A forest is a community of trees, shrubs, herbs, microorganisms, and animals, the trees being the most obvious living structures. Trees can survive under a wide range of climatic conditions, but forests generally occupy the moister, less frigid parts of the terrestrial biosphere. To different human cultures at different times, forests have been regarded as places of danger, security, economic opportunity, recreation, and aesthetic pleasure. They take part in natural processes of nutrient cycling and water purification, and otherwise help maintain a clean environment. Forests are important sources of many products. Forestry is the science, art, and technology of managing these forest resources. FORESTS

The large size and slow growth of trees make forests appear stable and permanent, but in fact they are dynamic sites of ongoing processes such as tree growth and death and soil formation. The tree species in a particular area are also constantly changing as species migrate and new trees invade disturbed areas. Climates themselves change, but this generally occurs so slowlyÑover tens or hundreds of yearsÑthat a given forest area appears to contain a constant group of species.

Ecology

The inhabitants of forest communities interact in complex ways. Trees compete with each other for sunlight, moisture, and mineral nutrients. These materials are necessary for photosynthesis, the process by which green plants produce organic compounds for energy to live and grow. As trees photosynthesize, they absorb carbon dioxide from the air and extract moisture from the soil. Trees help to retain water; heavy rains do not run rapidly off forest land. Natural or human activities that destroy forests result in increased runoff and in temporarily higher levels of carbon dioxide in the atmosphere. After this the growing forest increases the oxygen content of the atmosphere. A mature forest adds less oxygen to the atmosphere. A global research project designed to measure the overall influence of forests on the atmosphere of the Earth is in progress.

Trees also serve as temporary repositories for mineral nutrients in ecosystems; these nutrients accumulate in tree roots and thus are not easily washed away. Natural or human destruction of forests alters the nutrient cycles, especially in the case of the nitrogen cycle, where plants play a substantial role. Regrowth of young forests may increase the nitrogen added to the ecosystem. Trees take up the nutrients they need from the soil and from dead organic matter with the assistance of mycorrhizae (fungi that grow symbiotically on tree roots, obtaining food from the tree).

The process of soil development, aided by soil organisms, occurs in all forests. Microorganisms break down minerals in the soil and create passages for air and water movement, decomposing the remains of plants and animals and extracting and releasing nutrients. Depending on the climate, decomposition occurs at different rates. In cool or dry climates, organic matter will decompose slowly and a thick layer will develop, whereas in warm, moist climates, organic matter will decompose rapidly, releasing minerals that are quickly absorbed by plant roots. Little organic matter will accumulate.

After all or part of a forest is destroyed by a disturbance, such as fire or wind or avalanche, trees and other plants reinvade the area, halting erosion and nutrient loss and maintaining water quality. This series of changes in vegetation structure, known as ecological succession, will make the forest more suitable for some animals and plants and less suitable for others.

Depending on environmental conditions, different tree species will be dominant at different successional stages. The characteristic group of tree species in a given area is referred to as a forest type. Within each type, certain species may be found most commonly under specific soil and climate conditions and at certain times after a disturbance; these species are best evolved physiologically to compete under these conditions. In areas of recurrent fire, for example, fire-resistant trees will likely predominate.

Types of Forests

Tree species can be divided into six groups based on their evolutionary origins: Holarctic (originating in the Northern Hemisphere), Neotropic (originating in Central and South America), Paleotropic (originating in Africa and tropical Asia), Capensis (originating in southern Africa), Australian, and Antarctic. A species is found naturally only where it first developed or where it migrated thereafter. Pines are found naturally in the Northern Hemisphere and thus belong to the Holarctic group. Many species, however, have been deliberately introduced into other areas with similar climates; for example, pines are planted in many parts of the Southern Hemisphere, and eucalyptus, a tree genus native to Australia, is planted in other places.

Forest communities with different genetic backgrounds that grow under similar soil and climate conditions in different parts of the world have many of the same structural characteristics. Thus forests can be classified as major parts of many biomes. Taiga and boreal forests are coniferous forests with few species in areas of cool climates. Temperate deciduous forests are predominantly broadleaf forests in areas of moderate temperature and rainfall with cold winters. Subtropical evergreen forests are a combination of broadleaf and conifer forests in areas of sufficient rainfall and mild winters. Tropical rain forests are lush forests of complex structure with many species in warm, moist regions. Chaparral or sclerophyllous forests are thicketlike forests of shrubs and small trees in areas with mild winters and warm, dry summers. Tropical grasslands and savannas are grasslands with scattered trees that occur in warm regions with seasonal drought.

FORESTRY

Forestry involves the use and management of forest resources. Forest uses can be divided into two categories: nonconsumptive and consumptive. Nonconsumptive uses, which remove little from the forest, include watershed protection, wildlife and fish habitat, recreation, and aesthetic uses. These specific uses require that the forests be maintained in particular conditions with which some other nonconsumptive and consumptive uses are sometimes incompatible. Consumptive uses, by definition, involve the extraction of products from forests; this often requires the harvesting of trees. Forest resources are renewable, since new trees can grow after the old ones are cut.

Products

The use of forests to obtain wood, chemicals, and other products is consumptive. About half of the wood harvested in the world is used directly for fuel. Wood is the primary fuel source in developing countries; its use fluctuates with the cost of alternative energy sources. Wood has been used for lumber for construction purposes for thousands of years. Today, wood for structures primarily comes from straight, strong, conifer trees. Paper was first made from wood about 150 years ago, and it is still made primarily from wood. The cellulose fibers in wood can also be used to make rayon, photographic film, artificial sponges, synthetic lacquers, and other plastics. Wood might be more widely used in industry to produce plastics, except that petroleum, an alternative raw material, is cheaper than wood is.

Various chemicals are made from by-products of pulp and paper manufacture and from the independent distillation of wood; these include charcoal, acetic acid, methanol, various oils, and medicinal chemicals. Turpentine and tar may be obtained from destructive distillation or by scarring and scraping the wound of living pine trees. Maple sugar is obtained by taking the sap from the interior of living maple trees, and various trees provide other products.

Management

Forests are managed for a variety of objectives, ranging from carefully tended plantations to relatively natural areas of no cutting and minimal protection from disturbance. The intensity of management depends on the growth potential of the forest and various economic and political objectives. Even the most carefully tended forest plantations are not managed as intensively as most agricultural crops. Unlike agricultural crops, forest crops take many years to grow, even on the most productive soils. Often the products in demand change before the forest is suitable for a specific use; forest management needs to be flexible.

The ultimate unit of forest management is the "stand." A stand is a group of trees of uniform age, species, structure, and growth conditions. Stands vary in size from 0.4 to more than 40 ha (1 to 100 acres). The technology of manipulating stands is known as silviculture. Many silvicultural techniques mimic disturbances of some kind, often to remove existing trees or other vegetation in order to allow desired trees to become established and grow.

Four methods are used to remove trees from forest stands. The most radical is clearcutting, or the cutting of all the trees at one time, thus creating an even-age stand by planting or natural regeneration. The other methods are seed tree cutting, or the cutting of all the trees except for a few trees for reseeding, creating an even-age stand except for the seed trees; shelterwood cutting, or the removal of an old stand of trees in a series of cuttings extended over several years, which also creates an even-age stand; and selection cutting, or the removal of a few mature trees, usually repeatedly, over relatively short intervals, which creates an uneven-age stand.

Each system has its advantages and disadvantages; the proper method must be chosen on the basis of management goals and conditions at the stand in question. The system of logging the stand by clearcutting is appropriate where trees can become established and grow without shade. Where the clearcut area will be exposed to public view or to extreme temperatures, the conditions for forest regeneration are poor and the site can be aesthetically displeasing until the trees grow. Seed tree cutting is used in reforestation (discussed below). The shelterwood system is desired where extreme temperatures will inhibit growth of a new forest, and the selection system may be chosen where uneven-age stands are desired for some use and the regenerating species can grow in partial shade.

Inappropriate selection cutting of mixed-species forests in many parts of the world has left stands of diseased trees of little value that prevent vigorous trees from growing. The proper logging method should be chosen for a particular stand, or the stand may lose its usefulness or even become an erosion or fire hazard.

Silvicultural techniques constantly change with technological advances. They involve the use of fire, machinery, and chemicals for preparing stands for regeneration and for removing competing plants; nurseries for growing seedlings; genetic improvements resulting in more efficiently growing trees; fertilizers for increasing growth; and remote-control machines for pruning unwanted limbs.

It might seem odd to mention fire as one means of forest management, because the enormous destructiveness of great forest firesÑsuch as the one that swept Yellowstone National Park in 1988Ñis well known. Controlled fires, however, are useful in preparing ground for planting and in clearing the ground of weeds or fungal diseases that would harm seedlings. Controlled fires may also be used in attempting to block the course of great disaster fires. The majority of forest fires are caused by human carelessness, although many of the largest that sweep vast remote areas are produced by lightning. However damaging such natural fires are to human interests, they play a contributing role in forest evolution.

Conservation

Conservation is the planned management of natural resources to prevent their neglect, exploitation, and destruction. Forests provide each of the uses described earlier, but only under certain conditions. Forests have changed and will continue to change as trees grow and die, species migrate, and climates change. Often a forest is stressed by these changes, and the trees can become weakened and infected by insects or diseases, resulting in their death. Air pollution and water pollution created by human or natural activity can further damage trees. In northern Europe, many hectares of forests have been affected by acid rain.

One objective of conservation is the prevention of unintentional destruction of forests by disease, insects, and other agents. The other objective is the determination of management goals for each area of forest. Once the objectives of each stand are determined, the actual management requires the understanding of the natural sciences, long-term processes and history, and modern technologies. Deciding what values to conserve is a scientific, technological, and political subject. The decision requires the understanding of what natural and human activities will most readily destroy the stand and the knowledge of the most realistic uses, which entail both the private rights of the individual landowner and the public.

The objectives of conservation have changed along with changes in such related areas as the understanding of forest process, human values themselves, demands on the forest, availabilities of various resources, and technologies. Early forest conservation in North America was aimed at protecting forested areas from clearing for agricultural lands. Pine trees were conserved and harvested for making sailing ships. In the late 1800s and early 1900s forests were protected from fire, overharvesting, and overgrazing by the establishment of grazing laws, fire control practices, and harvesting regulations. Aesthetically unique areas and high-quality watersheds were set aside as national parks and forests. In the mid-20th century, unproductive farmland was converted to forests through the subsidizing of reforestation, thus halting erosion and providing for future forests. In the 1930s, southern U.S. forest industries began to grow seedlings on a large scale in forest tree nurseries and to replant large deforested areas.

In the past few decades, increases in mobility, leisure time, and disposable income have led to more interest in conserving forests for nonconsumptive purposes. In the United States, management objectives for national forests have shifted from timber production to multiple uses. Although this has become a source of controversy, more areas of public lands are mandated for nonconsumptive uses such as watershed and wildlife management and recreation. Various U.S. states have established or revised Forest Practices Acts to ensure that some uses of privately held lands are conserved.

Forests have been used for consumptive purposes throughout the world; in tropical regions, where forest soils grow rapidly, forest harvesting is occurring at a rapid rate. In parts of Africa, where the soils are easily eroded and the climate is unpredictable, forests and woodlands are being diminished. Agricultural practices may lead to deforestation under pressures of increasing population.

Three solutions to the deforestation problem have met with some success: the first involves the use of local people in forest management; the second involves "agroforestry," or the planting of trees in croplands and pastures; and the third involves the use of the financial resources of developed countries. The Food and Agriculture Organization of the United Nations has gained support for the protection of the world's forests and their role in rural development.

In the United States less than 5 percent of the virgin forests that used to blanket the country remain. In the face of population increases and continued industrialization, environmental activists in the United States have undertaken a constant watch to ensure that the remaining forests are conserved as humans increasingly alter the environment. As scientific knowledge of forest growth expands and a better understanding of detrimental effects of human activity develops, conservation efforts are working to turn the tide and prevent the demise of forests as sources of consumptive products, clean water, wildlife and fish habitats, and recreational areas. These efforts are being applied globally to prevent the neglect, exploitation, and destruction of forests. Nevertheless, the total amount of forest per 1,000 people declined from 11.4 Ü (4.4 a) in 1970 to 7.3 Ü (2.8 a) in 1998.

Chadwick D. Oliver

Bibliography: Attiwell, P. M., and Leeper, G. W., Forest Soils and Nutrient Cycles (1987); Bormann, F. H., and Likens, G. E., Pattern and Process in a Forested Ecosystem (1996); Drengson, A., Eco-Forestry: The Art and Science of Sustainable Forest Use (1997); Hutchinson, B. A., and Hicks, B. B., eds., The Forest-Atmosphere Interaction (1985); Jordan, C. F., Nutrient Cycling in Tropical Forest Ecosystems (1985); Mater, J., Reinventing the Forest Industry (1997); Nyland, R. D., Silviculture (1996); Oliver, C., and Larson, B. C., Forest Stand Dynamics (1996); Puri, G. S., et al., Forest Ecology, 2 vols. (1985Ð88); Sharpe, G. W., Introduction to Forestry and Renewable Resources, 6th ed. (1995); Smith, W. H., Air Pollution and Forests, 2d ed. (1989).

See also: fire prevention and control; hydrologic cycle.

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