Forest tent caterpillar disturbance: The importance of extreme events

Background/Question/Methods

The aspen-dominated mixedwoods of the boreal forest are subject to episodic disturbance associated with periodic, wave-like outbreaks of forest tent caterpillar, Malacosoma disstria Hbn.  Some of these disturbances can be dramatic, despite the tent caterpillar’s reputation as a benign defoliator.  The question is: why do some populations sometimes attain a density sufficient to cause forest disturbance?  What synchronizes these events across large areas?  Why don't all populations grow to the level of a forest-damaging disturbance?

Results/Conclusions

I examine historical defoliation records and tree-ring data across Canada in order to summarize outbreak spatiotemporal patterns and their consequences.   I link population data from studies in western Canada to forest impact studies in eastern Canada to derive a generic outbreak impact model that shows the importance of critical thresholds in determining when and where quasi-periodic population fluctuations may be amplified to the level of a severe forest disturbance.  I discuss both cause and effect, showing how the coupled insect-forest disturbance process may behave as a closed feedback loop, with extreme perturbation events acting as a major driver of productivity and successional dynamics.  This is a synthesis paper that will attempt to bridge the gap between this model system and other forest-insect systems.