...ure. They take part in natural processes of nutrient cycling and ater purification, and otherise help maintain a clean environment. Forests are important sources of many products. Forestry is the science, art, and technology of managing these forest resources.FORESTSThe large size and slo groth of trees make forests appear stable and permanent, but in fact they are dynamic sites of ongoing processes such as tree groth and death and soil formation. The tree species in a particular area are also constantly changing as species migrate and ne trees invade disturbed areas. Climates themselves change, but this generally occurs so slolyover tens or hundreds of yearsthat a given forest area appears to contain a constant group of species. EcologyThe inhabitants of forest communities interact in complex ays. Trees compete ith each other for sunlight, moisture, and mineral nutrients. These materials are necessary for photosynthesis, the process by hich green plants produce organic compounds for energy to live and gro. As trees photosynthesize, they absorb carbon dioxide from the air and extract moisture from the soil. Trees help to retain ater 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 groing 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 ashed aay. Natural or human destruction of forests alters the nutrient cycles, especially in the case of the nitrogen cycle, here plants play a substantial role. Regroth 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 ith the assistance of mycorrhizae fungi that gro symbiotically on tree roots, obtaining food from the tree.The process of soil development, aided by soil organisms, occurs in all forests. Microorganisms break don minerals in the soil and create passages for air and ater 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 ill decompose sloly and a thick layer ill develop, hereas in arm, moist climates, organic matter ill decompose rapidly, releasing minerals that are quickly absorbed by plant roots. Little organic matter ill accumulate.After all or part of a forest is destroyed by a disturbance, such as fire or ind or avalanche, trees and other plants reinvade the area, halting erosion and nutrient loss and maintaining ater quality. This series of changes in vegetation structure, knon as ecological succession, ill make the forest more suitable for some animals and plants and less suitable for others.Depending on environmental conditions, different tree species ill be dominant at different successional stages. The characteristic group of tree species in a given area is referred to as a forest type. ithin 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 ill likely predominate. Types of ForestsTree 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 here it first developed or here it migrated thereafter. Pines are found naturally in the Northern Hemisphere and thus belong to the Holarctic group. Many species, hoever, have been deliberately introduced into other areas ith 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 ith different genetic backgrounds that gro under similar soil and climate conditions in different parts of the orld 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 ith fe species in areas of cool climates. Temperate deciduous forests are predominantly broadleaf forests in areas of moderate temperature and rainfall ith cold inters. Subtropical evergreen forests are a combination of broadleaf and conifer forests in areas of sufficient rainfall and mild inters. Tropical rain forests are lush forests of complex structure ith many species in arm, moist regions. Chaparral or sclerophyllous forests are thicketlike forests of shrubs and small trees in areas ith mild inters and arm, dry summers. Tropical grasslands and savannas are grasslands ith scattered trees that occur in arm regions ith seasonal drought.FORESTRYForestry involves the use and management of forest resources. Forest uses can be divided into to categories nonconsumptive and consumptive. Nonconsumptive uses, hich remove little from the forest, include atershed protection, ildlife and fish habitat, recreation, and aesthetic uses. These specific uses require that the forests be maintained in particular conditions ith hich 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 reneable, since ne trees can gro after the old ones are cut. ProductsThe use of forests to obtain ood, chemicals, and other products is consumptive. About half of the ood harvested in the orld is used directly for fuel. ood is the primary fuel source in developing countries its use fluctuates ith the cost of alternative energy sources. ood has been used for lumber for construction purposes for thousands of years. Today, ood for structures primarily comes from straight, strong, conifer trees. Paper as first made from ood about 150 years ago, and it is still made primarily from ood. The cellulose fibers in ood can also be used to make rayon, photographic film, artificial sponges, synthetic lacquers, and other plastics. ood might be more idely used in industry to produce plastics, except that petroleum, an alternative ra material, is cheaper than ood is.Various chemicals are made from by-products of pulp and paper manufacture and from the independent distillation of ood these include charcoal, acetic acid, methanol, various oils, and medicinal chem...
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