Successional responses of aspen forests to successive FTC Outbreaks in northeastern Ontario

Background/Question/Methods

The recent forest tent caterpillar (Malacosoma disstria) (FTC) outbreaks from late 80s to early 2000s in northeastern Ontario have resulted in extensive mortality and decline in aspen (Populus tremuloides) stands. First detected in 2000 through aerial survey, the affected area covered over 500,000 ha by 2004. It is anticipated that the decline of overstory aspen, including balsam poplar (Populus balsamifera), will release understory conifers and accelerate stand succession from hardwood to conifer dominance. However, little quantitative information is available about how the affected stands will evolve over time and the implications of this decline for long-term wood supply for the affected areas, especially in stands with little regeneration of both hardwoods and conifers where heavy shrubs have developed following overstory decline. In the summer of 2006, overstory mortality, regeneration, and vegetation development were assessed in 48 aspen stands that included a range of: aspen (including balsam poplar) mortality - from low defoliation-induced mortality (mortality <35%; most live trees are healthy) to heavy defoliation-induced mortality (aspen mortality ≥70%; most live trees show signs of die back); (2) stand ages - from young (31-50 years old), mature (51-70 years old), to overmature (>70 years old); and (3) pre-decline stand compositions -  from pure aspen (hardwood, mostly aspen, ≥90%) to aspen-dominated (hardwood <90%).

Results/Conclusions

The results suggest that insect defoliation occurring from late 80s to mid 90s, and not the more recent outbreak from mid 90s to early 2000s, caused overstory aspen to decline. Hardwood regeneration (mainly aspen) and understory shrubs responded rapidly to foliage loss and mortality of overstory aspen resulting from insect defoliation. The regenerated aspen competed successfully with developing shrubs and was likely sufficient to compensate for the aspen trees lost to insect infestation. Based on the residual overstory and developing understory, and growth and mortality estimates from the literature, it is likely that the hardwood component will be maintained in pure aspen stands but will decrease in aspen-dominated stands over the long-term. The insect defoliation and resulting aspen decline accelerated the transition of aspen dominance to conifer dominance in declined stands through both enhanced conifer recruitment and growth, and reduced hardwood overstory. From a timber supply perspective, the decline by forest tent caterpillar defoliation could delay the availability of these stands for harvesting by 30 to 40 years.