodson, 1941; B l a i s et al., 1955). In both cases l a r g e bodies of water provided c o o l e r s p r i n g temperatures i n the adjacent areas, d e l a y i n g the hatch and host budburst u n t i l a f t e r the storm. The i n d i r e c t e f f e c t of temperature upon l a r v a l feeding may a l s o cause s i g n i f i c a n t l a r v a l m o r t a l i t y . Hodson (1941) found t h a t at temperatures below 15°C l a r v a e do l i t t l e or no feeding, and t h a t most of the d e f o l i a t i n g i s done on days when the d a i l y maximum temperature exceeded 23°C. Wellington (1952) observed t h a t l a r v a e were most a c t i v e i n warm, humid weather, and suggested t h a t years of moist, warm weather during the l a r v a l stage are favourable f o r p o p u l a t i o n i n c r e a s e s . By examining the maps of c y c l o n i c centres at 2 l o c a t i o n s , W e l l i n g t o n (1952) showed that i n the years p r i o r t o outbreaks there was an above average number of passages of warm, moist a i r masses during the l a r v a l stage. Ives -10-(1973) explored the r e l a t i o n s h i p between v a r i o u s temperature i n d i c e s and outbreaks, f o r 10 l o c a t i o n s , over a 40 year p e r i o d . He suggested that p o p u l a t i o n increases were preceded by a s i n g l e year (2 to 4 years p r i o r ) w i t h above average temperatures i n the 3 weeks f o l l o w i n g hatch, w h i l e p o p u l a t i o n decreases were preceded by a year w i t h below average temperatures (same year, or 1 to 2 years p r i o r ) . Hodson (1977) compared temperature records d u r i n g the 3 weeks f o l l o w i n g egg hatch t o c a t e r p i l l a r abundance over the course of 2 outbreaks at 2 l o c a t i o n s i n Minnesota. He found th a t temperatures were g e n e r a l l y above average 2 to 3 years p r i o r t o the s t a r t of outbreaks, and below average i n the years of decreasing abundance. F i n a l l y , l a r v a l m o r t a l i t y i n high d e n s i t y p opulations has a l s o been a t t r i b u t e d to l a t e i n s t a r s t a r v a t i o n , as l a r v a e can exhaust t h e i r food supply before t h e i r development i s complete (Hodson, 1977; S i p p e l l , 1957). Disease Nuclear p o l y h e d r o s i s v i r u s has been p r e v a l e n t i n some po p u l a t i o n s , w i t h e p i z o o t i c s (and subsequent l a r v a l m o r t a l i t y ) most o f t e n o c c u r r i n g i n populations t h a t have been at high d e n s i t i e s f o r s e v e r a l generations ( S t a i r s , 1972). V i r u s i s t r a n s m i t t e d on the t r e e s and egg masses (Clark, 1958), and the pupal p a r a s i t e Sarcophaga aldrichi may a l s o be capable of t r a n s m i t t i n g v i r u s ( S t a i r s , 1966). -11-P r e d a t i o n A study of b i r d p r edation during one year of a northern Minnesota outbreak showed th a t a wide v a r i e t y of b i r d s w i l l feed upon f o r e s t t e n t c a t e r p i l l a r l a r v a e (Fashingbauer et al., 1957). While m o r t a l i t y due t o p r e d a t i o n i s thought t o be n e g l i g i b l e at high d e n s i t i e s , l i t t l e i s known of i t s importance i n endemic years. Pupal M o r t a l i t y Several s t u d i e s of pupal p a r a s i t i s m have found t h a t r a t e s i n c r e a s e over the course of an outbreak (Hodson, 1941; Connola et al., 1957; S i p p e l l , 1957; Ives, 1971; W i t t e r et al., 1975; Hodson, 1977). The general consensus i s t h a t these r a t e s reach t h e i r highest l e v e l s only a f t e r a p o p u l a t i o n d e c l i n e has been i n i t i a t e d by some other f a c t o r . The dominant pupal p a r a s i t e i s the f l e s h f l y , Sarcophaga aldrichi Parker. While the p r e f e r r e d host of S. aldrichi i s the f o r e s t tent c a t e r p i l l a r , i t has a l s o been reared from the gypsy moth, Porthetria dispar, the s a t i n moth, Stilpnotia salicls, and the eastern spruce budworm, Choristoneura fumiferana (Arthur and Coppel, 1953). Adult f l i e s appear i n mid to l a t e May, w i t h the p r e l a r v i p o s i t i o n p e r i o d l a s t i n g 3-4 weeks. Once l a r v i p o s i t i o n begins the female f l y d e p o s i t s s i n g l e l a r v a i n t e r m i t t e n t l y onto the -12-cocoons of the f o r e s t tent c a t e r p i l l a r . A f t e r 8-12 days the f u l l y fed l a r v a drops to the ground, where i t forms a puparium and overwinters (Hodson, 1939). S i p p e l l (1957) recorded r a t e s of pupal p a r a s i t i s m at 16 d i f f e r e n t l o c a t i o n s across Ontario over the course of an outbreak. He found t h a t the r a t e at which cocoons were p a r a s i t i z e d by S. aldrichi went from an average of about 20% i n the f i r s t outbreak year to 75% i n the f o u r t h , w i t h r a t e s of t o t a l pupal p a r a s i t i s m f o r most s i t e s i n excess of 90% i n the l a t e r outbreak years. Hodson (1941) found t h a t r a t e s of p a r a s i t i s m i n c r e a s e d with d i s t a n c e from the centre of l o c a l outbreaks, and concluded t h a t S. aldrichi p o p u l a t i o n s increase r a p i d l y where there are high host numbers and then extend t h e i r range i n t o surrounding areas. A d u l t D i s p e r s a l L i t t l e i s known about the d i s p e r s a l p a t t e r n s of a d u l t moths. Hodson (1941) found t h a t a d u l t s of both sexes l i v e d f o r about 5 days, w i t h moth emergence g e n e r a l l y o c c u r r i n g over a two week p e r i o d . Brown (1965) found moths i n A l b e r t a t h a t had been c a r r i e d f o r at l e a s t 450 km by the t u r b u l e n t a i r a s s o c i a t e d w i t h a c o l d f r o n t , while Greenbank (1954) estimated another mass movement of moths to have occurred over a d i s t a n c e of about 80 km. -13-Summary The f o l l o w i n g f a c t o r s are c u r r e n t l y thought t o i n f l u e n c e the p o p u l a t i o n dynamics of the f o r e s t t e n t c a t e r p i l l a r i n Ontario: 1. L a r v a l feeding temperature There i s evidence suggesting t h a t temperature dur i n g the e a r l y l a r v a l feeding p e r i o d i s an important f a c t o r i n determining l a r v a l s u r v i v a l , w i t h warm temperatures being favourable t o p o p u l a t i o n i n c r e a s e s . The s t u d i e s p r o v i d i n g t h i s evidence have a l l been c o r r e l a t i v e , r e l a t i n g p a t t e r n s of outbreaks t o temperature records. However, r e s u l t s of such c l i m a t i c s t u d i e s are ofte n d i f f i c u l t t o i n t e r p r e t (Martinat, 1987; Myers, 1988), p a r t i c u l a r l y when the number of a s s o c i a t i o n s considered are high (such as Ives, 1973), or the number of observations are few (as w i t h W e l l i n g t o n , 1952; B l a i s et al., 1955; Hodson, 1977). 2. Overwintering temperature A second c l i m a t i c hypothesis i s t h a t pharate l a r v a l m o r t a l i t y i n c r e a s e s sharply i n those years when wi n t e r temperatures drop below -40°C (Witter et al., 1975). This f i n d i n g , however, i s based upon measurements made at only a s i n g l e l o c a t i o n , over the course of one outbreak. -14-3. Pupal p a r a s i t i s m The r a t e of pupal p a r a s i t i s m has been found t o r i s e over the course of an outbreak and may c o n t r i b u t e to outbreak c o l l a p s e s . I t i s not c l e a r , however, whether the p a r a s i t e s cause the pop u l a t i o n changes or simply respond to them. 4. Food supply and disease Disease and exhaustion of the food supply k i l l l a t e i n s t a r l a r v a e i n some high d e n s i t y populations and have c o n t r i b u t e d t o the c o l l a p s e of some outbreaks. 5. Adult d i s p e r s a l D e f o l i a t i o n maps f o r v a r i o u s outbreaks show an apparent year-to-year spread i n the area d e f o l i a t e d , suggesting that moth d i s p e r s a l may play an important r o l e i n determining the s p a t i a l p r o g r e s s i o n of outbreaks. -15-CHAPTER 3 PATTERN OF OUTBREAKS The f o l l o w i n g chapter presents the f o r e s t t e n t c a t e r p i l l a r d e f o l i a t i o n maps f o r Ontario from 1948 t o 1988. The f i r s t s e c t i o n o u t l i n e s the methods used t o generate these maps. This i s fol l o w e d by an assessment of the q u a l i t y of t h i s i n f o r m a t i o n : the r e l a t i o n s h i p between the maps and tr u e i n s e c t abundance, and the s c a l e at which the maps can be meaningfully i n t e r p r e t e d . F i n a l l y , the maps are used t o provide a p i c t u r e of the province-wide s p a t i a l and temporal p a t t e r n of outbreaks. D e f o l i a t i o n Maps Since 1948, rangers from the Forest Insect and Disease Survey at F o r e s t r y Canada's Great Lakes F o r e s t r y Centre have been mapping the extent of d e f o l i a t i o n by the f o r e s t t e n t c a t e r p i l l a r i n Ontario. This mapping i s done by f o l l o w i n g f l i g h t l i n e s 5-10 km apart back and f o r t h across areas suspected of being d e f o l i a t e d and sketc h i n g the areas t h a t show moderate t o severe d e f o l i a t i o n . The mapping i s done each year a f t e r the la r v a e have f i n i s h e d t h e i r feeding and before the host t r e e s r e f o l i a t e . L o c a t i n g the exact boundary between d e f o l i a t e d and non-d e f o l i a t e d areas i s somewhat s u b j e c t i v e , and v a r i e s -16-according t o each mapper's i n t e r p r e t a t i o n of "moderate to severe" and the t i m i n g of the f l i g h t w i t h respect to i n s e c t feeding and host r e f o l i a t i o n . I f a l a r g e area has been d e f o l i a t e d , most sketch mappers w i l l not t r y t o d e l i n e a t e n o n - d e f o l i a t e d stands (often non-host species) w i t h i n the general i n f e s t a t i o n boundaries. Conversely, small pockets of d e f o l i a t i o n i n l o c a t i o n s f a r from any previous d e f o l i a t i o n may not be recorded. Each ranger maps the d e f o l i a t i o n onto topographic base maps, u s u a l l y at a sc a l e of 1:100,000. The maps of a l l the rangers i n Ontario are then combined each year t o create a s i n g l e province-wide map, u s u a l l y at a s c a l e of 1:1,584,000 (1 inch=25 m i l e s ) . In some years areas of d e f o l i a t i o n were mapped according t o three l e v e l s of s e v e r i t y ( l i g h t , moderate and severe), while i n other years d e f o l i a t i o n was mapped according t o only one (moderate t o severe). To compare one year to the next, or one outbreak t o the next, I have dropped the l i g h t category from those maps where i t e x i s t e d , and combined the moderate and severe c a t e g o r i e s . The 41 annual maps were then d i g i t i z e d u s i n g the ARC/INFO Geographic Information System; the r e s u l t i n g maps are shown i n Figu r e 1. A l l of the maps i n t h i s t h e s i s are d i s p l a y e d using a Lambert Conformal p r o j e c t i o n (standard p a r a l l e l s 44.5° and 53.5°, c e n t r a l meridian -85°). -17-F i g u r e 1. A n n u a l maps o f t h e a r e a w i t h i n w h i c h moderate t o s e v e r e d e f o l i a t i o n has o c c u r r e d i n O n t a r i o , 1948-1988. Figure 1. (continued) Figure 1. (continued) Figure 1. (continued) km 0 500 1000 Maps and Insect Abundance The d e f o l i a t i o n maps provide only a coarse p i c t u r e of the changes i n abundance of the f o r e s t tent c a t e r p i l l a r i n Ontario. Before u s i n g the maps to make in f e r e n c e s about the dynamics of the i n s e c t i n Ontario, i t would be u s e f u l t o disc u s s the q u a l i t y of t h i s i n f o r m a t i o n . F i r s t c o n sider the r e l a t i o n s h i p between a p a r t i c u l a r l e v e l of d e f o l i a t i o n and f o r e s t tent c a t e r p i l l a r abundance each year. R e c a l l (from Chapter 2) t h a t 95% of the feeding occurs i n the 4th and 5th i n s t a r s (Hodson, 1941). As d e f o l i a t i o n i s mapped at the end of the l a r v a l p e r i o d each year, the l e v e l of d e f o l i a t i o n i s thus an index of the l a t e i n s t a r l a r v a l abundance. A study i n northern Minnesota, where l i k e much of Ontario t r e m b l i n g aspen i s the primary host species, determined t h a t each c a t e r p i l l a r consumes approximately 8.5 leaves i n i t s l i f e t i m e (Hodson, 1941). The study a l s o estimated the number of leaves per t r e m b l i n g aspen t r e e as a f u n c t i o n of t r e e diameter; t h i s v a r i e d from about 2 0 0 0 t o 2 0 0 0 0 f o r the range of t r e e diameters i n t h e i r study p l o t s (2.5 cm to 18 cm diameter at breast h e i g h t ) . In another northern Minnesota study, W i t t e r (1971) determined t h a t a t y p i c a l aspen dominated stand had an average t r e e diameter of 8.7 cm, which corresponds to an average of 1 0 0 0 0 leaves per t r e e . From these f i g u r e s one can estimate t h a t the number of l a r v a e r e q u i r e d t o completely d e f o l i a t e an - 2 2 -area would need to be about 1200 l a r v a e per host t r e e (10000 leaves per t r e e / 8.5 leaves per l a r v a e ) . This r e l a t i o n s h i p would vary somewhat from stand to stand according t o the s i z e s t r u c t u r e of i t s aspen component. I t may a l s o be q u i t e d i f f e r e n t i n the southern part of the province, where sugar maple and oak are a l s o important host species, as the l a r v a l feeding requirements f o r these other species may d i f f e r from those of t r e m b l i n g aspen. For example, a study of f o r e s t t e n t c a t e r p i l l a r feeding upon t u p e l o gum (Nyssa aquatica L.) t r e e s i n southern L o u i s i a n a found t h a t each l a r v a e consumed 3 times more l e a f area than was consumed f o r the t r e m b l i n g aspen i n Minnesota (Smith et al., 1986). These problems aside, we now have a rough estimate of the l a t e l a r v a l d e n s i t i e s r e q u i r e d to cause complete d e f o l i a t i o n i n an area. However, the sketch maps of Figure 1 do not show complete d e f o l i a t i o n , but r a t h e r areas w i t h i n which moderate to severe d e f o l i a t i o n has occurred. So what i s the r e l a t i o n s h i p between an area marked as moderately to s e v e r e l y d e f o l i a t e d and the percentage d e f o l i a t i o n ? Based upon the d i f f e r e n c e between the weights of leaves on d e f o l i a t e d and n o n - d e f o l i a t e d t r e e s , Ives (1971) found t h a t the t r u e d e f o l i a t i o n f o r stands c l a s s i f i e d from the a i r as moderately to s e v e r e l y d e f o l i a t e d ranged from 60-100%. This would correspond to l a r v a l d e n s i t i e s of at l e a s t -23-700-1200 l a r v a e per t r e e i n the areas t h a t are c l a s s i f i e d as moderately t o s e v e r e l y d e f o l i a t e d . Note t h a t t h i s i s the minimum l a r v a l d e n s i t y which w i l l cause moderate to severe d e f o l i a t i o n ; Hodson (1941) has rep o r t e d l a r v a l d e n s i t i e s as h i g h as 8000 per t r e e i n some stands. But the sketch mappers f i l t e r out s m a l l e r pockets of d e f o l i a t e d and n o n - d e f o l i a t e d areas; they do not always d e l i n e a t e n o n - d e f o l i a t e d areas w i t h i n l a r g e r i n f e s t a t i o n s and can miss s m a l l e r i s o l a t e d pockets of d e f o l i a t i o n . This i m p l i e s t h a t the u n c e r t a i n t y i n l a r v a l d e n s i t i e s a s s o c i a t e d w i t h a p a r t i c u l a r l e v e l of d e f o l i a t i o n i s s c a l e dependent. At a stand l e v e l the maps have a r e l a t i v e l y weak l i n k t o l a r v a l d e n s i t i e s ; when a p a r t i c u l a r stand i s mapped as d e f o l i a t e d , i t does not n e c e s s a r i l y mean tha t there were high l a r v a l d e n s i t i e s . This suggests t h a t the maps w i l l be of more use f o r l a r g e r s c a l e d analyses. I t i s d i f f i c u l t t o decide upon the minimum s c a l e of a n a l y s i s , however, without knowing more about the f i l t e r i n g process of the rangers. Because of the s c a l e at which the rangers f l y t h e i r f l i g h t l i n e s (5-10km), and the s c a l e at which the province-wide maps were produced (1:1,584,000), i n t e r p r e t i n g the maps at a s c a l e as f i n e as 10km by 10km would be of l i t t l e v a l u e . A s c a l e of about 100km by 100km i s probably more ap p r o p r i a t e . F i n a l l y , w h i l e the maps give us a crude measure of the absolute abundance of l a t e i n s t a r l a r v a e f o r an area i n any given year, they are probably best used as a measure of the -24-change i n abundance at a p a r t i c u l a r l o c a t i o n from one year t o the next. For example, consider a 100km by 100km area f o r which 50% of the area i s d e f o l i a t e d one year and only 10% of the area i s d e f o l i a t e d the year a f t e r . Although there may be co n s i d e r a b l e u n c e r t a i n t y regarding the absolute d e n s i t y of l a r v a e each year,, one can be more c e r t a i n t h a t there was a decrease i n abundance between mappings. Pattern of D e f o l i a t i o n The province-wide time s e r i e s of d e f o l i a t i o n shows tha t there have been three outbreaks since 1948, w i t h a f o u r t h c u r r e n t l y underway (Figure 2 ) . At t h i s s c a l e the f o r e s t t e n t c a t e r p i l l a r would appear t o be a p e r i o d i c d e f o l i a t o r , the peak years of d e f o l i a t i o n having occurred at e x a c t l y 13 year i n t e r v a l s . Each outbreak has c o n s i s t e d of 4-6 years of i n c r e a s i n g d e f o l i a t i o n , f o l l o w e d by roughly the same number of decreasing years. F i g u r e 3 shows the d i f f e r e n c e i n s p a t i a l extent of each of these outbreaks. By o v e r l a y i n g the four maps of Figure 3, one sees t h a t while the p a t t e r n of d e f o l i a t i o n appears to be p e r i o d i c at a province-wide s c a l e , there are many areas that have had only one or two outbreaks through t h i s 41 year p e r i o d (Figure 4). In f a c t there would appear to be a number of regions w i t h i n which the dynamics, at l e a s t q u a l i t a t i v e l y , appear to be q u i t e s i m i l a r over the f u l l 41 -25-3 0 0 0 0 0 - i J 2 0 0 0 0 0 or cc CD < 1 0 0 0 0 0 H 1 9 4 0 1 9 5 0 1 9 6 0 1 9 7 0 Y e a r 1 9 8 0 1 9 9 0 Figure 2. Total area within which moderate to severe d e f o l i a t i o n has occurred each year i n Ontario, 1948-88 - 2 6 -/ L - 1 19 8 3 - 81 km i i — i 500 1000 F i g u r e 3 . Composite maps showing the t o t a l extent of the moderate to severe d e f o l i a t i o n for each of the four province-wide outbreaks from 1948-88. - 2 7 -Figure 4. The number o f o u t b r e a k s t h a t have o c c u r r e d t h r o u g h o u t t h e p r o v i n c e from 1948-88. (Note: t h i s map was c r e a t e d by o v e r l a y i n g t h e 4 maps o f F i g u r e 3.) -28-year p e r i o d . These are most r e a d i l y seen from the 1972-82 map of F i g u r e 3, where there are three q u i t e separate outbreak regions and a f o u r t h area (North-Centre) w i t h no d e f o l i a t i o n . This d i f f e r e n c e between regions suggests that the dynamics ( i . e . the p e r i o d i c i t y , s e v e r i t y and d u r a t i o n of outbreaks) may be a f u n c t i o n of the s c a l e and l o c a t i o n at which they are measured. To explore t h i s f u r t h e r I chose four l o c a t i o n s across the province, each of which was centered w i t h i n one of four general outbreak regions (Figure 5). Given these four l o c a t i o n s I then c r e a t e d a time s e r i e s of d e f o l i a t i o n f o r three s p a t i a l s c a l e s . Using c e l l s of 3 d i f f e r e n t s i z e s (100x100 km, 200x200 km and 400x400 km), centered at each of the four l o c a t i o n s , I c a l c u l a t e d the p r o p o r t i o n of land area w i t h i n a c e l l t h a t was d e f o l i a t e d each year. The r e s u l t s of t h i s a n a l y s i s are shown i n Figure 6. Looking f i r s t across the rows of t h i s f i g u r e , one sees that the p a t t e r n of d e f o l i a t i o n f o r a p a r t i c u l a r l o c a t i o n does not change a p p r e c i a b l y over s c a l e s of 100-400 km. Looking down the columns, however, one sees t h a t the dynamics do change from one region to the next. While the North-West region e x h i b i t s the province-wide p e r i o d i c i t y , the other regions s u f f e r e d l i t t l e or no d e f o l i a t i o n i n at l e a s t one of the f i r s t three outbreaks. Whenever d e f o l i a t i o n does -29-Figure 5. The loc a t i o n of the c e l l s defining the 4 outbreak regions (North-West, North-Centre, North-East and South-East) at each of 3 scales (100x100 km, 200x200 km and 400x400 km). -30-100km x 100km 200km x 200km 400km x 400km i c .o 3? o M— CD Q rl 1 I A L l fil / J L N-W N - C — J 4 —rT. N-E S-E 1940 1860 1860 1970 1980 1990 1940 1960 I960 1970 1980 1990 1940 1950 1960 1970 1B80 1990 Year F i g u r e 6. The proportion of land area d e f o l i a t e d w i t h i n each of the 4 outbreak regions, at 3 d i f f e r e n t s c a les. See Figure 5 f o r d e t a i l s of region and s c a l e l o c a t i o n s . occur i n a regio n , however, the p a t t e r n of d e f o l i a t i o n i s roughly synchronized across the e n t i r e p r o v i n c e . The p o p u l a t i o n dynamics of the f o r e s t t e n t c a t e r p i l l a r appear t o be homogeneous over areas t h a t are w i t h i n approximately 50-200km of each other. While the r i s e s and f a l l s are somewhat synchronous over d i s t a n c e s greater than t h i s , the s e v e r i t y of outbreaks would appear t o vary c o n s i d e r a b l y at t h i s l a r g e r s p a t i a l s c a l e . This suggests th a t f a c t o r s which are homogeneous over t h i s s m a l l e r (200 km) s c a l e , yet vary from one region to the next w i t h i n the province, may p l a y an important r o l e i n shaping the observed p a t t e r n of outbreaks. Summary The areas w i t h i n which d e f o l i a t i o n by the f o r e s t tent c a t e r p i l l a r has occurred each year have been mapped i n Ontario from 1948 to 1988. These maps provide an index of the l a t e l a r v a l d e n s i t i e s each year, and are best used as a measure of r e l a t i v e changes i n abundance f o r l a r g e r s c a l e d analyses. The p a t t e r n of d e f o l i a t i o n i n Ontario suggests t h a t , w h i l e outbreaks appear p e r i o d i c a l l y at a province-wide s c a l e , there i s considerable v a r i a t i o n i n t h e i r s e v e r i t y from region t o region and outbreak to outbreak. -32-CHAPTER 4 CLIMATE AND FOREST COMPOSITION Recent reviews suggest that s t u d i e s of the f o r e s t tent c a t e r p i l l a r provide evidence of how weather can pla y an important r o l e i n t r i g g e r i n g f o r e s t i n s e c t outbreaks (Martinat, 1987; Wallner, 1987). In p a r t i c u l a r there i s evidence t o suggest t h a t both the temperature through the e a r l y l a r v a l feeding p e r i o d , and the minimum temperature through the winter, may determine when and where outbreaks occur i n Ontario (see Chapter 2 ) . For the most p a r t , however, these s t u d i e s have i n v o l v e d observations at only a few p o i n t s i n space and time. In the f o l l o w i n g chapter, 41 years of la r g e s c a l e d d e f o l i a t i o n data w i l l be used i n conjunction w i t h m a c r o - c l i m a t i c and f o r e s t composition i n f o r m a t i o n to explore some of the p r e v i o u s l y developed hypotheses regarding the r o l e of c l i m a t e i n outbreaks. The a n a l y s i s w i l l begin by examining the r e l a t i o n s h i p between the year-to-year changes i n d e f o l i a t i o n and both the temperature during l a r v a l feeding and the minimum ov e r w i n t e r i n g temperature. The long-term p a t t e r n of outbreaks w i l l then be compared to p a t t e r n s of cl i m a t e and host a v a i l a b i l i t y . -33-Larval Feeding Temperature Several s t u d i e s have provided evidence l i n k i n g the r i s e s and f a l l s of outbreaks to the temperature to which the e a r l y i n s t a r l a r v a e are exposed (Table I ) . With the exception of the study by Ives (1973), whose r e s u l t s are somewhat d i f f i c u l t to i n t e r p r e t , these f i n d i n g s are based upon only a few i s o l a t e d observations of p o p u l a t i o n changes. The l o c a l nature of t h i s work makes i t d i f f i c u l t to assess the s i g n i f i c a n c e of such a f a c t o r over a l a r g e r s p a t i a l and temporal s c a l e . While unfavourable c l i m a t e may be capable of i n d u c i n g higher l a r v a l m o r t a l i t y i n a p a r t i c u l a r l o c a t i o n or year, i t i s not c l e a r whether i t has determined changes i n abundance i n Ontario. To i n v e s t i g a t e t h i s r e l a t i o n s h i p f u r t h e r I w i l l use two sources of i n f o r m a t i o n : annual maps of the areas w i t h i n which moderate to severe d e f o l i a t i o n occurred i n Ontario from 1948 t o 1988 (as o u t l i n e d i n Chapter 3), and d a i l y maximum and minimum temperature data, from the Atmospheric Environment S e r v i c e of Environment Canada, f o r a network of 20 s t a t i o n s across the province over t h i s same p e r i o d (Figure 7 ) . Two c r i t e r i a were used i n s e l e c t i n g the c l i m a t e s t a t i o n s to be used i n t h i s a n a l y s i s . F i r s t , s t a t i o n s were s e l e c t e d such t h a t they were approximately 200 km apart, although t h i s was not always p o s s i b l e i n the northern part -34-Table I Studies p r o v i d i n g evidence of the r e l a t i o n s h i p between f o r e s t tent c a t e r p i l l a r outbreaks and the temperature through the e a r l y l a r v a l feeding p e r i o d . Reference Location Extent of Findings Observations Hodson (1941) B l a i s et al. (1955) Ives (1973) Hodson (1977) Northern Minnesota Manitoba & N-W Ontario Ontario & P r a i r i e s Minnesota 2 l o c a t i o n s 1 outbreak 2 l o c a t i o n s 1 outbreak several l o c a t i o n s and outbreaks 2 l o c a t i o n s 2 outbreaks Cold, wet weather f o r 3 weeks a f t e r hatch; high l a r v a l m o r t a l i t y . Prolonged c o l d and wet f o l l o w i n g hatch; outbreak c o l l a p s e d . Years p r i o r to s t a r t of outbreaks warmer than years p r i o r to c o l l a p s e s . Warm i n years p r i o r to s t a r t of outbreaks cool i n years p r i o r t o c o l l a p s e s . Figure 7. The l o c a t i o n of the 20 c l i m a t e s t a t i o n s . S o l i d boxes i n d i c a t e the 4 s t a t i o n s whose r e s u l t s w i l l be d i s p l a y e d throughout the remainder of the t h e s i s . of the p r o v i n c e . Second, s t a t i o n s were s e l e c t e d based upon the completeness of t h e i r record from 1948 t o 1988. To extend the p e r i o d of record f o r those s t a t i o n s which were e i t h e r m i s s i n g records or were replaced over time w i t h a s t a t i o n nearby, s t a t i o n s w i t h i n 20 km of each other were t r e a t e d as a s i n g l e l o c a t i o n . Given such a data set, the f i r s t step I f o l l o w e d i n t e s t i n g the importance of the e a r l y l a r v a l feeding temperature was to p r e d i c t the s t a r t of the l a r v a l stage f o r each of the 20 c l i m a t e s t a t i o n s and 41 years using a simple hatch model. I then c a l c u l a t e d a measure of how favourable the s p r i n g temperature was through the e a r l y p a r t of the l a r v a l p e r i o d f o r each s t a t i o n and year. This c l i m a t i c index was then compared to changes i n d e f o l i a t i o n , to see i f c l i m a t i c a l l y favourable l o c a t i o n s and years corresponded w i t h i n c r e a s e s i n abundance and unfavourable ones w i t h decreases. Hatch model Two models p r e d i c t i n g the hatch date of eggs from d a i l y maximum and minimum temperature data have been developed f o r the f o r e s t t e n t c a t e r p i l l a r (Ives, 1973; Hodson, 1977); both use a heat u n i t (or degree-day) concept t o p r e d i c t egg development. Such models are widely used f o r p r e d i c t i n g i n s e c t development (Wagner et al., 1984) and are based upon the f o l l o w i n g p r i n c i p l e s : there i s some temperature -37-t h r e s h o l d below which no egg development w i l l occur; whenever the temperature exceeds t h i s t h r e s h o l d , the r e l a t i o n s h i p between temperature and the r a t e of development i s l i n e a r ( A l l e n , 1976). For hatch t o occur the eggs must accumulate a f i x e d number of heat u n i t s , where the heat u n i t s are c a l c u l a t e d as the degree-days above the development t h r e s h o l d . Both of the e x i s t i n g models, were developed u s i n g recorded hatch dates t o estimate the parameters of the model. The model developed by Ives (1973), u s i n g records from A l b e r t a , p r e d i c t s that hatch w i l l occur each year once the number of degree-days above a t h r e s h o l d of 4.4°C (40°F), s t a r t i n g on A p r i l 27th, exceeds 61.1°C-days (110°F-days). The model developed by Hodson (1977), u s i n g records from Minnesota (which borders north-western O n t a r i o ) , has d i f f e r e n t parameters. This model r e q u i r e s an accumulation of 44.4°C-days (80°F-days), above a t h r e s h o l d of 7.8°C (46°F), and beginning March 1st. Both models c l a i m t o be able to p r e d i c t the hatch date t o w i t h i n 2-3 days of the t r u e date. As n e i t h e r model had been t e s t e d i n Ontario, I compared p r e d i c t e d and recorded hatch dates, f o r both models, at v a r i o u s l o c a t i o n s and times across the p r o v i n c e . A t r i a n g u l a r approximation (Ives, 1973) was used t o c a l c u l a t e the degree-days from d a i l y maximum and minimum temperature data. For those t e s t l o c a t i o n s more than 50km from a climate s t a t i o n , hatch dates were estimated u s i n g a d i s t a n c e -38-weighted average of the p r e d i c t e d values f o r the two nearest s t a t i o n s . Table I I compares the a c t u a l hatch dates to those p r e d i c t e d by the Minnesota model. With one exception the p r e d i c t e d hatch dates f o r t h i s model are w i t h i n 5 days of the a c t u a l dates. As the eggs from a s i n g l e egg band g e n e r a l l y hatch over the course of a 2-3 day p e r i o d (Raske, 1974), i t would be d i f f i c u l t to p r e d i c t the hatch date of the f o r e s t t e n t c a t e r p i l l a r , u sing only macroclimatic temperature data, w i t h much greater accuracy. The A l b e r t a model, which does not begin accumulating degree-days u n t i l A p r i l 27th, c o n s i s t e n t l y p r e d i c t e d hatch dates w e l l a f t e r the recorded ones. This model's l a t e s t a r t i n g date f o r heat u n i t accumulation probably e x p l a i n s i t s poor performance, as hatch dates before May 1st are q u i t e common i n Ontario. The Minnesota model was t h e r e f o r e used to p r e d i c t the hatch dates at each of the 20 c l i m a t e s t a t i o n s from 1948 to 1988. The p r e d i c t e d dates f o r any one s t a t i o n vary over a range of about 30-40 days from year to year (Figure 8). The s p a t i a l v a r i a t i o n i n hatch dates i s shown by the d i s t r i b u t i o n of long-term mean hatch dates across the province (Figure 9). Note the delay i n hatch date t h a t occurs as one moves towards the g e n e r a l l y c o o l e r northern and c e n t r a l p a r t s of the province. -39-Table I I Actual hatch dates and those predicted by the Minnesota model for various times and locations i n Ontario. Location Year Actual Hatch Date Actual Model Difference Source Date1" Date (Model-Actual) Fort F r a n c e s ^ 1967 Witter et al. (1972) May 22 May 18 -4 1968 May 1 May 2 + 1 Fort F r a n c e s ^ 1969 May 2 Apr 30 -2 1970 Mattson and Erikson (1978) May 13 May 18 +5 1971 May 7 May 6 -1 1972 May 12 May 10 -2 1973 May 7 May 6 -1 1974 May 17 May 18 + 1 Cedar Lake 1953 Blais et al. (1955) May 8 May 7 -1 Nagagami 1966 Hall (1967) May 24 May 24 0 Bancroft 1967 Livesey (1968) May 1 May 12 + 11 Earlton 1974 MacLeod et al. (1975) May 18 May 22 + 4 Muskoka 1978 Anonymous (1978) May 8 May 13 +5 Muskoka 1988 G. Howse (pers. comm.) May 3 May 6 + 3 Actual date represents estimated date of 50% hatch. As the average time between start and 90% hatch i s 2 days (Raske, 1974) , 1 day was added to the date of those sources reporting only the hatch sta r t . Actual hatch dates were recorded near International F a l l s , Minnesota (about 10 km from Fort Frances, Ontario). Kenora (1) Year Figure 8. The p r e d i c t e d hatch date each year f o r 4 of the 20 cl i m a t e s t a t i o n s (the s t a t i o n numbers from F i g u r e 7 are given i n b r a c k e t s ) . Hatch dates are d i s p l a y e d as J u l i a n days beginning March 1st. Shading shows the hatch date each year r e l a t i v e to May 8 (day 70), the mean f o r a l l s t a t i o n s and years. -41-Figure 9. The mean hatch date, 1948-88, f o r each of the 20 c l i m a t e s t a t i o n s . Dates are d i s p l a y e d as J u l i a n days beginning March 1st. -42-Feeding degree-days The next step was to c a l c u l a t e an index of how favourable the temperature was f o r feeding i n the e a r l y p a r t of the l a r v a l stage. The index I chose i s based upon s e v e r a l f i n d i n g s . F i r s t , l a b o r a t o r y experiments and f i e l d o bservations suggest t h a t l a r v a e do not feed at temperatures below 15°C, and most d e f o l i a t i o n occurs on days when temperatures are i n excess of about 23°C (Hodson, 1941). Furthermore W e l l i n g t o n (1950), i n a study of the r e l a t i o n s h i p between a i r temperatures and the surface temperatures of f o l i a g e , found that at temperatures between 15°C and 26°C t r e m b l i n g aspen leaves were no more than 1.6°C above the a i r temperature (whereas c o n i f e r o u s f o l i a g e was as much as 8°C h i g h e r ) . This suggests t h a t , f o r f o r e s t t e n t c a t e r p i l l a r , m a croclimatic temperature observations are a r e l a t i v e l y good measure of the temperature to which l a r v a e are exposed w h i l e feeding. The index I decided t o use i s s i m i l a r t o t h a t p r e v i o u s l y developed by Ives (1973); i t uses a heat u n i t concept t o determine the number of degree-days t h a t accumulate each year above a t h r e s h o l d feeding temperature. Using d a i l y maximum and minimum temperature data, the index i s c a l c u l a t e d as the number of degree-days above a t h r e s h o l d of 15°C (or "feeding" degree-days) that accumulate during the f i r s t 3 weeks a f t e r hatch. A high value i n d i c a t e s a warm, or favourable e a r l y l a r v a l feeding p e r i o d while a low -43-value i n d i c a t e s a l e s s favourable year. Only 3 weeks a f t e r hatch are considered as t h i s i s how long newly hatched l a r v a e are thought to be able to s u r v i v e without food (Smith and Raske, 1968) . Based upon the p r e d i c t e d hatch dates I c a l c u l a t e d the feeding degree-days t h a t have accumulated at each of the 2 0 s t a t i o n s from 1948 t o 1988. U n l i k e the hatch dates, however, a strong north-south or east-west gradient i n t h e i r long-term means does not e x i s t (Figure 10). However, the number of feeding degree-days at each s t a t i o n v a r i e s c o n s i d e r a b l y from one year t o the next (Figure 11). Comparing c l i m a t e and d e f o l i a t i o n Next I compared the d e f o l i a t i o n maps to the feeding degree-days f o r each s t a t i o n and year (or " s t a t i o n - y e a r " ) . As the d e f o l i a t i o n i s mapped at the end of the l a r v a l stage each year ( u s u a l l y mid- to l a t e - J u n e ) , i t provides an i n d i c a t i o n of each year's l a t e i n s t a r l a r v a l abundance. Whether or not the l a t e l a r v a l d e n s i t i e s i n an area i n c r e a s e d or decreased between years can be seen by comparing the p r o p o r t i o n of a f i x e d s i z e d area that i s d e f o l i a t e d i n one year (say, year t-1) t o the p r o p o r t i o n of t h a t same area d e f o l i a t e d i n the next year (year fc). I c a l c u l a t e d the area d e f o l i a t e d f o r each s t a t i o n - y e a r by c e n t e r i n g c e l l s t hat were 100km by 100km i n s i z e at each of the 20 s t a t i o n s and c a l c u l a t i n g the p r o p o r t i o n of the F i g u r e 10. The mean number of feeding degree-days, 1948-88, f o r each of the 20 cl i m a t e s t a t i o n s . Kenora ( 1 ) 120 - i 1940 1950 1960 1970 1980 1990 Armstrong (5) 120 -l 80 -1940 1950 1960 1970 1980 1990 Figure 11. The feeding degree-days each year f o r 4 of the 20 cl i m a t e s t a t i o n s . Shading shows the degree-days each year r e l a t i v e to 25°C-days, the mean f o r a l l s t a t i o n s and years. -46-land area i n each c e l l t hat was d e f o l i a t e d each year. The importance of the l a r v a l feeding temperature i n determining p o p u l a t i o n increases or d e c l i n e s was t e s t e d by comparing the change i n d e f o l i a t e d area, from year t-1 t o year t , t o the feeding degree-days i n year t f o r each s t a t i o n - y e a r . F i g u r e 12 shows the time s e r i e s of feeding degree-days and d e f o l i a t i o n f o r 4 of the 20 s t a t i o n s (see Appendix B f o r remaining 16 s t a t i o n s ) . I f the l a r v a l feeding temperature i s important i n determining when and where outbreaks r i s e and f a l l , one would expect higher feeding degree-days f o r those s t a t i o n - y e a r s where the area d e f o l i a t e d i n c r e a s e d from year-to-year than f o r those s t a t i o n - y e a r s where the d e f o l i a t i o n decreased. A simpler way of examining t h i s r e l a t i o n s h i p i s t o p l o t the change i n d e f o l i a t i o n , from year t-1 t o year t , against the feeding degree-days i n year t f o r a l l s t a t i o n s and years. R e c a l l , however, th a t i n Chapter 3 I suggested the l a r v a l d e n s i t i e s could be as high as 500-1000 l a r v a e per host t r e e and s t i l l not show any moderate to severe d e f o l i a t i o n . So i f the d e f o l i a t i o n remains the same from one year t o the next, i t i s very d i f f i c u l t t o know whether or not the l a r v a l d e n s i t i e s increased, decreased, or remained the same. For t h i s reason I compared only those s t a t i o n -years f o r which there was a non-zero change i n d e f o l i a t i o n from year t-1 to year t . - 4 7 -Kenora (1) j i i i i i i \j 1940 1950 1960 1970 1980 1990 North Bay (14) 1940 1950 1960 1970 1980 1990 Y e a r F i g u r e 12. The feeding degree-days and the p r o p o r t i o n of area d e f o l i a t e d each year f o r 4 of the 20 cl i m a t e s t a t i o n s . Shading shows the degree-days each year r e l a t i v e t o 25°C-days, the mean f o r a l l s t a t i o n s and years. Dark l i n e shows the percent d e f o l i a t i o n . -48-F i g u r e 13 shows the feeding degree days f o r years of i n c r e a s i n g and decreasing d e f o l i a t i o n at a l l 20 s t a t i o n s , from 1948-88. Again, i f the feeding temperature were determining p o p u l a t i o n change, one would expect to see decreases i n d e f o l i a t i o n a s s o c i a t e d w i t h low degree-day years and/or increases a s s o c i a t e d w i t h h i g h degree-day years. This p l o t shows no s i g n of such a r e l a t i o n s h i p . There were s e v e r a l s t a t i o n - y e a r s w i t h very few degree-days and i n c r e a s e s i n d e f o l i a t i o n , and others w i t h high degree-day t o t a l s and d e f o l i a t i o n decreases. S e n s i t i v i t y a n a l y s i s To t e s t the s e n s i t i v i t y of these r e s u l t s to changes i n the measure of abundance, I v a r i e d the s c a l e over which the d e f o l i a t i o n was measured. This i n v o l v e d changing the s i z e of the 100km by 100km s t a t i o n centered c e l l t o s i z e s of 50km by 50km and 200km by 200km. The r e s u l t s of these changes shows t h a t the l a c k of a r e l a t i o n s h i p between feeding degree-days and changes i n d e f o l i a t i o n i s not s e n s i t i v e t o small changes i n the s c a l e over which the d e f o l i a t i o n i s measured (Figure 14) . Next I looked at the e f f e c t of changes i n the c l i m a t i c measure. The feeding degree-day index i s based upon 3 parameters: the s t a r t of the l a r v a l p e r i o d (as p r e d i c t e d by the hatch model), the feeding t h r e s h o l d temperature (15°C) and the p e r i o d of i n f l u e n c e (3 weeks). To t e s t the -49-Degree Days F i g u r e 13. R e l a t i o n s h i p between the change i n percentage of d e f o l i a t e d area and the feeding degree-days, f o r a l l s t a t i o n s and years w i t h a non-zero change i n d e f o l i a t i o n . V e r t i c a l l i n e shows the mean number of feeding degree-days f o r a l l s t a t i o n s and years. -50-CD CJ) c CO x: O c o ]i o Q 1 0 0 0 - — : • 1 0 0 • • • • • • • • • • : • • • Alj:: _•___:.„ • • • •• • « • • • • a • • • • • • • ., - .. . t. . • 0 5 0 1 0 0 0 • 1 0 0 5 0 Degree Days (a) 1 0 0 B . (b) 1 0 0 Figure 14. The e f f e c t of varying the c e l l size on the rel a t i o n s h i p between the change in percentage of de f o l i a t e d area and the feeding degree-days, for a l l stations and years with a non-zero change i n d e f o l i a t i o n . Area d e f o l i a t e d i s calculated at c e l l sizes of: (a) 50km by 50km; (b) 200km by 200km. - 5 1 -robustness of the r e s u l t s to these v a l u e s , I repeated the a n a l y s i s u s i n g more conservative estimates of each parameter. This i n v o l v e d the f o l l o w i n g changes to my index: the s t a r t of the feeding p e r i o d was delayed f o r one week a f t e r the p r e d i c t e d hatch date; the feeding t h r e s h o l d was lowered from 15°C t o 10°C; the p e r i o d f o r accumulating degree-days was extended from 3 to 4 weeks. For each of these changes I s t i l l found no r e l a t i o n s h i p between incre a s e s and decreases i n d e f o l i a t i o n and the temperature through the l a r v a l p e r i o d (Figure 15). Years p r i o r t o increases To t h i s p o i n t my a n a l y s i s has compared changes i n d e f o l i a t i o n to the l a r v a l feeding temperature f o r the same year. Given the high l a r v a l d e n s i t i e s r e q u i r e d t o b r i n g about n o t i c e a b l e d e f o l i a t i o n , i t i s q u i t e p o s s i b l e t h a t the most important c l i m a t i c i n f l u e n c e occurs s e v e r a l years before any d e f o l i a t i o n i s recorded. Both W e l l i n g t o n (1952) and Ives (1973) have t e s t e d t h i s idea f o r the f o r e s t tent c a t e r p i l l a r by examining the cli m a t e i n the years p r i o r to d e f o l i a t i o n . E x p l o r i n g t h i s p o s s i b i l i t y i n v o l v e s d i s t i n g u i s h i n g between those years i n which d e f o l i a t i o n f i r s t began t o incr e a s e (or decrease) at a p a r t i c u l a r s t a t i o n , and subsequent i n c r e a s i n g (or decreasing) years. " I n i t i a l i n c r e a s e " years were s a i d to occur f o r a s t a t i o n i n year t -52--100 (a) 200 CD CC c CO _c O c o ~M o o Q (b) 200 (c) 200 Degree Days Figure 15. The e f f e c t of changing the c l i m a t i c measure on the r e l a t i o n s h i p between the change i n percentage of d e f o l i a t e d area and the feeding degree-days, f o r a l l s t a t i o n s and years w i t h a non-zero change i n d e f o l i a t i o n . Feeding degree-days are c a l c u l a t e d u s i n g : (a) 1 week delay a f t e r hatch; (b) feeding t h r e s h o l d of 10°C; (c) 4 week p e r i o d of i n f l u e n c e . V e r t i c a l l i n e s show the mean number of feeding degree-days f o r a l l s t a t i o n s and years. -53-when there was an increase i n d e f o l i a t i o n from year t-1 to year t, w i t h no increase i n d e f o l i a t i o n at t h a t s t a t i o n i n the previous three years (years t-3 to t - 1 ) . S i m i l a r l y , " i n i t i a l decrease" years were those years showing a decrease i n d e f o l i a t i o n i n the current year and no decreases i n previous 3 years. I f the feeding temperature does i n i t i a t e e i t h e r the r i s e or the d e c l i n e of outbreaks, one would expect t o see higher feeding degree-days i n the years preceding i n i t i a l i n c r e ase years than i n the years preceding i n i t i a l decrease years. To explore t h i s p o s s i b i l i t y I used a 4 year average (years t-3 to t i n c l u s i v e ) of feeding degree-days at each s t a t i o n as my c l i m a t i c index f o r year t . I then compared the new index values f o r those s t a t i o n - y e a r s showing i n i t i a l i n c r e a s e s t o those showing i n i t i a l decreases (Figure 16). These r e s u l t s show no d i f f e r e n c e between the 4 year average of feeding degree-days p r i o r to r i s e s and those p r i o r t o c o l l a p s e s of outbreaks. Twenty of the 39 i n i t i a l i ncrease years (51%) were preceded by a 4 year average t h a t was above the mean f o r a l l s t a t i o n s and years. S i m i l a r l y , 21 of 39 i n i t i a l decrease years (54%) were preceded by a 4 year average above the mean. -54-10 I 1 1 1 1 1 1 1 r 10 I 1 1 1 1 i 1 ' i i - 2 0 - 1 5 - 1 0 - 5 0 5 1 0 1 5 2 0 2 5 3 0 Degree Days - M e a n F i g u r e 16. The d i s t r i b u t i o n o f 4-year averages o f f e e d i n g degree-days p r i o r t o : (a) i n i t i a l i n c r e a s e y e a r s ; (b) i n i t i a l d e c r e a s e y e a r s . Degree-days are d i s p l a y e d r e l a t i v e t o 25°C-days, t h e mean number o f f e e d i n g degree-days f o r a l l s t a t i o n s and y e a r s . -55-Overwintering Temperature The second c l i m a t i c f a c t o r examined was the o v e r w i n t e r i n g temperature. There are two s t u d i e s which together suggest t h a t t h i s could i n f l u e n c e p o p u l a t i o n change. Hanec (1966) showed th a t a seasonal v a r i a t i o n i n g l y c e r o l content allows f o r e s t t e n t c a t e r p i l l a r eggs to be supercooled t o temperatures as low as -41°C, below which they w i l l f r e e z e . A subsequent study of f o r e s t t e n t c a t e r p i l l a r p o p u l a t i o n dynamics i n northern Minnesota (Witter et al., 1975) found egg m o r t a l i t y , from f a c t o r s other than p a r a s i t i s m and i n f e r t i l i t y , t o vary from 0-65% over a 9 year p e r i o d . Comparing egg m o r t a l i t y t o the c o l d e s t temperature through the previous wi n t e r , a sharp r i s e i n m o r t a l i t y f o r those years i n which the temperature dropped below -40°C was discovered. Egg m o r t a l i t y f o r the 5 years f o r which the temperature d i d not drop below -4 0°C ranged from 0-10%, whi l e i n the 4 years where temperatures dropped below t h i s t h r e s h o l d , m o r t a l i t y ranged from 10-65%. This suggests t h a t the minimum o v e r w i n t e r i n g temperature may p l a y an important r o l e i n the c o l l a p s e of f o r e s t t e n t c a t e r p i l l a r outbreaks (Witter, 1979). However evidence i s based upon observations at a s i n g l e l o c a t i o n over the course of a s i n g l e outbreak, supplemented by i s o l a t e d r e p o r t s of high egg m o r t a l i t y at other l o c a t i o n s ( P r e n t i c e , 1954; Gautreau, 1964). To examine the r o l e of the -56-minimum o v e r w i n t e r i n g temperature as an i n f l u e n c e on p o p u l a t i o n change over my 41 year study p e r i o d i n Ontario, I used the d e f o l i a t i o n maps along w i t h d a i l y minimum temperature data. As Wellington (1950) has shown tha t the d a i l y minimum temperature of f o r e s t t e n t c a t e r p i l l a r egg masses i s g e n e r a l l y w i t h i n 1°C of the a i r temperature, m a c r o c l i m a t i c temperature observations should provide a good measure of the minimum temperatures experienced by the eggs. To t e s t the importance of the minimum o v e r w i n t e r i n g temperature I determined the minimum d a i l y temperature each w i n t e r , from 1947/1948 t o 1987/1988, f o r each of the 20 c l i m a t i c s t a t i o n s across the province. I then compared the change i n d e f o l i a t i o n , from one year t o the next, t o the minimum o v e r w i n t e r i n g temperature f o r each s t a t i o n - y e a r . F i g u r e 17 shows the long-term means of the minimum o v e r w i n t e r i n g temperatures at each of the 20 s t a t i o n s . There i s a l a r g e r e g i o n to the north and centre of the province where temperatures, on average, f a l l below the -40°C t h r e s h o l d . For much of the province, however, temperatures r a r e l y drop below -40°C. Fi g u r e 18 shows the time s e r i e s of minimum o v e r w i n t e r i n g temperatures and the area d e f o l i a t e d each year f o r 4 of the 20 s t a t i o n s (see Appendix C f o r remaining 16 s t a t i o n s ) . I f the ove r w i n t e r i n g temperature i s an important f a c t o r i n causing c o l l a p s e s of outbreaks, then one would expect t o see decreases i n d e f o l i a t i o n f o r those years with -57-F i g u r e 17. The mean of the minimum ov e r w i n t e r i n g temperature, 1948-88, f o r each of the 20 c l i m a t e s t a t i o n s . -58-1940 Kenora (1) 1950 1960 1970 1980 100 1990 CD i ~C0 CD CL E CD -10 -40 -70 1940 -10 -, -40 --70 1940 Armstrong (5) 1950 1960 1970 Kapuskasing (9) 1980 1950 1960 1970 1980 100 50 1990 J 100 50 0 1990 55 c .o H o CD Q -10 - i 1940 North Bay (14) 1950 1960 1970 1980 100 1990 Year F i g u r e 18. The minimum overwintering temperature and the p r o p o r t i o n of area d e f o l i a t e d each year f o r 4 of the 20 cl i m a t e s t a t i o n s . For each year t, temperature i s the minimum f o r the wi n t e r of year t-1 t o year t . Shading shows the temperature r e l a t i v e to -40°C. Dark l i n e shows the percent d e f o l i a t i o n . - 5 9 -lower o v e r w i n t e r i n g temperatures, i n p a r t i c u l a r those below -40°C. Furthermore, one might expect t o see fewer increases i n d e f o l i a t i o n f o r those s t a t i o n s and years where the temperature dropped below t h i s t h r e s h o l d . A simple way of e x p l o r i n g t h i s i s to p l o t the change i n d e f o l i a t i o n , from year t-1 to t , against the minimum temperature o c c u r r i n g between these d e f o l i a t i o n mappings (which I r e f e r to as the minimum temperature f o r year t ) . As w i t h the p l o t s f o r the l a r v a l feeding temperature, t h i s i s done only f o r those s t a t i o n - y e a r s f o r which there was a non-zero change i n d e f o l i a t i o n from one year to the next. Fi g u r e s 19 and 20 show the r e s u l t s of such a p l o t f o r a l l 20 s t a t i o n s , from 1948 t o 1988, w i t h d e f o l i a t i o n c a l c u l a t e d at 3 d i f f e r e n t s p a t i a l s c a l e s . When one considers a l l of the outbreaks i n the province through t h i s p e r i o d , there i s no apparent r e l a t i o n s h i p between the o v e r w i n t e r i n g temperature and decreases i n d e f o l i a t i o n . Regardless of the s c a l e over which the d e f o l i a t i o n i s measured, one sees s e v e r a l s t a t i o n - y e a r s w i t h minimum temperatures w e l l below -40°C and increases i n d e f o l i a t i o n . Furthermore, there are many s t a t i o n - y e a r s f o r which the temperatures were w e l l above -40°C and yet d e f o l i a t i o n s t i l l decreased s u b s t a n t i a l l y . -60-100 CD CD c CTJ JZ O a _ o ~M o o Q • • • • w • • • • _ f • • • 0 • y . . • • • • • - 1 0 0 - 6 0 -50 - 4 0 -30 -20 - 1 0 Temperature Figure 19. R e l a t i o n s h i p between the change i n percentage of d e f o l i a t e d area and the minimum o v e r w i n t e r i n g temperature, f o r a l l s t a t i o n s and years w i t h a non-zero change i n d e f o l i a t i o n . Area d e f o l i a t e d i s c a l c u l a t e d at a c e l l s i z e of 100km by 100km. V e r t i c a l l i n e shows the -40°C t h r e s h o l d . -61-1 0 0 CD CD c CO o c o 3i o "CD O - 1 0 0 1 0 0 (a) 0 - • - 1 0 0 (b) Temperature Figure 20. The e f f e c t of varying the c e l l size on the re l a t i o n s h i p between the change in percentage of def o l i a t e d area and the minimum overwintering temperature, for a l l stations and years with a non-zero change i n d e f o l i a t i o n . Area d e f o l i a t e d i s calculated at c e l l sizes of: (a) 50km by 50km; (b) 200km by 200km. -62-F o r e s t Composition and Long-Term Climate R e c a l l t h a t the s e v e r i t y of outbreaks has d i f f e r e d c o n s i d e r a b l y from one region to another across the province (see F i g u r e 4 i n Chapter 3). There are many p a r t s of the province, f o r example, th a t have been d e f o l i a t e d i n 3 or 4 outbreaks s i n c e 1948, while other areas have shown no n o t i c e a b l e d e f o l i a t i o n through t h i s p e r i o d . To t h i s p o i n t I have only compared c l i m a t e t o the year-to-year changes i n f o r e s t t e n t c a t e r p i l l a r p o p u l a t i o n s . This s e c t i o n ' s m u l t i -outbreak a n a l y s i s w i l l explore the dynamics at a d i f f e r e n t temporal s c a l e , and look at why some p a r t s of the province have had many outbreaks while others have had none. To do t h i s I compared the long-term s p a t i a l p a t t e r n of d e f o l i a t i o n i n the province t o the p a t t e r n of l a r v a l feeding temperatures, o v e r w i n t e r i n g temperatures, and f o r e s t composition. This was accomplished u s i n g 3 sources of i n f o r m a t i o n : annual d e f o l i a t i o n maps, d a i l y maximum and minimum temperature data ( f o r 20 c l i m a t i c s t a t i o n s ) , and a s i n g l e province-wide f o r e s t i nventory. Host a v a i l a b i l i t y The f i r s t stage i n t h i s a n a l y s i s was t o assess the a v a i l a b i l i t y of host across the province. This was accomplished using a f o r e s t inventory provided by F o r e s t r y Canada's Forest Resource Data Program (f o r more d e t a i l s see -63-Anonymous, 1988). The inventory was d e r i v e d from stand l e v e l f o r e s t type maps; such maps g e n e r a l l y use a e r i a l photography, i n conjunction w i t h f i e l d sampling, t o d e l i n e a t e and c l a s s i f y stands. The stand l e v e l maps were aggregated to a g r i d system w i t h c e l l s corresponding t o p r o v i n c i a l townships i n the south and east, and w i t h 10km by 10km c e l l s i n the north and west (Figure 21). The inventory was compiled u s i n g the most recent a v a i l a b l e stand data, as of 1986, f o r each p a r t of the province. F i g u r e 22 shows that most c e l l s were i n v e n t o r i e d between 1972 and 1986. For each stand i n the inventory v a r i o u s a t t r i b u t e s are recorded. Those r e l e v a n t to my a n a l y s i s i n c l u d e d : 1. Fo r e s t Type The f o r e s t type i s c l a s s i f i e d a ccording t o one of 3 c a t e g o r i e s : softwood (76-100% of the canopy i s c o n i f e r o u s ) ; mixedwood (26-75% c o n i f e r o u s ) ; hardwood (0-25% c o n i f e r o u s ) . 2. Predominant Genus The most abundant genus i n the stand. This i s c l a s s i f i e d according to 12 c a t e g o r i e s , of which the deciduous c a t e g o r i e s are: poplar (Populus s p e c i e s ) ; b i r c h (Betula s p e c i e s ) ; maple (Acer s p e c i e s ) ; other broadleaved species. 3. Area The stand's f o r e s t e d area. -64-F i g u r e 2 2 . The y e a r i n w h i c h t h e s t a n d i n f o r m a t i o n was r e c o r d e d f o r e a c h c e l l i n t h e f o r e s t i n v e n t o r y . -66-R e c a l l (from Chapter 2) t h a t the host p r e f e r r e d by the f o r e s t t e n t c a t e r p i l l a r i n the northern and western p a r t s of the province i s t r e m b l i n g aspen (Populus tremuloides); f u r t h e r south t h i s preference s h i f t s to a l s o i n c l u d e sugar maple, Acer saccharum, and red oak, Quercus rubra ( S i p p e l l , 1957; F o r e s t Insect and Disease Survey, pers. comm.). The f i r s t step i n a s s e s s i n g the d i s t r i b u t i o n of host across the province was t o determine the p r o p o r t i o n of each c e l l t h a t was c l a s s i f i e d as f o r e s t e d . Figure 23 shows t h a t , f o r most of the p r o v i n c e , 80% or more of the area i s f o r e s t e d . In the southern p a r t of the province, however, there i s a drop i n the f o r e s t e d area. The land use here i s p r i m a r i l y urban and a g r i c u l t u r a l , and the f o r e s t e d areas t h a t do e x i s t are g e n e r a l l y s m a l l , i s o l a t e d woodlots. The next step was t o determine the composition of the f o r e s t e d areas. Using the f o r e s t type a t t r i b u t e , I estimated the p r o p o r t i o n of each c e l l ' s area t h a t had a s i g n i f i c a n t deciduous component. This was accomplished by c a l c u l a t i n g the p r o p o r t i o n of the f o r e s t e d area i n each c e l l t h a t was c l a s s i f i e d as e i t h e r mixedwood or hardwood. Figure 24 shows t h a t most of the province has a s u b s t a n t i a l deciduous component, although the p r o p o r t i o n of stands t h a t are mixed or deciduous increases as one moves south. To get a p i c t u r e of the breakdown of t h i s deciduous f o r e s t by genus, I determined the p r o p o r t i o n of t o t a l deciduous dominated area t h a t was dominated by each of the four genus c a t e g o r i e s . For -67-Figure 23. P r o p o r t i o n o f e a c h c e l l ' s a r e a c l a s s i f i e d as non-f o r e s t e d . - 6 8 -Figure 24. P r o p o r t i o n o f e a c h c e l l ' s f o r e s t e d a r e a t h a t i s c l a s s i f i e d as e i t h e r mixedwood o r hardwood ( i . e . >25% o f t h e f o r e s t c a n o p y i s d e c i d u o u s ) . - 6 9 -p o p l a r t h i s was c a l c u l a t e d as: (area c l a s s i f i e d as predominantly poplar)  (area c l a s s i f i e d as pred. poplar + b i r c h + maple + other) This p r o p o r t i o n was c a l c u l a t e d i n the same way f o r each of the other 3 deciduous c a t e g o r i e s , p r o v i d i n g an estimate of the p r o p o r t i o n of the deciduous t r e e component i n each c e l l a t t r i b u t a b l e to each genus. Figure 25 shows t h a t , through most of the northern and western p a r t of the prov i n c e , p o p l a r dominates the f o r e s t ' s deciduous component. In southern Ontario, however, maple i s much more s i g n i f i c a n t . Given t h a t t r e m b l i n g aspen i s the most common of the poplar species i n Ontario, and sugar maple the most common maple (Rowe, 1972) , t h i s a n a l y s i s suggests t h a t there i s host a v a i l a b l e across most of the pr o v i n c e . The only exception i s the southern p a r t of the province where, although the f o r e s t i s p r i m a r i l y deciduous, there i s very l i t t l e f o r e s t e d area. D e f o l i a t i o n and c l i m a t e To compare the long-term patterns of d e f o l i a t i o n t o those of the two c l i m a t i c i n d i c e s , I f i r s t c a l c u l a t e d a long-term measure of the d e f o l i a t i o n f o r each c l i m a t e s t a t i o n . The measure I s e l e c t e d was the t o t a l number of years of d e f o l i a t i o n , from 1948 to 1988. This was c a l c u l a t e d as the number of years i n which non-zero d e f o l i a t i o n was -70-F i g u r e 25. Breakdown o f t h e d e c i d u o u s component i n e a c h c e l l by g enus. C a l c u l a t e d as t h e p r o p o r t i o n o f e a c h c e l l ' s d e c i d u o u s d o m i n a t e d a r e a t h a t i s d o m i n a t e d by: (a) p o p l a r ; (b) b i r c h ; (c) maple; (d) o t h e r b r o a d l e a v e d s p e c i e s . -71-F i g u r e 25. (continued) -72-F i g u r e 25. (continued) F i g u r e 25. (continued) -74-reported w i t h i n a 100km by 100km c e l l centered upon each of the 20 s t a t i o n s . I a l s o c a l c u l a t e d long-term means, usi n g annual values from 1948 to 1988, f o r both the number of feeding degree days and the minimum o v e r w i n t e r i n g temperature at each s t a t i o n . F i g u r e 2 6 shows the long-term s p a t i a l p a t t e r n of d e f o l i a t i o n , feeding degree-days and minimum ov e r w i n t e r i n g temperature. There i s no apparent r e l a t i o n s h i p between the long-term mean number of feeding degree-days and the number of years of d e f o l i a t i o n . However there i s some evidence t h a t the areas w i t h mean minimum ov e r w i n t e r i n g temperatures near or below -40°C are d e f o l i a t e d l e s s o f t e n . In the a n a l y s i s of host a v a i l a b i l i t y I showed t h a t there i s r e l a t i v e l y l i t t l e f o r e s t e d area i n the southern p a r t of the province. This could e x p l a i n the southern l i m i t of d e f o l i a t i o n seen so c l e a r l y i n Chapter 2 (see Figure 4), and i n the map of long-term d e f o l i a t i o n (Figure 26). Given t h a t the f i v e southern-most c l i m a t i c s t a t i o n s f a l l w i t h i n t h i s u r b a n / a g r i c u l t u r a l r e g i o n (see Fi g u r e s 7 and 23), i f one p l o t s the long-term d e f o l i a t i o n a g a i n s t each of the c l i m a t i c i n d i c e s f o r the remaining s t a t i o n s , a r e l a t i o n s h i p between the minimum ov e r w i n t e r i n g temperature and the frequency of d e f o l i a t i o n i s more evident (Figure 27). Once again, there i s s t i l l no r e l a t i o n s h i p between the long-term p a t t e r n of d e f o l i a t i o n and feeding degree-days. -75-Figure 26. The long-term patterns of d e f o l i a t i o n and climate, 1948-88, for each of the 20 climate stations: (a) t o t a l years of d e f o l i a t i o n ; (b) mean number of feeding degree-days; (c) mean minimum overwintering temperature. -76-2 0 -i (a) c o _l—I o M— CD Q CO ca CD 10 0 15 2 0 2 5 3 0 Degree Days — i 3 5 20 (b) c o ^ — » o Q o CO \_ crj CD > 10 - 5 0 - 4 0 - 3 0 Temperature -rB 8-- 2 0 Figure 27. R e l a t i o n s h i p between the number of years of d e f o l i a t i o n and the long-term c l i m a t e , 1948-88, f o r the 20 cl i m a t e s t a t i o n s : (a) mean feeding degree-days; (b) mean minimum ov e r w i n t e r i n g temperature. Open c i r c l e s represent the s t a t i o n s w i t h i n the non-forested p a r t of southern Ontario. - 7 7 -Summary This a n a l y s i s shows no r e l a t i o n s h i p between yea r - t o -year changes i n d e f o l i a t i o n and e i t h e r the l a r v a l feeding temperature or the minimum ove r w i n t e r i n g temperature. The long-term s u s c e p t i b i l i t y of an area, as measured by the number of years of non-zero d e f o l i a t i o n , v a r i e s c o n s i d e r a b l y across the pr o v i n c e . While t h i s s p a t i a l v a r i a t i o n i n s u s c e p t i b i l i t y appears t o be u n r e l a t e d t o the mean l a r v a l f eeding temperature, i t may be r e l a t e d t o both the p r o p o r t i o n of an area that i s f o r e s t e d , and the mean annual minimum winter temperature. - 7 8 -CHAPTER 5 DISCUSSION The l a c k of a r e l a t i o n s h i p between the two c l i m a t i c f a c t o r s and the year-to-year dynamics of outbreaks appears to be i n c o n s i s t e n t w i t h the p r e d i c t i o n s of past s t u d i e s . I w i l l begin t h i s chapter w i t h some comments on how such a c o n t r a d i c t i o n might a r i s e . I w i l l then examine the p o s s i b l e r o l e of c l i m a t e and f o r e s t composition i n determining the long-term s u s c e p t i b i l i t y of d i f f e r e n t p a r t s of the province. F i n a l l y , the r o l e of other f a c t o r s i n determining when and where outbreaks occur w i l l be discussed. Larval Feeding Temperature As i n t h i s a n a l y s i s , a l l of the past s t u d i e s l i n k i n g the l a r v a l feeding temperature t o r i s e s and c o l l a p s e s i n outbreaks have been c o r r e l a t i v e . Such s t u d i e s are always prone t o d i s c o v e r i n g spurious r e l a t i o n s h i p s : the fewer the number of observations, the gr e a t e r the chance of f i n d i n g a r e l a t i o n s h i p t h a t does not e x i s t . But even i f the r e l a t i o n s h i p s uncovered i n such s t u d i e s do e x i s t , i t becomes i n c r e a s i n g l y d i f f i c u l t t o g e n e r a l i z e the f i n d i n g s beyond the bounds of the study as the number of observations decreases. For three of the past s t u d i e s t h a t have suggested the importance of the l a r v a l feeding temperature, the -79-conclusions have been based upon observations through one or two outbreaks at only one or two l o c a t i o n s (Hodson, 1941; B l a i s et al., 1955; Hodson, 1977). As an example, consider the one study f o r which I a l s o have data: t h a t of B l a i s et al. (1955), which l i n k e d the c o l l a p s e of an outbreak i n north-western Ontario t o c o l d weather s h o r t l y a f t e r hatch. F i g u r e 28 shows the time s e r i e s of feeding degree-days and d e f o l i a t i o n f o r Sioux Lookout, a s t a t i o n w i t h i n t h e i r study area. From t h i s we see very few degree-days and a drop i n d e f o l i a t i o n i n 1953, the year of t h e i r study, which agrees w i t h t h e i r f i n d i n g s . This p a t t e r n disappears, however, when one looks at more p o i n t s i n space and time. I f the l a r v a l f eeding temperature were an important f a c t o r i n causing outbreak c o l l a p s e s , one would expect t o see fewer increases i n d e f o l i a t i o n than decreases f o r those s t a t i o n s and years w i t h low feeding degree-day accumulations. While Figure 13 showed s e v e r a l d e c l i n e s f o r s t a t i o n - y e a r s w i t h few degree-days, as i n Sioux Lookout i n 1953, i t showed j u s t as many d e f o l i a t i o n i n c r e a s e s i n the c o o l e r years. Furthermore, a l a r g e number of outbreaks c o l l a p s e d i n years w i t h high degree-day t o t a l s . This suggests t h a t there are f a c t o r s other than the temperature during l a r v a l feeding t h a t c o n t r i b u t e t o outbreak d e c l i n e s . -80-Figure 28. The f e e d i n g degree-days and t h e p r o p o r t i o n o f a r e a d e f o l i a t e d each y e a r f o r S i o u x Lookout ( s t a t i o n 3 ) . Outbreak c o l l a p s e was l i n k e d t o t h e t e m p e r a t u r e d u r i n g l a r v a l f e e d i n g i n 1953. Shading shows t h e degree-days each y e a r r e l a t i v e t o 25°C days, t h e n mean f o r a l l s t a t i o n s and y e a r s . Dark l i n e shows t h e p e r c e n t d e f o l i a t i o n . -81-Years p r i o r t o increases A p o s s i b l e e x p l a n a t i o n f o r the discrepancy between my f i n d i n g s and those of previous s t u d i e s i s the l o c a l nature of the observations i n the previous s t u d i e s . However not a l l s t u d i e s have been so l i m i t e d i n the number of o b s e r v a t i o n s . Ives (1973) used records of outbreaks and temperature f o r 10 s t a t i o n s i n the P r a i r i e provinces and Ontario, from 1930 to 1970, t o show t h a t outbreaks were preceded by a s i n g l e year (2 t o 4 years e a r l i e r ) w i t h a high number of feeding degree-days. This study had, i n my view, provided the most con v i n c i n g evidence of the importance of the l a r v a l feeding temperature i n determining the year-to-year dynamics of the f o r e s t t e n t c a t e r p i l l a r . Does t h i s same p a t t e r n appear w i t h my data? To check t h i s f o r the 20 s t a t i o n s i n my study, I l e t " i n i t i a l i n c r e a s e " years (as d e f i n e d i n Chapter 4) d e f i n e the beginning of an outbreak at a s t a t i o n . S i m i l a r l y , I l e t " i n i t i a l decrease" years define the end of outbreaks, those being years showing a decrease i n d e f o l i a t i o n w i t h no decreases i n the previous 3 years. For every i n i t i a l i n c r e a s e and decrease year (year t ) , I then determined the highest annual degree-day value i n the previous 2-4 years (years t-2, t-3 and t-4) . This allowed me t o determine the highest value of the feeding degree-days i n the 2-4 years p r i o r t o both the beginning and end of outbreaks. -82-Figure 29 shows the d i s t r i b u t i o n of values f o r the highest annual feeding degree-day value i n the 2 - 4 years p r i o r to i n i t i a l i ncrease and decrease years, f o r a l l 20 s t a t i o n s . T h i r t y of the 36 i n i t i a l i ncrease years (83%) were preceded by at l e a s t one year of above average degree-days. This would appear to be c o n s i s t e n t w i t h the Ives hypothesis t h a t there be a s i n g l e year of favourable feeding temperatures p r i o r to the beginning of outbreaks. However the d i s t r i b u t i o n of maximum feeding degree-days p r i o r t o i n i t i a l decrease years shows the same p a t t e r n . Here 35 of the 40 i n i t i a l decrease years (or 88%) were preceded by one or more above average degree-day years. Years w i t h above average feeding degree-days occur j u s t as o f t e n p r i o r t o the end of outbreaks as they do p r i o r t o s t a r t . This suggests t h a t over a p e r i o d of 3 successive years, the p r o b a b i l i t y of having at l e a s t one warm year may be q u i t e high. To examine t h i s more c l o s e l y , I looked at how the p r o b a b i l i t y of having an above average feeding degree-day year v a r i e d as a f u n c t i o n of the p e r i o d of successive years considered. I determined the highest annual feeding degree-day value through a l l p o s s i b l e periods of 1 t o 4 consecutive years, using the feeding degree-days c a l c u l a t e d at a l l s t a t i o n s and years. F i g u r e 30 shows t h a t as one i n c r e a s e s the l e n g t h of the p e r i o d over which the maximum annual value i s determined, the chance of f i n d i n g an above average value i n c r e a s e s d r a m a t i c a l l y . I f the s t a r t of - 8 3 -Degree Days - M e a n Figure 29. The d i s t r i b u t i o n of maximum feeding degree-day values i n a period 2-4 years p r i o r to: (a) i n i t i a l increase years; (b) i n i t i a l decrease years. Degree-days are displayed r e l a t i v e to 25°C-days, the mean number of feeding degree-days for a l l stations and years. -84-lQ CO .O O C L 1.0 -i 0.8 0.6 0.4 -0.2 0.0 (a) maximum (b) average o —r 3 i 4 5 Period (years) Figure 30, The p r o b a b i l i t y of having an above average feeding-degree day value as a f u n c t i o n of the p e r i o d of successive years considered. The two measures are: (a) the maximum annual value i n the p e r i o d ; (b) the average value over the p e r i o d . - 8 5 -outbreaks bear no r e l a t i o n s h i p t o the feeding temperatures i n the preceding years, one would s t i l l expect to f i n d a s i n g l e warm year i n the preceding 2-4 years f o r 80% of the cases. This c o u l d e x p l a i n the d i f f e r e n c e i n conclusions regarding the e f f e c t of the l a r v a l feeding temperature of t h i s study and t h a t of Ives (1973). An a l t e r n a t i v e way of examining the e f f e c t of temperature p r i o r t o outbreaks i s t o use an average of the feeding degree days over a f i x e d number of years p r i o r t o the s t a r t of d e f o l i a t i o n (as i n W e l l i n g t o n , 1952). I f the feeding degree-days are above average i n one or more of the years p r i o r t o the s t a r t of an outbreak, the m u l t i - y e a r average should a l s o be above average. U n l i k e the previous measure, however, such an average i s not b i a s e d by the l e n g t h of the p e r i o d over which i t i s measured (Figure 30). I t i s f o r t h i s reason t h a t i n Chapter 4 I chose to t e s t the i d e a t h a t the feeding temperature may i n i t i a t e outbreaks, i n the years p r i o r t o the f i r s t n o t i c e a b l e d e f o l i a t i o n , using the average of the feeding degree-days i n the 4 years p r i o r t o i n i t i a l i n c r e a s e s . R e c a l l t h a t the r e s u l t s of t h i s a n a l y s i s (Figure 16) showed no d i f f e r e n c e between the 4 year average of feeding degree days p r i o r t o the r i s e s (51% above the mean) and the d e c l i n e s (54% above the mean), p r o v i d i n g f u r t h e r evidence t h a t there are f a c t o r s other than the temperature d u r i n g l a r v a l feeding t h a t c o n t r i b u t e t o outbreak r i s e s and d e c l i n e s . -86-Indices and Insects Although there does not appear t o be any r e l a t i o n s h i p between changes i n d e f o l i a t i o n and my feeding temperature index, one cannot assume th a t the feeding temperature has no r o l e i n shaping the year-to-year dynamics of outbreaks. As i n a l l b i o c l i m a t i c s t u d i e s of t h i s form, i t i s always p o s s i b l e t h a t the measure of c l i m a t e I have chosen does not represent the feeding c o n d i t i o n s as experienced by the i n s e c t . The index used here was chosen t o match t h a t of previous s t u d i e s (Ives, 1973; Hodson, 1977) and was based upon f i e l d observations of hatching r a t e s (Hodson, 1977), feeding h a b i t s (Hodson, 1941) and s t a r v a t i o n experiments (Smith and Raske, 1968). The s e n s i t i v i t y a n a l y s i s served to explore how contingent my r e s u l t s were upon v a r i o u s assumptions regarding the i n f l u e n c e of temperature on the feeding l a r v a e . F i r s t l y , w h i l e the hatch model i s able t o p r e d i c t the hatch date t o w i t h i n a few days, i t i s not c l e a r at e x a c t l y what p o i n t a f t e r hatch the l a r v a l feeding begins t o be i n f l u e n c e d by c l i m a t e . This prompted me t o delay the p e r i o d over which I c a l c u l a t e d the feeding degree-days f o r a week a f t e r hatch, and t o t r y extending i t from 3 t o 4 weeks. Furthermore, the temperature below which the l a r v a e w i l l not feed i s not known p r e c i s e l y . To account f o r t h i s I t r i e d -87-lowering the feeding t h r e s h o l d from 15°C t o 10°C. I a l s o t e s t e d the s e n s i t i v i t y of my f i n d i n g s to the choice of s c a l e over which the d e f o l i a t i o n (and thus abundance) i s measured f o r each s t a t i o n and year. The choice of 100km by 100km was d i c t a t e d by the nature of the d e f o l i a t i o n data. The r e s o l u t i o n of the d e f o l i a t i o n data makes i n t e r p r e t a t i o n of the maps at s c a l e s f i n e r than t h i s d i f f i c u l t (see Chapter 2), w h i l e s c a l e s much l a r g e r than t h i s (say 1000km by 1000km) would not capture the r e g i o n a l v a r i a t i o n i n both d e f o l i a t i o n and c l i m a t e . Thus while l a r g e changes i n the s c a l e of a n a l y s i s were not considered, s m a l l e r changes i n the area over which the d e f o l i a t i o n was measured were examined. None of these changes produced any n o t i c e a b l e d i f f e r e n c e s i n the c l i m a t e - d e f o l i a t i o n r e l a t i o n s h i p s . This suggests t h a t the f i n d i n g s are r e l a t i v e l y i n s e n s i t i v e t o some of the assumptions of the i n d i c e s . Overwintering Temperature As w i t h the s t u d i e s t h a t have suggested the importance of the l a r v a l feeding temperature, evidence f o r the r o l e of the o v e r w i n t e r i n g temperature i n the c o l l a p s e of outbreaks i s p r i m a r i l y c o r r e l a t i v e . The strongest evidence comes from records of egg m o r t a l i t y through a s i n g l e outbreak, from 1965 t o 1972, along the Minnesota-Ontario border (Witter et - 8 8 -al., 1 9 7 5 ) . These observations were a l l recorded near Fort Frances, Ontario, for which the time series of d e f o l i a t i o n and overwintering temperatures were calculated as part of t h i s study (Figure 3 1 ) . The d e f o l i a t i o n and overwintering temperature data seem to agree with Witter's findings, and are indeed suggestive of a r e l a t i o n s h i p between the overwintering temperature and the decline of outbreaks. The r e l a t i o n s h i p disappears, however, when one looks at collapses across the entire province over the l a s t 4 1 years. If the overwintering temperature were an important factor i n causing the decline of outbreaks, one would expect to see fewer years of increasing d e f o l i a t i o n than decreasing d e f o l i a t i o n for years with temperatures f a l l i n g below - 4 0 ° C . Figure 1 9 showed that while some declines do occur i n years with minimum overwintering temperatures below - 4 0 ° C , such as those from 1 9 6 5 - 1 9 7 2 at Fort Frances, increases i n d e f o l i a t i o n occurred just as often as decreases i n these colder years. The large number of declines that occurred at station-years with temperatures well above - 4 0 ° C i s a further i n d i c a t i o n of the importance of factors other than the overwintering temperature i n causing population declines. - 8 9 -1940 1950 1960 1970 1980 1990 Year Figure 31. The minimum ove r w i n t e r i n g temperature and the p r o p o r t i o n of area d e f o l i a t e d each year f o r Fort Frances ( s t a t i o n 2 ) . High egg m o r t a l i t y and low minimum ov e r w i n t e r i n g temperatures were recorded i n 1966, 1968 and 1972. Shading shows the temperature r e l a t i v e t o -40°C. Dark l i n e shows the percent d e f o l i a t i o n . -90-Host A v a i l a b i l i t y Outbreaks appear to be r e s t r i c t e d t o regions i n the prov i n c e w i t h continuous t r a c t s of deciduous and mixed f o r e s t s . While the cli m a t e and species composition of southern Ontario appear t o be conducive t o outbreaks, the fragmented nature of i t s f o r e s t e d areas appears t o l i m i t t h e i r s e v e r i t y . F urther evidence of the p o s s i b i l i t y of outbreaks o c c u r r i n g i n t h i s area, i f s u f f i c i e n t f o r e s t cover e x i s t e d , i s the re p o r t s of l o c a l outbreaks throughout the United States (Hodson, 1941; Connola et al., 1957; Rejmanek et al., 1987). Outbreaks have been as severe i n the c o n i f e r o u s dominated f o r e s t s of north-western Ontario (where t r e m b l i n g aspen i s the primary host) as i n the more s o u t h e r l y mixed and deciduous f o r e s t s (where sugar maple dominates). Wherever there i s continuously f o r e s t e d area, there would appear t o be a s u f f i c i e n t l y high p r o p o r t i o n of host t r e e s to support an outbreak. This homogeneity of host a v a i l a b i l i t y across the pro v i n c e suggests t h a t species composition i s not a major f a c t o r i n determining the s u s c e p t i b i l i t y of a region to f o r e s t t e n t c a t e r p i l l a r outbreaks. This f i n d i n g i s s u r e l y s c a l e dependent, however. I f one were to assess the s u s c e p t i b i l i t y of i n d i v i d u a l stands, f o r example, one might expect a higher p r o p o r t i o n of the t r e e s t o be a l l host or -91-non-host, and thus f i n d the species composition t o have a gr e a t e r i n f l u e n c e upon the stand's s u s c e p t i b i l i t y . Long-Term Climate While there i s no apparent r e l a t i o n s h i p between the long-term mean l a r v a l feeding temperature and the s e v e r i t y of outbreaks across the province, there i s some i n d i c a t i o n of a r e l a t i o n s h i p between the minimum o v e r w i n t e r i n g temperature and s e v e r i t y of outbreaks. The n o r t h - c e n t r a l p a r t of the province, w i t h annual minimum temperatures t h a t f r e q u e n t l y f a l l below -40°C, has shown l i t t l e d e f o l i a t i o n from 1948 t o 1988. Note t h a t one should t r e a t the r e s u l t s of t h i s long-term comparison of o v e r w i n t e r i n g temperatures and outbreak s e v e r i t y w i t h c a u t i o n . As the c l i m a t e at any one s t a t i o n i s r e l a t e d t o that at the neighbouring ones, the measures at each of the 20 s t a t i o n s are not independent. Such a u t o c o r r e l a t i o n amongst v a r i a b l e s tends to exaggerate the presence of r e l a t i o n s h i p s ( C l i f f and Ord, 1981). Given the absence of a r e l a t i o n s h i p between the o v e r w i n t e r i n g temperature and the year-to-year p o p u l a t i o n dynamics, how c o u l d the o v e r w i n t e r i n g temperature i n f l u e n c e an area's long-term s u s c e p t i b i l i t y t o outbreaks? I t may be t h a t w h i l e w i n t e r m o r t a l i t y r a t e s are r a r e l y high enough to be the primary cause of outbreak c o l l a p s e s throughout most of the province, they may be high enough i n the n o r t h --92-c e n t r a l p a r t of the province t o prevent d e n s i t i e s from ever reaching l e v e l s t h a t would cause n o t i c e a b l e d e f o l i a t i o n . Given f e c u n d i t i e s of about 150 t o 200 eggs per a d u l t (Hodson, 1941; W i t t e r et al., 1975) and a sex r a t i o of approximately 1:1 ( S i p p e l l , 1957; W i t t e r , 1979), generation m o r t a l i t y r a t e s of clo s e t o 99% are r e q u i r e d t o b r i n g about a d e c l i n e i n abundance. The highest r e p o r t e d o v e r w i n t e r i n g m o r t a l i t y r a t e f o r f o r e s t t e n t c a t e r p i l l a r i s 65%, which occurred i n a year w i t h a minimum temperature of -43°C (Witter et al., 1975). Furthermore, the s u p e r c o o l i n g p o i n t of -40°C provides only an i n d i c a t i o n of the temperature at which m o r t a l i t y becomes s i g n i f i c a n t ; the d i r e c t e f f e c t of low temperatures on egg m o r t a l i t y i s not known. The l e t h a l l i m i t of low temperatures f o r h i b e r n a t i n g i n s e c t s i s not f i x e d ; the p r o b a b i l i t y of f r e e z i n g i n c r e a s e s as the temperature decreases and the exposure time incre a s e s ( S a l t , 1961). As such, areas which experience only b r i e f periods of temperatures below -40°C may s t i l l have q u i t e h i g h egg s u r v i v a l . Eggs w i t h i n areas w i t h longer, more frequent p e r i o d s of exposure, such as the n o r t h - c e n t r a l p a r t of the province, w i l l have a higher p r o b a b i l i t y of f r e e z i n g . This might f u r t h e r e x p l a i n the reduced s u s c e p t i b i l i t y of such areas t o d e f o l i a t i o n . A c o n t r o l l e d experiment, measuring egg m o r t a l i t y i n r e l a t i o n t o exposures of v a r y i n g extremes and du r a t i o n s , would help assess the p o t e n t i a l r o l e of the ov e r w i n t e r i n g -93-temperature i n l i m i t i n g the s e v e r i t y of outbreaks. As the s u p e r c o o l i n g p o i n t of the eggs v a r i e s s e a s o n a l l y , the m o r t a l i t y r a t e may a l s o be a f u n c t i o n of both the t i m i n g of the exposure and the p r i o r temperatures. These f a c t o r s could a l s o be accounted f o r when c a l c u l a t i n g the temperature-m o r t a l i t y r e l a t i o n s h i p . I f the minimum overwintering temperature does i n f l u e n c e the s e v e r i t y of f o r e s t tent c a t e r p i l l a r outbreaks i n Ontario, one would expect to see a s i m i l a r p a t t e r n f o r other p a r t s of Canada. D e f o l i a t i o n data, s i m i l a r t o t h a t used i n t h i s study, e x i s t s f o r the P r a i r i e provinces as f a r back as 1923 ( H i l d a h l and Reeks, 1960; Ives, 1971) . A good t e s t of t h i s s u s c e p t i b i l i t y hypothesis would be to compare temperature data across these provinces t o t h e i r p a t t e r n s of outbreak s e v e r i t y . I f the d i s t r i b u t i o n of minimum winter temperatures does i n f l u e n c e the s u s c e p t i b i l i t y of regions t o f o r e s t t e n t c a t e r p i l l a r outbreaks, one would a l s o expect i t t o be important i n determining the long-term p a t t e r n of outbreaks f o r other f o r e s t d e f o l i a t o r s . One example of a f o r e s t i n s e c t showing a s i m i l a r r e l a t i o n s h i p between temperature and outbreaks i s the winter moth, Operophtera brumata, i n Scandinavia. MacPhee (1967) has shown th a t w i n t e r moth egg m o r t a l i t y i n c r e a s e s s i g n i f i c a n t l y when temperatures are near i t s s u p e r c o o l i n g p o i n t of -35°C, w i t h 10% m o r t a l i t y o c c u r r i n g at -32.2°C and 100% m o r t a l i t y -36.7°C. When the -94-long-term p a t t e r n of d e f o l i a t i o n by the winter moth i n Norway and Sweden (from 1862-1967) was compared t o the d i s t r i b u t i o n of mean minimum winter temperatures, outbreaks were found t o be concentrated w i t h i n regions w i t h mean minimum winter temperatures above -33°C (Tenow, 1972). Furthermore, w i t h i n l a r g e r areas of d e f o l i a t i o n there were of t e n b e l t s of undamaged f o r e s t along r i v e r s and l a k e s , where higher o v e r w i n t e r i n g m o r t a l i t y was a t t r i b u t e d t o l o c a l accumulations of c o l d a i r (Tenow, 1981). O t h e r F a c t o r s My a n a l y s i s so f a r i n d i c a t e s t h a t , w h i l e c l i m a t e and f o r e s t composition may determine the s u s c e p t i b i l i t y of an area t o f o r e s t t e n t c a t e r p i l l a r outbreaks, they are not s o l e l y r e s p o n s i b l e f o r year-to-year changes i n abundance. The f o l l o w i n g s e c t i o n discusses the p o s s i b l e importance of other f a c t o r s i n determining the year-to-year dynamics of outbreaks. Synchrony of outbreaks In Chapter 3 I showed how r i s e s and f a l l s of outbreaks occur i n a r e l a t i v e l y synchronous manner across the e n t i r e province (see F i g u r e 6). This suggests t h a t a c l i m a t i c f a c t o r (or f a c t o r s ) p r i m a r i l y r e s p o n s i b l e f o r these dynamics should a l s o act synchronously over a s i m i l a r s p a t i a l extent. -95-As c l i m a t e at one l o c a t i o n i n space and time i s o f t e n r e l a t e d t o t h a t at neighbouring l o c a t i o n s (Figure 32), c l i m a t i c f a c t o r s could conceivably cause synchronous changes i n abundance over q u i t e l a r g e areas. One way t o examine the p o t e n t i a l of a c l i m a t i c f a c t o r f o r causing such synchronous changes i n abundance i s to examine the degree t o which i t s neighbouring values are c o r r e l a t e d . I f a f a c t o r i s p r i m a r i l y r e s p o n s i b l e f o r the year-to-year changes i n abundance, one would expect the s p a t i a l extent over which i t s neighbouring values are c o r r e l a t e d to match the extent over which the changes i n abundance are synchronized. In g eneral, i f high values f o r a p a r t i c u l a r v a r i a b l e at one l o c a l i t y are c o r r e l a t e d w i t h high values nearby, the v a r i a b l e i s s a i d t o e x h i b i t p o s i t i v e s p a t i a l a u t o c o r r e l a t i o n . I f high and low values a l t e r n a t e , the s p a t i a l a u t o c o r r e l a t i o n i s negative, w h i l e i f the values are independent the a u t o c o r r e l a t i o n w i l l be zero. Consider t h a t a c l i m a t i c index i s a random v a r i a b l e , X, t h a t has been measured at n p o i n t s i n space and time. The value f o r a p a r t i c u l a r l o c a t i o n i ( i n space and time) i s then x^. A simple model of the s p a t i a l interdependence among the {x^} would be ( C l i f f and Ord, 1981): xi = P £ w±j Xj + e± j J J where -96-CO >« CO Q CD CD te> CD Q 1940 1940 (a) 3 1 0 km 1950 1960 1970 1980 1990 (b) 1 4 8 0 km 1950 1960 1970 1980 1990 Year F i g u r e 32. The feeding degree-days each year f o r p a i r s of neighbouring and d i s t a n t s t a t i o n s . S t a t i o n s are separated by: (a) 310 km; (b) 1480 km. - 9 7 -WJ_J = 1 i f i i s s p a t i a l l y adjacent t o j , 0 otherwise; {£^} are independent and i d e n t i c a l l y d i s t r i b u t e d v a r i a t e s ; p i s a measure of the o v e r a l l s p a t i a l a u t o c o r r e l a t i o n . The most commonly used measure of s p a t i a l a u t o c o r r e l a t i o n i s Moran's I s t a t i s t i c ( C l i f f and Ord, 1981), which i s expressed as: I = ( II w±1 c.. ) / (s 2 II ii1 ) where c i j = ( x i ~ (xj ~ ' x = I X£ / n ; i s 2 = I (x± - x)2 / n . i With high p o s i t i v e a u t o c o r r e l a t i o n I w i l l be c l o s e t o 1; w i t h no a u t o c o r r e l a t i o n the exp e c t a t i o n of I i s - l / ( n - l ) (Upton and F i n g l e t o n , 1985), which approaches 0 f o r l a r g e values of n. The adjacency weights d e f i n e the dis t a n c e over which the a u t o c o r r e l a t i o n i s measured. For example, t o measure the a u t o c o r r e l a t i o n at a d i s t a n c e of 100 km, one might set the WJ_J t o 1 f o r a l l i and j t h a t are 50 t o 150 km apart (and i n the same y e a r ) . One can then generate a s p a t i a l correlogram, showing the a u t o c o r r e l a t i o n as a f u n c t i o n of d i s t a n c e , by c a l c u l a t i n g Moran's I f o r a range of d i s t a n c e c l a s s e s . -98-As a measure of the s c a l e over which the l a r v a l feeding temperature and the overwintering temperature might cause synchronous changes i n abundance, I c a l c u l a t e d the s p a t i a l correlogram f o r the annual values of both these c l i m a t i c i n d i c e s . As the 20 c l i m a t i c s t a t i o n s used t o t h i s p o i n t in' the a n a l y s i s were chosen so as t o be about 200 km apart, they provide l i t t l e data f o r c a l c u l a t i n g the a u t o c o r r e l a t i o n at d i s t a n c e s of 0 t o 200 km. To generate a correlogram across the f u l l range of d i s t a n c e s , I used 88 c l i m a t i c s t a t i o n s across the province. Although many of these s t a t i o n s had only enough temperature data t o generate index values f o r a few years, they s t i l l p rovided v a l u a b l e i n f o r m a t i o n regarding the a u t o c o r r e l a t i o n at s m a l l e r d i s t a n c e c l a s s e s . F i g u r e 33 shows the r e s u l t i n g correlograms f o r the annual values of the feeding degree-days and minimum ov e r w i n t e r i n g temperatures. For the o v e r w i n t e r i n g temperature index, the r e g u l a r gradient i n values (from n o r t h t o south) leads t o the c h a r a c t e r i s t i c p a t t e r n of high negative a u t o c o r r e l a t i o n s at the l a r g e r d i s t a n c e c l a s s e s (Sokal and Oden, 1 9 7 8 ). For both i n d i c e s there i s l i t t l e p o s i t i v e a u t o c o r r e l a t i o n between the values recorded at l o c a t i o n s separated by more than 500 km. For the north-western and southern p a r t s of the province, which are more than 1000 km apart, the feeding and o v e r w i n t e r i n g temperatures each year are thus g e n e r a l l y - 9 9 -Distance (km) Figure 33. The s p a t i a l c o r r e l o g r a m s f o r t h e a n n u a l v a l u e s o f : (a) f e e d i n g degree-days; (b) minimum o v e r w i n t e r i n g t e m p e r a t u r e . -100-u n r e l a t e d . I f temperature was s o l e l y r e s p o n s i b l e f o r the year-to-year dynamics, one would have expected the synchrony to have been l i m i t e d to a range of about 500 km. This provides f u r t h e r evidence of the importance of other f a c t o r s being r e s p o n s i b l e f o r l i n k i n g the dynamics of outbreaks across the pr o v i n c e . Years of i n c r e a s i n g abundance Why then have the outbreaks appeared and disappeared so synchronously across the e n t i r e province? Consider f i r s t the incr e a s e phase of outbreaks. I f one overlays p a i r s of d e f o l i a t i o n maps f o r successive years, the f o l l o w i n g p i c t u r e emerges (see Appendix A): small pockets of d e f o l i a t i o n f i r s t appear i n a few i s o l a t e d l o c a t i o n s (often r e f e r r e d t o as " e p i c e n t r e s " - see Wallner, 1987); the e p i c e n t r e s u s u a l l y appear synchronously across the province ( w i t h i n 1-2 years of each o t h e r ) ; the l o c a t i o n s of the e p i c e n t r e s vary from outbreak t o outbreak; the d e f o l i a t i o n i n subsequent years spreads r a p i d l y outwards. This spreading p a t t e r n of d e f o l i a t i o n i s s i m i l a r t o th a t observed f o r the eastern spruce budworm (Hardy et al., 198 6) and suggests that adult d i s p e r s a l may p l a y an important r o l e i n the r i s e phase of outbreaks. This would a l s o e x p l a i n the e a r l i e r f i n d i n g t h a t outbreaks do not appear i n the p a r t of the province w i t h only s m a l l , i s o l a t e d patches of host. Although l i t t l e i s known of the d i s p e r s a l -101-response of forest tent c a t e r p i l l a r moths, there are reports of moths being transported hundreds of kilometres (Brown, 1965; Raske, 1976) . Such a process could p o t e n t i a l l y homogenize the pattern of outbreaks over a r e l a t i v e l y large scale. Simulation studies of the population dynamics of the spruce budworm i n New Brunswick have suggested that moth disp e r s a l plays an important role i n that system's synchronous pattern of outbreaks (Clark, 1979). While the d e f o l i a t i o n maps are suggestive of the presence of epicentres, from which outbreaks then spread, more extensive records of abundance i n the non-outbreak years would help to better e s t a b l i s h t h e i r existence. Light trap records of endemic population l e v e l s between outbreaks in Minnesota indicate that low densities of moths were present i n a l l of the endemic years over much of the area previously d e f o l i a t e d (Hodson, 1977). Apparently populations do not go l o c a l l y extinct between outbreaks, but rather remain at low l e v e l s throughout the endemic years. One could e s t a b l i s h a network of pheromone traps, as has been done previously i n A t l a n t i c Canada (Pendrel, 1984), and monitor i t over a period of years p r i o r to the next outbreak i n Ontario. This may provide some valuable information regarding the presence of epicentres. One could also set up an egg mass c o l l e c t i o n program, over the same network of stations, to provide further abundance information i n the early epidemic years. -102-Given t h a t these e p i c e n t r e s do e x i s t , and can be l o c a t e d i n the e a r l y stages of an outbreak, one could b e t t e r assess the r o l e of f o r e s t composition and c l i m a t e i n t r i g g e r i n g outbreaks. Furthermore, the d e f o l i a t i o n maps cou l d be used t o generate models f o r p r e d i c t i n g the yea r - t o -year p r o g r e s s i o n of outbreaks (as i s c u r r e n t l y being done by F o r e s t r y Canada f o r the hemlock looper i n Newfoundland - M. Power, pers. comm.). Developing b i o l o g i c a l l y meaningful p r e d i c t i v e models f o r the f o r e s t t e n t c a t e r p i l l a r , however, w i l l r e q u i r e a f a r b e t t e r understanding of the adu l t moth d i s p e r s a l response. Years of decreasing abundance Consider the n o n - c l i m a t i c f a c t o r s t h a t have been suggested as important i n causing outbreaks t o d e c l i n e : pupal p a r a s i t i s m , disease and the food supply. C e r t a i n l y S i p p e l ' s (1957) extensive record of increases i n pupal p a r a s i t i s m by the f l e s h f l y , Sarcophaga aldrichi, over the course of an outbreak suggest t h a t t h i s f a c t o r c o u l d p l a y an important r o l e i n the d e c l i n e of outbreaks. Rates of pupal p a r a s i t i s m i n excess of 90% i n the l a t e outbreak years have f r e q u e n t l y been recorded i n Ontario and Minnesota (Hodson, 1941; S i p p e l l , 1957; Hodson, 1977). Very l i t t l e data have been c o l l e c t e d f o r the other two f a c t o r s , however, making i t d i f f i c u l t t o assess t h e i r r o l e i n causing d e c l i n e s . I f higher S. aldrichi a t t a c k r a t e s -103-o c c u r r e d f o r t h o s e pupae t h a t were p r e v i o u s l y weakened by s t a r v a t i o n or d i s e a s e , then the h i g h r a t e s o f p a r a s i t i s m would o v e r s t a t e the f l y ' s c o n t r i b u t i o n t o p o p u l a t i o n d e c l i n e s . I t i s q u i t e p o s s i b l e t h a t two or more such d e n s i t y dependent f a c t o r s ac t t o g e t h e r t o b r i n g about t h e end o f an o u t b r e a k . Food s u p p l y , p u p a l p a r a s i t i s m and d i s e a s e c o u l d a l l c o n c e i v a b l y cause the synchronous d e c l i n e s shown i n C h a p t e r 3. The d i s p e r s a l o f S. aldrichi c o u l d p o t e n t i a l l y l i n k the dynamics o f n e i g h b o u r i n g a r e a s , as c o u l d d i s e a s e t r a n s m i s s i o n (which, as n u c l e a r p o l y h e d r o s i s v i r u s can be t r a n s m i t t e d by Si aldrichi, may i n t u r n be l i n k e d t o the f l y ' s d i s p e r s a l ) . The food s u p p l y , g i v e n t h a t the outbreak dynamics are a l r e a d y homogenized by p r o c e s s e s such as d i s p e r s a l , c o u l d a l s o become s y n c h r o n o u s l y l i m i t i n g over l a r g e a r e a s . The d e f o l i a t i o n maps a lone w i l l do l i t t l e t o s e p a r a t e the e f f e c t s o f these f a c t o r s ; what i s r e q u i r e d i s more i n f o r m a t i o n c o n c e r n i n g t h e i r v a r i a t i o n over the course o f an o u t b r e a k . - 1 0 4 -CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS The r e s u l t s of t h i s t h e s i s suggest t h a t d e f o l i a t i o n maps are a v a l u a b l e source of in f o r m a t i o n i n the study of f o r e s t i n s e c t p o p u l a t i o n dynamics. When combined w i t h other l a r g e s c a l e d i n f o r m a t i o n they provide a unique view of the dynamics of such systems. The value of these maps has been i l l u s t r a t e d i n two ways. F i r s t l y , the maps have been used t o examine the importance of f a c t o r s t h a t were p r e v i o u s l y i d e n t i f i e d as important through only a l i m i t e d number of s m a l l e r s c a l e d observations. This was done i n asse s s i n g the r o l e of both the l a r v a l feeding temperature and the o v e r w i n t e r i n g temperature i n determining year-to-year changes i n abundance. The r e s u l t s of t h i s a n a l y s i s suggest t h a t these year-to-year dynamics cannot be s o l e l y a t t r i b u t e d t o the v a r i a t i o n i n these two c l i m a t i c f a c t o r s . The maps were a l s o used to explore the importance of f a c t o r s t h a t might go unnoticed at smaller s c a l e s . A long-term a n a l y s i s examined the r o l e of the l a r v a l feeding temperature and the overwintering temperature i n determining the s u s c e p t i b i l i t y of d i f f e r e n t areas t o outbreaks. The r e s u l t s of t h i s a n a l y s i s suggest t h a t the o v e r w i n t e r i n g temperature may pl a y an important r o l e i n determining t h i s long-term s u s c e p t i b i l i t y . Further study of the r e l a t i o n s h i p - 1 0 5 -between c o l d temperatures and egg m o r t a l i t y , f o r the f o r e s t t e n t c a t e r p i l l a r and other f o r e s t i n s e c t s , would help g r e a t l y i n asse s s i n g the overwintering temperature's importance. The synchrony of outbreaks, and t h e i r apparent yea r - t o -year spread, suggests t h a t adult d i s p e r s a l may l i n k the dynamics of outbreaks over l a r g e areas. Developing b i o l o g i c a l l y meaningful models of the d i s p e r s a l process, such as those of C l a r k (197 9) f o r the spruce budworm, would help to b e t t e r assess i t s importance. This, however, w i l l r e q u i r e a f a r b e t t e r understanding of the f o r e s t t e n t c a t e r p i l l a r ' s d i s p e r s a l response. T e s t i n g these models w i l l a l s o r e q u i r e more inf o r m a t i o n regarding the s p a t i a l and temporal p a t t e r n of i n s e c t d e n s i t i e s i n the p r e - d e f o l i a t i o n years. F i n a l l y , the d e f o l i a t i o n maps suggest t h a t outbreaks may begin i n a few d i s t i n c t e p i c e n t r e s . While d e f o l i a t i o n may f i r s t appear only at these s o - c a l l e d e p i c e n t r e s , the high l a r v a l d e n s i t i e s r e q u i r e d to b r i n g about any n o t i c e a b l e d e f o l i a t i o n i n d i c a t e t h a t outbreaks are w e l l underway before any d e f o l i a t i o n i s recorded. B e t t e r records of endemic p o p u l a t i o n l e v e l s would help to e s t a b l i s h the e x i s t e n c e of these e p i c e n t r e s , and allo w one to b e t t e r assess the importance of other f a c t o r s , such as c l i m a t e and f o r e s t composition, i n t r i g g e r i n g t h e i r development. -106-CHAPTER 7 LITERATURE CITED A l l e n , J.C. 1976. A modified s i n e wave method f o r c a l c u l a t i n g degree days. Environmental Entomologist 5: 388-396. Anonymous, 1978. Spring Survey B u l l e t i n . Canadian F o r e s t r y S e r v i c e , Great Lakes Forest Research Centre. /Anonymous, 1988. Canada's Forest Inventory 1986. F o r e s t r y Canada: Ottawa. Arthur, A.P. and H.C. Coppel 1953. Studies on di p t e r o u s p a r a s i t e s of the spruce budworm, C h o r i s t o n e u r a fumiferana (Clem.). I. Sarcophaga a l d r i c h i Park. Canadian J o u r n a l of Zoology 31: 374-391. B l a i s , J.R. 1968. Regional v a r i a t i o n i n s u s c e p t i b i l i t y of eastern North American f o r e s t s to budworm a t t a c k based on h i s t o r y of outbreaks. F o r e s t r y C h r o n i c l e 44: 17-23. B l a i s , J.R. 1983. Trends i n the frequency, extent, and s e v e r i t y of spruce budworm outbreaks i n eastern Canada. Canadian J o u r n a l of Forest Research 13: 539-547. B l a i s , J.R., R.M. P r e n t i c e , W.L. S i p p e l l and D.R. Wallace 1955. E f f e c t s of weather on the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria Hbn., i n c e n t r a l Canada i n the s p r i n g of 1953. Canadian Entomologist 87: 1-8 . Brown, A.W.A. 1938. Forest t e n t c a t e r p i l l a r i n Ontario 1931-1938. Entomological Society of Ontario Annual Report 69: 37-42. Brown, C.E. 1965. Mass t r a n s p o r t of f o r e s t t e n t c a t e r p i l l a r moths by a c o l d f r o n t . Canadian Entomologist 97: 1073-1075. C l a r k , E.C. 1958. Ecology of the polyhedroses of t e n t c a t e r p i l l a r s . Ecology 39: 132-139. Cl a r k , W.C. 1979. S p a t i a l s t r u c t u r e r e l a t i o n s h i p i n a f o r e s t i n s e c t system: s i m u l a t i o n models and a n a l y s i s . M i t t e i l u n g e n der Schweizerischen Entomologischen G e s e l l s c h a f t 52, 235-257. C l i f f , A.D. and Ord, J.K. 1981. S p a t i a l Processes: Models and A p p l i c a t i o n s . P ion: London. -107-Connola, D.P., W.E. Waters and W.E. Smith 1957. The development and a p p l i c a t i o n of a s e q u e n t i a l sampling plan f o r f o r e s t tent c a t e r p i l l a r i n New York. New York State Museum and Science B u l l e t i n 366. Duncan, D.P. and A.C. Hodson 1958. Influence of the f o r e s t t e n t c a t e r p i l l a r upon the aspen f o r e s t s of Minnesota. Forest Science 4: 71-93. Fashingbauer, B.A., A.C. Hodson and W.H. M a r s h a l l 1957. The i n t e r - r e l a t i o n s of a f o r e s t t e n t c a t e r p i l l a r outbreak, song b i r d s and DDT a p p l i c a t i o n . F l i c k e r 29: 132-143, 146-147. Gautreau, G.J. 1964. Unhatched f o r e s t t e n t c a t e r p i l l a r eggbands i n northern A l b e r t a a s s o c i a t e d w i t h l a t e s p r i n g f r o s t . Canadian Department of F o r e s t r y , B i -monthly Progress Report 20: 3. Greenbank, D.O. 1954. H i s t o r y of the spruce budworm i n New Brunswick w i t h reference t o c l i m a t e and d i s p e r s a l . I n t e r i m T e c h n i c a l Report, D i v i s i o n of F o r e s t r y , B i o l o g y Department, A g r i c u l t u r e Canada. H a l l , K.C. 1967. Report of the f o r e s t research t e c h n i c i a n s , Port Arthur d i s t r i c t , 1966. Canadian F o r e s t r y S e r v i c e , Ontario Region, Forest Research Laboratory, Information Report O-X-51. Hanec, W. 1966. Cold hardiness i n the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria Hbn. Jo u r n a l of Insect Physiology 12: 1443-1449. Hardy. Y.J. 1984. How do outbreaks s t a r t ? I n: Proceedings, new and improved techniques f o r monitoring and e v a l u a t i n g spruce budworm po p u l a t i o n s . United States Department of A g r i c u l t u r e , Forest S e r v i c e , Northeastern S t a t i o n , General Technical Report NE-88. Hardy, Y., M. M a n v i l l e and D.M. Schmitt 1986. An a t l a s of spruce budworm d e f o l i a t i o n i n Eastern North America: 1938-80. United States Department of A g r i c u l t u r e , Miscellaneous P u b l i c a t i o n No. 1449. H i l d a h l , V. and I.M. Campbell 1975. Forest t e n t c a t e r p i l l a r and the P r a i r i e p r ovinces. Canadian F o r e s t r y S e r v i c e , Northern Forest Research Centre, Report NOR-X-135. H i l d a h l , V. and W.A. Reeks 1960. Outbreaks of the f o r e s t t e n t c a t e r p i l l a r and t h e i r e f f e c t s on stands of tre m b l i n g aspen i n Manitoba and Saskatchewan. Canadian Entomologist 92: 199-209. -108-Hodson, A.C. 1939. Sarcophaga aldrichi Parker as a p a r a s i t e of Malacosoma disstria Hbn. Journal of Economic Entomology 32: 396-401. Hodson, A.C. 1941. An e c o l o g i c a l study of the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria Hbn., i n northern Minnesota. U n i v e r s i t y of Minnesota A g r i c u l t u r a l Experiment S t a t i o n Technical B u l l e t i n 148. Hodson, A.C. 1977. Some aspects of f o r e s t t e n t c a t e r p i l l a r p o p u l a t i o n dynamics. U n i v e r s i t y of Minnesota A g r i c u l t u r a l Experiment S t a t i o n T e c h n i c a l B u l l e t i n 310: 5-16. Hodson, A.C. and C.J. Weinman 1945. Factors a f f e c t i n g recovery from diapause and hatching of eggs of the f o r e s t t e n t c a t e r p i l l a r , Malacosoma d i s s t r i a Hbn. U n i v e r s i t y of Minnesota A g r i c u l t u r a l Experiment S t a t i o n Technical B u l l e t i n 170: 3-31. Ives, W.G.H. 1971. The f o r e s t t e n t c a t e r p i l l a r i n A l b e r t a . Canadian F o r e s t r y S e r v i c e , Northern Forest Research Centre, I n t e r n a l Report NOR-4. Ives, W.G.H. 1973. Heat u n i t s and outbreaks of the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria. Canadian Entomologist 105: 529-543. Livesey, F. 1968. Report of the f o r e s t research t e c h n i c i a n s , Tweed d i s t r i c t , 1967. Canadian F o r e s t r y S e r v i c e , Ontario Region, Forest Research Laboratory, Information Report O-X-59. MacLeod, L.S., J . Hook, F. Livesey 1975. Forest i n s e c t and disease surveys i n the Northern region of Ontario, 1974. Canadian F o r e s t r y S e r v i c e , Ontario Region, Forest Research Laboratory, Information Report O-X-222. MacPhee, A.W. 1967. The winter moth Operophthera brumata (Lepidoptera: Geometridae), a new pest a t t a c k i n g apple orchards i n Nova S c o t i a , and i t s c o l d h a r d i n e s s . Canadian Entomologist 99: 829-834. M a r t i n a t , P.J. 1987. The r o l e of c l i m a t i c v a r i a t i o n and weather i n f o r e s t i n s e c t outbreaks. In P. Barbosa and J.C. Shultz eds. Insect Outbreaks. Academic Press: San Diego, pp. 241-268. Mattson, W.J. and G.W. Er i c k s o n 1978. Degree-day summation and hatching of the f o r e s t tent c a t e r p i l l a r , Malacosoma disstria (Lepidoptera: Lasiocampidae). Great Lakes Entomologist 11: 59-61. -109-M o r r i s , R.F. (ed.) 1963. The dynamics of epidemic spruce budworm po p u l a t i o n s . Memoirs of the Entomological S o c i e t y of Canada 31. Myers, J.H. 1988. Can a general hypothesis e x p l a i n p o p u l a t i o n c y c l e s of f o r e s t Lepidoptera? Advances i n E c o l o g i c a l Research 18: 179-242. O ' N e i l l , R.V., D.L. DeAngelis, J.B. Waide, T.F.H. A l l e n 1986. A H i e r a r c h i c a l Concept of Ecosystems. Monographs i n P o p u l a t i o n Biology 23 (R.M. May ed.), P r i n c e t o n U n i v e r s i t y Press, P r i n c e t o n , New Jersey. Pendrel, B.A. 1984. Population d i s t r i b u t i o n of the f o r e s t t e n t c a t e r p i l l a r (Malacosoma disstria) i n the Maritimes 1984 d e s c r i b e d through pheromone t r a p p i n g . T e c h n i c a l Note No. 137, Maritimes Forest Research Centre. P r e n t i c e , R.M. 1954. Decline of populations of the f o r e s t t e n t c a t e r p i l l a r i n c e n t r a l Saskatchewan. Canadian Department F o r e s t r y , Bi-monthly Progress Report 10 (5) : 2. Raske, A.G. 1974. Hatching r a t e s of f o r e s t t e n t c a t e r p i l l a r i n the l a b o r a t o r y . Canadian F o r e s t r y S e r v i c e Bi-monthly Research Notes 30(4): 24-24. Raske, A.G. 1975. Cold-hardiness of f i r s t i n s t a r l a r v a e of the f o r e s t t e n t c a t e r p i l l a r . Canadian Entomologist 107: 75-80. Raske, A.G. 1976. Forest t e n t c a t e r p i l l a r moths found i n Newfoundland. Canadian F o r e s t r y Service Bi-monthly Research Notes 32(1): 1-2. Rejmanek, M., J.D. Smith and R.A. Goyer 1987. P o p u l a t i o n dynamics of the f o r e s t t e n t c a t e r p i l l a r (Malacosoma disstria) i n a water tupelo (Nyssa Aquatica) f o r e s t : a s i m u l a t i o n model. E c o l o g i c a l M o d e l l i n g 39: 287-305. Rowe, J.S. 1972. Forest Regions of Canada. Canadian F o r e s t r y S e r v i c e P u b l i c a t i o n No. 1300. S a l t , R.W. 1961. P r i n c i p l e s of i n s e c t c o l d - h a r d i n e s s . Annual Review of Entomology 6: 55-74. S i p p e l l , W.L. 1957. A study of the f o r e s t t e n t c a t e r p i l l a r and i t s p a r a s i t e complex i n Ontario. Unpulished Ph.D. Thesis, U n i v e r s i t y of Michigan, Ann Arbor, Michigan. S i p p e l l , W.L. 1962. Outbreaks of the f o r e s t t e n t c a t e r p i l l a r , a p e r i o d i c d e f o l i a t o r of broad-leaved t r e e s i n Ontario. Canadian Entomologist 94: 408-416. -110-Smith, G.J. and A.G. Raske 1968. S t a r v a t i o n experiments w i t h f i r s t i n s t a r f o r e s t t e n t c a t e r p i l l a r l a r v a e . Canadian F o r e s t r y S e r v i c e Bi-monthly Research Notes 24: 39. Smith, J.D., R.A. Goyer and J.P. Woodring 1986. I n s t a r determination and growth feeding i n d i c e s of the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria (Lepidoptera: Lasiocampidae), reared on tupelo gum. Annals of the Entomological Society of America 79: 304-307. Sokal, R.R. and N.L. Oden 1978. S p a t i a l a u t o c o r r e l a t i o n i n b i o l o g y . I . Methodology. B i o l o g i c a l J o u r n a l of the Linnean S o c i e t y 10: 199-228. S t a i r s , G.R. 1966. Transmission of v i r u s i n t e n t c a t e r p i l l a r p o p u l a t i o n s . Canadian Entomologist 98: 1100-1104. S t a i r s , G.R. 1972. Pathogenic microorganisms i n the r e g u l a t i o n of f o r e s t i n s e c t p o p u l a t i o n s . Annual Review of Entomology 17: 355-372. Stehr, F.W. and E.F. Cook 1968. A r e v i s i o n of the genus Malacosoma Hiibner i n North America (Lepidoptera: Lasiocampidae): Systematics, b i o l o g y , immatures and p a r a s i t e s . United States N a t i o n a l Museum B u l l e t i n 276. Tenow, 0. 1972. The outbreaks of Oporinia autumnata Bkh. and Operophthera spp. (Lep., Geometridae) i n the Scandinavian mountain chain and northern F i n l a n d 1862-1968. Zoologiska Bidrag Fran Uppsala, Supplement 2. Tenow, 0. 1981. Topoclimatic l i m i t a t i o n s t o the outbreaks of Epirrita (=0porinia) automnata (Bkh.) (Lepidoptera: Geometridae) near the f o r e s t l i m i t of the mountain b i r c h i n Fennoscandia. In P. Morisset and S. Payette eds. Tree-Line Ecology. Proceedings of the Northern Quebec Tree-Line Conference, Nordicana No. 47. Centre d'etudes nordiques, U n i v e r s i t e L a v a l , Quebec, pp. 159-164 . Upton, G.J. and B. F i n g l e t o n 1985. S p a t i a l data a n a l y s i s by example. Volume 1: P o i n t P a t t e r n and Q u a n t i t a t i v e Data. John Wiley & Sons: Chichester. Wagner T.L., H.I. Wu, P.J. Sharpe, R.M. S c h o o l f i e l d and R.N. Coulson 1984. Modeling i n s e c t development r a t e s : a l i t e r a t u r e review and a p p l i c a t i o n of a b i o p h y s i c a l model. Annals of the Entomological S o c i e t y of America 77: 208-225. Wallner, W.E. 1987. Factors a f f e c t i n g i n s e c t p o p u l a t i o n dynamics: d i f f e r e n c e s between outbreak and non-outbreak spec i e s . Annual Review of Entomology 32: 317-340. - I l l -W e l l i n g t o n , W.G. 1950. E f f e c t s of r a d i a t i o n on the temperature of i n s e c t s and h a b i t a t s . S c i e n t i f i c A g r i c u l t u r e 30: 209-234. Wel l i n g t o n , W.G. 1952. A i r mass c l i m a t o l o g y of Ontario North of Lake Huron and Lake Superior before outbreaks of the spruce budworm and the f o r e s t t e n t c a t e r p i l l a r . Canadian Journa l of Zoology 30: 115-127. Wetzel, B.W., H.M. Kulman and J.A. W i t t e r 1973. E f f e c t s of c o l d temperatures on hatching of the f o r e s t t e n t c a t e r p i l l a r . Canadian Entomologist 105: 1145-1149. W i t t e r , J.A. 1971. Bionomics of the f o r e s t t e n t c a t e r p i l l a r , Malacosoma disstria Hbn. Unpublished Ph.D. Thesis, U n i v e r s i t y of Minnesota, St. P a u l . W i t t e r , J.A. 197 9. The f o r e s t t e n t c a t e r p i l l a r (Lepidoptera: Lasiocampidae) i n Minnesota: a case h i s t o r y review. Great Lakes Entomologist 12: 191-197. W i t t e r , J.A. and H.M. Kulman 1972. M o r t a l i t y f a c t o r s a f f e c t i n g eggs of the f o r e s t t e n t c a t e r p i l l a r . Canadian Entomologist 104: 705-710. W i t t e r , J.A., H.M. Kulman, and A.C. Hodson 1972. L i f e t a b l e s f o r the f o r e s t tent c a t e r p i l l a r . Annals of the Entomological Society of America 65: 25-31. W i t t e r , J.A., W.J. Mattson and H.M. Kulman 1975. Numerical a n a l y s i s of a f o r e s t t e n t c a t e r p i l l a r outbreak i n northern Minnesota. Canadian Entomologist 107, 837-854. -112-APPENDIX A Successive year overlays of d e f o l i a t i o n maps Appendix A provides a p i c t u r e of the year t o year changes i n the s p a t i a l p a t t e r n of d e f o l i a t i o n . For each p a i r of successive years, from 1948-49 t o 1987-88, the d e f o l i a t i o n maps have been o v e r l a i d . The r e s u l t i s a s e r i e s of 40 maps c l a s s i f i e d as f o l l o w s ( for a year t-1 and year t o v e r l a y ) : area d e f o l i a t e d only i n year t - 1 ; P area d e f o l i a t e d only i n year t ; • area d e f o l i a t e d i n both year t-1 and year t . i -113--114--115--116--117-- 1 1 8 --119--120--121--122--123-APPENDIX B Feeding degree-days and d e f o l i a t i o n f o r 16 stations Appendix B shows the feeding degree-days and the p r o p o r t i o n of area d e f o l i a t e d each year, from 1948-88, f o r 16 of the 20 c l i m a t e s t a t i o n s . The other 4 s t a t i o n s are shown i n F i g u r e 12. Shading shows the feeding degree-days r e l a t i v e t o 25°C-days, the mean f o r a l l s t a t i o n s and years. Dark l i n e shows the percent d e f o l i a t i o n . -124-1940 Fort F rances (2) 1950 1960 1970 1980 100 1990 1940 Sioux Lookout (3) 1950 1960 1970 1980 100 •50 1990 120 T 80 40 0 1940 1950 Centra l Patricia (4) 1960 1970 1980 r 100 Thunder B a y (6) 100 - 5 0 Year - 1 2 5 -Moosonee (7) 100 CO >-ed O CD CD CD CD Q Hornepayne (8) 1980 Chapleau (10) 100 100 -50 1940 1950 1960 1970 1980 0 1990 C _o C) Q Earlton (11) 100 Y e a r -126-120 80 40 0 1940 Sault Ste . Marie (12) 1960 1970 1980 100 50 0 1990 CO >> CO Q CD CD i _ CD CD Q 1950 Gore B a y (13) 1970 1980 Muskoka (15) 100 100 -50 \o c o 3§ o "CD Q Ot tawa (16) 100 - 50 Year -127-1940 Wiarton (17) 1950 r 100 CO >> CO Q CD CD i _ CO CD Q Peterborough (18) 1970 London (19) 100 -50 100 I S c o i i o C^D Q 1970 Woodslee (20) 1980 100 Year -128-APPENDIX C Minimum overwintering temperature and d e f o l i a t i o n f o r 16 stations This appendix shows the minimum overwintering temperature and the proportion of area d e f o l i a t e d each year, from 1948-88, for 16 of the 20 climate stat i o n s . The other 4 stations are shown i n Figure 18. Shading shows the temperature r e l a t i v e to -40°C. Dark l i n e shows the percent d e f o l i a t i o n . -129--10 -40 --70 1940 Fort F rances (2) J3-i 1950 1960 1970 1980 r 100 50 1990 -10 -40 -70 1940 1950 Sioux Lookout (3) r 1960 1970 1980 100 50 M - 0 1990 -10 -40 -70 1940 1950 Central Patricia (4) 1960 1970 r 100 mjt uimij w* - 50 IL 1980 •0 1990 c O n o o Q -10 -, Thunder B a y (6) 100 50 1990 -130-•10 - l -40 •70 1940 Moosonee (7) 1950 1960 1970 1980 100 -50 1990 -10 -i -40 -70 1940 Homepayne (8) 1950 1960 1970 1980 100 50 *H-0 1990 -10 - i -40 -70 1940 1950 Chapleau (10) 1960 1970 1980 100 50 •0 1990 c _o M o M— CD Q Earlton (11) r 100 Year 10 -40 --70 1940 Sault Ste. Marie (12) 1950 1960 1970 1980 100 50 •0 1990 -10 -i 1940 1950 Gore Bay (13) 1960 1970 1980 100 1990 -10 -40 -70 1940 Ix 1950 Muskoka (15) 1960 1970 1980 r 100 50 1990 -10 n 1940 Ottawa (16) 100 -50 Year -132-1940 Wiarton (17) 1950 100 CD ZS "5 CD CL E CD 1940 -10 -40 -Peterborough (18) 1950 1960 1970 London (19) 1980 100 1990 100 • 50 oN C o —^< o % -70 1940 1950 1960 1970 1980 1990 -10 - i -40 Woodslee (20) 100 •50 -70 1940 1950 1960 1970 1980 •0 1990 Year -133-APPENDIX D Deta i l s of the data analysis D e f o l i a t i o n Data The d e f o l i a t i o n maps were provided by F o r e s t r y Canada's Forest Insect and Disease Survey (FIDS) at the Great Lakes F o r e s t r y Centre (GLFC) i n Sault Ste. Marie, O n t a r i o . The Ontario FIDS create a province-wide d e f o l i a t i o n map each year by combining a l l of the i n d i v i d u a l ranger sketch maps onto a s i n g l e Ontario base map. From 1948 t o 1974 the s c a l e of the province-wide base maps was 1:1,267,200 (1 i n c h = 20 m i l e s ) , w h i l e the s c a l e from 1975 t o 1988 was 1:1,584,000 (1 i n c h = 25 m i l e s ) . A l l o r i g i n a l base maps used a Lambert Conformal p r o j e c t i o n (standard p a r a l l e l s of 44.5° and 53.5°, c e n t r a l meridian of -85°). The d i f f e r e n t l e v e l s of d e f o l i a t i o n r e p o r t e d on the o r i g i n a l maps v a r i e d from one year t o the next. Some years showed only one category of d e f o l i a t i o n (moderate t o severe) w h i l e others showed as many as three c a t e g o r i e s ( l i g h t , moderate and severe). To standardize these maps I ignored the areas marked as " l i g h t " and combined the other areas i n t o a s i n g l e "moderate to severe" category. There were two years f o r which no province-wide composite maps e x i s t e d : 1958 and 1985. Maps f o r these years were r e - c r e a t e d u s i n g the annual Ontario FIDS ranger -134-r e p o r t s ; w h i l e the 1985 re p o r t s showed some d e f o l i a t i o n , no d e f o l i a t i o n was reported i n 1958. A l l of the d e f o l i a t i o n maps were d i g i t i z e d u s i n g the ARC/INFO Geographic Information System. The r e g i o n a l maps f o r 1985 were combined and transformed t o the same p r o j e c t i o n as those of the other years. For d i s p l a y purposes a l l of the maps were overlayed upon an Ontario base map provided by the FIDS Technology Development Group at F o r e s t r y Canada's Petawawa N a t i o n a l F o r e s t r y I n s t i t u t e . To generate the d e f o l i a t i o n time s e r i e s i n t h i s t h e s i s , a 10 km by 10 km g r i d system was created f o r the pr o v i n c e . By o v e r l a y i n g the Ontario base map on t h i s g r i d I determined the land area w i t h i n each of the g r i d ' s 10 km by 10 km c e l l s . I then determined the d e f o l i a t e d area each year, f o r each c e l l , by o v e r l a y i n g each year's d e f o l i a t i o n map on the g r i d system. To generate a time s e r i e s f o r a p a r t i c u l a r area, I then summed the land and d e f o l i a t e d areas of a l l the c e l l s corresponding t o the area i n question. The percent d e f o l i a t i o n over the area f o r each year t was then c a l c u l a t e d as: (% d e f o l i a t i o n ) t = ( t o t a l d e f o l i a t e d a r e a ) t x 100% ( t o t a l l a n d a r e a ) t The percent d e f o l i a t i o n f o r each of the 20 s t a t i o n s and 41 years, at a s c a l e of 100 km by 100 km, i s given i n Table D-I. -135-Temperature Data D a i l y maximum and minimum temperature data were provided by the Canadian Climate Centre, a d i v i s i o n of Environment Canada's Atmospheric Environment S e r v i c e (AES). A catalogue showing a l l of the AES s t a t i o n l o c a t i o n s and approximate periods of record was used t o s e l e c t 88 s t a t i o n s across the pro v i n c e . These were s e l e c t e d t o give a network of s t a t i o n s t h a t would provide continuous temperature data, from 1947-88, f o r the e n t i r e p r o v i nce. To extend the p e r i o d of record f o r those s t a t i o n s which were e i t h e r m i s s i n g records or were rep l a c e d over time w i t h a s t a t i o n nearby, s t a t i o n s w i t h i n 20 km of each other were t r e a t e d as a s i n g l e l o c a t i o n . The completeness of the record f o r each s t a t i o n was then determined by ranking the s t a t i o n s according t o the number of years f o r which the feeding degree-days and the minimum ov e r w i n t e r i n g temperature could be evaluated. For the feeding degree-days, a s t a t i o n and year (or "s t a t i o n - y e a r " ) was deemed t o have s u f f i c i e n t data i f a continuous record of d a i l y maximum and minimum temperatures e x i s t e d from March 1 t o June 30. S i m i l a r l y , a s t a t i o n - y e a r q u a l i f i e d f o r c a l c u l a t i n g the minimum ove r w i n t e r i n g temperature i f a continuous r e c o r d of d a i l y minimum temperatures e x i s t e d from December 1 (of the previous year) t o March 31. -136-Two c r i t e r i a were then used to s e l e c t those s t a t i o n s t h a t would be used f o r f u r t h e r a n a l y s i s . F i r s t , s t a t i o n s were s e l e c t e d such t h a t they were approximately 200 km apart. Second, s t a t i o n s were s e l e c t e d based upon the completeness of t h e i r record. This l e d t o the network of 20 s t a t i o n s used i n t h i s t h e s i s (Table D - I I ) . The c a l c u l a t e d hatch dates, feeding degree-days and minimum o v e r w i n t e r i n g temperatures f o r each of the 20 s t a t i o n s and 41 years are given i n Tables D - I I I , D-IV and D-V r e s p e c t i v e l y . The minimum o v e r w i n t e r i n g temperature f o r each s t a t i o n - y e a r i s the minimum temperature recorded at the s t a t i o n between December 1 and March 31. Hatch dates and feeding degree-days were c a l c u l a t e d u s i n g the formulas given i n Chapter 4. A t r i a n g u l a r approximation was used t o estimate the degree-days each day from maximum and minimum temperature data. This approximation uses the f o l l o w i n g formula (Ives, 1973): degree days = (max + min) - th th < min; 2 = (max - th) mm < th < max; 2 (max - min) = 0 th > max; where max = d a i l y maximum temperature; min = d a i l y minimum temperature; th = t h r e s h o l d temperature. -137-Table D-I Proportion of area defoliated each year, 1948-88, for the 20 climate stations. Values are the percentage of t o t a l land area defoliated, at a scale of 100 km by 100 km. Station numbers are those given in Figure 7. Stn. Year # 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 1 0 1 0 3 80 35 0 0 0 0 0 0 0 4 50 98 100 100 0 0 0 2 0 0 2 21 97 70 0 0 0 0 0 0 0 0 7 56 67 88 48 54 15 3 0 31 66 89 93 0 0 0 0 0 0 0 0 0 94 62 95 90 0 0 0 4 0 0 0 0 0 0 0 0 1 2 0 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 19 18 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0 57 49 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 7 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8 0 0 3 15 53 87 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 9 0 0 14 21 59 96 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 1 23 43 88 29 0 0 0 0 0 0 0 0 0 0 0 0 0 0 11 0 0 0 4 11 67 96 92 31 1 0 0 0 0 0 0 0 0 0 0 0 12 0 11 47 95 74 1 0 0 0 0 0 0 0 0 0 0 0 5 5 9 7 13 0 9 46 99 83 0 0 0 0 0 0 0 0 0 0 0 5 11 8 25 29 14 0 0 8 37 79 92 4 0 0 0 0 0 0 0 0 0 0 15 20 0 0 15 1 10 36 98 24 4 0 0 0 0 0 0 0 0 0 1 6 16 3 0 0 16 0 0 0 2 35 85 0 0 0 0 0 0 0 0 0 0 0 8 47 0 0 17 0 0 0 0 74 31 12 0 0 0 0 0 0 0 0 0 0 0 0 0 0 18 0 0 31 34 22 21 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Table D-I (continued) Stn. Year # 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 1 0 0 0 0 0 0 0 43 81 100 14 0 0 0 0 0 0 0 0 0 2 18 20 22 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 1 30 3 0 0 0 0 1 0 0 0 20 100 95 0 0 0 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 28 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 2 6 6 11 8 6 4 7 0 0 0 0 7 0 0 0 0 0 0 1 4 14 1 0 0 0 0 0 0 0 0 0 0 8 0 0 0 0 0 2 18 12 6 2 0 0 0 0 0 0 0 0 0 0 9 0 0 0 0 0 0 53 62 64 90 5 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 11 0 0 0 0 13 16 22 0 1 2 0 4 0 0 0 12 19 26 29 0 12 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 0 2 4 13 11 0 0 0 0 0 0 3 16 16 3 2 0 0 0 0 0 1 12 56 14 0 0 0 0 1 2 31 30 33 10 0 0 0 0 0 0 0 11 72 90 15 0 0 0 0 0 0 11 77 84 0 0 0 0 0 0 0 0 0 29 67 16 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0 1 17 0 0 0 0 0 0 0 40 66 0 0 0 0 0 0 0 0 0 0 1 18 0 0 0 0 0 0 4 17 15 0 0 0 0 0 0 0 0 0 5 31 19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Table D-II The l o c a t i o n of the Atmospheric Environment Se r v i c e (AES) cli m a t e s t a t i o n s used i n the a n a l y s i s . S t a t i o n numbers are those given i n Figure 7. Stn. AES L a t i t u d e Longitude Name # Id. deg. min. deg. min. 1 6034075 49 48 94 22 KENORA A 2 6022475 48 37 93 25 FORT FRANCES 3 6037775 50 7 91 54 SIOUX LOOKOUT A 4 6011305 51 30 90 9 CENTRAL PATRICIA 4 6016525 51 27 90 12 PICKLE LAKE 5 6040325 50 17 88 54 ARMSTRONG A 6 6048261 48 22 89 19 FT WILLIAM PT ARTHUR A 7 6075425 51 16 80 39 MOOSONEE 8 6053570 49 14 84 48 HORNEPAYNE 9 6073960 49 25 82 26 KAPUSKASING 10 6061358 47 50 83 26 CHAPLEAU 10 6061359 47 50 83 26 CHAPLEAU 10 6061361 47 49 83 21 CHAPLEAU A 11 6072225 47 42 79 51 EARLTON A 12 6057589 46 32 84 30 SAULT STE MARIE 12 6057590 46 32 84 20 SAULT STE MARIE 2 12 6057592 46 28 84 30 SAULT STE MARIE 13 6092925 45 53 82 34 GORE BAY A 14 6085700 46 22 79 25 NORTH BAY A 15 6115525 44 58 79 18 MUSKOKA A 16 6106000 45 19 75 40 OTTAWA A 17 6119500 44 45 81 6 WIARTON A 18 6166416 44 20 78 19 PETERBOROUGH 18 6166418 44 14 78 21 PETERBOROUGH A 19 6144475 43 2 81 9 LONDON A 20 6139600 42 13 82 44 WOODSLEE -140-Table D-III Predicted hatch date each year, 1948-88, for the 20 climate stations. Hatch dates are given as the Julian days beginning March 1st. Station numbers are those given i n Figure 7. Stn. Year # 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 1 73 61 85 69 55 68 83 58 89 61 71 70 75 81 77 68 65 71 84 86 73 2 64 60 52 68 82 57 86 60 50 64 74 80 66 59 64 67 83 79 63 3 75 62 88 74 57 69 94 58 91 66 73 84 76 81 76 72 65 71 87 88 75 4 94 72 77 91 78 77 64 78 92 86 5 83 73 88 78 62 78 95 71 97 71 77 87 81 88 78 86 69 79 91 93 87 6 81 61 85 75 58 69 84 64 87 67 72 76 78 80 78 77 75 73 85 90 74 7 94 83 88 80 83 102 98 73 101 91 95 85 83 96 88 86 67 84 85 93 75 8 78 77 88 78 66 84 97 74 80 88 77 78 65 71 87 92 75 9 75 64 84 75 70 80 82 60 98 68 75 70 79 87 79 64 70 83 90 75 10 66 83 66 76 82 62 96 59 75 71 77 77 76 65 85 89 75 11 74 64 83 75 70 70 75 60 91 59 68 71 77 76 76 77 63 70 83 90 69 12 74 65 93 63 61 70 82 91 68 72 74 75 75 77 66 72 84 88 73 13 71 65 82 68 61 70 78 60 91 66 73 72 77 76 75 77 66 71 85 89 73 14 65 65 81 69 61 70 72 61 90 57 67 68 67 75 73 73 62 69 83 86 53 15 55 63 76 64 58 68 61 55 82 55 52 67 56 74 60 66 59 68 81 79 47 16 55 63 71 63 57 67 62 60 75 55 51 64 64 70 64 65 61 68 76 75 47 17 56 62 77 68 59 62 55 76 55 64 67 60 74 60 64 58 69 81 62 47 18 56 59 74 63 55 67 60 51 75 56 51 64 61 73 65 75 62 45 19 51 57 67 60 52 65 52 44 72 54 50 56 54 72 57 48 53 66 66 46 44 20 65 59 50 49 42 64 53 49 49 48 67 56 34 48 63 51 40 35 Table D-III (continued) Stn. Year # 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 1 62 83 73 71 78 82 71 70 56 67 86 56 68 63 79 53 60 67 47 63 2 61 79 67 71 67 79 73 59 55 67 83 53 66 61 71 56 60 65 45 62 3 67 84 75 73 80 88 76 75 63 77 87 60 75 82 57 63 72 48 64 4 87 90 86 78 78 68 78 88 62 82 82 89 70 75 76 51 65 5 89 92 89 78 87 95 79 83 73 80 91 63 82 79 6 76 83 76 78 83 85 74 74 64 78 87 62 76 69 88 73 70 73 51 69 7 100 94 90 83 8.7 96 81 94 82 88 79 81 93 68 89 75 67 80 8 84 79 84 74 83 85 74 85 83 91 65 83 76 95 50 63 72 75 9 88 83 89 78 80 88 74 77 66 79 79 73 82 68 83 59 63 67 51 75 10 88 81 87 78 80 82 78 79 64 82 68 90 67 61 51 69 11 83 74 76 77 70 83 72 71 62 75 74 65 80 67 80 59 70 60 49 70 12 79 61 78 78 71 81 78 69 64 78 77 65 80 72 81 72 64 59 49 70 13 . 83 75 78 78 68 82 73 70 62 77 72 65 81 72 83 68 62 59 50 69 14 76 64 76 77 63 79 71 49 52 75 69 63 80 66 75 60 61 58 48 66 15 65 61 76 75 53 60 71 49 50 74 57 63 48 72 61 56 47 67 16 65 61 72 74 53 60 67 49 50 70 57 62 44 60 67 60 57 50 46 66 17 68 61 76 77 52 59 71 48 51 76 69 66 48 65 75 68 54 56 48 66 18 58 59 69 73 52 58 68 48 47 74 56 62 47 62 67 61 54 51 46 65 19 52 59 69 67 50 57 66 47 43 72 55 64 41 59 62 60 51 45 46 54 20 47 57 55 61 46 48 63 45 41 63 53 55 35 55 58 46 Table D-IV Feeding degree-days each year, 1948-88, for the 20 climate stations. Degree-days are in °C-days. Station numbers are those given in Figure 7. Stn. Year # 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 1 49 13 34 25 47 12 17 27 99 10 28 11 30 61 18 8 38 12 22 44 8 2 18 22 47 19 21 35 90 16 2 11 30 57 16 10 36 22 24 36 3 3 45 7 26 28 26 7 55 20 101 12 19 56 25 32 21 12 30 13 18 40 11 4 89 4 7 45 29 19 20 6 41 15 5 40 9 18 23 6 6 43 18 66 11 2 30 16 30 15 19 7 9 18 28 11 6 47 8 22 25 7 7 22 17 54 10 15 18 14 25 9 14 13 9 18 28 5 7 18 29 17 24 22 35 68 13 33 25 18 27 29 15 22 7 11 17 14 37 21 8 19 9 15 24 9 6 67 13 28 20 23 26 24 10 12 38 20 9 31 5 28 48 14 7 24 14 68 10 12 22 49 17 24 30 9 16 41 25 10 11 22 9 7 19 17 67 17 10 21 34 36 42 38 15 47 17 11 26 14 40 45 14 18 10 15 65 9 10 29 45 2 55 40 38 23 27 58 15 12 19 8 16 13 5 14 21 56 9 16 29 11 40 37 29 16 22 45 8 13 8 8 26 19 4 8 10 10 62 8 11 14 23 12 33 22 22 7 17 56 7 14 3 13 55 32 4 18 12 15 62 5 8 28 27 8 55 20 30 28 28 63 6 15 12 31 33 23 9 26 13 22 30 20 3 31 15 15 49 6 30 36 38 36 4 16 8 34 38 29 10 41 14 26 22 26 3 47 49 25 54 9 41 44 43 33 5 17 3 32 23 28 4 4 12 11 13 8 29 20 17 37 2 28 29 31 1 3 18 7 37 26 29 11 28 16 27 16 22 2 25 19 32 37 33 3 6 19 9 51 19 29 15 29 11 22 37 36 8 19 18 32 60 13 28 50 18 4 5 20 30 36 33 16 17 27 44 12 16 34 28 72 5 30 58 3 6 11 Table D-IV (continued) Stn. Year # 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 1 4 59 15 74 23 21 29 31 49 47 41 39 23 10 8 1 19 51 26 22 2 8 44 12 92 13 36 50 13 49 54 28 25 23 15 9 2 23 24 26 31 3 11 49 22 77 23 22 35 52 59 42 34 33 35 15 4 14 59 17 16 4 9 64 38 19 51 73 37 26 40 21 44 46 9 5 48 5 17 5 5 42 39 47 16 16 21 59 45 23 27 35 18 27 6 13 28 14 41 26 22 28 22 31 28 25 35 26 13 21 20 10 43 8 10 7 15 40 33 12 12 37 20 65 22 11 15 12 20 4 20 25 14 11 8 13 37 34 53 35 33 52 83 17 39 23 26 18 102 4 8 40 32 9 11 31 53 51 25 43 50 42 49 33 18 32 29 33 17 7 11 39 3 25 10 20 34 39 57 19 31 50 19 33 28 30 44 4 16 4 24 11 26 13 18 55 12 37 59 25 46 67 23 24 39 31 5 4 18 15 12 33 12 13 5 17 51 8 25 38 13 42 48 12 25 27 32 6 8 18 12 11 28 13 17 9 13 36 7 22 34 9 22 34 3 20 23 18 9 3 8 12 5 17 14 18 6 17 50 5 39 59 1 6 64 20 20 37 23 5 3 16 21 12 18 15 8 14 25 50 2 2 67 4 9 56 25 21 5 9 5 31 14 24 16 11 20 34 62 6 5 93 9 15 80 26 22 2 32 11 8 27 22 21 29 17 11 12 16 34 5 1 47 9 8 43 18 13 3 25 4 8 18 29 9 16 18 15 25 33 47 5 12 71 11 13 67 24 25 3 29 9 5 21 21 16 23 19 13 34 25 39 13 13 69 15 17 62 31 32 2 45 10 7 37 18 16 12 20 23 52 13 36 18 17 59 34 34 21 37 19 8 51 10 60 Table D-V Minimum overwintering temperature each year, 1948-88, for the 20 climate stations. Temperatures are in °C. Station numbers are those given i n Figure 7. Stn. Year # 48 49 50 51 52 53 54 55 56 57 58 59 60 61 1 -36. 1 -40. 0 -40. 6 -38. 9 -36.1 -35 .0 -38 .9 -36 .1 -37.2 -37.2 -33. 9 -35. 6 -33. 3 -37 .2 2 -38. 9 -37. 8 -41. 1 -33.9 -34 .4 -40 .0 -37 .2 -39.4 -38.3 -33. 3 -37. 8 -31. 7 -37 .2 3 -37. 2 -42. 2 -41. 7 -41. 7 -39.4 -38 .3 -40 .0 -37 .2 -39.4 -41.7 -38. 3 -38. 3 -34. 4 -38 .3 4 -50 .0 -40.6 -45. 6 -53. 9 5 -46. 1 -42. 8 -47. 8 -48. 3 -42.8 -40 .0 -49 .4 -42 .2 -41.1 -50.0 -44. 4 -45. 0 -42. 8 -41 .7 6 -38. 3 -34. 4 -40. 6 -41. 1 -32.2 -32 .8 -35 .0 -36 .7 -33.3 -38.3 -31. 7 -34. 4 -32. 2 -33 .3 7 -46. 7 -36. 1 -46. 7 -41. 7 -38.3 -35 .6 -41 .7 -42 .8 -38.3 -45.0 -42. 8 -38. 3 -40. 0 -42 .8 8 -46. 1 -45. 0 -43. 9 -44. 4 -38.9 -37 .2 -35.6 -41. 1 -42 .8 9 -42. 2 -41. 1 -42. 2 -38. 9 -37.2 -36 .7 -42 .8 -41 .7 -37.8 -42.2 -40. 0 -37. 2 -39. 4 -41 .7 10 -40. 0 -43. 9 -45. 0 -41,1 -38 .9 -42 .2 -42.8 -45.0 -42. 8 -42. 2 11 -39. 4 -33. 9 -40. 6 -45. 0 -41.7 -35 .0 -40 .0 -40.0 -39.4 -33. 9 -35. 6 -36. 1 -36 .1 12 -38. 9 -28. 9 -36. 7 -28.9 -28 .9 -30 .6 -26.1 -29. 4 -28. 3 -23. 9 13 -30. 0 -26. 1 -30. 0 -31. 7 -33.3 -25 .6 -27 .8 -26 .1 -23.9 -32.2 -27. 8 -31. 1 -26 .7 14 -35. 6 -31. 7 -36. 7 -35. 6 -34.4 -30 .0 -32 .2 -29 .4 -32.2 -34.4 -28. 3 -33. 9 -32. 2 -28 .9 15 -36. 1 -30. 6 -32. 2 -38. 9 -34.4 -27 .8 -31 .7 -30 .6 -34.4 -32.2 -28. 3 -35. 6 -33. 9 -33 .3 16 -33. 3 -25. 0 -30. 6 -33. 3 -31.1 -25 .0 -29 .4 -29 .4 -30.6 -35. 6 -25. 0 -30. 0 -27. 8 -28 .3 17 -30. 0 -20. 0 -22. 2 -22. 8 -23.3 -18 .9 -23 .3 -24 .4 -22.8 -23.9 -23. 3 -27. 2 -23. 9 -26 . 1 18 -33. 3 -25. 6 -33. 3 -28.3 -21 .1 -27 .8 -27 .2 -27.2 -32.2 -25. 0 -31. 1 -30. 0 -27 .2 19 -26. 7 -20. 0 -22. 8 -24. 4 -20.0 -21 .1 -20 .6 -23 .3 -22.2 -23.3 -25. 6 -25. 0 -21. 1 -26 .1 20 -15. 6 -21. 1 -21.7 -17 .2 -18 .9 -19 .4 -20.0 -25.6 -20. 6 -20. 6 -21. 1 -23 .3 T a b l e D-V (continued) Stn. Year # 62 63 64 65 66 67 68 69 70 71 72 73 74 75 1 - 3 8 . 3 - 4 0 . 6 - 3 2 . 8 - 3 4 . 4 - 4 0 . 6 - 3 6 . 7 - 4 1 . 7 - 3 3 . 3 - 3 7 . 2 - 3 6 . 1 - 4 1 . 7 - 3 3 . 3 - 3 8 . 3 - 3 3 . 9 2 - 3 8 . 9 - 3 7 . 8 - 3 5 . 6 - 4 0 . 6 - 4 3 . 3 - 4 0 . 6 - 4 1 . 1 - 3 7 . 2 - 3 8 . 3 - 3 5 . 6 - 4 1 . 1 - 3 8 . 9 - 3 6 . 1 - 3 3 . 3 3 - 3 8 . 3 - 4 3 . 3 - 3 8 . 9 - 4 1 . 1 - 4 6 . 1 - 4 1 . 7 - 4 3 . 3 - 3 4 . 4 - 3 8 . 3 - 3 7 . 2 - 4 1 . 1 - 4 1 . 1 - 4 5 . 0 - 3 9 . 4 4 - 4 3 . 9 - 4 7 . 2 - 4 2 . 2 - 4 7 . 2 - 4 8 . 3 - 5 1 . 2 - 3 8 . 9 - 4 2 . 2 - 4 6 . 1 - 4 2 . 8 - 4 8 . 3 5 - 4 4 . 4 - 4 9 . 4 - 4 2 . 8 - 4 3 . 9 - 4 4 . 4 - 4 6 . 1 - 4 8 . 3 - 3 9 . 4 - 4 7 . 2 - 4 3 . 9 - 4 2 . 8 - 4 3 . 3 - 4 7 . 8 - 4 8 . 9 6 - 3 6 . 7 - 3 8 . 3 - 3 1 . 7 - 3 7 . 8 - 3 5 . 0 - 3 9 . 4 - 3 8 . 3 - 3 0 . 0 - 4 0 . 0 - 3 4 . 4 - 3 7 . 2 - 3 4 . 4 - 3 6 . 7 - 3 5 . 0 7 - 4 0 . 6 - 4 2 . 2 - 3 8 . 3 - 4 0 . 0 - 3 9 . 4 - 4 6 . 1 - 4 3 . 3 - 3 8 . 3 - 4 3 . 3 - 4 4 . 4 - 4 1 . 7 - 3 8 . 9 - 4 3 . 3 - 4 2 . 2 8 - 4 1 . 7 - 4 2 . 2 - 4 4 . 4 - 4 1 . 1 - 4 1 . 7 - 4 2 . 8 - 3 7 . 8 - 4 3 . 9 - 4 1 . 7 - 4 0 . 6 - 4 0 . 6 - 4 1 . 1 - 4 2 . 8 9 - 4 4 . 4 - 4 4 . 4 - 4 0 . 6 - 4 4 . 4 - 4 5 . 6 - 3 8 . 3 - 4 2 .8 - 4 0 . 6 - 4 0 . 6 - 4 0 . 0 - 4 3 . 3 - 4 2 . 2 10 - 4 4 . 4 - 4 0 . 0 - 4 1 . 7 - 4 0 . 0 - 4 1 . 7 - 4 4 . 4 - 4 3 . 9 - 4 1 . 1 - 4 2 .8 - 4 0 . 0 - 4 4 . 4 - 4 0 . 0 - 4 3 . 3 - 4 0 . 6 11 - 4 5 . 0 - 4 1 . 7 - 3 6 . 7 - 4 1 . 1 - 4 2 . 2 - 4 1 . 7 - 3 9 . 4 - 3 8 . 3 - 4 2 . 2 - 4 1 . 7 - 3 9 . 4 - 4 0 . 6 - 4 2 . 2 - 3 8 . 9 12 - 3 5 . 0 - 3 2 . 2 - 3 0 . 6 - 3 4 . 4 - 2 9 . 4 - 3 5 . 0 - 3 3 . 9 - 2 4 . 4 - 3 5 . 0 - 3 5 . 0 - 3 5 . 6 - 3 0 . 6 - 3 5 . 0 - 3 2 . 2 13 - 3 0 . 0 - 3 2 . 8 - 2 6 . 7 - 3 2 . 2 - 2 8 . 9 - 3 2 . 8 - 3 3 . 3 - 2 5 . 6 - 3 2 . 2 - 3 2 . 8 - 3 0 . 6 - 2 6 . 7 - 2 9 . 4 - 2 6 . 7 14 - 3 3 . 9 - 3 1 . 1 - 3 1 . 7 - 3 5 . 0 - 3 1 . 7 - 3 6 . 1 - 3 7 . 8 - 3 1 . 7 - 3 3 . 3 - 3 2 . 2 - 3 6 . 1 - 3 1 . 1 - 3 0 . 0 - 3 2 . 8 15 - 3 3 . 9 - 3 4 . 4 - 3 4 . 4 - 3 6 . 1 - 3 3 . 9 - 3 5 . 6 - 3 5 . 0 - 3 0 . 0 - 3 7 . 2 - 3 6 . 7 - 3 9 . 4 - 3 6 . 1 - 3 1 . 1 - 3 3 . 9 16 - 3 2 . 2 - 2 8 . 3 - 2 7 . 8 - 2 9 . 4 - 2 7 . 2 - 3 1 . 7 - 3 2 . 8 - 3 0 . 6 - 2 8 . 9 - 3 2 . 2 - 3 0 . 0 - 2 7 . 8 - 2 7 . 2 - 2 9 . 4 17 - 3 2 . 8 - 3 1 . 1 - 2 3 . 3 - 2 6 . 7 - 2 6 . 1 - 2 7 . 2 - 2 5 . 6 - 1 8 . 9 - 2 5 . 0 - 2 6 . 7 - 2 5 . 6 - 2 6 . 1 - 2 7 . 2 - 2 2 . 8 18 - 3 3 . 9 - 2 8 . 3 - 2 5 . 0 - 3 0 . 0 - 3 4 . 4 - 2 6 . 7 - 3 1 . 7 - 3 5 . 6 - 3 1 . 7 - 2 7 . 2 - 3 1 . 1 - 2 5 . 6 19 - 2 2 . 2 - 2 5 . 6 - 2 3 . 3 - 2 3 . 9 - 2 5 . 0 - 2 4 . 4 - 2 2 . 8 - 2 2 . 8 - 3 1 . 7 - 2 5 . 6 - 2 3 . 3 - 2 3 . 9 - 2 3 . 9 - 2 0 . 0 20 - 2 3 . 3 - 2 6 . 1 - 2 1 . 1 - 2 2 . 2 - 2 2 . 2 - 2 0 . 6 - 2 2 . 2 - 1 9 . 4 - 2 3 . 9 - 2 2 . 8 - 2 5 . 0 - 2 1 . 1 - 2 2 . 8 - 2 0 . 6 Table D-V (continued) Stn. Year # 76 77 78 79 80 81 82 83 84 85 86 87 88 1 - 3 6 . 1 - 3 6 . 0 - 3 3 . 6 - 3 7 . 6 - 3 4 . 9 - 3 4 . 6 - 3 8 . 7 - 3 0 . 1 - 3 8 . 0 - 4 0 . 0 - 3 3 . 5 - 3 5 . 1 - 3 4 . 4 2 - 3 6 . 7 - 3 7 . 2 - 3 6 . 1 - 4 0 . 0 - 3 3 . 5 - 3 8 . 0 - 3 7 . 5 - 3 2 . 5 - 3 7 . 5 - 3 8 . 5 - 3 6 . 0 - 3 6 . 0 - 3 4 . 5 3 - 3 6 . 1 - 3 9 . 5 - 3 6 . 7 - 3 9 . 4 - 3 5 . 7 - 4 2 . 0 - 4 3 . 2 - 3 9 . 0 - 4 0 . 9 - 4 2 . 0 - 3 7 . 4 - 3 6 . 4 - 3 7 . 9 4 - 4 3 . 9 - 4 8 . 3 - 4 3 . 3 - 4 0 . 6 - 3 9 . 8 - 4 3 . 3 - 4 0 . 5 - 4 1 . 9 - 4 1 . 1 - 4 3 . 8 - 4 2 . 2 - 3 8 . 6 5 - 4 1 . 7 - 4 4 . 4 - 4 0 . 7 - 4 5 . 0 - 4 3 . 9 - 3 9 . 7 - 4 7 . 7 6 - 3 1 . 1 - 3 7 . 9 - 3 7 . 3 - 3 9 . 0 - 3 1 . 6 - 3 6 . 9 - 4 0 . 4 - 3 5 . 4 - 3 4 . 6 - 3 6 . 4 - 3 5 . 7 - 3 3 . 7 - 3 6 . 7 7 - 3 8 . 9 - 4 0 . 0 - 3 9 . 6 - 4 1 . 5 - 3 4 . 8 - 4 1 . 6 - 4 0 . 3 - 4 0 . 2 - 3 6 . 9 - 3 8 . 3 - 3 4 . 6 - 3 9 . 6 8 - 4 2 . 2 - 3 8 . 0 - 4 0 . 0 - 3 9 . 0 - 4 0 . 0 - 4 3 . 0 - 3 9 . 5 9 - 4 7 . 2 - 4 0 . 6 - 4 0 . 0 - 4 2 . 0 - 3 6 . 0 - 4 2 . 0 - 4 7 . 0 - 3 9 . 0 - 4 4 . 0 - 3 9 . 0 - 3 9 . 5 - 3 8 . 0 - 3 9 . 0 10 - 4 1 . 1 - 4 2 . 0 11 - 4 2 . 2 - 3 8 . 4 - 4 0 . 1 - 3 9 . 9 - 3 0 . 4 - 4 1 . 2 - 4 4 . 8 - 3 7 . 4 - 4 1 . 3 - 3 6 . 3 - 3 6 . 1 - 3 4 . 7 - 3 5 . 7 12 - 3 0 . 6 - 3 5 . 0 - 2 9 . 4 - 3 8 . 7 - 3 0 . 1 - 3 5 . 9 - 3 6 . 8 - 2 6 . 6 - 3 2 . 9 - 3 2 . 3 - 3 5 . 9 - 3 0 . 0 - 2 8 . 4 13 - 3 3 . 3 - 3 5 . 2 - 2 8 . 7 - 3 6 . 5 - 2 8 . 0 - 3 2 . 2 - 3 6 . 9 - 2 5 . 2 - 3 2 . 6 - 3 5 . 3 - 2 8 . 0 - 2 7 . 2 - 2 9 . 2 14 - 3 7 . 2 - 3 2 . 8 - 2 8 . 5 - 3 4 . 5 - 3 0 . 6 - 3 7 . 7 - 3 8 . 4 - 2 6 . 3 - 3 3 . 5 - 2 9 . 5 - 3 2 . 4 - 3 2 . 3 - 3 3 . 3 15 - 3 8 . 3 - 3 9 . 8 - 3 2 . 6 - 4 1 . 5 - 3 4 . 3 - 3 8 . 4 - 3 9 . 2 - 3 1 . 5 - 3 5 . 0 - 3 3 . 0 - 3 7 . 8 16 - 3 1 . 7 - 2 7 . 9 - 2 5 . 4 - 2 9 . 3 - 2 6 . 0 - 3 5 . 2 - 3 0 . 7 - 2 3 . 6 - 2 9 . 4 - 2 6 . 2 - 2 6 . 7 - 2 7 . 4 - 2 9 . 4 17 - 2 9 . 4 - 3 6 . 4 - 3 1 . 6 - 3 4 . 8 - 3 0 . 7 - 3 3 . 0 - 2 6 . 5 - 2 1 . 4 - 2 7 . 5 - 2 5 . 3 - 2 4 .8 - 2 0 . 4 - 2 2 . 6 18 - 3 6 . 1 - 3 7 . 1 - 3 3 . 9 - 3 7 . 8 - 2 9 . 1 - 3 6 . 2 - 3 2 . 0 - 2 6 . 1 - 3 7 . 9 - 2 6 . 5 - 2 9 . 6 - 2 9 . 5 - 2 8 . 6 19 - 3 0 . 6 - 2 5 . 7 - 2 9 . 5 - 2 5 . 9 - 2 1 . 7 - 2 6 . 5 - 2 6 . 1 - 1 9 . 3 - 2 8 .8 - 2 4 . 9 - 2 5 . 2 - 2 3 . 7 - 1 9 . 6 20 - 2 2 . 2 - 2 4 . 4 - 2 5 . 5 - 2 2 . 0 - 2 0 . 0 - 2 3 . 0 - 2 6 . 0 - 1 5 . 0 - 2 7 . 0 - 2 6 . 0 

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