Phenological shifts in trembling aspen (Populus tremuloides) one year after defoliation by the forest tent caterpillar (Malacosoma disstria)

Shifts in budbreak directly following years of heavy defoliation have been documented in several studies, and may influence susceptibility to herbivory by altering phenological synchrony with early-spring insect defoliators. Such shifts, however, have been documented in relatively few tree species, and have yet to be considered in context with chemical defense allocation, or the timing of spring insect emergence. To address these issues, we established a common garden of 12 trembling aspen genotypes (Populus tremuloides) and observed the effects of forest tent caterpillar defoliation (Malacosoma disstria) and nutrient availability implemented during the prior growing season on budbreak phenology and phenolic glycoside concentrations during the subsequent year. Defoliation and low nutrient treatments caused significant delays in budbreak and significant declines in phenolic glycoside concentrations. Comparison of tree phenological shifts with forest tent caterpillar emergence degree-day models revealed that defoliation-induced delays caused trembling aspen to break bud during periods of higher herbivore density. These results suggest that budbreak delays may increase the susceptibility of trembling aspen to herbivory during multi-year insect outbreaks, potentially contributing to insect population building and tree mortality.