Much of the information in this section comes from the
Interagency Fire Education Initiative. For more information, contact:
Ecological Communications Lab
School of Natural Resources
The Ohio State University
2021 Coffey Road
Columbus Ohio 43210
Telephone: (614) 292-9828
Fax: (614) 292-7432
People usually do not perceive a forest that has just been burned as beautiful. But fire is a part of the natural world, just like the wind, the rain, and other natural forces. Wildfires are an essential feature of ecosystems; both plants and animals are well adapted to fires and benefit from fire. Fire is an agent of change performing a variety of functions and producing a range of effects.
Fire ecology is a branch of ecology that concentrates on the origins, cycles, and future stages of wildland fire. It probes the relationship of fire with living organisms and their environment. Three concepts provide the basis for fire ecology
1) Fire Dependence: This concept applies to species of plants that rely on the effects of fire to make the environment more hospitable for their regeneration and growth.
2) Fire History: This concept describes how often fires occur in a geographical area. Fire scars, or a layer of charcoal remaining on a living tree as it adds a layer of cells annually, provide a record that can be used to determine when in history a fire occurred.
3) Fire Regime: Fire regime is a generalized way of integrating various fire characteristics, such as the fire intensity, severity, frequency, and vegetative community.
4) Fire Adaptation: This concept applies to species of plants that have evolved with special traits contributing to successful abilities to survive fires at various stages in their life cycles. For example, serotinous cones, fire resistant bark, fire resistant foliage, or rapid growth and development enable various kinds of plants to survive and thrive in a fire prone environment.
One major effect of fire is a change in soil nutrients and soil temperature. Fire may be a chief factor maintaining productivity in colder soils where the lack of nutrients is a major factor limiting plant growth. Fires release nitrogen and other nutrients from woody vegetation back into the soil in the form of mineral-rich ash, which makes them readily available for new plant growth.
Plant regeneration begins almost immediately following a fire. At any given location, vegetation develops over time in orderly stages called succession. Each successive stage is determined by climate, soil conditions, available sunlight, and natural disturbances such as wildland fire. For example, throughout Alaska’s boreal forest fast-growing herbs and grasses, such as fireweed, cottongrass, and bluejoint grass are the first plants to recover a burned area. Within a few years, shrubs and saplings, such as blueberry, aspen, and willow also flourish on the site. These may dominate the burn area for 10 to 30 years, but eventually, the birch, willow, and aspen grow into young trees and many shrubs are shaded out and replaced with an understory of shade tolerant spruce. This successional stage may last for many decades, but 70 to 100 years following a fire the birch and aspen are slowly replaced by spruce trees. The spruce forest is the final, or climax stage of the boreal forest. The forest floor is now covered with moss layer and berry-producing plants. Accumulated litter and thick tree growth in the understory make these forests susceptible to fire, starting the cycle of disturbance and recovery over again.
Although fire may destroy individual trees and understory plants, the species themselves are well adapted to survive. In many cases, this is accomplished through a high regeneration capacity. For example, black spruce is not fire resistant and may be killed by a fire because of its thin bark. However, the serotinous cones that remain closed while on the tree they are opened by the heat of a fire. After a fire burns through a black spruce stand, a multitude of seeds are dispersed from the open cones to the forest floor. In addition, because this species usually grows in wet low-land areas the chance of complete destruction is much reduced. Birch and aspen produce thousands of wind-borne seeds in neighboring unburned stands and are the usual colonizers after a fire. White spruce, and upland species, is very susceptible to destruction by fire but regenerates from seed blown in. Balsam poplar which typically grows along rivers and streams is the most fire resistant because bark near the ground on mature trees may be four inches thick. The respouting of shrubs and grasses from underground stems, stumps, or roots is another fire adaptation. Species like aspen, willow, and fireweed commonly regrow in this manner.
The effects of fire on habitat- an animal’s surroundings or home- are generally more significant than the effects on animals themselves. Forests of different ages support different kinds of wildlife. Different types of birds and mammals seeking food and shelter are attracted to different types of forest types. All of these animals need a variety of resources to provide shelter, food, water, and space. These resources are often found on the borders of two or more plant communities, such as meadow/black spruce or birch/shrubland. These “edges” are created by fires and other disturbances and are beneficial to maintaining a healthy wildlife habitat. Small fires that occur in an area create more “edges” than one large burn. In fact a majority of species generally do best in forests that provide a combination of habitats.
Many wildlife species thrive on the occurrence of fire. The grasses, seedling shrub, and trees that reestablish burned areas provide an ideal environment for many small seed-eating mammals and birds, such as voles and sparrows. This abundance of small prey attracts predators like foxes, hawks, and weasels. Burned trees provide sites for cavity nesting birds like flickers, kestrels, and chickadees, while woodpeckers thrive on the insects that inhabit fire-killed trees.
As the natural habitat changes from low-growing plants and shrubs to small trees and larger shrubs, the forest’s wildlife community changes accordingly. The tall shrubs and tree saplings which are found in an area 5-30 years after a fire provide excellent shelter and forage for a great variety of wildlife. New nesting sites abound for more bird species. Abundant grasses and shrubs create perfect cover for ground-nesting birds and ever increasing small mammals. More predators follow to prey on the small animals. Berries can also become abundant after a burn, particularly in upland areas, providing an excellent food source for wildlife species.
By the time that the forest has “matured” into birch, aspen, and spruce stands, many of the grasses, saplings, and smaller shrubs have been shaded out. Animals that relied on these plants have moved elsewhere. Some new animals can be found, but in the later stages, as the hardwoods are replaced by spruce trees, these old “mature” coniferous forests support fewer number and species of wildlife and are generally less productive.
When fires naturally, a vegetation mosaic of different forest types is created. This provides a greater diversity of vegetation and consequently a greater diversity of wildlife species.Although the common conception is that fire is a destroyer of the natural environment, the opposite is actually true, where a carefully planned prescribed burning program can be beneficial and even enhance the health of an ecosystem. Prescribed fires can reduce the amount of combustible fuel buildup that can cause larger more destructive fires. Other benefits of prescribed fire include: insect pest control, removal of undesirable plants competing for nutrients, addition of nutrients from ash, and removal of sunlight inhibiting brushy undergrowth. However, incorrectly managed prescribed fires can have very adverse effects causing excessive soil heating, loss of nutrients, and removal of woody debris needed to protect seedlings. Wildfires are suppressed in developed and high-fuel areas where intense fire could destroy a plant community or human built structures. Modern fire policy permits the burning of some natural fires and recognizes the use of prescribed fire as a management tool.
There are six different vegetative communities that are adapted to fire in the U.S.
1) Tallgrass Prarie: Midwest
The tallgrass prairies cover parts of Nebraska, Illinois, Iowa, and Kansas, and extend into the more eastern states in the Midwest. Tallgrass prairie is made up of grasses, forbs, shrubs, and trees, and is further characterized by relatively moist soils. Prairies depend on fire to maintain the ecosystem stability and diversity. One benefit of fire in this community is the elimination of invasive plants, and because grass provides a low quantity of fuel, grassland fires usually are easily controlled and suppressed if necessary. Timing is also of crucial importance as plant recovery following a prairie fire is fastest in the sprig and fall when moisture is high and plants are not producing seeds. If fire were excluded, the tallgrass prairie would vanish and shrubs, trees, and exotic grasses would dominate the ecosystem. Before European settlement of the grassland, naturally occurring fire helped maintain the grasslands. Today many of the prairies that remain are managed by prescribed burns.
2) Chaparral: California and Southwest
Chaparral is a general term that applies to various types of brushland found in southern California and the southwestern U.S. This community contains the most flammable type of vegetation found in the U.S., as well as many species well adapted to fire and some even promote fire. One chaparral plant, Ceanothus, has leaves that are coated with flammable resins, seeds that require intense heat for germination, and roots that are specially adapted to enable the plant to grow in areas that were recently burned. The leaves of other chaparral plants that contain flammable oils and resins in their leaves also remain small in size, adding to their flammability. These plants sprout quickly, grow, and spread rapidly. In addition, their heat-resistant seeds are able to remain dormant yet viable in ground litter, and contribute to the ability of chaparral to recover quickly following a fire. Furthermore, burning releases many of the nutrient that are locked up in the chaparral, and these nutrients are recycled back into the soil. With age, chaparral plants become less productive but are not overtaken by invading species. Fire in this type of community serves to replace older plants with younger, more productive ones of the same species rather than to eliminate exotic species and replace them with native ones, as is the case in the tallgrass prairies.
3) Ponderosa Pine: Interior West
Ponderosa can co-exist in a mixed forest, particularly in combination with Douglas-fir, or as a pure forest type. Residing among a mixture of grass, forbs, and shrubs, it generally receives less than 25 inches or less of rain a year. After 5 years of competing with fire prone grass, the ponderosa begins to grow a thick bark, deep roots, and lose their lower limbs to become less susceptible to fire and decrease the possibility of a fire climbing to the crown. Conifers, including the poderosa pine, are most flammable in the spring when their old needles are dry and new needles have not yet grown. In the fall, when the needles have dried out, conifers again are susceptible to fire. Fire in ponderosa pine forests, as in Chaparral communities, serves to replace older plants with younger ones of the same species. Historically, fires in ponderosa pine communities burned naturally on a cycle of one every 5 to 25 years.
4) Douglas-Fir: Pacific Northwest
Found in areas of the northwest including Oregon, Washington, and British Columbia, the Douglas Fir favors being in a mixed forest with climates that provide over 50 inches of rain. Douglas-fir regenerate readily on sites that are prepared by fire. In fact, nearly all the natural stands of Douglas-fir in the U.S. originated following fire. One of the main benefits of fire in these forest communities is the removal of fuel and consequent reduction of the chance of severe crown fires.
5) Loblolly and Shortleaf Pine: The Southeast
After several years of initial growth, loblolly and shortleaf pine are able to withstand surface fires. However, they are not as highly resistant to fire as many other tree species. The major benefits of fire in this ecosystem include the creation of a favorable environment for seedlings and hindrance of an invasion by competing species.
6) Jack Pine: Great Lake States
Jack pine is found amongst a variety of trees, brush, forbs, and grass. Jack pine has a unique relationship with fire. Unlike many other tree species, jack pine does not drop all of its seeds as they ripen. The majority of the seeds remain in closed cones that stay on the branches for many years. When a fire occurs, the thick cone protects the jack pine seed from the intense heat. That heat, though, opens the scales of the cone and releases the seed onto the ground where fire has removed much of the existing vegetation, preparing the site for the new seedlings. Fire then serves to prepare a seedbed, reduce competition from other plants and release the jack pine seed. Prescribed fire also is used to reduce fuel levels and prepare sites for seeding. The timing of the burning is tied to the life cycle of the pines; fall is the season of choice.