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

Community response to disturbance is influenced by species life history and the nature of disturbance. In this study we investigated the effects of cl earcutting on the early successional changes of an understory species commu nity in the balsam fir-yellow birch forest type of the Lower St. Lawrence R egion of Quebec, Canada. Clearcuts of 5, 10 and 20 years were sampled along with mature stands. In each of these communities, ten circular quadrats of 400 m(2) were established where the percent cover of herb, fern, and shrub species was estimated. Along a 150-m transect starting in the middle of re ach quadrat, the seedling density of commercial tree species was measured i n ten 4-m(2) plots. Detrended Correspondence Analysis (DCA; cover values) w as used to describe the early successional changes in the understory commun ity and a Correspondence Analysis (CA) was used with presence/absence data to evaluate the compositional differences. The results revealed a time grad ient associated with understory community changes, in which the first 10 ye ars of successional changes are characterized by the dominance of shade int olerant species. A decrease in the floristic heterogeneity was associated w ith clearcut logging. Small herb species with a shallow root system were mo st affected by logging. For commercial tree seedlings, the changes followin g clearcutting were characterized by a decrease over time in the density an d 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 sugges ted that clearcut logging in the balsam fir-yellow birch forest type may co ntribute to rapidly shifting the seedling understory composition towards do minance by shade tolerant species. This situation may be related to the dom inance of mountain maple in the understory of old clearcuts that contribute to reducing the available light in the understory, thus increasing mortali ty of shads intolerant seedling species. Clearcutting may also have deleter ious effects on some understory herb species that need particular condition s before re-establishing in a site. Logging tends to reduce micro-site hete rogeneity over large extended areas-a situation that may preclude the re-es tablishment of some late successional herb species.