Spatial analysis of large-scale patterns of forest tent caterpillar outbreaks

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Abstract
A spatial analysis of forest tent caterpillar (Malacosoma disstria Hübner) outbreak data from Ontario indicates that previous studies examining the influence of forest heterogeneity and climate on outbreak vulnerability were pseudoreplicated. We adopted a spatially explicit modeling framework to re-examine, at several spatial scales, the influence of forest heterogeneity on outbreak duration, and also tested for effects of other plausible environmental factors including winter temperature, spring degree-day accumulation, and elevation. Of these, forest heterogeneity was the strongest predictor of the number years of defoliation recorded over three outbreak cycles. Nevertheless, correlations were inconsistent across districts and were often weak. We suggest that this is due either to the involvement of other factors or to neighbourhood effects associated with contagious population processes such as insect dispersal. The precise role of climatic perturbations in governing outbreak dynamics remains unclear but may be elucidated through studies at larger and smaller spatial scales.