Intentional burning, usually in forests, prairies, and savannas, in order to maintain desired ecological conditions or prevent major wildfires that could result from accumulated dry, woody debris. Prescribed burning, also called controlled burning, has become a widely recognized component of modern ecosystem management in recent decades as ecologists and managers have recognized the role of fire in natural ecosystems. In practice, though, prescribed burning is still used only on a small portion of public and private lands. In a forest, prescribed burning is usually aimed at removing accumulated fuel—fallen branches, pine needles, and other dry debris. In a prairie or savanna environment , prescribed burns may be used to prevent encroachment of trees, to reduce populations of nonnative grassland species , and to enhance the health of native grassland vegetation that is adapted to periodic fires.
The principle of prescribed burning is that fire is a normal component of ecosystems, and that frequent small fires can serve both to maintain species diversity and to prevent large, catastrophic wildfires. Under natural conditions wildfires, usually ignited by lightning, occur frequently in many forest and grassland biomes. Proponents of prescribed burning argue that these natural fires would usually be small and cool, occurring too frequently to allow excessive accumulation of fallen branches, dead grass, and other fuel. Frequent, low-intensity fires should do little damage either to mature trees or to the root systems of most plants. At the same time, by clearing away debris every few years, and by a natural fire regime should provide fresh ground, exposed to sunlight and freshly fertilized by ashes, to encourage new germination and plant growth.
Deliberate burning is also an age-old method of maintaining grazing lands in many parts of the world. Periodic forest burning to maintain pasturage and to clear farm plots has probably been practiced for millennia in Africa and South America. In North America, indigenous peoples used fire to maintain open forests and prairies that provide grazing for game animals such as deer and bison . In some areas, especially New England and the rural South, early European immigrants to the New World adopted Indian methods of burning. These practices differ from what is usually considered a prescribed burn, however, because they are often carried out without the planning and the careful fire control usually exercised in a controlled or prescribed burn. In most of North America, traditional burning practices have also disappeared as more and more forests have come under professional management for timber production.
The introduction of prescribed burning to professional land management strategies in the United States is often attributed to Aldo Leopold , who pointed out in 1924, while he was serving as a forester and firefighter in Arizona, that fire control programs were producing brush-choked forests susceptible to extreme wildfire . Leopold argued that decades of fire suppression had actually increased the probability of severe fires, and that forests were healthier when subject to regular, natural fires. Since Leopold began arguing for prescribed burning in the 1920s, ecosystem managers have widely recognized the ecological benefits of fire. The amount of prescribed burning carried out still remains modest, however. Between 1983–1994, prescribed burns were carried out on only about 0.1% of public lands in the United States each year, as compared to 0.3% burned annually by wildfires in the same interval.
Proponents of prescribed burning sometimes argue fire suppression techniques--bulldozed fire breaks, tree felling, air drops of chemical fire retardants, and other measures--cause more damage than the fires they are meant to control. In addition, these expensive, and dangerous, tactics may ultimately have little effect. Prescribed burn proponents argue that most large natural fires die out as a result of rainfall, cooler weather, or a lack of fuel, not because of the efforts of fire fighters. These opponents of standard military-style fire suppression programs propose that it would be more effective to use fire control budgets for planned burns than for impressive but useless shows of force against large forest fires. However events such as the Yellowstone National Park fires of 1988, a series of highly publicized fires that led to loud criticism from the media and politicians that Park officials were negligent in their duties to protect the park, demonstrate that the public is unlikely to settle for inactivity during a forest fire. During fires such as those in Yellowstone, the public is quick to criticize officials for doing nothing even if there is nothing to be done. Most of us would rather see a campaign to fight fires even if wastes money and causes serious damage to the forest.
Benefits of Prescribed Fires
Fire has many ecological benefits. By clearing patches of forest, fire allows tree seeds to germinate. The seedlings of many trees, including Douglas fir (Pseudotsuga menziesii ) and white pine (Pinus strobus ), both prized for timber, require the strong sunlight of an open patch in the forest to begin growing. Other species, such as jack pine (Pinus banksiana ) require intense heat, such as that produced by a fire or extremely hot sun, to release the seeds from their resin-sealed cones. These trees are considered fire-adapted, because they reproduce best, or only, when seedlings can grow in the full sun and fresh ashes of recently burned patches of forest. Many prairie grasses and flowers are also fire adapted, thriving best when periodic fires remove dead stem litter and release stored nutrients to the soil .
Fire plays an important role in cleaning out damaged or diseased regions of a forest. Where parasites , fungus, or disease have damaged stands of trees, a fire can locally eliminate the pest population, reducing the risk of parasite or disease spread, at the same time as it removes standing dead trees and makes room for new seedlings to replace the damaged trees. Land managers in southern Alaska, faced with the worst spruce beetle outbreak ever in the United States, have recently begun to consider using prescribed fire to control beetles.
Undesirable species encroaching on grasslands and savannas can also be controlled with fire. Opportunistic trees and brush such as aspen, buckthorn, juniper and sumac, introduced European grasses, as well as nuisance species such as poison oak and poison ivy can all be set back by periodic burning. At the same time, prescribed burns can rejuvenate fire-adapted plant populations by clearing debris and releasing nutrients to the soil. In a burned area increased sunlight can extend the growing season, an important advantage in cold climates. In addition, periodic patchy burns help maintain habitat diversity by producing a mosaic of landscape types and different-aged tree stands. Because many animal species utilize a variety of environments for feeding and nesting, a mosaic landscape maintained by fire can benefit wildlife populations.
A prescribed burn is a fire set under conditions that should allow control of the fire. Usually this means identifying the limits of an area that should be burned and creating or identifying fire breaks that will prevent the uncontrolled spread of the fire once it has been lit. Often a backfire is used. A backfire is line of fire burned on the downwind edge of the area to be burned. The backfire is kept low and forced to burn upwind until it has created a swath of land clear of any combustible fuel. Flank fires, similarly, clear combustible materials from strips along the flanks of the burn area. The main fire, moving forward in a downwind direction, will die out due to a lack of fuel when it reaches the blacked zone created by the backfire. In addition to these precautions, prescribed burning usually requires selecting a day with high humidity and light winds, to keep the fire relatively low.
Ideally areas should be burned periodically in order to mimic the pattern of natural fires. Fire ecologists and managers usually attempt to burn areas with the frequency they estimate would occur in local ecosystems under natural conditions. In some areas natural fire frequency might be as often as every 2-3 years; elsewhere natural fires might normally occur as infrequently as every 10-20 years. Or managers may simply burn any areas in which debris has accumulated to dangerous levels.
Prescribed Natural Fires
Sometimes a prescribed burn policy means establishing a rule to let naturally ignited fires, usually started by lightning, burn themselves out. When rules establish that fires should be allowed to burn, fires that occur are referred to as prescribed natural fires. "Let-burn" policies are widely applied in national parks and forests in the United States. Seventy-five percent of Yosemite National Park , for example, is treated as a prescribed natural fire zone. In large part this is because it is more politically palatable to allow nature to take its course than it is to deliberately burn a park. In addition, it is less costly to let fires burn naturally than to maintain a burn crew, design burn plans and fire breaks, and carry out and monitor a regular program of prescribed burns. Usually a prescribed natural fire policy sets limits on the conditions and size of fires that will be allowed to burn, and any natural fires that do occur are closely monitored to ensure that they do not become too large or dangerous.
History of Prescribed Burning in the United States
Two factors influenced the development of prescribed burning in the United States. One was the observation that despite decades of fire suppression, the number and intensity of wildfires has actually increased since the advent of major fire suppression programs. One United States Forest Service study showed that total acreage burned, after declining from 1917 to 1955, increased steadily from the 1960s to the 1990s. The other factor was a transition in ecological theory toward understanding ecosystems as complex and dynamic, not static structures that should be maintained a single, undisturbed, optimal climax state. The shift in ecological theory developed partly from the observation that ecosystems maintained rigorously by managers were not necessarily healthier, than those that underwent periodic catastrophic events such as floods and fires. In fact, according to this theory, ecosystems may require periodic change and disruption in order to remain healthy.
The Nature Conservancy , which maintains scattered patches of native prairie, savanna, and other ecosystems, was one of the pioneers of prescribed burning. Since it conducted its first prescribed burn on a North Dakota prairie in the 1970s, the Nature Conservancy has expanded its prescribed burning program to a current level of over 40,000 acres (16,400 ha) of native habitat each year. The organization's objectives are controlling invasive plant species and maintaining the vigor of native fire-adapted plant communities.
Controversy over Prescribed Burns
Prescribed burning is controversial for several reasons. First, decades of education about fire suppression, both in the general public and among land management professionals, makes burning seem intuitively wrong to many people. Cultural icons such as Smokey Bear and Bambi, both fictional characters who suffered from forest fires, have symbolized the trauma and destruction of fires. The idea that fire might rejuvenate a forest is often harder to explain than the idea that fire destroys a forest. Second, people living near forests and grasslands fear that a controlled burn may escape and damage their property--an event that occurs rarely but is always a potential risk. Property owners are understandably nervous about the possibility of a fire escaping. On the other hand, prescribed burns have sometimes been used to protect property. A number of homes adjacent to the Wenatchee National Forest in Washington State were spared from the Tyee Creek fire of 1994 because previous burns had reduced fuel levels in the surrounding forest.
In addition to objections to fire danger, the smoke resulting from a prescribed fire is unpleasant and sometimes hazardous. There can be a real health risk for people with lung ailments or asthma if they are exposed to concentrated smoke. In addition, a deliberately set fire often violates air quality laws, making fires technically illegal in many populated areas. If the fire occurs near a road, visibility is also reduced, so that the road must often be closed while a burn is in progress. Although the smoke produced by a series of prescribed fires may be less than the smoke from a single large wildfire, prescribed fires are sensitive because the individual or group responsible for the fire is also liable for any damage or injury it causes.
Controversy over the use of prescribed burns has become especially serious in areas where human settlement is encroaching on natural areas. Where cities and suburbs spread into forested areas or reach the edges of preserves that are managed with fire, residents of new subdivisions usually object to the potential risks and inconvenience imposed by fire near their property. This problem is especially severe in the chaparral country of southern and central California. Chaparral is a fire-adapted ecosystem with highly combustible, dry vegetation that normally burns as every 30 years or so. As suburban development, with its high real estate values, moves into the chaparral, the economic and environmental costs of fire control have risen sharply. In wildlife and habitat preserves near metropolitan areas land managers have sometimes had to abandon successful fire management programs because of suburban sprawl.
[Mary Ann Cunningham Ph.D. ]
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