Most Alaska soils are immature, cold, and acidic. Except for the lowlands of southeastern and south central Alaska, and portions of the lowlands in the Interior, most Alaska soils are permanently frozen, a condition called permafrost. At a certain depth in the ground, the soil remains perpetually frozen. This level is known as the permafrost table and the ground beneath is known as the inactive layer because it never thaws. The soil above, however, freezes and thaws every year, and in the process the soil is constantly churned. The permafrost table is impervious to water. Therefore the surface in much of the Interior, western, and Arctic Alaska is waterlogged and contains numerous but usually shallow lakes often called thaw lakes, in spite of the region’s low precipitation. Thaw lakes form when large blocks of ground ice contained in the inactive layer thaw and leave a hole in the surface, which fills with water. On the Arctic coastal plain these lakes are generally rectangular in shape, whereas in the Interior and western Alaska they are usually oval in form. Such lakes fill in with vegetation over time, and in the Interior eventually with trees. Thus the lakes are not permanent, but are constantly changing, with new lakes forming and others being filled in with vegetation. About 80 percent of Alaska contains permanently frozen ground. Of this, over half is called continuous permafrost, that is, has an active layer of only a few inches to a foot or so in depth. The remainder is called discontinuous permafrost, where the active layer may be many feet in depth.
Forests cover about a third of Alaska’s land area. The most important commercial species of trees are birch, Sitka spruce (the state tree), western hemlock, black spruce, and white spruce.
The Alaska Panhandle is a land of forests. The mild climate and heavy rainfall promote dense tree growth. The huge Tongass National Forest is an area where young saplings compete for space with trees that are centuries old. Trees found there include the western hemlock, Sitka spruce, canoe cedar, and yellow cedar (also called Alaska cedar), which are all conifers. The forest floor is carpeted with berry-producing plants and moss.
South central Alaska south of the Alaska Range is also heavily forested, but the trees are usually smaller and there is a transition from Sitka spruce to white and black spruce. The largest stand of timber is in the Chugach National Forest.
Southwestern Alaska is almost entirely treeless, except for Sitka spruce and some cottonwood on Kodiak Island and a few stunted birches and willows found in the Aleutian Islands. However, grasses grow luxuriantly in the cool wet climate. Flowers bloom in great variety and include the forget-me-not (the state flower), anemone, lupine, paintbrush, and marsh marigold in boggy areas, and the dwarf rhododendron on the hillsides. In the Interior region, vegetation must adapt itself to short, warm summers and long, cold winters. Trees grow slowly, and their root systems must be shallow because they cannot penetrate the permafrost. Toward the west the trees become sparse and are replaced by wet tundra. Similarly, the mountain slopes contain tundra in the Interior. Cleared areas are often brilliant with fireweed in the summer months. Principal trees found in this region are black and white spruce, paper birch, tamarack, aspen, Alaskan larch, and balsam poplar. There are expanses of bogs called muskeg, and grasslands, where many species of wild flowers, berries, and shrubs occur. Arctic Alaska contains primarily tundra vegetation with tall brush and some forests in stream valleys. Tundra consists of mosses, lichens, and grasses 3 to 5 cm (1 to 2 in) high, sedges and heather up to 20 cm (8 in) high, and willows taller than an average adult person.
Tundra is characteristic of the northlands around the Arctic Ocean and of other areas in Alaska above the timberline. Colorful flowers carpet the tundra during the brief summer weeks when the sun never sets, and plants grow well although the soil thaws less than 30 cm (1 ft) before the long winter returns. Here and there, dwarf willows are found. Encarta "USA" © Emmanuel BUCHOT, Encarta, Wikipedia
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