National Geographic : 1972 Mar
Whatis tundra? IKE A FRAYED and frozen col lar atop the northland's forest wrap, tundra rings the Arctic Ocean (left). Under the name alpine tundra, similar hardy vegetation leapfrogs far southward on the frigid heights of mountains. A land of paradoxes, the Arctic tundra has been likened to desert in amount of precipi tation, yet water soaks large areas of its surface. Permafrost forms a seal that blocks absorp tion, and flatness bars runoff. Despite its apparent same ness, the tundra contains varied nt Ice Polygons Strangely uniform patterns occur when intense cold contracts the ground, opening fissures. During spring, water enters the cracks and freezes. Repeated cracking, filling, and freezing causes ice wedges to form. worlds, depending upon surface moisture, temperature, and winds-near-desolate areas of lichens and lemmings; more fa vored zones of cranberries and dwarf willows; and relatively hospitable expanses of grasses and sedges with willow thickets tall enough to hide a moose. Just as the Eskimo of the tun dra traditionally avoided the forest, so the tree line defines the range of most tundra crea tures. Only a handful of ani mals, such as the caribou and the red fox, are equally at home in the forest and on the plain. Solifluction Permafrost prevents absorption of surface water, so the topsoil remains saturated. During summer thaws the soil may flow slowly downhill in a movement known as solifluction. Perimafrost Beneath the tundra lies a zone that remains below 32° F. the year round-the permafrost, as much as 2,000 feet thick in parts of Alaska's North Slope. Atop it rests the shallow "active layer" that thaws in summer and sustains tundra life.