Forest.

Forest. I INTRODUCTION Forest, plant community, predominantly of trees or other woody vegetation, occupying an extensive area of land. In its natural state, a forest remains in a relatively fixed, self-regulated condition over a long period of time. Climate, soil, and the topography of the region determine the characteristic trees of a forest. In local environments, dominant species of trees are characteristically associated with certain shrubs and herbs. The type of vegetation on the forest floor is influenced by the larger and taller plants, but because low vegetation affects the organic composition of the soil, the influence is reciprocal. Disturbances such as a forest fire or timber harvesting may result in a shift to another forest type (see Forest Fires; Lumber Industry). Left undisturbed, ecological succession will eventually result in a climax forest community (see Ecology). Human intervention is practiced to maintain some desirable forest types. II CLASSIFICATION Forests may be divided into the following eight general types on the basis of leaf characteristics and climate. 1. Deciduous forests of the temperate regions are the typical formation of the eastern United States. Two subtypes exist; forests of the same latitude in the northern and southern hemispheres are radically different, probably due to the continental climate of the northern hemisphere and the oceanic climate of the southern. 2. Deciduous monsoon forests are characteristic of Bengal and Myanmar (formerly known as Burma) and common throughout Southeast Asia and India; they are also found along the Pacific coastal regions of Mexico and Central America. The climate is characterized by heavy daily rainfall, seasonally relieved by dry periods during which the trees shed their leaves. 3. Tropical savanna forests are found in regions such as the campos of Brazil, where forest and grassland meet. Savannas, which occur widely in Africa and South America, are dominated by grasses and sedges, with open stands of widely spaced trees that are frequently thorny. Some savannas are created by fire or by grazing and browsing mammals (see Savanna). 4. Northern coniferous forests form a worldwide belt in subarctic and alpine regions of the northern hemisphere. Gnarled scrub trees dominate at the northern tree line and on mountaintops (see Tundra). Spruce and fir trees are characteristic of the more northerly forests; pine, larch, and hemlock dominate farther south. These forests usually occupy formerly glaciated regions and occur in association with lakes, bogs, and rivers. 5. Tropical rain forests are characteristic of central Africa and the Amazon watershed. Plant growth is profuse, and because the fall and regrowth of leaves occur gradually throughout each year, the forest is always active. Tree species are highly diverse but usually have smooth, straight trunks and large, simple leaves. Large vines are common, but the tangled growth of a jungle occurs only where the normal forest area has been abused or at a river's edge. 6. Temperate evergreen forests are found in the subtropical regions of North America and the Caribbean islands that have a warm maritime climate. The type is best developed along the Gulf Coast and in the Florida Everglades. The characteristic trees are live oak, magnolia, palms, and bromeliads. 7. Temperate rain forests, with broad-leaved evergreen trees, are common on Mediterranean coasts. Rainfall may be low, but the ocean-cooled air is moisture laden, and fogs are frequent. In the United States the temperate West Coast rain forests are dominated by hemlock, cedar, spruce, fir, and redwood. 8. Tropical scrub forests occur in regions of slight rainfall, bordering wetter forests (see Chaparral). III UNITED STATES FORESTS Three major forest areas exist in the United States. The western forests of the Rocky Mountains and the Pacific coast are coniferous and contain Douglas fir, ponderosa pine, western white pine, Engelmann spruce, and white fir. More than half of the softwood lumber yield of the United States comes from the productive Douglas fir forests of the Pacific Northwest. The South Atlantic and Gulf states account for most of the remaining softwood lumber, chiefly from longleaf, shortleaf, loblolly, and slash pines. Hardwoods, yielding about one-fourth of the total production, are found in the eastern half of the United States, with particularly dense stands in the area surrounding the Mississippi and Ohio river valleys. Among the many hardwood species are oaks, black walnut, yellow poplar, and sugar maple. More than one-fourth of the forest area of the United States is under the administration of the Forest Service. Beginning in 1891 with a single area in Wyoming, the National Forest System had by the late 1980s expanded to more than 77 million hectares (191 million acres) in 44 states, Puerto Rico, and the Virgin Islands. The earliest national forests, called forest reserves, were established through reservation of public lands. Present national forests, whose boundaries are established by Congress, cover areas that include about 17 percent privately held land, which the federal government is acquiring gradually. Almost every state has a state forester, whose duties involve administration and protection of state forestlands (see Forests, National and State). IV CANADIAN FORESTS The forestlands of Canada stretch across the continent in a belt varying from about 800 to 950 km (about 500 to 590 mi) in width. Forests producing usable timber cover about 3 million sq km (about 1.2 million sq mi). Coniferous trees make up four-fifths of the total, and deciduous trees the rest. The belt of conifers that extends from the Atlantic coast westward and northwest to Alaska includes spruce, balsam, fir, and pine, with poplar and white birch the principal deciduous species represented. South of this coniferous belt are the mixed forests of the Great Lakes, St. Lawrence, and Acadian regions. Here the major conifers are eastern white and red pine, eastern hemlock, spruce, cedar, and fir. Also present are deciduous yellow birch, maple, oak, and basswood trees. On the western coast, in British Columbia, the large coniferous species provide more than one-fourth of all wood cut in Canada annually. Most important are cedar, hemlock, spruce, fir, and Douglas fir. More than 90 percent of productive forestland is publicly owned. Its administration and protection is largely vested in the various provincial governments, which lease the forests to private industry. In the three Maritime provinces (Nova Scotia, New Brunswick, and Prince Edward Island), however, a sizable proportion of the forestland is privately owned. On the federal level, the Canadian Forestry Service of the Department of the Environment promotes improved management of forest resources and better products, in addition to providing advisory services to the provincial forestry administrations. V DISEASES AND PESTS Insects and diseases are a continuing menace to forests. Various insects, such as the gypsy moth, the tussock moth, and the spruce budworm, devastate extensive areas by defoliation. Other insects serve as carriers for the causative agents of diseases that destroy trees. Parasitic tree diseases may be caused by bacteria, fungi, viruses, and nematodes, or by such parasitic plants as the mistletoe or dodder. Noninfectious diseases of trees include sunscald; drought injury; root drowning, or suffocation; nutritional excesses or deficiencies; winter injury; and injury from smoke, gases, and fumes. See articles on individual trees. See also Conservation; Diseases of Plants; Forestry. Contributed By: David L. Adams Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.