Environment and biogeography of the western boreal forest

The western boreal forest of North America (Manitoba through Alaska) has a typical boreal climate, but the largely sedimentary Interior Plains and the northern Cordillera (part of which was ice-free in the Pleistocene) are physiographically and geologically very different from the Canadian Shield that underlies most of the eastern boreal forest. The mountainous nature of much of the region, with pronounced topography, local climates, aspect differences, sharp drainage, and elevational clines, gives some of its boreal landscapes a different character from those of the Shield. The region's forests are structurally consistent with boreal forests everywhere, but floristic differences are apparent, with the loss of eastern species and the addition of Cordilleran and Beringian elements. The southern margins of the western boreal forest also have a distinctive character, as they merge with either the Cordilleran subalpine forests or with aspen parkland and grassland—not with Great Lakes-St. Lawrence conifer-hardwood forests.Several different biogeographic or ecological land classifications have been applied to the western boreal forest. Although there are a variety of classifications, several common themes are apparent. Broad-scale zonation reflects 1) latitudinal change (boreal forest to subarctic woodland to forest-tundra ecotone, from south to north), 2) physiography and biogeography (Plains/Shield to northern Cordillera to Alaska-Beringia, from east to west), and 3) the physiographic vs. vegetation bias of the classification. Regardless of higher-level, broad-scale differences, the various classifications tend to be consistent at the site level, and therefore all have some value and utility for foresters.