Spatial analysis of response of trembling aspen patches to clearcutting in black spruce-dominated stands

Abstract

While expansion of intolerant hardwoods, including trembling aspen, has been recognized as a problem by boreal forestmanagers in eastern Canada, the problem has mostly been associated with the boreal mixedwood and, with a few exceptions,the spatial aspects of the phenomenon have not been well documented in the scientific literature. The objective ofthis study is to evaluate how the density and size of aspen patches influence the change in density, size and persistence ofpatches following clearcutting of black spruce forests in the claybelt region of northwestern Quebec. To do this, we finelyinterpreted: 1) aerial photos taken between 1970 and 1979 of 12 mature black spruce-dominated stands containing varyingsizes and densities of aspen patches prior to being clearcut harvested and 2) aerial photos taken between 1992 and1995 of the 12 resulting clearcuts, 11 to 18 years after harvesting. Sketch maps of pre- and post-harvest aspen presencewere rasterized and changes in aspen patch size and cover were determined. As well, the probabilities of aspen colonizationwith increasing distance from pre-harvest patches were calculated for each site. Black spruce forests of the claybeltcontaining no aspen prior to harvesting are relatively resistant to aspen invasion. However, when forests do contain aspenpatches with cover values ≤25%, almost 60% of these patches increase to a higher density class. Aspen patches less than 2ha in area had about a 50% probability of disappearing following clearcutting whereas larger patches had about a 96%chance of persisting as a result of clearcutting. In the black spruce forest of the claybelt, thick organic layers appear to havea negative effect on aspen recruitment. Distances of regenerating aspen from pre-harvest patches suggest that recruitmentfrom seed is more important than previously thought. Key words: encroachment, trembling aspen, clearcutting, black spruce, claybelt, aerial photography