I.

Introduction

Tundra, biome in the Northern hemisphere lying above the zone of northern coniferous forests (taiga) and below the polar oceans and terrestrial regions of permanent ice and snow. Alpine tundra is a climatically similar type of environment found above the altitudinal limits of tree growth in temperate regions. The Arctic tundra stretches in an unbroken strip across the northern latitudes of Eurasia and North America and encompasses Greenland, Iceland, the islands of the Canadian archipelago, the Pribilof Islands, Nunivak, St Matthew and St Lawrence islands, Wrangel Island, the New Siberian Islands, Severnaya Zemlya, Novaya Zemlya, Franz Josef Land, Svalbard, and numerous smaller islands. In North America, most of the tundra lies above 60^o N, while in Eurasia it tends to occur above 70^o N. The more northerly extent of taiga in Eurasia is a reflection of warmer summers in the vast Eurasian continental interior.

Alternate freezing and thawing of the ground on a seasonal cycle and the presence of a permanently frozen subsoil layer, the permafrost, are the unique features that distinguish Arctic tundra from polar regions to the north and taiga to the south. Freeze-thaw cycles often give rise to ground characterized by cracks, polygons, hummocks, knolls, frost boils, and earth stripes resulting from differential movement of soil, stones, and rocks, combined with a steady downward creep (solifluction) of the soil mantle.