Land cover

The Arctic lands are commonly subdivided into the High and Low Arctic based on broad climatic, geographical and biological grounds (Bliss, 1997) although many ecologists recognize five subzones (Walker et al., 2002). As a general statement, the High Arctic is characterized by more severe environmental conditions than the Low Arctic, reflected in the type and distribution of vegetation.

Land in the High Arctic is characterized by tundra, a Finnish term for treeless upland. More generally, tundra refers to the treeless regions north of the Arctic treeline. The most extreme High Arctic tundra landscape falls under the category of polar desert, which implies both cold and a lack of moisture. Good examples are provided by the Canadian Arctic Archipelago and Peary Land in northeast Greenland. The concept of polar desert was introduced by the German geographer Passarge (1920). Subsequently, the term has been applied in a geobotanical context by Alexandrova (1970). Figure 2.12 shows the extent of polar desert as interpreted by Charlier (1969). Korotkevich (1972) includes ice caps and the Greenland Ice Sheet in the polar desert category.

Field studies of polar desert sites on 10 islands in the Canadian Arctic Archipelago showed only 2-3% plant cover (Bliss, 1997). The brief summers are relatively warm, but scant moisture permits only a few cushion-form plants, mosses and lichens to grow in niches with favorable microclimates, particularly at sites where snow banks provide meltwater. In these favored areas, which occupy about 3-5% of the landscape, about 25-35% of the surface is typically vegetated. Vegetation is suppressed under

Figure 2.12 Distribution of Arctic polar desert (dark shading) and approximate southern limit of tundra (bold line) (adapted from Charlier, 1969, also see Webber, 1974, courtesy of N. Saliman, NSIDC, Boulder, CO).

semi-permanent snow banks. As discussed by Ives (1962) and Locke and Locke (1977) for north-central Baffin Island, it is possible that the limited cover of lichens and vascular plants is attributable to the widespread presence of snow banks during the Little Ice Age (150-400 years ago).

Part of the polar and subpolar desert surface is covered with fell fields (felsenmeer), comprising angular blocks derived from freeze-thaw splitting of the bedrock, and gravel. As the first author can attest, such fell fields should be climbed only with extreme caution. In other regions, stones and fine sediments are often organized as patterned ground. This may consist of stone circles and ice-wedge polygons up to a few meters across, or stone stripes on sloping surfaces (French, 1996).

As for less extreme forms of tundra, in the North American Arctic, Bliss (1997) distinguishes wet meadow (graminoid-moss) tundra comprising dwarf-shrub heaths and polar semi-deserts of cushion plants, cryptograms and herbs. In northern Russia, the so-called "typical" tundra (Chernov and Matveyeva, 1997) is predominantly moss and lichen covered (polar semi-desert according to Bliss), with sedges and dwarf shrubs, and dry heaths. The vertical structure is about 20 cm in height.

In the Low Arctic of both northern continents, vegetation covers 80-100% of the surface. Shrubs are an important component, together with sedges and grasses. There is also a wide range in the vertical structure. Several species of alder, birch and willow in the tall shrub tundra reach 2-5 m height, whereas in low shrub tundra, dwarf birch and dwarf willow are only 40-60 cm high. Where the canopy is more open, heath and tussock species are widespread. At the southern margin of the Low Arctic tundra, there is a forest-tundra transition or ecotone. Tree "islands" and gallery forest are found along major watercourses such as the Mackenzie, Khatanga and Lena rivers, extending to 71° N to 72° N. The forest-tundra ecotone is some 50-100 km wide. This is a result of the effects on the forest limit of climatic fluctuations over the last 4000-5000 years, and recurrent fires that destroy slow-growing lichens and mosses (Nichols, 1976; Elliott-Fisk, 1983). South of the ecotone, we enter the boreal forest (or taiga, a Russian term).

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