TUNDRA

treeless, level or rolling ground in polar regions (Arctic tundra) or on high mountains (alpine tundra), characterized by bare ground and rock or by such vegetation as mosses, lichens, small herbs, and low shrubs. The plant life of tundras tends to be greenish brown in colour, and species succession takes place slowly. The foggy tundras found along coastal areas produce matted and grassy swards. Algae and fungi are found along rocky cliffs, and rosette plants grow in rock cornices and shallow gravel beds. In the drier inland tundras, spongy turf and lichen heaths develop. Tundra climates vary, the most severe being the Arctic regions where polar deserts fluctuate in temperature extremes from 40 F (4 C) at midsummer to -25 F (-32 C) during the winter months. Alpine tundra has a more moderate climate, with cool summers and moderate winters (rarely falling below 0 F in winter). The freezing climate of the Arctic produces a layer of permanently frozen soil, called the permafrost, which can reach soil depths of between 300 and 1,500 feet (90 and 456 m). An overlying layer of soil alternates between freezing and thawing with seasonal variations in temperature. The permafrost layer exists only in Arctic tundra, but both Arctic and alpine tundras have a freeze-thaw layer. Because Arctic tundras receive extremely long periods of daylight and darkness (lasting between one and four months), biological rhythms tend to be adjusted more to variations in temperature than to the amount of sunlight available for photosynthesis.