Tundra, a major zone of treeless level or rolling ground found in cold regions, mostly north of the Arctic Circle (Arctic tundra) or above the timberline on high mountains (alpine tundra). Tundra is known for large stretches of bare ground and rock and for patchy mantles of low vegetation such as mosses, lichens, herbs, and small shrubs. This surface supports a meagre but unique variety of animals. The Finns called their treeless northern reaches the tunturi, but the concept of a vast frozen plain as a special ecological realm called tundra was developed by the Russians.
One constant factor shaping the tundra is alternate freezing and thawing of the ground. Along with the factors mentioned above, this freeze-thaw cycle sets the tundra apart from two ecosystems frequently found adjacent to it the icy polar barrens on the one hand and the evergreen taiga on the other. Permafrost perennially frozen ground is a significant feature of the Arctic tundra; however, it does not typically occur in alpine regions.
The global extent of the tundra biome is considerable, accounting for roughly 10 percent of Earth’s surface. The southern limit of Arctic tundra follows the northern edge of the coniferous forest belt. In North America this line lies above latitude 60° N, while in Eurasia most of it occurs north of 70° N—except in eastern Siberia, where it extends southward to 60° N in Kamchatka. The northward bulge of forest in Eurasia is a result of the warmer summers that occur over that large contiguous landmass.
Alpine tundra covers approximately 3 percent of Earth’s land surface, and it is mostly found in the Northern Hemisphere. This habitat can be found in mountainous areas worldwide, occurring at high elevations where temperatures are too low and winds are too strong for the growth of trees. The average elevation where alpine tundra occurs is generally higher near the Equator than at the poles.
Tundra climates vary considerably. The most severe occur in the Arctic regions, where temperatures fluctuate from 4 °C (about 40 °F) in midsummer to –32 °C (–25 °F) during the winter months. Alpine tundra has a more moderate climate: summers are cool, with temperatures that range from 3 to 12 °C (37 to 54 °F), and winters are moderate, with temperatures that rarely fall below –18 °C (0 °F).
Unlike other biomes, such as the taiga, the Arctic tundra is defined more by its low summer temperatures than by its low winter temperatures.
Over most of the Arctic tundra, annual precipitation, measured as liquid water, amounts to less than 38 cm (15 inches), roughly two-thirds of it falling as summer rain. The remainder falls in expanded form as snow, which can reach total accumulations of 64 cm (25 inches) to (rarely) more than 191 cm (75 inches).