Tundra

figure 21-2

This herd of caribou lives in the treeless tundra in Canada. The photo shows the tundra in the summer, after the top layer of soil has thawed enough for plants to grow.

This herd of caribou lives in the treeless tundra in Canada. The photo shows the tundra in the summer, after the top layer of soil has thawed enough for plants to grow.

The tundra (TUHN-druh) is a cold and largely treeless biome that forms a continuous belt across northern North America, Europe, and Asia. It is the largest and northernmost biome, covering about one-fifth of the world's land surface. Permafrost, a permanently frozen layer of soil under the surface, characterizes the tundra. Even the surface soil above the permafrost remains frozen for all but about eight weeks of the year.

Trees do not usually grow in the tundra because the winters are long and extremely cold and permafrost prevents their roots from penetrating far into the soil. The tundra receives little precipitation and has a very short growing season of about two months. Cold temperatures slow down decay, so few nutrients are available in the soil. For these reasons, most tundra plants are small and grow slowly. Grasses, sedges, and mosses are common. Animals that inhabit the tundra include caribou, musk oxen, snowy owls, arctic foxes, lemmings, and snowshoe hares. Figure 21-2 shows some caribou in the Canadian tundra. In the brief summer, the upper layers of ice and soil thaw, creating ponds and bogs. Swarms of insects appear, and ducks, geese, cranes, other waterfowl, and predatory birds arrive in great numbers to feed.

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