Mosses, sedges, and lichens are common, while few trees grow in the tundra. Vegetation in the tundra has adapted to the cold and the short growing season. Much of the arctic has rain and fog in the summers, and water gathers in bogs and ponds. That's less than most of the world's greatest deserts! Still, the tundra is usually a wet place because the low temperatures cause evaporation of water to be slow. Precipitation in the tundra totals 150 to 250 mm a year, including melted snow. In the tundra summers, the top layer of soil thaws only a few inches down, providing a growing surface for the roots of vegetation. This permafrost is a defining characteristic of the tundra biome. The temperatures are so cold that there is a layer of permanently frozen ground below the surface, called permafrost. Tundra winters are long, dark, and cold, with mean temperatures below 0☌ for six to 10 months of the year. Temperatures are frequently extremely cold, but can get warm in the summers. Tundra is also found at the tops of very high mountains elsewhere in the world. Much of Alaska and about half of Canada are in the tundra biome. Tundra is found in the regions just below the ice caps of the Arctic, extending across North America, to Europe, and Siberia in Asia. It also receives low amounts of precipitation, making the tundra similar to a desert.
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