Wildfire

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Wildfire Summary

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Wildfire

Few natural forces match fire for its range of impact on the human consciousness, with a roaring forest fire at one extreme and a warming and comforting campfire or cooking flame at the other. Along with earth, water, and air, fire is one of the original "elements" once thought to comprise the universe, and it has frightened and fascinated people long before the beginning of modern civilization. In nature, fire both destroys and renews.

Fire is an oxidation process that rapidly transforms the potential energy stored in chemical bonds of organic compounds into the kinetic energy forms of heat and light. Like the much slower oxidation process of decomposition, fire destroys organic matter, creating a myriad of gases and ions, and liberating much of the carbon and hydrogen as carbon dioxide and water. A large portion of the remaining organic matter is converted to ash which may go up in the smoke, blow or wash away after the fire, or, like the humus created by decomposition, be incorporated into the soil.

Although fire often is considered bad and was once thought to destroy natural ecosystems, modern scientists have recognized the importance of fire in ecological succession and in sustaining certain types of plant communities. Fires can help maintain seral successional stages, prolonging the time for the community to reach climax stage. Some ecosystems depend on recurring fire for their sustainability; these include many prairies, the chaparral of mediterranean climatic regions, pine savannas of the Southeastern United States, and long-needle pine forests of the American West. Fire controls competing vegetation, such as brush in grasslands, prepares new seed beds, and kills harmful insects and disease organisms. Nonetheless, diseases sometimes increase after fire because of increased susceptibility of partially burned, weakened trees.

The effects of fire on an ecosystem are highly variable, and they depend on the nature of the ecosystem, the fire and its fuel, and weather conditions. In climatic regions where natural fires occur seasonally, grasslands usually are the first ecosystems to burn because the fuel they supply has a large surface area to volume ratio, which allows it to dry rapidly and ignite easily. Grassland fires tend to burn quickly, but they release little energy compared to fires with heavier fuel types. As a result, the effect on soil properties normally is minor and short-lived. The intense greening of a grassland as it recovers from a burn is due largely to the flush of nutrients released from mature and dead plants and made available to new growth. Grassland fires have been set intentionally for many generations in the name of forage improvement.

Naturally caused brushland fires, including the infamous chaparral fires of the southwestern United States, usually start somewhat later in the season than the first grass fires, and they normally have more intense, longer lasting impacts. These fires burn very rapidly but with far more thermal output than grassland fires, because fuel loading is five to fifty times greater.

The season for forest fires normally begins somewhat later than that for grass or brush fires. The fuel in a mature forest, which may be 100 times greater than that of a medium density brushland, requires more time to dry and become available for combustion. When the forest burns, the ground may or may not be intensely heated, depending on the arrangement of fuels from the ground to the forest canopy. Under a hot burn with the heavy ground fuel found in some forests, heat can penetrate mineral soil to a depth of 12 in (30 cm) or more, significantly altering the physical, chemical, and biological properties of the soil. When the soil is heated, water is driven out; soil structure, which is the small aggregations of sand, silt, clay, and organic matter, may be destroyed, leaving a massive soil condition to a depth of several inches.

Forest and brushland soils often become hydrophobic, or water repellent. A hydrophobic layer a few inches thick commonly develops just below the burned surface. This condition is created when the fire's heat turns organic matter into gas and drives it deeper into the soil, where it then condenses on cooler particle surfaces. Under severe conditions, water simply beads and runs off this layer, like water applied to a freshly waxed car. Soil above the hydrophobic layer is highly susceptible to sheet and rill erosion during the first rains after a fire. Fortunately, it is soon broken up by insects and burrowing animals, which have survived the fire by going underground; they penetrate the layer, allowing water to soak through it.

Forest fires decrease soil acidity, often causing pH to increase by three units (e.g., from 5 to 8) before and after the fire. Normally, conditions return to prefire levels in less than a decade. Fires also transform soil nutrients, most notably converting nutritive nitrogen into gaseous forms that go up in the smoke. Some of the first plants to recolonize a hotly burned area are those whose roots support specialized bacteria that replenish the nutritive nitrogen through a process called nitrogen fixation. Large amounts of other nutrients, including phosphorus, potassium, and calcium, remain on the site, contributing to the so-called "ashbed effect." Plants that colonize these fertile ashbeds tend to be more vigorous than those growing outside of them. When heating has been prolonged and intense in areas such as those under burning logs, stumps, or debris piles, soil color can change from brown to reddish. Fires hot enough to cause these color changes are hot enough to sterilize the soil, prolonging the time to recovery.

In the absence of heavy ground fuels, so much of the energy of an intense forest fire may be released directly to the atmosphere that soils will be only moderately affected. This was the case in the great fires at Yellowstone National Park in 1988, after which soil scientists mapped the entire burn area as low or medium intensity with respect to soil effects. Although soils on some sites did suffer intense heating, these were too small and localized to be mapped or to be of substantial ecological consequence, and most of the areas recovered quickly after the fires.

Although fire is vital to the long-term health and sustainability of many ecosystems, wildfires take numerous human lives and destroy millions of dollars of property each year. Controlling these destructive fires means fighting them aggressively. Fire suppression efforts are based on the fact that any fire requires three things: heat, fuel, and oxygen. Together, these make up the three legs of the so–called "fire triangle" known to all fire fighters. The strategy in all fire fighting is to extinguish the blaze by breaking one of the legs of this triangle. An entire science has developed around fire behavior and the effects of changing weather, topography, and fuels on that behavior.

Competing conceptions of the costs and benefits of wildfires have led to conflicting fire management and suppression objectives, most notably in the national parks and national forests. The debate over which fires should be allowed to burn and which should be suppressed undoubtedly will continue for some time.

A stand of trees bursts into flames in Grand Teton National Forest in 1988. (Corbis-Bettmann. Reproduced by permission.)