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 tree-cutting by the timber industry may result in a shift to another forest type. Left undisturbed, ecological succession will eventually result in a climax forest community. Human intervention, in the form of forest conservation and management, is practised to maintain some desirable forest types.

II

Classification

Forests may be divided into the following general types on the basis of leaf characteristics and climate.

Deciduous forests of the temperate zone are the typical formation of much of Europe and eastern North America. Two sub-types exist; forests of the same latitude in the Northern and Southern hemispheres are radically different, probably because of the continental climate of the Northern hemisphere and the oceanic climate of the Southern.

Deciduous monsoon forests are characteristic of Bengal and Myanmar and common throughout south-eastern 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.

Tropical savannah forests are found in regions such as the campos of Brazil, where forest and grassland meet. Savannahs, 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 savannahs are created by fire or by grazing and browsing mammals.

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 in the landscape known as tundra, and on mountain tops. 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.

Tropical rainforests are characteristic of all land masses which lie within the tropics, such as western Africa and the Amazon watershed. Plant growth is profuse, and because the fall and regrowth of leaves occur continuously throughout the 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.

Mediterranean broad-leaved evergreen forests are found around Mediterranean coasts, where warm, wet winters follow hot, dry summers. Evergreen oaks and olive trees are dominant and, despite the description “broad-leaved”, coniferous species form an important element in this forest type.

Temperate rainforests are restricted to regions where rainfall is high and the ocean-cooled air is moisture laden, and fogs are frequent. In the United States and Canada the temperate west coast rainforests are dominated by hemlock, cedar, spruce, fir, and sequoia.

Various other, more specific forest types can be distinguished, some of them of restricted occurrence, such as cloud forest and mangrove swamp.

III

Diseases and Pests

Insects and diseases are a continuing menace to forests. Various insects devastate extensive areas by defoliation, while others serve as carriers for the causative agents of plant diseases that destroy trees. Tree diseases may be caused by bacteria, fungi, viruses, and nematodes (see Diseases of Plants). Non-infectious 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.