Site factors contribute to aspen decline and stand vulnerability following a forest tent caterpillar outbreak in the Canadian Clay Belt

Abstract

Following the recent forest tent caterpillar (Malacosoma disstria Hbn.; FTC) outbreaks in the Canadian Clay Belt, several trembling aspen (Populus tremuloides Michx.) stands that have expressed variable mortality levels did not regenerate and were invaded by speckled alder (Alnus rugosa (DuRoi.) Sprengel.). The present study has been initiated to identify the causes of aspen dieback and of suckering inhibition, and to test silvicultural treatments to reinitiate stands having high stocking levels. A total of 84 plots were established in 2009 along a gradient of tree mortality based on the residual aspen live basal area (ALBA) in stands that had a high crown closure prior the last FTC outbreak. Treatments included winter and early summer harvest prior to the 2010 growing season, with or without alder removal in the winter of 2011. Results indicate that alder removal is not needed to regenerate the stands, that apical dominance of the residual trees played an important role in the inhibition of suckering but that ALBA was the main factor leading to higher sucker densities, suggesting that the root system was affected in the most degraded stands. A clear threshold of 11m2ha−1 ALBA has been evidenced above which stands can be regenerated. Below this threshold, suckering is too low to regenerate to productive stands. Moreover, this study is the first to show a strong and positive association between aspen mortality, humus layer thickness and water table height. Aspen was more vulnerable to FTC defoliation on sites combining a high water table and a thick humus layer. Because the basal area increment of residual trees has been lower since the 1950s on those sites, aspen dieback following the last FTC outbreak appears to be a symptom of a slower, long-term process of aspen decline. From a management viewpoint, it is suggested to convert these degraded sites to conifer stands.