Twenty-year responses of aspen stands to forest tent caterpillar defoliation and overstory dieback in Northeastern Ontario, Canada

Abstract

Forest tent caterpillar (Malacosoma disstria Hbn.) is a major forest defoliator in North American boreal forests. This pest periodically affects hardwood trees such as trembling aspen (Populus tremuloides Michx.) and balsam poplar (Populus balsamifera L.) across large areas, causing mortality and altering stand attributes and long-term dynamics. In this study, I quantified the responses of aspen stands to forest tent caterpillar defoliation and overstory dieback, including short-term (5 years after dieback) and long-term (20 years later) dynamics. Results indicate that affected stands fully recovered to pre-dieback density but not basal area. Defoliation caused overstory dieback that stimulated understory growth and regeneration but the hardwood regeneration (mostly aspen) did not adequately replace hardwood trees lost to defoliation, resulting in decreased hardwood composition relative to unaffected stands. The high density in affected stands suggested possible further basal area recovery as regeneration and released understory conifers grew into main canopy, following the general trends of boreal mixedwood succession. The results support earlier projections based on residual stand attributes shortly after dieback and reported boreal species growth and mortality rates that indicate forest tent caterpillar defoliation and subsequent overstory dieback would accelerate aspen stand transition to conifer dominance and delay the availability of stands for harvesting by 40–50 years as stands recover.