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 fires - such as the one that swept Yellowstone National Park in 1988 - is 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 km2 (4.4 mi2) in 1970 to 7.3 km2 (2.8 mi2) in 1998.