Arctic limits for tree growth vary from area to area; the most northerly is about 70 °N, though in western Alaska and Siberia it dips sharply south due to the cold Bering Sea and does the same around Hudson Bay in Canada. A broad transitional zone separates the lowland boreal forest from the arctic tundra (Sirois, 1992; see Section 1.6.1). The northernmost part of this zone is the forest-tundra with scattered patches of krummholz or stunted trees (Fig. 3.16), with larger trees along rivers and in sheltered sites, set in a matrix of tundra. This extends south to the open boreal forest or 'lichen woodland', whose open groves of erect trees are underlain by a rich lichen carpet (mainly Cladonia spp.). The proportion of tree cover to lichen mat increases southwards until reaching the forest line, where trees cover at least 50% of the landscape. The arctic timberline differs dramatically from its alpine equivalents in its great width across the lowlands (150 km or more), in contrast to a very sharp cutoff of up to 0.5 km or so on a mountain. Alpine timberlines are sometimes showered with tree seeds from stands below; in contrast regeneration from seed is rare in the arctic timberline. A number of important species of the undergrowth are abundant in the arctic timberline zone, including crowberry Empetrum nigrum, narrow-leaf Labrador tea Ledum decumbens, cloudberry Rubus chamaemorus, bog blueberry Vaccinium uliginosum and cranberry V. vitis-idaea.
Spruce are by far the most common trees of the arctic timberline of North America. Black spruce Picea mariana grows primarily on poorly drained boggy sites that are prevalent in the lowlands underlain by permafrost. This tree, which readily assumes a stunted or krummholz form on exposed sites, regenerates vegetatively by layering where the lower branches root into the abundant moss and become independent trees. In contrast white spruce, P. glauca grows on well-drained soils, especially along streams where the permafrost has been melted back by the flowing water, and usually forms a single erect trunk at the treeline. It mostly reproduces from seed.
The arctic and alpine timberlines, along with drought-timberlines (see Section 3.5.1), are of particular interest in marking the boundaries of the world's woodlands and forests, whose zonation is controlled by a number of factors. These include climate (especially the wind, water and temperature regimes), fire, soil type, altitude and biotic factors including the ever-extending influence of humans. Woodlands and forests include the most diverse communities known; the mechanisms which maintain this diversity are considered in Chapter 6.
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