FOREST

The Role of Forests

Forests provide habitat for a wide variety of plants and animals and perform many other important functions that affect humans. Photosynthesis is the chemical process in the leaves that uses sunlight and carbon dioxide to produce energy-supplying sugars for the tree or plant—in the process the foliage of the plants and trees gives off pure oxygen for breathing. Forests also prevent erosion, the wearing away of soil by wind and rain. In bare lanscapes with little or no vegetation, heavy rains fall uniformly over large areas and can wash soil into rivers and streams and cause landslides and flooding. This leads to ecosystems that are deprived of both water and soil, which are quickly carried away in rivers and streams. In forested areas the forest canopy (treetops) intercepts and gradually re-distributes precipitation that would otherwise cause this flooding and erosion—some of the precipitation flows down the bark of the trunks as stemflow, the rest percolates through the branches and foliage as throughfall. This slower and nonuniform distribution of the rain ensures that soil and water will not be immediately carried away. In addition, the roots of the trees and other vegetation hold the soil in place and prevent flooding and clouding of streams and rivers. Forests also increase the ability of the land to capture and store valuable water. The canopy is especially efficient at capturing water from fog—condensed, cloudlike water vapor—which it distributes, like precipitation, into the vegetation and soil. Water stored in tree roots, trunks, stems, and foliage, as well as the soil of the forest floor, enables forests to maintain an even flow of water in rivers and streams in times of heavy precipitation or drought.

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.

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.

1.      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.

2.       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.

3.      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.

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.