a dense growth of trees, together with other plants, covering a large area of land. The science concerned with the study, preservation, and management of forests is forestry. A forest is an ecosystem—a community of plants and animals interacting with one another and with the physical environment (see ecology). The forests of the world are classified in three general types, or formations, which are primarily expressions of the climate in which the vegetation grows.
The tropical hardwood forests, including rain forests, occur throughout the lowland areas of the tropics—especially along the routes of rivers in Central and South America and in central and W Africa—and in the East Indies, the Malay Peninsula, and parts of India, Indochina, and Australia. They are characterized by an annual rainfall of 160–400 in. (406–1,000 cm) annually, with an average temperature of at least 80 degrees Fahrenheit (27 degrees Celsius), and support a great diversity of plant life. The foliage is a luxuriant and interlaced community from ground level to the tree canopies, and the trees support the omnipresent woody vines (see liana) and air plants (see epiphyte). Although some tropical forests are deciduous, most tropical trees are considered evergreen because their leaves are not shed simultaneously at a certain season; however, they are believed to drop and renew their leaves sporadically each year. Even though they cover only 7% of the earth's landmass, about one half of the planet's species live there.
The temperate hardwood forests of North America, Europe, and Asia are marked by seasonal rainfall distribution. The trees, typically species of beech, maple, ash, oak, elm, and basswood, are deciduous but are often mixed with conifers, especially in areas of poorer soil. The temperate hardwood forests overlap the boreal, or northern, conifer forest belts, which encircle the earth in the subarctic and cool, temperate regions south of the treeless tundra. The vegetation is typically fir and spruce in northern regions and at higher altitudes, and pine, larch, and hemlock in southern regions and at lower altitudes. In transitional areas, especially where there is a pronounced season without rain (e.g., the chaparral and tropical mountain slopes), scrub forests are frequently found in which the trees are more widely spaced and grasses intervene. Nontropical rain forests exist in New Zealand, Tasmania, Chile, and the Pacific coast of North America.
In the United States east of the prairies are the northern (boreal) forest belt, in which sugar maple, beech, and birch mix with the conifers; the hardwood forest belt, a typical temperate forest; and the warmer southern forest belt, encompassing many stands of smaller pines and cypress thickets. In the chiefly coniferous Rocky Mt. forest belt, the Ponderosa pine is most common. The Pacific forest belt has the heaviest stands of trees in America and probably in the world. The characteristic redwood and giant sequoia mingle with Douglas fir and other species.
In early times the only nonforested areas of the earth were those where the land was either excessively dry (e.g., the plains and deserts) or excessively wet (e.g., the swamps). Where the environment was favorable, forests extended from the equator to the timber line, i.e., as far as those regions in the extreme north or at high altitudes where generally there is perpetual snow. Climatic conditions favor the continued expansion of the forests as the ice cap continues to recede and the timber line to withdraw, since the forests, with their mammal and bird inhabitants, move into formerly glaciated regions. However, the favorable natural conditions are more than countered by forest clearing by humans and through fire. About 30% of the world is forested today, but the ratio between forest and population varies immensely. More than one half of the world's softwood timber (the major forest product) comes from North America and Europe—an area with only a fourth of the world's population. Yet the Mediterranean countries have been cleared of most of their forests for centuries, and the forested area of the United States has shrunk in 300 years from about one half to one third of the total land acreage. The United States and Canada share about 15% of the world's forests; Russia contains about 20%, Africa has 20%, and Latin American has 24%.
The chief economic product of forests is timber, but the economic benefits, in terms of climate control, pollution abatement, and wildlife maintenance, have rarely been calculated. The economic importance of nontimber forest products is also increasing. The forest is also vital as a watershed. Because of the thick humus layer, loose soil, and soil-retaining powers of the trees' long roots, forests are vitally important for preserving adequate water supplies. Almost all water ultimately feeds from forest rivers and lakes and from forest-derived water tables. In addition, the forest provides shelter for wildlife, recreation and aesthetic renewal for people, and irreplaceable supplies of oxygen and soil nutrients. Deforestation, particularly in the tropical rain forests, has become a major environmental concern, as it can destabilize the earth's tempeature, humidity, and carbon dioxide levels. Efforts to control deforestation, including those at the 1992 United Nations Conference on Environment and Development, remain ineffective.
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