Ecology, Fire

Fire has been an agent of change in nearly every terrestrial vegetation type on Earth, shaping both the species composition and structure. The probability of occurrence and the effects of fire vary widely depending upon the amount of fuel present, topography, climate, sources of ignition, and present species composition of the area. Fires may be severe, causing great mortality of existing plants and significantly changing the species composition of the burned area, or they may have little impact on the composition and consume only the dry dead plant material present. They may burn intensely as fast-moving fires with flame lengths greater than 25 meters, or they may occur as slow-moving fires with flame lengths less than 0.5 meter. These sources of variation influence the effects that fire has on the vegetation and the ecological role of fire.

Prior to human intervention, fires occurred very frequently in some vegetation types. Fire history studies in temperate grasslands in Africa and North America, some ponderosa pine forests of western North America, and longleaf pine forests of southeastern North America indicate that fires occurred at an average of less than every ten years. When fires occur frequently, the vegetation's composition becomes dominated by fire-adapted (tolerant) species. Consequently, when fires occur, there is only minor change in species composition and the vegetation quickly recovers to the preburn condition, often within five years or less. The more frequently fire occurs on an area, the more it becomes dominated by fire-adapted species.

In other vegetation types, the fires may occur many decades or even centuries apart. Fires were naturally very infrequent in many arid areas where the fuels are not sufficient to carry a fire, or in humid areas where the fuels are seldom adequately dry to burn. Very often the effects of fire in these areas is long lasting and the vegetation may not recover for many centuries. When fires occur infrequently, fire-intolerant species become established, and the effects of fire are much more lasting. Even in these communities, however, fire may play important ecological roles in the functioning of the ecosystem .

Ecologists are becoming increasingly aware of the importance of fire in the maintenance and functioning of ecosystems. Rather than focusing on a single fire event, it is useful to apply the concept of a fire regime when considering the effects of fire on large areas over a long time. A fire regime describes the typical fire characteristics when applied to a landscape over many burning cycles. Fire regimes include characteristics such as frequency (how often the fire occurs over time), size, intensity (the rate at which fire consumes fuel and releases heat), severity (the effects of fire on the biota and soil), continuity (the degree to which unburned areas remain within the fire's perimeter), pattern (where fire typically occurs on the landscape), and variability in the previous characteristics. Modern humans have changed the fire regimes for many areas of the world. When fire regimes change, ecosystems and fire effects change in ways that are often not desirable. For instance, when fires are less frequent, they burn more intensely.

Creation and Maintenance of Vegetation Composition and Diversity

Fire and other disturbances typically kill some plants and alter the competitive relationships between species. The initial postburn community is composed of those species that survive the fire and those that can efficiently migrate to the site. Community succession gradually modifies the postburn environment of the site and the composition changes in response to the changing environment. Species and, in some cases, the community may be replaced by later successional species and communities. Thus, fire plays a critical role in maintaining or creating new habitat for those species that are adapted to fire occurrence. For example, in many areas ponderosa pine-bunchgrass vegetation will gradually change to a forest dominated by other tree species, such as Douglas-fir, in the absence of fire. Periodic fires maintain the ponderosa pine-bunchgrass vegetation. A similar situation exists for many temperate grasslands, where in the absence of fire, grasslands are replaced through succession by forest or woodland vegetation.

In some instances, the vegetation itself must be removed by fire in order for the environment necessary for that vegetation to be maintained. For example, big sagebrush grassland often occurs in a fire-maintained mosaic with juniper woodland vegetation in western North America. In the absence of fire, the juniper woodland vegetation will replace the sagebrush grassland through community succession. Young juniper that are establishing within the sagebrush grassland can be readily killed by fire. Sagebrush will also be removed by the fire and a community composed of grasses and other herbaceous plants will become established initially. Sagebrush seedlings that cannot become established within a dense juniper woodland, however, will establish within the grassland and thus, over time, fire maintains the sagebrush grassland vegetation in the landscape.

Cycling of Organic Matter and Nutrients

In many areas of the world, the rates of plant biomass production exceed the rates of biomass decomposition. In these cold or dry areas, bio-mass tends to gradually increase over time as succession and plant growth occur. Accumulation of biomass, particularly dead biomass, has many effects on the ecosystem. The nutrients essential to plant growth become increasingly concentrated in plant tissue and unavailable for subsequent plant growth. This may result in deficiencies of some essential nutrients and the reduction in biomass production. Fire rapidly cycles these nutrients and makes them available for future plant growth. In this way fire may help maintain the productivity of the ecosystem. The combustion process, however, results in the volatilization of some elements such as nitrogen and carbon, and these are lost to the atmosphere. Nitrogen is replaced in the ecosystem through nitrogen fixation and other processes of the nitrogen cycle.

Aboriginal Humans and Fire

In addition to its use for heating, lighting, and cooking, fire was the first tool that primitive peoples had to manipulate the environment on a broad scale to better meet their purposes. Fire has been used by hunter-gatherer societies to promote the production of certain wild crops (such as seeds: wild rice, sunflower, balsamroot, and mesquite beans; tubers: camas and bracken; berries: blueberry and blackberry; and nuts: acorns and chestnuts), increase the nutritional quality of forage for wild animals, create desirable habitat for game species, decrease the natural migration rates of game species allowing for increased hunting possibilities, control problem tick and insect populations, open travel corridors, and reduce fire hazard and enemy hiding cover in the vicinity of campsites.

Aboriginal people have also used fire for driving game species into traps or to hunters, long-distance signaling, warfare, and ceremonial purposes. Some peoples had the tradition of setting large fires in hopes that it would induce rain. Pastoralists used fire to clear pastures of trees and shrubs, increase forage production, improve forage nutritional quality, and decrease parasites affecting their livestock. Early agricultural cultures used fire to clear natural vegetation to facilitate cultivation , remove organic crop residue, and fertilize fields by cycling nutrients. In addition, many fires were likely set by accident from cooking fires. Thus, human culture has had a long association and evolution with fire.

Use of Fire as a Land Management Tool

The intentional use of fire to achieve a land management objective is often referred to as prescribed burning. The fire is prescribed in the sense that the specific area, burning conditions, and expected results are identified prior to ignition. In addition, specific land management objectives are developed that justify the use of fire. Weather conditions (such as wind, temperature, relative humidity, and fuel moisture) and ignition patterns are selected that allow the land manager to control fire spread and achieve desirable effects on the vegetation. The management objectives of today's prescribed burning remain very similar to many of the aboriginal people's uses. The most common objectives include: creating or maintaining habitat for wild and domestic plants and animals, controlling undesirable plants, increasing the nutritional quality of forage for wild and domestic herbivores , reducing fire hazard through fuel reduction, and increasing nutrient cycling rates. Fire continues to be extensively used as a land treatment by hunter-gatherer, pastoral, and agricultural peoples around the world to clear vegetation, improve pastures, and remove crop residue.

Natural fire programs are employed in some national parks and wilderness areas to maintain the ecosystem in nonhuman-affected conditions as much as possible. Natural fires are those that have a nonhuman ignition source, primarily lightning. Prior to initiating a natural fire program, land managers develop a plan that identifies the conditions under which lightning-ignited fires will be allowed to burn without direct fire suppression control measures being taken. Since the weather conditions or location of any specific fire cannot be precisely predicted, however, the expected results of fire are usually described in more general terms than for human-ignited fire. The objectives of these fires usually includes having fire play a natural role in the ecological processes of the ecosystem.

see also Chaparral; Coniferous Forests; Ecology; Grasslands.

Stephen C. Bunting

Bibliography

Agee, James K. Fire Ecology of Pacific Northwest Forests. Covello, CA: Island Press, 1993.

Biswell, Harold H. Prescribed Burning in California Wildlands Vegetation Management. Berkeley, CA: University of California Press, 1989.

Boyd, Robert, ed. Indians, Fire and the Land in the Pacific Northwest. Corvallis, OR: Oregon State University Press, 1999.

Bunting, Stephen C. "The Use and Role of Fire in Natural Areas." In National Parks and Protected Areas: Their Role in Environmental Protection, ed. R. Gerald Wright. Cambridge, UK: Blackwell Science, 1996.

Pyne, Stephen J. Fire in America: A Cultural History of Wildland and Rural Fire. Princeton: Princeton University Press, 1988.

——. World Fire: The Culture of Fire on Earth. Seattle, WA: University of Washington Press, 1995.

——, Patricia L. Andrews, and Richard D. Laven. Introduction to Wildland Fire, 2nd ed. New York: John Wiley & Sons, 1996.

Fire Ecology

Fire is one of the leading natural forces that has shaped nearly all land-based ecosystems for several thousand years. Fire is especially important in regulating the species composition of vegetation. Fire is particularly important in forests of cold northern regions, such as Canada and Siberia, and in savannas , grasslands, and shrubby vegetation types in temperate and tropical regions, such as Australia and California.

Fire has become more important in tropical forests due to human disturbance. Fires are caused naturally by lightning and by people both accidentally and intentionally for management purposes. Native peoples around the world have used fire to maintain favored vegetation types and manage wildlife.

Surviving Fire

Some plant species have adaptations that allow them to survive or reproduce after fire. Survival adaptations include sprouting from underground roots (aspen, grasses), sprouting from stumps (birches, oaks), and growing thick bark that insulates trees from fire (many species of pine and oak). Adaptations that lead to reproduction after fire include serotinous cones on species such as jack pine and lodgepole pine. Serotinous cones are held high in the forest canopy in closed condition and do not shed their seeds unless scorched by fire. Buried seeds of some species survive in the forest floor until fire kills the tree canopy, allowing sufficient light to stimulate seed germination (pin cherry and some geraniums).

Some plant species have none of these adaptations to fire. They survive fire in refuges, such as rocky areas without continuous fuel to carry fires, or wetlands that rarely burn. They may also grow in parts of the world where rainfall is frequent and fires are rare.

Natural vegetation types of the world experience several "fire regimes," or characteristic occurrence of fire in terms of the frequency and intensity of fire. A fire regime with very frequent, low-intensity fire (two- to ten-year recurrence) occurs in moderately dry climates supporting grasslands and savannas around the world. Frequent, low-intensity fires (ten- to forty-year recurrence) constitute the fire regime in many temperate and boreal forests dominated by tree species with thick bark, especially oaks and pines. The fires kill invading tree species with thin bark while allowing oak and pine to survive.

A fire regime of moderately frequent, high-intensity fire (thirty- to one-hundred-year recurrence) occurs in oak and manzanita-dominated chaparral in California and in dry boreal forests dominated by trees with serotinous cones, such as jack pine in North America. These high-intensity fires kill the forest from the ground up and initiate a new, young forest. Infrequent, high-intensity fires (one-hundred- to five-hundred-year recurrence) occur in many conifer forests of the Rocky Mountains and wetter parts of the boreal forest.

"Helpful" Fire

A number of ecosystems have a regime in which fires are rare, including hemlock and sugar maple forests of eastern North America, arctic tundra, and very dry deserts. Surprisingly, fire may still be important in these systems. For example, lightning strikes in maple forests of Michigan sometimes burn a fraction of an acre of forest, called a spot fire. These spot fires are usually invaded by oak trees, which then live for up to three hundred years. These spot fires have a long-lasting impact where they occur and they enhance biodiversity by maintaining fire-dependent oak as a component of the forest landscape where big fires never occur.

Many vegetation types around the world require fire for their maintenance over time, and they are replaced by different vegetation in the absence of fire. People have suppressed fire during the nineteenth and twentieth centuries in many parts of the world. Fire suppression in savannas and prairie remnants has allowed invasion by forest in many cases. Restoration of prairies requires the use of "prescribed fire," purposely set by people, to reestablish the fire regime required by the prairie plants.

Pine forests throughout the United States (ponderosa pine in the west, white pine in the east) were formerly kept in a parklike condition with open understories by the occurrence of surface fires. After several decades of fire suppression, these forests have accumulated a high density of trees, including the invasion of other species such as spruce and fir. The buildup of high fuel loads and smaller trees that can function as a ladder to carry fire into the crowns of large pines means that fires become more intense than in the past, possibly too intense to be controlled by fire fighters, and intense enough to kill the old pines.

Fire and Wildlife

Fires kill relatively few numbers of wildlife species directly. The major impact of fires on wildlife is that it alters their habitat. Any substantial alteration in habitat is sure to affect some species positively and others negatively. For example, if an old-growth boreal forest of pine, spruce, and fir is replaced by a young aspen forest after a fire, then a whole suite of conifer-dependent birds, such as spruce grouse, gray jay, and boreal chickadee, will fare poorly after the fire. Conversely, birds that prefer young aspen forest such as ruffed grouse will increase in population.

According to the National Office of Fire and Aviation, more than 57,000 acres of land in the United States burned as a result of prescribed fires in 2000. Oregon and Idaho had the most, with an estimated 12,400 and 10,300, respectively.

Forest fires generally only consume 10 to 20 percent of the wood in tree trunks, leaving many standing dead trunks referred to as snags. Snags are good habitat for woodpeckers that seek insects living within the dead wood and cavity-nesting birds that use the cavities excavated by the woodpeckers. Deer and elk also prefer young post-fire forests, whereas the pine marten prefers mature forests.

If a major forest fire were to burn an entire forest, for example, an entire national park or wildlife refuge, then all of the habitat after the fire would be young forest, and those species that lived in mature forests could be excluded from the park. Conversely, if there were never any fires, those species of wildlife that require young, regenerating forests would be excluded.

An ideal solution to this problem is to have relatively small fires occur on a regular basis so that a mix of young, middle-aged, and mature habitat is always present to accommodate all species of wildlife that could live in the area. This concept is known as landscape diversity. The distribution and size of fires on the landscape over time is, together with human disturbance such as logging, the most important factor in determining landscape diversity and the consequent ability of the landscape to provide for a variety of wildlife.

see also Adaptation; Forest, Temperate; Grassland

Lee E. Frelich and Peter B. Reich

Bibliography

Johnson, Edward A. Fire and Vegetation Dynamics: Studies from the North American Boreal Forest. Cambridge, UK: Cambridge University Press, 1992.

Pyne, Stephen, J. Fire in America: A Cultural History of Wildland Fire and Rural Fire. Princeton, NJ: Princeton University Press, 1982.

Whelan, Robert J. The Ecology of Fire. Cambridge, UK: Cambridge University Press, 1995.