Abstract for M156

Alaskan-Yukon North American Boreal Forest & Woodland
Forêts et terres boisées boréales nord américaines d’Alaska et du Yukon

M156 describes upland boreal forests and woodlands of northwestern North America, ranging from northern British Columbia into Yukon, Northwest Territories and Alaska. Forest canopies can be evergreen coniferous, broad-leaved cold-deciduous or a conifer – broad-leaved mixture. Stands of M156 become patchy and more open with increased elevation, where they often occur as tree islands or widely spaced trees in a shrubland matrix. On wind-exposed sites, trees develop characteristic krummholtz forms in response to physical damage by extreme cold and blowing snow and ice crystals. These forests and woodlands are maintained on the landscape by stand-replacing fire, with most parts of the range experiencing short (<100 years) to intermediate (100-270 years) regional fire cycles. Dominant tree species in most of the Canadian range include white spruce (Picea glauca), black spruce (P. mariana), trembling aspen (Populus tremuloides), lodgepole pine (Pinus contorta var. latifolia) and, at higher elevations, subalpine fir (Abies lasiocarpa). Understories range from dense, species-rich shrub and herb conditions to sparse shrub and herb layers with continuous feathermoss and/or lichen ground cover. Common understory species include willows (Salix spp.), shrub birches (Betula nana and/or B. glandulosa), black crowberry (Empetrum nigrum), common Labrador tea (Rhododendron groenlandicum), fireweed (Chamerion angustifolium), lingonberry (Vaccinium vitis-idaea), twinflower (Linnaea borealis), Arctic lupine (Lupinus arcticus), Altai fescue (Festuca altaica), common bearberry (Arctostaphylos uva-ursi), northern comandra (Geocaulon lividum), reindeer (Cladina spp.) and clad (Cladonia spp.) lichens, and stairstep moss (Hylocomium splendens).

M156 occurs within a mostly subhumid continental boreal climate, with long cold winters and short cool summers. Mean annual temperatures range from about -1°C to -6°C. Annual precipitation varies between approximately 300 mm and 800 mm, depending on latitude, longitude and elevation. M156 forests and woodlands occur from valley bottom to treeline in most of Yukon (up to 1450 mASL) but, in northern British Columbia, southeastern Yukon and southwestern Northwest Territories, they are found at elevations above approximately 800-1100 mASL. Regional geologic and topographic features of the Cordilleran physiographic region produce an array of local site conditions. Except for some areas in western Yukon and southwestern Northwest Territories, all parts of the range experienced late Pleistocene glaciation. Soils are mostly Brunisols and Luvisols developed in glacial surficial materials, although Cryosols occur on cold moist sites in the northern part of the range.

Two subtypes distinguish low elevation boreal forests (CM156a [Low Montane Alaskan-Yukon Boreal Forest]) and higher elevation boreal woodlands (CM156b [High Montane Alaskan-Yukon Boreal Woodland]).