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.
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.
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
Forestry involves the use and management of forest resources.
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.
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
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
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
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
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
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.
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.
Ultimele referate adaugate
- Mihai beniuc - „poezii"
- Mihai eminescu - student la berlin
- Mircea Eliade - Mioara Nazdravana (mioriţa)
- Chirita in provintie de Vasile Alecsandri -expunerea subiectului
- Dragoste de viata de Jack London
|Ion Luca Caragiale
- Triumful talentului… (reproducere) de Ion Luca Caragiale
- Fantasticul in proza lui Mircea Eliade - La tiganci
- „Personalitate creatoare” si „figura a spiritului creator” eminescian
- Enigma Otiliei de George Calinescu - geneza, subiectul si tema romanului
- Arta literara in romanul Ion, - Liviu Rebreanu