CLIMATE AND THE SOUTHERN LIMIT OF THE WESTERN CANADIAN BOREAL FOREST

Four species of boreal forest conifers share a similar southern limit of natural distribution in the three Prairie Provinces of western Cana da. The southern boundaries of boreal forest and aspen parkland were c ompared with geographic patterns of several climate variables to provi de a preliminary assessment of how global climate change could affect forest distribution in the future. Forest zonation corresponded most c losely with climatic moisture regimes (annual precipitation minus pote ntial evaporation). In contrast, thermal characteristics of climate (m ean July and annual temperature, growing degree-days) showed an incons istent relationship with forest zonation. It is postulated that moistu re limitations prevent conifer regeneration south of the present limit of natural distribution. Alternatively, the more arid climates south of the boreal forest may have promoted higher fire frequencies histori cally, thus preventing conifers from achieving sufficient longevity to regenerate. The driest areas of boreal forest in the region occur at low elevations in west-central Manitoba, throughout Saskatchewan and A lberta, and the southwestern Mackenzie District, Northwest Territories ; these areas may be most vulnerable to increased climatic dryness. Cl imatically induced losses of forest cover from these low-elevation are as could eventually lead to the fragmentation of the boreal forest in western Canada.