The influence of forest composition and climate on outbreak characteristics of the spruce budworm in eastern Canada

The spruce budworm (Choristoneura fumiferana Clemens) is perhaps the single most important disturbance agent in Canada's eastern forests. Climate and forest composition are dominant factors in spruce budworm outbreak dynamics through their direct influences on the pest, its natural enemies, and its hosts, and through their influence on the multitrophic interactions that are important in outbreak dynamics. A combination of four climate variables, three forest composition variables, and one location variable explained 60% of the multivariate variability in outbreak characteristics (duration and severity) in eastern Canada. Outbreak duration was most strongly influenced by April-May accumulation of degree-days; outbreak severity was most strongly influenced by the extreme maximum temperatures of April-May. The basal area of balsam fir had a stronger influence than that of black spruce on duration and on severity. Both outbreak characteristics declined in more northerly locations. Under a projected future (2011-2040) climate scenario the largest increases in outbreak duration and severity are predicted to occur on the Gaspe Peninsula and the north shore of the St. Lawrence River (Quebec). The largest decreases in duration and severity are predicted to occur in southern Ontario and along the Bay of Fundy in New Brunswick and Nova Scotia. The predicted average change in outbreak duration is around -1.3 years. The predicted average change in outbreak severity is only slightly different from zero (around -1.5% defoliation).