Tundra occupies vast areas of the earth's land surfaces (about 20% in all), primarily above the Arctic Circle (Fig. 26.2). It has the appearance of being treeless, although miniature willow and birch only 2.5 to 50 centimeters (1 inch to 2 feet) tall do survive in some areas. Another characteristic of arctic tundra is the presence of permafrost (permanently frozen soil) at a depth of from 10 to 20 centimeters (from 4 to 8 inches) to about 1 meter (3 feet 3 inches) below the surface. The level of the permafrost determines the depth to which plant roots can penetrate. Precipitation averages less than 25 centimeters (10 inches) a year, but because of the permafrost, it is largely held at or near the surface of the land. There is a very short growing season of only 2 to 3 months, with frost possible on any day of the year. However, temperatures can soar to 27°C (81°F) or higher during a long midsummer day. The vegetation of arctic tundra is dominated by dwarf shrubs, sedges, grasses, lichens, and mosses. During the brief growing season, tiny perennial
taiga (northern coniferous forest) | desert
[»ff^ti temperate deciduous forest [11118 mountain forest tropical rain forest
Figure 26.1 Major biomes of North America.
plants produce brightly colored flowers and form brilliant mats over the topsoil, which is largely organic and generally only 5.0 to 7.5 centimeters (2 to 3 inches) deep.
Patches of alpine tundra occur above timberline in mountains below the Arctic Circle. Permafrost is frequently absent from alpine tundra areas, and grasses, sedges, and herbs tend to predominate.
The biome is exceptionally fragile. A car driven across the tundra compresses the soil sufficiently to kill plant roots, and the tracks are evident many years later. Occasionally, sheep grazing tundra have been observed to pull up patches of the matted vegetation, leaving exposed edges. High winds then can catch the exposed edges and rip away larger segments of mat, leaving barren patches called blowouts.
Animals of the tundra include lemmings, the little rodents that reproduce prodigiously every 3 or 4 years and then decline in numbers. The arctic fox and snowy owl also fluctuate in numbers, the fluctuations being correlated with the size of the populations of lemmings that constitute their principal food. Other animals of the tundra include caribou, polar bears (near the coast), shrews, marmots, ptarmigans, loons, plovers, jaegers, and arctic terns.
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