ON THE DURATION AND DISTRIBUTION OF FOREST TENT CATERPILLAR OUTBREAKS IN EAST-CENTRAL CANADA

摘要

An analysis of forest tent caterpillar (Malacosoma disstria Hbn.) defoliation records from Ontario and Quebec indicates that outbreaks recur periodically and somewhat synchronously (r=0.51) in the two provinces, with six inter-provincial-scale cycles having been observed over the period 1938-2002. When the entire spatiotemporal range of observed defoliation is considered, it appears that, at the local stand level, individual outbreaks tend to last for less than a year on average. Within the three core areas where all six cycles were observed (Dryden, Sudbury, Temiscamingue), individual outbreaks tended to last for 2.6+or-0.5 years. The seemingly small difference between two versus three years of detectable defoliation at the local stand level appears to be critical, as this determines whether annual rates of stem mortality are sufficient to produce obvious signs of forest decline. Infestations lasting three years or longer normally occur in ~45% of the stands within the relatively small core outbreak areas. However not all infestations behave "normally", in the sense of being the product of a regionally synchronized population cycle. For example, we show how a reversing, traveling wave of forest tent caterpillar outbreaks in northern Ontario in the 1990s generated an unusually long-lasting infestation along the Highway 11 corridor - an outbreak which resulted in a regional-scale decline of trembling aspen. This demonstrates how incomplete synchronization of forest insect population cycles can lead to overlapping waves of outbreak that may result in large-scale forest disturbance.