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The Goal of Forestry
It is the chief goal of forestry to devise methods for felling trees that provide for the growth of a new forest crop and to ensure that adequate seed of desirable species is shed onto the ground and that conditions are optimal for seed germination and the survival of saplings. The basic rule of timber management is sustained yield; that is, to cut each year a volume of timber no greater than the volume of wood that grew during that year on standing trees.
Desirable timber species are usually those of the native climax vegetation (see ecology) that can perpetuate themselves by natural succession, although at times (intentionally or unintentionally) a forest may not represent the climax vegetation—such as the pine of the SE United States, which grows faster than, and has replaced, the hardwoods destroyed by fire and logging. The Douglas fir of Western forests is encouraged because it is more valuable than the climax vegetation of mixed conifers that tends to establish itself in the absence of human intervention. Planting trees of different sizes (either because of species or of age) prevents crowding and insures maximal growth for the given area. Extermination of diseases and insect pests is standard forestry practice.
The management of forest fires has developed into an independent and complex science costing exceeding $2 billion annually at times in the United States. Because of the extremely rapid spreading and customary inaccessibility of fires once started, the chief aim of this work has long been prevention. However, despite the use of modern techniques (e.g., radio communications, rapid helicopter transport, and new types of chemical firefighting apparatus) some 7 million acres of forest are still burned annually on average. Of these fires, about two thirds are started accidentally by people, almost one quarter are of incendiary origin, and more than 10% are due to lightning.
Modern firefighting practice now recognizes that fires caused by lightning are an important tool of nature. Such fires do away with dead underbrush and diseased areas of growth, leaving clear areas for new growth of grass and new generations of trees. Some trees, it has been found, cannot grow without the aid of fire. The cones of the jack pine, for example, need exposure to intense heat to release seed. Other species, such as the Douglas fir and the sequoia, cannot flourish in shaded areas but need the open sunlit space cleared by fire. For such reasons lightning-caused fires in many cases—especially in wilderness areas far from habitation—are now permitted to burn but are carefully monitored and kept under control. In some cases, controlled burns, fires set by forest management personnel that are then monitored and managed, are also used to clear areas of dead and diseased growth, in order to promote new, healthy growth and prevent more intense, catastrophic wildfires.
The potential commercial value of the land lost to human-caused fire cannot be calculated: aside from the loss of timber, the damage is inestimable in terms of land rendered useless by ensuing soil erosion and flooding, elimination of wildlife cover and forage, and the loss of water reserves collected by a healthy forest. The increasingly complex interface of human habitation and wildlands that has developed since the late 20th cent. has also made wildfires a greater hazard to human life and property. Large-scale fires also are a source of air pollution that can present a significant health hazard, and the often uncontrolled use of burning to clear tropical forestland for farming and ranching has made pollution from forest fires a significant recurring problem at certain times of the year in some regions.
The Forest Service and Environmental Debate
See S. W. Allen, An Introduction to American Forestry (3d ed. 1960); D. M. Smith, The Practice of Silviculture (7th ed. 1962); C. H. Stoddard, Essentials of Forestry Practice (2d ed. 1968).
The term wildfire refers to all uncontrolled fires that burn surface vegetation (grass, weeds, grainfields, brush, chaparral, tundra, and forest and woodland); often these fires also involve structures. In addition to the wildfires, several million acres of forest land are intentionally burned each year under controlled conditions to accomplish some silvicultural or other land-use objective or for hazard reduction.
Most wildfires are caused by human beings, directly or indirectly. In the United States less than 10% of all such fires are caused by lightning, the only truly natural cause. In the West (the 17 Pacific and Rocky Mountain states) lightning is the primary cause, with smoking (cigarettes, matches, and such) the second most frequent. Combined they account for 50 to 75% of all wildfires. In the 13 southern states (Virginia to Texas) the primary cause is incendiary. This combined with smoking and debris burning make up 75% of the causes. The 20 eastern states have smoking and debris burning as causing close to 50% of all wildfires. Miscellaneous causes of wildfires are next in importance in most regions. The other causes of wildfires are machine use and campfires. Machine use includes railroads, logging, sawmills, and other operations using equipment.
The manner in which fuel ignites, flame develops, and fire spreads and exhibits other phenomena constitutes the field of fire behavior. Factors determining forest fire behavior may be considered under four headings: attributes of the fuel, the atmosphere, topography, and ignition. A forest fire may burn primarily in the crowns of trees and shrubs (a crown fire); primarily in the surface litter and loose debris of the forest floor and small vegetation (a surface fire); or in the organic material beneath the surface litter (a ground fire). The most common type is a surface fire.
The U.S. Forest Service has developed a National Fire Danger Rating System (NFDRS) to provide fire-control personnel with numerical ratings to help them with the tasks of fire-control planning and the suppression of specific fires. The system includes three basic indexes: an occurrence index, a burning index, and a fire load index. Each of these is related to a specific part of the fire-control job. These indexes are used by dispatchers in making decisions on setting up firefighting forces, lookout systems, and so forth.