Interactions between climate, trembling aspen, and outbreaks of forest tent caterpillar in Alberta

Periodic outbreaks of the forest tent caterpillar, Malacosoma disstria Hbn., occur at six to sixteen year intervals throughout Canada, causing severe defoliation of host-trees. There are many environmental factors and natural enemies that influence survival and reproduction, but it is unclear how these conspire to produce large-scale synchronized population fluctuations.; A localized outbreak of forest tent caterpillars in central Alberta was monitored beginning in 1995. Populations started declining in 1995 and widespread collapse occurred during the summer of 1996. Many eggs failed to hatch in the spring of 1996, probably as a result of cold winter temperatures. Rates of parasitism were high in the spring and summer of 1996, and populations became scarce in 1997. Poor egg hatch in 1996 thus seems to have assisted in precipitating parasitism-driven collapse of this tent caterpillar population.; Historical outbreaks of forest tent caterpillar were reconstructed for the province of Alberta by measuring ring widths of sections of trembling aspen, Populus tremuloides Michx. Aspen ring width chronologies spanning the period 1837–1998 indicated a persistent pattern of decadal outbreaks. Outbreaks were less regularly periodic and less well-synchronized in Alberta than they appear to be in Ontario, but they were not chaotic. Outbreaks were more frequent, less well-synchronized, and less stable in the aspen parkland than in the boreal forest.; High-resolution outbreak reconstruction in the Cooking Lake area showed that decadal outbreaks are often split into spatially and temporally discrete pulses, such that localized outbreaks occur at five to seven year intervals.; Boreal and parkland outbreak cycles were modeled as a stochastic host-parasitoid interaction perturbed by winter temperature. There was weak evidence that monthly winter temperature acts as a phase-locking or period-forcing mechanism on outbreak cycles. The effect of temperature, however, seems to be more complicated than this model assumes.; A theoretical model of spatiotemporal outbreak patterns at Cooking Lake suggested that sub-decadal outbreak asynchrony may be driven by forest fragmentation. Winter temperature may help to further de-synchronize outbreaks, but a better model is required to simulate the effects of daily winter temperature on egg survival and density-dependent susceptibility and vulnerability of eggs to winter-kill. A synthetic conceptual model is offered as a starting point.