Early understory successional changes following clearcutting in the balsam fir-yellow birch forest

Abstract

Community response to disturbance is influenced by species life history and the nature of disturbance. In this study we investigated the effects of clearcutting on the early successional changes of an understory species community in the balsam fir-yellow birch forest type of the Lower St. Lawrence Region of Québec, Canada. Clearcuts of 5, 10 and 20 years were sampled along with mature stands. In each of these communities, ten circular quadrats of 400 m2 were established where the percent cover of herb, fern, and shrub species was estimated. Along a 150-m transect starting in the middle of each quadrat, the seedling density of commercial tree species was measured in ten 4-m2 plots. Detrended Correspondence Analysis (DCA; cover values) was used to describe the early successional changes in the understory community and a Correspondence Analysis (CA) was used with presence/absence data to evaluate the compositional differences. The results revealed a time gradient associated with understory community changes, in which the first 10 years of successional changes are characterized by the dominance of shade intolerant species. A decrease in the floristic heterogeneity was associated with clearcut logging. Small herb species with a shallow root system were most affected by logging. For commercial tree seedlings, the changes following clearcutting were characterized by a decrease over time in the density and stocking of shade intolerant and mid-tolerant species (principally white and yellow birches). In the older clearcuts, sugar maple appeared to be the dominant commercial species in the seedling understory. The results suggested that clearcut logging in the balsam fir-yellow birch forest type may contribute to rapidly shifting the seedling understory composition towards dominance by shade tolerant species. This situation may be related to the dominance of mountain maple in the understory of old clearcuts that contribute to reducing the available light in the understory, thus increasing mortality of shade intolerant seedling species. Clearcutting may also have deleterious effects on some understory herb species that need particular conditions before re-establishing in a site. Logging tends to reduce micro-site heterogeneity over large extended areas—a situation that may preclude the re-establishment of some late successional herb species.