Spruce budworm defoliation patterns during outbreak rise are influenced by tree species, insecticide spraying, and spatial autocorrelation

Spruce budworm (SBW; Choristoneura fumiferana Clem.) outbreaks are an important natural disturbance in North America, killing trees over millions of hectares. We related 11 years of SBW defoliation in 87 plots in Gaspé Peninsula, Québec, to 23 stand, site, and climate variables. Defoliation was consistently ordered among host species: balsam fir > white spruce > black spruce. Within the relatively small 200 km2 study area, cluster analyses resulted in four and 10 clusters for balsam fir cumulative and current defoliation, respectively; variation in cumulative defoliation converged over 11 years. Current defoliation was significantly spatially autocorrelated among plots within stands, but autocorrelation weakened at distances >2500m. Cumulative defoliation was significantly related to insecticide spraying, minimum and maximum summer temperature, and interactions between SBW larvae per branch versus hardwood and white spruce basal area. Tree species, insecticide spraying, and number of defoliating SBW larvae were the main determinants of defoliation. Results showed much higher local spatial variability in current defoliation patterns than previous studies, but over the course of an outbreak, cumulative defoliation patterns converged. Cumulative defoliation patterns similar to these, assigned based on local defoliation severity, can be input into defoliation-based growth models to predict impacts on growth and survival.