Wildfire
Wildfire
Wildfire is a periodic ecological disturbance, associated with the rapid combustion of much of the bio-mass of an ecosystem. Once ignited by lightning or by humans, the biomass oxidizes as an uncontrolled blaze, until the fire either runs out of fuel or is quenched. Wildfire is best known as a force affecting forests, although savanna, chaparral, prairie, and tundra also burn. A large wildfire can kill mature trees over an extensive area, after which a process of ecological recovery ensures, called secondary succession. Fire can be an important factor affecting the nature of ecological communities. In the absence of wildfire or other catastrophic disturbances, relatively stable, climax communities tend to develop on the landscape, the nature of which is determined by climate, soil, and the participating biota. However, intervening wildfires can arrest this process, so that the climax or other late-successional communities are not reached.
The nature of wildfire
Wildfire is especially frequent in ecosystems that experience seasonal drought, for example, boreal forests, temperate pine forests, tall-grass prairie, chaparral, and savanna. Wildfires can be very extensive, and
in aggregate they affect tremendous areas of landscape each year. For example, an average of about 8 million acres (3 million ha) of forest burns each year in Canada, and in some years it exceeds 25 million acres (10 million ha). Most of those fires are started by humans, although most of the actual burned area is ignited naturally by lightning. The natural fires predominantly affect northern, non-commercial forests, where most fires are not actively quenched by humans, so that individual burns can exceed 2.5 million acres (one million ha) in area. However, even vigorously fought fires can be enormous, as was the case of the famous Yellowstone fires of 1988, which burned more than 1.2 million acres (0.5 million ha), including 45% of Yellowstone National Park. Even moist tropical rainforest will occasionally burn, as happened over more than 7.4 million acres (3.0 million ha) of Borneo during relatively dry conditions in 1982-1983.
Fire causes a number of changes in soil quality. Depending on the intensity of the burn, much of the organic matter and litter of the forest floor may be consumed, and mineral soil may be exposed. The combustion of organic matter results in a large emission of carbon dioxide to the atmosphere, along with gaseous oxides of nitrogen derived from the oxidation of organic nitrogen, and sooty particulates. The layer of ash that deposits onto the soil surface is of a basic quality, so soil acidity is temporarily decreased after a fire. The ash also supplies large quantities of certain nutrients, especially calcium, magnesium, potassium, and phosphorus, some of which leaches from the site. Often, post-fire soils are relatively fertile for several years as a net effect of these physical and chemical changes, and plant growth can be rather lush until the intensity of competition increases when the canopy closes again.
Post-fire succession
When an ecosystem is disrupted by a wildfire, it can quickly suffer an intensive mortality of its dominant species, along with disruptions of its physical ecological structure and other damages. However, except in the case of rare, extremely intense fires, some plants survive the disturbance, and these can contribute to the post-fire regeneration that immediately begins.
Plant species vary greatly in the strategies they have evolved to survive wildfire, and to regenerate afterwards. Often, the below-ground tissues of certain plants can survive the fire even though the above-ground biomass was killed by combustion or scorching, and the regeneration may then occur through stump or root-sprouting. In North America, trembling aspen (Populus tremuloides), other woody plants, and many understorey herbs commonly display this sort of survival and regeneration strategy.
Other plants may survive the fire as long-lived seeds that are buried in the forest floor, and are stimulated to germinate by post-fire environmental conditions. Species such as pin cherry (Prunus pensylvanica) and red raspberry (Rubus strigosus) can regenerate vigorously from this so-called buried seedbank. A few conifers maintain their seedbank in persistent, aerial cones, which are stimulated to open by the heat of the burn, so that seeds are released to the fire-prepared seedbed immediately afterwards. These fire-adapted tree species often form even-aged stands after fire, as is the case of knobcone pine (Pinus attenuata) in the southwestern United States, and jack pine (Pinus banksiana) farther to the north.
In other cases, species may invade the burned site, by dispersing from unburned communities nearby. Species with light, windblown seeds are especially efficient at colonizing burned sites, as is the case for white birch (Betula papyrifera), fireweed (Epilobium angustifolium), and various species in the aster family, such as goldenrod (e.g., Solidago rugosa ).
The post-fire ecological recovery that is manifest in the regenerating vegetation is a type of secondary succession. In the absence of another wildfire, or some other catastrophic disturbance of the stand, the post-fire secondary succession often restores an ecosystem similar to the one present prior to the fire.
If the return frequency of natural wildfire is shorter than the time required for a climax ecosystem to develop, then disturbance by fire may be important in maintaining the land in an earlier successional stage. For example, most of the region of North America that supported a tall-grass prairie was climatically suitable for the development of an oak (Quercus spp. ) dominated forest. It was only the periodic burning of the prairie that prevented the encroachment of shrubs and trees, and maintained the natural prairie. Today, tall-grass prairie is an endangered ecosystem, because most of its original area has been converted to agricultural purposes. To maintain the ecological integrity of the few, small remnants of tall-grass prairie that remain in protected areas, these areas must be deliberately burned to prevent them from successionally turning into forest.
Even stands of the giant sequoia (Sequoiadendron giganteum ) appear to require periodic ground fires of a particular intensity if that community type is to be sustained over the longer term. Fire helps to maintain stands of this species, by reducing fuel loads and thereby preventing catastrophic crown fires that could kill mature trees, and by optimizing recruitment of sequoia seedlings.
Management of fires
Sometimes, to achieve particular ecological objectives, fire may be used as a tool in ecosystem management. The use of prescribed burns to maintain prairie was previously described, but similar practices have also been used to manage other ecological communities, and even some species. For example, prescribed burning is an essential component of the management strategy used to maintain an appropriate habitat of jack pine required by Kirtland’s warbler (Dendroica kirtlandii ), an endangered bird that only nests in northern Michigan. Prescribed burning is also used in forestry in some regions, to reduce the quantities of slash left after logging operations, to prepare a suitable seedbed for particular species of trees, or to
KEY TERMS
Secondary succession —A succession that follows any disturbance that is not so intense as to eliminate the regenerative capabilities of the biota. Secondary succession occurs on soils that have been modified biologically, and on sites where plants survived the disturbance. In contrast, primary succession occurs on a bare substrate that has not previously been influenced by organisms.
Succession —A process of ecological change, involving the progressive replacement of earlier communities with others over time, and generally beginning with the disturbance of a previous type of ecosystem.
prevent large build-ups of fuel that could lead to a more catastrophic wildfire.
To protect stands of timber that are important commercially or for other reasons, many agencies actively engage in fire protection activities. Fire protection is usually achieved by attempting to prevent humans from starting uncontrolled blazes, by using prescribed burns to prevent dangerous accumulations of large quantities of fuel, and by quenching fires that are accidentally or deliberately ignited. However, fire is a natural, ecological force, and even the greatest efforts of humans are not always capable of preventing or quenching large fires. This fact is occasionally brought to our attention when uncontrollable conflagrations destroy homes, commercial timber, or forest in protected areas such as parks.
See also Disturbance, ecological.
Resources
BOOKS
Gurevitch, Jessica, et. al. The Ecology of Plants. Sunderland, MA: Sinauer Associates, 2002.
PERIODICALS
Christensen, N.L., et al. “Interpreting the Yellowstone Fires of 1988.” BioScience, 39 (1989): 678-685.
OTHER
Johnston, Nicholas. “California Firefighters Focus on Two Worst Blazes.” October 30, 2003. Sandiegosource. <http://www.sddt.com/News/article.cfm?SourceCode=20031030fat> (accessed on December 3, 2006).
Bill Freedman