In the eastern Canadian boreal forest, the term hardwood expansion or encroachment (enfeuillement in French) generally refers to the phenomenon of increasing cover of intolerant hardwoods, notably aspen (Populus tremuloides), following clearcutting. Such expansion is perceived as a local (stand-level) process that is reflected at a larger, regional scale. The objective of this study was to characterize the increase in aspen cover following clearcutting in the clay belt of Abitibi, in northwestern Quebec. Regional data from 3 province-wide forest inventories (1970, 1980, and 1990) conducted by the Quebec Ministry of Natural Resources and Wildlife were analyzed for 2 ecological regions, the Abitibi Plain (western balsam fir-white birch bioclimatic subdomain) and the Matagami Lake Plain (western black spruce-feathermoss bioclimatic subdomain). An assessment was done of forest cover transformations following clearcutting over 3 decades based on time since harvesting, surface deposit/drainage, and ecological region. Results show that mesic clay sites are particularly susceptible to aspen encroachment and that the overall rate of encroachment from the 1970s to the 1990s in the black spruce and balsam fir regions has been about 30%. Moreover, despite the greater abundance of poorly drained and organic sites, less favourable to hardwood establishment, in the more northerly black spruce region, the 2 regions appear to present the same sensitivity (or propensity) to intolerant hardwood expansion. On sensitive sites, management of black spruce forests should include adaptive practices to limit increases of the hardwood component.

Nomenclature: Marie-Victorin, 1995.