III.

Flora

The Arctic tundra is, by definition, treeless. However, although the tree line in atlases is depicted as a firm line between green conifers and brown tundra, in practice it is often difficult to locate the boundary to within 200 km (124 mi) in the field. Deciding what constitutes a tree is not as straightforward as it might seem, for species which occur as tall specimens in the taiga may occur as stunted, shrub-like, and dwarfed forms within the forest/tundra transition zone. Trees are arbitrarily defined as those tall enough to stand proud of the winter snows, a convention which allows boundaries to be plotted for each of the dozen or so species typically found at the forest edge, and an average tree line to be calculated. Even so, tongues and isolated rafts of trees penetrate far into the tundra in sheltered valleys and hollows, while elevated or swampy ground isolates islands of tundra vegetation within the forest edge.

The number of species of plant on the tundra proper is usually small compared with temperate regions. Arctic tundra is characterized by tough, low-growing dwarf shrubs, including heather, birch, willow, bilberry, and crowberry, and a carpet of sedges, grasses, rushes, mosses, and lichens. Sedges and mosses tend to dominate between hummocks whereas, on slightly elevated sites, often only 15 cm to 60 cm (6 in to 24 in) above the waterlogged soils, willows, grasses, and rushes are more common. Low temperatures combined with a short growing season limit annual primary (plant) productivity to 0.1-0.4 kg/sq m, a level similar to that of many deserts.

Alpine tundra communities consist of mat-forming and cushion-forming plants adapted to heavy snows, gusting winds, and widely fluctuating temperatures. In higher mountain areas, mosses and lichens manage to grow on exposed rock surfaces. Vascular plants usually die out at or just below the line of permanent snow. Annual primary productivity varies from 0.05 kg/sq m in windswept habitats, to 0.15 kg/sq m in alpine meadows, and 0.3 kg/sq m in dwarf shrub communities.