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Wildfires

WILDFIRES

WILDFIRES have always shaped the landscape. Many are not caused by humans, but by lightning, which is a major cause of wildfires, particularly in the West. In the twentieth century, governments at all levels tried to suppress wildfires. In the second half of the century, the environmental movement introduced the notion that wildfires were ecologically beneficial, and in 1972 the National Park Service adopted, experimentally, a policy of letting some wildfires burn; in 1976 the policy was adopted generally and became known as the "let-burn" policy. In 1978 the U.S. Forest Service adopted the same policy. Most such fires, designated "prescribed fires," burned less than 100 acres. In years of drought, however, there were major problems. In 1977, 175,000 acres of California wilderness burned in the Marble Cone fire. In 1979 wildfires in California, Oregon, Idaho, Montana, and Wyoming burned more than 600,000 acres. In 1981 a swamp fire in Florida consumed 300,000 acres. Three years later 250,000 acres of Montana forest and range burned, and in 1987, 200,000–300,000 acres in Oregon burned.

The worst wildfire season since World War II came in 1988; more than 6 million acres burned, 2.2 million of them in Alaska. Fires around Yellowstone National Park were shown on television and had a major public effect. As drought in the West persisted, wildfires continued to pose major risks. Yosemite National Park closed temporarily in 1990 because of a nearby wildfire. In 1991 a wildfire raced through a residential section of Oakland, Calif., killing at least twenty-four people. In 1993 a spectacular wildfire in the brushland north and south of Los Angeles burned the houses of many celebrities and caused nearly $1 billion in damage. In 1994 more than fourteen fire-fighters were killed fighting wildfires in Colorado and elsewhere in the West; acreage burned again exceeded one million acres. The extension of suburban development into wilderness areas in the 1990s made the fire risk to property and human life even more acute, ensuring that debates over fire management would continue to preoccupy both homeowners and policy leaders into the next century.

BIBLIOGRAPHY

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

Nancy M.Gordon/l. t.; a. r.

See alsoConservation ; Disasters ; Fire Fighting ; Forest Service, United States ; Interior, Department of the ; Western Lands .

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wildfire

wild·fire / ˈwīldˌfīr/ • n. 1. a large, destructive forest- or brush-fire that spreads quickly. 2. hist. a combustible liquid such as Greek fire that was readily ignited and difficult to extinguish, used esp. in warfare. 3. less common term for will-o'-the-wisp. PHRASES: spread like wildfire spread with great speed: the news had spread like wildfire.

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wildfire

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Wildfire

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 socalled "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.

See also Biomass; Biomass fuel; Deforestation; Ecological productivity; Forest management; Forest Service; Old-growth forest; Soil survey

[Ronald D. Taskey ]


RESOURCES

BOOKS

Barbour, M. G., J. H. Burk, and W. D. Pitts. Terrestrial Plant Ecology. Menlo Park, CA: Benjamin/Cummings, 1980.

OTHER

Greater Yellowstone Pos-Fire Resource Assessment Committee, Burned Area Survey Team. Preliminary Burned Area Survey of Yellowstone National Park and Adjoining National Forests. 1988.

Lotan, J. E., et al. Effects of Fire on Flora. Forest Service General Technical Report WO16. Washington, DC: U. S. Government Printing Office, 1981.

National Wildfire Coordinating Group. Firefighters Guide. NFES 1571/PMS 414-1. Boise: Boise Interagency Fire Center, 1986.

Wells, C. G., et al. Effects of Fire on Soil. Forest Service General Technical Report WO7. Washington, DC: U. S. Government Printing Office, 1979.

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Wildfire

Wildfire

The nature of wildfire

Post-fire succession

Management of fires

Resources

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 Kirtlands 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

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Wildfire

Wildfire

Wildfires in the West
Dangerous science: How wildfires happen
Consequences of wildfires
A possible connection between wildfires and global warming
Fighting fires: The technology connection
A matter of survival
For More Information

On August 5, 1949, fifteen young men parachuted into the upper part of Mann Gulch in the wilderness of Montana. Mann Gulch drains westward into the Missouri River about 20 miles (30 kilometers) north of Helena, Montana. The area still remains a rugged, roadless wilderness. The young men were "smokejumpers," firefighters trained to parachute into the area of a wildfire, a large, uncontrolled fire in grass, brush, or trees. At first, this wildfire seemed easy to control and they expected to have it put out by the next day. One of the survivors is quoted in the official report:

I took a look at the fire and decided it wasn't bad. It was burning on top the ridge and I thought it would continue on up the ridge. I thought it probably wouldn't burn much more that night because it was the end of the burning period (for that day) and it looked like it would have to burn down across a little saddle before it went uphill any more.

The smokejumpers started down the canyon, planning to work the flank of the fire. They were joined by a forest ranger from the nearby Meriwether campground, who had been working alone to try to contain the fire.

As the crew worked their way down the canyon, things went terribly wrong. The fire had burned down the slope in front of them and also jumped to the other side of the gulch. The crew encountered a strong wind blowing up the canyon and bringing the fire in their direction. They turned and tried to run for safety up the ridge to the north of the gulch, dropping their gear as they ran.

Only three of the original crew survived. The crew chief stopped and deliberately started a fire in the dry grass and brush. After this fire had spread over a wider area, he stepped through the flames into the middle of the burned area and lay down. Such a fire is now known as an escape fire, but at the time, none of the other crew members trusted this tactic and continued to run up the slope. Two crew members ran straight up the hill and managed to squeeze through a gap in the rocks on top of the ridge.

They lay down in the middle of a bare rock slope on the other side of the ridge. The other thirteen perished in the fire.

The Mann Gulch disaster remained the worst fire tragedy to strike the United States Forest Service until 1994, when an inferno on Storm King Mountain near Glenwood Springs, Colorado, killed fourteen firefighters, three of them smokejumpers. A small, lightning-caused fire had blown up into an intense firestorm, the most violent type of wildfire. In Mann Gulch and on Storm King Mountain, young men and women lost their lives fighting dangerous, unpredictable wildfires.

WORDS TO KNOW

backfire:
a small fire set by firefighters in the path of an oncoming wildfire to burn up the fuel before the main fire arrives, thus blocking it.
crown fire:
a fire that spreads through the tree-tops, or crown, of a forest.
fire line:
a strip of ground, cleared of all combustible material, that is dug by firefighters to stop the advance of a wildfire. Also called control line.
fire triangle:
the combination of three elements required for any fire: fuel, oxygen, and heat.
firestorm:
also called a blowup, it is the most explosive and violent type of wildfire.
ground fire:
a fire that burns beneath the layer of dead plant material on the forest floor.
hotshot:
a specialized firefighter who ventures into hazardous areas and spends long hours battling blazes.
oxidation:
a chemical reaction involving the combination of a material with oxygen.
prescribed burn:
a planned, controlled fire that clears flammable debris from the forest floor.
Pulaski:
a combination ax and hoe that is used by firefighters to clear brush and create a fire line. It was invented by forest ranger Edward Pulaski in 1903.
regeneration:
the process of making or starting anew.
smokejumper:
a specialized firefighter who parachutes to strategic locations from airplanes to battle wildfires.
spotting:
the starting of new fires, called spot fires, by sparks and embers that drift ahead of an advancing wildfire.
surface fire:
a fire with a visible flame that consumes plant material and debris on the forest floor.
troposphere:
the lowest atmospheric layer, where clouds exist and virtually all weather occurs.
wildfire:
a large, uncontrolled fire in grass, brush, or trees.

A wildfire may begin with just a spark from a campfire or a lightning strike. If it occurs in a forested area, the fire can spread to nearby logs or bark, then climb up the branches of small, dry neighboring trees. From

there it can jump to taller trees and spread among the treetops. As long as fuel is available, a wildfire will continue to burn.

In the United States, more than three hundred wildfires occur every day. People are responsible for accidentally starting nine out of ten wildfires with campfires, burning leaf piles, cigarettes, or other sources. The most common natural cause of wildfires is lightning.

The regions most affected by wildfires are wilderness areas. As homes, resorts, and suburbs encroach further and further into wildlands, however, the danger to human life and property due to wildfire increases.

Notes of a Montana resident during fire season

The following report from wildfire-weary Montana was written by Mary Stange, a professor at Skidmore College and overseer of the Crazy Woman Bison Ranch. The account, titled "Tinder Dry: The Old West," was published in USA Today on August 24, 2000.

"We had dry lightning again last night. It's a phenomenon that occurs in various places, but nowhere so dramatically as in Big Sky country during fire season.

"Turgid storm clouds crowd the evening sky. Rumbling ominously, they wait until dark to unleash volleys of blue-white energy. Then the rainless lightning comes, fast and ferocious, in several forms: Some strikes streak across the sky from one cloud to another; some explode within the clouds, suggesting enormous Chinese lanterns; and some, too many, slice jagged paths down from sky to ground. The snapping sound these vertical strikes make has roughly the same effect on the nerves as the buzz of a rattlesnake. These are the strikes that spawn wildfires.

"We ranch in southeastern Montana. It's not the part of the state that the river runs through, or where the nationally reported blazes currently are raging. Those fires, along the Bitterroot Valley and elsewhere in the Rocky Mountain end of this vast state, are hundreds of miles away from here.

Nevertheless, we've been smelling their smoke for weeks now, and seeing it in the form of a yellow-brown haze. This haze has an interesting side effect: spectacular sunrises and sunsets. In ranch country, every cloud seems to have an ironic lining.

We've had our share of fires in this part of the state, too; we always do this time of year. But this has been an especially rough summer, with unremitting high temperatures and tinder-dry vegetation. Our neighbors had a lightning-caused grass fire this past week; fortunately, they got on it quickly before it could take out more than a piece of one pasture. They were lucky. Thousands of national-forest acres burned earlier this month near Ashland, about 60 miles west of us."

Wildfires in the West

A combination of chronic drought, record-breaking high temperatures, strong winds, and the firefighting policies of the U.S. Forest Service are widely considered to be the primary cause of a series of devastating wildfire seasons in the western United States. In addition to a greater number of fires, more people and property have been placed at risk

because of the recent increase in construction of homes, resorts, and other buildings in and near wilderness areas.

As the population increases and urban areas in the west expand into canyons and up the slopes of foothills, homes and property are more likely to be endangered by wild land fires. In these circumstances, firefighters are forced to divert their energies from the main task of fighting forest fires in order to protect property. People also make unwise choices in construction techniques, such as building their homes right among the trees. In the humid and well-watered East, this does not pose much risk, but in the relatively arid West, homes should be built well away from forest and brush.

The western United States burns in 2000

In the summer of 2000, the western United States experienced one of its worst fire seasons. Hundreds of fires charred a total of 6.7 million acres (2.7 million hectares) in twelve states throughout the West. Most of the area that burned was federal forestland. At least eight firefighters and two civilians died in the blazes. Hundreds of

homes were destroyed and thousands of residents had to be evacuated. Nationwide (including Alaska), over 8.4 million acres were burned in 2000.

The major fires of 2000

The 2000 fire season began in May, when a fire swept through the grounds of the federal nuclear laboratory at Los Alamos, New Mexico. The wildfire began as a prescribed burn, deliberately set by Forest Service employees to clear the forest of flammable undergrowth, but was swept out of control by high winds. The fire burned 47,000 acres (18,800 hectares) and two hundred homes before being extinguished.

The worst fires of the summer of 2000 occurred in Montana, where close to 900,000 acres (360,000 hectares) burned, and the blazes were battled by an army of eleven thousand firefighters. Montana's giant fires were concentrated in its southwest portion. The scenic Bitterroot Valley was hardest hit.

In Idaho, more than 200,000 acres (80,000 hectares) burned in the Salmon-Challis National Forest alone. In South Dakota, 64,640 acres (25,860 hectares) burned in the Black Hills National Forest, making it the largest fire in the forest's recorded history. Colorado experienced five major fires. Two of them were in Mesa Verde National Park, forcing the park's closure at times. A third fire took place southwest of Denver, and a fourth burned east of Rocky Mountain National Park. Each charred more than 10,000 acres (4,000 hectares).

One of the last major fires of the season, and the fifth major fire for Colorado that year, consumed 1,087 acres (435 hectares) near Boulder. The fire burned for one week in mid-September. It was believed to have been started by a campfire, despite the fact that any type of open fire was prohibited throughout much of the parched state. A combination of temperatures above 90°F (32°C), low humidity, and shifting winds caused the blaze to spread rapidly through the dry grass and pine needles. The smoke from the fire blanketed Boulder and its surrounding canyons and foothills. Firefighters dug fire lines (areas cleared of combustible material) on the mountainsides and spread fire retardant on the flames to keep the fire out of Boulder. A mountain neighborhood of about two hundred people on the outskirts of town was evacuated as a precaution, but the fire was held in check by firefighters and the evacuees' homes were spared.

Fighting the fires of 2000

Thousands of volunteers, military personnel, and others assisted professional firefighters in battling the blazes during the peak of the summer fire season. Firefighters came from Canada, Mexico, and as far away as New Zealand and Australia. In all, more than twenty-five thousand people were involved in battling the blazes. They used more than 230 helicopters and airplanes to dump hundreds of thousands of gallons of water and fire retardant on the flames. The cost of battling the fires was about $18 million per day; the summer's firefighting expenses were over $1.3 billion.

Military personnel were heavily involved in the firefighting effort throughout the summer. Approximately five hundred soldiers from Fort Hood in Texas were sent to fight a fire in Payette National Forest in Idaho. Another 712 members of the Army National Guard and 213 members of the Air National Guard were spread across ten western states fighting fires. In Southern California, Army National Guardsmen used helicopters to douse flames with suspended buckets that each held as much as 1,300 gallons (5,000 liters) of water. In Montana, more than four hundred National Guard members were called upon to perform mop-up operations after fires had been reduced to embers.

Wildfires continue to wreak havoc

Prior to 2006, the worst fire years on record were 1963 (7.1 million acres or 2.9 million hectares), 1988 (7.4 million acres or 3.0 million hectares), 2000 (8.4 million acres or 3.4 million hectare), and 2005 (8.7 million acres or 3.5 million hectares). However, in late 2006, a new record was set. Over 9.8 million acres (4.0 million hectares) were burned by at least 96,000 separate fires.

The unusual fire seasons have been blamed on a combination of extreme drought, record high temperatures, strong winds, and the total fire suppression policy of the U.S. Forest Service. From 1910 through the 1970s, the agency attempted to extinguish every forest fire, regardless of location or cause. This policy helped to create an overabundance of fallen dead timber and other dead and decaying material on the forest floor. According to critics, if forest fires were allowed to occur naturally, this material would be removed by relatively small fires that would not harm the standing timber. Due to total fire suppression, the accumulated material now almost guarantees that every large fire will be a forest disaster.

The summer and fall fires of recent years also endangered more people and property than in previous decades in part because of the recent increase in construction of homes, resorts, and other buildings in and near wilderness areas. Firefighters have been forced to divert their energies from the main task of fighting forest fires in order to protect property.

Major fires since 2000

Wildfires continued to rage across the United States in the years following 2000. Records were set, only to be broken the next or following year. However, 2001 was a relatively quiet year for wildland fires. The number of fires, eighty-four thousand, was relatively high, but the total acreage was relatively low at 3.6 million acres (1.5 million hectares).

Things turned ugly again in 2002, another record-setting year, with over seventy-three thousand fires burning 7.2 million acres (2.9 million hectares). That fire season started with the Rodeo-Chediski fire in Arizona that burned 467,000 acres (189,000 hectares) and four hundred structures. It threatened, but did not burn the town of Show Low, Arizona. That fire was followed by the Hayman fire in Pike National Forest, Colorado, that burned 137,000 acres (56,000 hectares). The Hayman fire caused nine firefighter deaths, and destroyed six hundred structures. Then there was the Florence/Sour Biscuit Complex fire in Oregon that burned 500,000 acres (200,000 hectares). ("Complex fire" is the term used by the forest service to describe a group of separate fires in the same general area that are handled together.) Firefighting in 2002 cost federal agencies a staggering $1.6 billion.

The year 2003 was not as bad in total number of fires or acreage burned. There were sixty-three thousand fires that burned 3.9 million acres, at a total cost to federal agencies of $1.3 billion. The Cramer fire in Idaho burned 13,000 acres (5,000 hectares) and cost the lives of two forest service firefighters.

Despite the relative small number of fires in 2003, disaster struck in California. The Cedar fire, fanned by Santa Ana winds, caused much damage due to its nearness to San Diego. This fire was the second largest wildfire in the history of California and one of 15 fires that started in late October. The Cedar fire burned a total of 721,800 acres (292,100 hectares) and destroyed 3,640 homes. Fifteen lives were lost, including one firefighter and one migrant worker. The fire was intentionally set by a hunter who had become lost. He claimed he was trying to signal rescuers.

The year 2004 was even worse than 2002 in total area burned, with sixty-five thousand fires burning 8.1 million acres (3.3 million hectares). However, the bulk of the area burned was in Alaska, so the total cost to federal agencies of $890 million was relatively low. Many Alaskan wild-land fires are in remote, inaccessible areas and are often allowed to burn out naturally unless they threaten homes or other structures. Alaska fires during 2004 burned over 6.4 million acres.

The total number of fires in 2005 was only sixty-seven thousand, but the total acreage was 8.7 million acres (3.5 million hectares). One of the more unusual aspects of the 2005 season was that wildfires broke out across Oklahoma, Texas, and the Southern Plains in late December, well past the usual fire season. Some of these fires continued to burn through January of 2006.

The largest wildland fire in 2005 was the Cave Creek Complex fire, which was started by a lightning strike on June 21. The fire burned more than 248,000 acres (100,000 hectares) and destroyed eleven structures. It involved 580 firefighters in ten crews. They used ten engines, three bulldozers, and eight helicopters to fight the fire. It was the largest fire ever recorded in the Sonoran Desert. The total cost to the federal government for fire suppression in 2005 was $876 million.

All records are broken in 2006

In 2006, there were over ninety-six thousand fires that burned a staggering 9.8 million acres (4.8 million hectares). Unlike 2004, most of the fires in 2006 were in the continental forty-eight states. Alaska fires totaled only about 270,000 acres (110,000 hectares). The fire season was also exceptionally long. The first wildfires of 2006 were fires in Texas and Oklahoma that had started in 2005 and were still burning. Through the spring and into the summer of 2006, severe drought conditions persisted throughout the West. Coupled with record-setting high temperatures, the conditions were right for an extreme fire season.

In June, fire activity started to increase sharply, with fires in Alaska, the Rocky Mountains, and the Southwest. The Brinns fire threatened 621 homes near Sedona, Arizona. Modular Airborne Fire Fighting Systems (large cargo aircraft carrying three-thousand-gallon tanks for water or fire retardant) were ordered to support the Southwest Area. Toward the end of June, Northern California had become the area of national priority reporting thirteen fires greater than five hundred acres.

In July, the Sawtooth Complex in Southern California destroyed 221 structures. The Winters fire in Nevada burned 238,458 acres (96,575 hectares) of grass and sagebrush, but destroyed no homes. The Tripod Complex threatened nine hundred structures, with only two storage sheds actually destroyed.

In early August, firefighting support from the U.S. military and from Canada, Australia, and New Zealand was requested. The Crystal fire burned 223,570 acres with two structures destroyed. The Columbia Complex, at 109,000 acres, destroyed twenty-eight structures. The Devil's Den fire near Oak City, Utah, was a relatively small and easily controlled fire, but it still took the life of one firefighter. Even small fires can be dangerous in rugged terrain.

In September the season continued with several large fires, including the Day fire in Southern California that burned 162,702 acres destroying one home, and the Derby Complex in Montana that burned 223,570 acres and destroyed 26 homes. By the end of October, when the fire season normally wanes, there had been fifteen fires of more than 100,000 acres (40,000 hectares) reported in eight states.

In the late summer and early fall of 2006, an arsonist set a series of fires across Southern California. The last one set, known as the Esperanza fire, was started on October 26, 2006. It cost the lives of five U.S. Forest Service firefighters who were trying to protect a house from the fire. The house was destroyed. The fire burned for six days and consumed 40,200 acres (16,300 hectares), and destroyed thirty-four homes and twenty outbuildings.

In 2006, according to the National Incident Information Center, the highest number of fire starts on a single day occurred July 25 with 548 new fires. During the height of the fire season there were 687 residences, 65 commercial buildings, and 1,436 outbuildings destroyed. There were twenty-five fire-related fatalities. Despite multiple burn bans in effect and widespread publicity about the dangers of burning, 69,515 fires (consuming a total of 3,980,881 acres or 1,612,257 hectares) were human-caused.

The last fire of 2006 was the Curly Horse fire in the Tucson district of Arizona. The fire burned 1,562 acres (637 hectares) before it was contained on December 18, 2006. In all, the fires of 2006 cost the federal government well over $1.5 billion.

Wildfires in the boreal forest in Alaska

The boreal forest stretches around the globe from Alaska, across Canada, into Northern Europe and Scandinavia, across Russia, and on into Siberia, where it is called the taiga. Fire is a natural part of the boreal forest ecosystem. Every year, both small- and large-scale wildland fires are ignited by lightning and by humans. The boreal forest and the animals within the forest are well adapted to these fire-caused changes. Without the routine occurrence of fire, organic matter builds up, the permafrost level rises, and ecosystem productivity declines. Fire, as an agent of change, rejuvenates and maintains these systems by removing some or all of the insulating material, thus warming the soil and increasing the availability of nutrients.

Part of the huge increase in wildland fires in recent years in the United States is due to wildland fires in the vast stretches of wildlands and forests in Alaska. Fires in these inaccessible regions are often allowed to burn naturally. The Alaska Division of Forestry has four categories of fire suppression response: critical, full, modified, and limited. Critical protection is provided when human lives are threatened or valuable or historic properties are at risk. Limited protection consists of just watching the fire. This level is provided when the expense of a higher level of protection is not justified or when the fire might benefit wildland ecosystems. During the 2005 Alaska fire season, when 4.7 million acres (1.9 million hectares) burned, 3.6 million acres (1.5 million hectares) or 77 percent were given only limited protection.

The five largest single fires in the U.S. since 1997 were in Alaska, and four of these fires were in 2004. The largest wildland fire ever recorded in Alaska or anywhere in the United States was the Taylor Complex fire on Alaska State Forest lands. It burned 1.3 million acres (530,000 hectares) of primarily boreal forest in 2004.

In an average year in Alaska, around 600,000 acres (200,000 hectares) burn. This is an easily manageable level of wildland fire that is necessary to sustain the health of the boreal forest. However, wildfires have dramatically increased in recent years. In the first years of the twenty-first century, Alaska experienced some of the worst wildfire years on record due to drought and record high temperatures. These conditions may have weakened the trees. Rising temperatures have also caused outbreaks of insect pests such as the spruce bark beetle, which is reproducing at twice its normal rate in the warmer climate. These factors are all likely due to climate change, which seems to have more dramatic effects in the arctic and subarctic regions of the globe than elsewhere. As a result, the boreal forest may be undergoing irreversible changes.

Laying blame for wildfires

Western governors, timber company officials, environmentalists, and the federal government have been quick to assign blame for the western fires. The governors of Wyoming, Montana, Utah, South Dakota, Idaho, and Oregon, along with timber company owners, blamed the federal government for its policies strictly limiting logging on federal lands and the ban on building new roads, which may act as fire breaks. They believed that the lack of logging had left too much fuel to burn in the forests. The federal government blamed the past Forest Service policy of putting out every fire, thus allowing brush to accumulate on forest floors and resulting in more crowded, but less healthy, stands of trees. The environmentalists blamed timber companies for harvesting large, fire-resistant trees and replacing them with small fire-prone and disease-prone trees. Everyone blamed developers and wealthy home builders for constructing houses in sensitive wilderness areas.

In 2000, the governors and the federal government came to a compromise: the Forest Service would step up controlled burns and other efforts to clear small, fire-prone trees. (At the time, efforts to thin the forest were being applied on just 6 percent of national forest land.) In addition, the federal government would provide the six states listed above with $700 million each to develop their own fire prevention plans.

Health effects of smoke from wildfires

In 2000, smoke from wildfires hung over much of the western United States. The smoke was worst over Montana, eastward through the Dakotas, and into Iowa. In some places, ash fell like snow so that a car parked overnight would be covered with a layer of ash in the morning. Not only was the smoke and soot bad for people's spirits, it was dangerous for their health.

Many localities set up air-quality hotlines that informed residents of the dangers of going outside. Young children, elderly people, and people with asthma or heart conditions, those most vulnerable to the effects of the smoke, spent many long summer days indoors. In parts of Montana's Bitterroot Valley, all residents were warned not to undertake strenuous activity outdoors. In Missoula, Montana, the air pollution level during the smokiest days measured 500 micrograms per cubic meter of air—more than sixteen times the normal level.

Health professionals treated patients for shortness of breath, headaches, nausea, and dizziness due to the smoke. Throughout the smoky region, pharmacies reported selling more than twice the normal number of inhalers (devices used by people with asthma or other respiratory conditions). Some doctors and nurses expressed concern about the potential, but unknown, long-term health consequences of smoke inhalation.

The air pollution from wildfires can be distributed widely over the globe and the effects of particulate matter in the troposphere are still not well understood. While wildfires contribute a huge amount of carbon dioxide to the atmosphere, thereby increasing the greenhouse effect, the particulate matter may partially shade Earth's surface, which would tend to cool the surface. Atmospheric models suggest that these concentrations

of sooty particles could increase absorption of incoming solar radiation during winter months by as much as 15 percent.

The Peshtigo fire

On October 8, 1871, Peshtigo, Wisconsin, was the location of the deadliest wildfire in the history of the United States. An estimated 1,200 people died as the fire consumed 2,400 square miles (6,140 square kilometers) of forest. By a strange twist of fate, the city of Chicago burned to the ground on that same night. Although the Chicago fire claimed only around three hundred victims, Chicago received almost all of the public's attention. It wasn't until weeks after the Peshtigo fire that the nation learned of the tragedy in the Wisconsin woodlands.

The Peshtigo fire had been preceded by months of drought. The days had been bone dry between a summer rain shower on July 8 and the fire three months later. Throughout September, sporadic fires had begun breaking out in the woods. Residents fought some of the small fires and built fire lines around lumber mills. During the first week of October, the nearby fires in the woods outside of Peshtigo raised temperatures in the town and blanketed it with a layer of haze. The sun appeared as a dull, copper-colored sphere.

On the evening of October 8th, the smoke grew thicker in Peshtigo. Although the town's residents didn't know it, the fires in the nearby woods had merged into a massive inferno that was coming their way. Winds gusted up to 80 miles (130 kilometers) per hour. A deafening roar filled the air. Flaming objects cast ahead of the fire landed in Peshtigo's sawdust streets quickly igniting them.

"In less than five minutes there was fire everywhere," wrote one survivor. "The atmosphere quickly got unbearably warm, and the town was enveloped by a rush of air as hot as though it were issued from a blast furnace. The wind lifted the roofs on houses, toppled chimneys, and showered the town with hot sands and live coals."

Many people were burned alive by the rapidly advancing wall of fire. Others were trampled by stampeding cattle or chose to take their own lives rather than suffer a fiery death. In just one hour the fire leveled the town. One of the few places that residents found refuge from the fire was the Peshtigo River, which ran

through the middle of the town. Once the fire had moved on to the northeast, hundreds of people emerged from the river—stunned and singed, but alive.

The ash and carbon dioxide from a fire would normally be deposited in Earth's troposphere. However, a large wildfire can sometimes cause the formation of a pyrocumulonimbus cloud. This is a type of cumulonimbus cloud (a vertical cloud that may produce wind, hail, and heavy rain) whose vertical development is further amplified by the rapidly rising hot air from the fires. These clouds can have all of the severe weather normally expected from a cumulonimbus cloud and, in addition, can inject soot, ash, carbon dioxide, and carbon monoxide directly into the stratosphere. Once in the stratosphere, these combustion byproducts could be carried around the globe.

Dangerous science: How wildfires happen

Fire is a type of chemical reaction called oxidation, which is the combination of oxygen with material such as wood or leaves. Fire, which is a particularly rapid and violent form of oxidation, releases heat, light, carbon dioxide, carbon monoxide, water, and smoke into the air and leaves ashes on the ground. Other forms of oxidation, such as rotting wood and rusting iron, are much slower processes.

Ingredients of a fire

Any fire requires the following three ingredients: fuel, oxygen, and heat. These three elements are called the fire triangle. Oxygen, which makes up about 21 percent of the air we breathe, is always readily available for wildfires. Wood and other plant matter contains a variety of different flammable materials, such as carbohydrates, proteins, and oils. Wood also contains plastic-like resins. The oxygen chemically combines with these materials within the fuel. Heat from the fire also breaks down carbohydrates and other materials to produce carbon and hydrogen, which burn and produce carbon dioxide and water vapor. Other substances such as oils and resins in the fuel are turned into vapor by the heat. This vapor burns, releasing more carbon dioxide and water vapor. Partial oxidation of the carbon produces toxic carbon monoxide.

The fuel for wildfires is usually dry vegetation, such as brush, grass, pine needles, leaves, and moss. The heat source (lightning, for instance) dries out the material and raises the temperature to the point where it ignites, or starts burning. A wildfire will continue burning as long as fuel, oxygen, and heat are available.

Once a fire gets started, it provides its own heat. The leading edge of a fire heats materials it encounters, causing them to ignite. After the leading edge has passed through an area, it leaves behind red-hot embers. The embers may continue to smolder for days or even weeks after flames are no longer visible, sometimes igniting new fires.

Certain conditions encourage the start of wildfires. For instance, wildfires are an especially bad problem during times of drought, when dry vegetation is plentiful, as well as when temperatures are high. Wet material is harder to ignite and burns more slowly than dry material, because the water in the substance must be heated and evaporated before the material can catch fire.

Causes of fire

The most common natural cause of wildfires is lightning. Another natural heat source is the burning material spewed out by volcanic eruptions. Throughout the United States, however, about 90 percent of all wildfires are started by human activity, some due to carelessness, some deliberate. Some wildfires are set by the combined actions of nature and humans. Earthquakes or windstorms, for example, can down power lines and start blazes.

Worldwide, lightning is a significant cause of wildfires. Each day the planet is struck by about one hundred thousand strokes hot enough to start a fire. Lightning is responsible for around ten thousand brushfires

and forest fires each year in North America, particularly in the western United States, western Canada, and Alaska. About 80 percent of these fires are small, consuming just 10 acres (4 hectares) or less.

Lightning is a short-lived, bright flash of light produced by a 100-million volt electrical discharge. It heats the air through which it travels to around 50,000°F (28,000°C), a temperature even hotter than the surface of the Sun. At any given moment, there are approximately one hundred lightning flashes occurring worldwide.

Lightning is most likely to start fires in areas that are unusually dry. For example, a jagged bolt of lightning strikes the top of a tall tree. The strike explodes the tree and sends a shower of flaming fragments outward and downward. The embers land on and ignite dry leaves, twigs, and pine needles. The flames grow larger, spread to a small tree, and then upward to a taller tree. The flames touch neighboring trees, setting them afire. A wildfire is born.

The human factor

About 90 percent of all wildfires in the United States are started by human activity. People start wildfires with unattended campfires or the burning of leaves, for example. Fireworks are another major starter of wildfires; they spark about fifty thousand fires a year in the United States. Cigarette butts carelessly tossed into the woods can start fires. Downed power lines and chainsaws also give off sparks that ignite fires. A few fires are caused by arson, when a person deliberately starts a fire.

The western United States entered an era of human-induced wildfires in the 1800s, as pioneers pushed westward across the continent. Burning embers from railroad steam engines were cast into the forests and grasslands. Flames from loggers' piles of burning brush spread to trees. Explosives used by miners ignited vegetation. The campfires of hunters and trappers were left to smolder, setting the ground material afire.

Building too close to fire zones

In recent decades, people in the United States have made the danger of wildfires worse by moving into rural or wooded areas where flammable vegetation is plentiful. Many houses and resorts in scenic areas have been built with little regard to fire danger. Building in forested areas poses new challenges to firefighters. While firefighters were previously concerned with protecting the forests, they now have the added burden of protecting people and property.

Nowhere is this situation better illustrated than in southern California, where construction of homes in or near forests or scrublands has occurred at a record pace. "Sixty-one percent of California is covered with wildlands," stated Karen Terill, spokesperson for California's Department of Forestry and Fire Prevention, in a published report by Leigh Wood titled Fires. "Fire is just part of California's ecological makeup. California was built to burn."

Due to the combination of hot, dry Santa Ana winds, high temperatures, dry vegetation, and scarce rain—especially during the summer months—much of Southern California is very susceptible to fire. Since the start of Southern California's building boom, wildfires have caused billions of dollars in property damage and claimed scores of lives. Following are some examples of wildfire-related calamities in the state:

  • In October 1991, following five years of drought (the longest in California's recorded history), wildfire invaded Oakland and Berkeley. The flames were whipped by strong winds into 100-foot (30-meter) towers. In a ten-hour period, approximately three thousand homes were destroyed and twenty-five people were killed. Estimates of property damage ran as high as $1.5 billion.
  • In the hot, dry month of October 1993, twenty-six separate firestorms (several of them ignited by people) raged across California, from Ventura County to the Mexican border. The fires burned more than 140,000 acres (56,000 hectares), destroyed more than one thousand homes, and caused four deaths. Damages totaled more than $1 billion. One month later, fires fanned by winds of 100 miles (160 kilometers) per hour ravaged the outskirts of the exclusive beach community of Malibu.
  • In October 1995, a wildfire in Marin County consumed 12,354 acres (4,942 hectares), including forty-five homes.
  • In October 1996, fires in Malibu and Harmony Grove destroyed more than 150 homes and burned more than 41,000 acres (16,400 hectares).
  • In July 2002, the human-caused McNally fire near Tulare burned 150,696 acres (61,031 hectares) and destroyed seventeen structures.
  • In October 2003, the Cedar fire, set by a lost hunter, burned 273,246 acres (110,665 hectares) near San Diego, destroyed 4,847 structures, and took fifteen lives. The Simi fire burned 108,204 acres (43,823 hectares) and destroyed 300 structures. The arsonist-set Old fire burned 91,281 acres (36,969 hectares), destroyed 1,003 structures, and caused six deaths.
  • In September 2006, the Day fire burned 162,702 acres (65,894 hectares) and destroyed eleven structures.

Types of fire

There are three types of wildfire: ground fire, surface fire, and crown fire. A ground fire is a fire that burns the organic matter on the forest floor, and burns into the underlying soil, such as a peat fire. It differs from a surface fire by being invulnerable to wind. Since oxygen is in short supply, these fires burn slowly and produce a lot of white smoke, but typically show no visible flame. They may burn for weeks on end, undetected, weaving an unpredictable pattern through the forest floor. In rare instances, a slow-burning ground fire can ignite a subterranean bed of coal. The coal beds beneath the town of Centralia, Illinois, were accidentally ignited when the town burned trash in a pit. The coal beds have been slowly burning for more than forty years. At Burning Mountain Nature Reserve, in northeastern New South Wales, Australia, a coal seam has been burning for an estimated five thousand years.

While ground fires usually burn out on their own, occasionally one ignites dry plant material or debris on the surface. In that case it becomes a surface fire. A surface fire has visible flames, but can smolder for hours before any flames appear. There is a range of risk in surface fires, from slow-moving fires that can be extinguished by hand tools and water to rapidly moving fires that pose great risk. They can travel at speeds up to 30 miles (48 kilometers) per hour, consuming dead wood and other material on the forest floor. When the debris on the forest floor is loosely packed, and therefore full of air that provides oxygen, the surface fire spreads more rapidly and burns hotter than it does when the debris is densely packed. While surface fires are the most common type of wildfires, they generally do not harm trees.

In very dry conditions, a surface fire can climb the branches of a small tree like a ladder. The flame may then spread to the tops of taller, adjacent trees, becoming a crown fire. (The crown, or canopy, of a forest consists of the treetops.)

Crown fires are the most harmful type of wildfires, destroying everything in their path. They present the greatest challenge to firefighters and are sometimes impossible to bring under control. The situation is intensified if high winds are present, providing increased oxygen and drying out wet, unburned materials. High winds push flames from tree-top to treetop and can even thrust fire across a road, a small body of water, or cleared land. A small fire in the presence of strong winds rapidly becomes a large fire.

The wind carries flaming branches or embers up to 1 mile (1.6 kilometers) ahead of a crown fire to start new fires. The fires started in this manner, called spot fires, can jump over fire lines to make firefighters retreat and start fighting again in a new location.

Firestorms

A firestorm, also called a blowup, is the most explosive and violent type of wildfire. A firestorm develops when a wildfire comes upon large quantities of dry wood or other plant material at the same time that strong, low-level winds are blowing. The fire suddenly explodes into a swirling bundle of flames containing powerful drafts and whirlwinds (caused by the rising of the hot air and the rushing in of cool air to take its place). This creates even stronger winds that make the fire burn hotter. Trees can explode as the moisture within them is turned to steam and rapidly expands.

A firestorm sucks loose objects into itself. As the flames whip ahead, they snap off trees. The whirlwind, which can be as great as 500 feet (150 meters) in diameter, tosses around trees like matchsticks. Firebrands (burning chunks of wood), sometimes thrown for miles, create spot fires where they land. Those spot fires grow and may merge with the advancing firestorm.

Firestorms build upward dramatically, fueling their own growth. Their heat rises into a column up to 5 miles (8 kilometers) tall, and oxygen is sucked into the bottom of the column, fueling the upward growth of the inferno. Gas bubbles rise within the column and explode in a display of fireworks.

The movement of firestorms is unpredictable and erratic, and their heat is extraordinary. Those factors, combined with the shower of embers they eject, make firestorms uncontrollable. All firefighters can do is stay out of their path and wait for them to become tamed naturally—either due to rain, a lack of fuel, or calmed winds. When the firestorm settles down, firefighters can move in to confront the flames. It was a firestorm that took the lives of fourteen firefighters on Storm King Mountain near Glenwood Springs, Colorado, in 1994.

Wildfire seasons

The times of year in which wildfires are most likely to occur vary from region to region throughout the world. In North America alone there are fifteen different fire seasons. In general, the fire season for a given area is the time of year when temperatures are highest and precipitation is lowest. Thunderstorms (with lightning, which ignites fires) and high winds are most common during warm periods.

In the region of the United States west of the Rocky Mountains, for instance, fire season normally starts in the middle of the summer when rain is scarce, making for dry forests. However, the fire season in the Northern Rockies has been starting earlier in recent years, due to early spring snow melts followed by warm days. In Southern California and in Florida's Everglades, fire season is in the dry months of fall and winter. In the northeastern United States and Canada, fire season comes twice each year: in the spring after the snow melts and in the late fall when rainfall is scarce.

The Santa Ana winds

The Santa Ana winds are warm, dry gusting winds from the east or northeast that create a major wildfire hazard in southern California. These winds occur between the months of October and February, peaking in intensity in December, and blow, on average, at a speed of 40 miles (64 kilometers) per hour and gust 57 to 115 miles (92 to 185 kilometers) per hour. According to the National Weather Service, in order to be classified as "Santa Anas" the wind must be at least 29 miles (46 kilometers) per hour.

Santa Ana winds originate over the elevated plateau of the Mojave Desert and wind their way through the San Gabriel and San Bernadino mountains. They gain speed as they travel through the canyons and reach tremendous speeds in the Santa Ana Canyon (for which the winds are named). The winds then blow onto the foothills of the Los Angeles Basin and the San Fernando Valley. This air, which originates over the desert, is dry at the outset and becomes warmer and drier as it descends. Santa Ana winds bring on heat waves throughout southern coastal California, with temperatures of 100°F (38°C) or higher.

As the Santa Ana winds travel across the dry, scrubby Southern California vegetation of manzanita shrubs, tall grass, oak and eucalyptus trees, they further dry these plants and turn them into perfect brush-fire fuel. The Santa Ana winds create conditions such that a single spark can set off a wildfire. Once the fire begins, the winds fan the flames into an inferno. The Santa Anas are also known for changing direction rapidly, thus spreading fire to new areas.

Consequences of wildfires

Wildfires have a devastating effect on the landscape. They blacken and topple trees, clear away all greenery, and cover the ground with a layer of ash. They drive most wildlife out of the region and kill the animals that are unable to escape. They muddy clear rivers and streams with soil, charcoal, and ash, harming the fish and other animals that live in the water or drink from it. Wildfire can create lasting change. In the absence of trees, grasses may take hold and turn the area into a prairie. Erosion commonly occurs as a result of the loss of trees and shrubs that once held the soil in place, and landslides are a frequent menace once the burning has stopped.

Wildfires claim the lives of many people each year—most of them firefighters. Surprisingly, most wildfires do not take a large toll on larger animal populations. Birds fly out of the burning areas and other animals are able to run or walk away from the flames. Some rodents, such as squirrels and mice, dig holes underground and wait out the fire. In general, animals face a bigger threat from the scarcity of food in winter than they do from wildfire.

The positive aspect of wildfires

The consequences of wildfires are not entirely negative. Fire is part of a natural cycle of forest regeneration (the process of making or starting anew). Periodic fires clear out diseased, dead, and decaying trees and material on the forest floor, creating a mineral-rich layer of ash. If the dead material is not removed from time to time, however, it can accumulate until it finally succumbs to fire; then it burns out of control and causes a great deal more destruction. In either case, though, the minerals left behind fortify the soil and enhance new plant growth.

Furthermore, when the leaves of the canopy are burned off, more sunlight reaches the forest floor and supports new growth. Fire also returns carbon dioxide to the air, thus aiding photosynthesis (the chemical process by which plants convert carbon dioxide and water into carbohydrates, releasing oxygen as a by-product, in the presence of sunlight).

Shortly after a fire sweeps through an area, a blanket of grasses, wildflowers, shrubs, and other plants appears. Deer, elk, and other wildlife return to the young forest and feed on the nutritious, tender green shoots. Woodpeckers fly to fallen trees and dig for insects beneath the bark. Bluebirds and tree swallows make their nests in dead trees. Owls and hawks return to burned areas and have a clearer view of prey on the forest floor. Studies show that three times as many plants and animals populate a forest after a fire as before one. If you walk through a natural area just a few years after a fire, it may be impossible to tell that the fire ever happened there.

Certain tree species, such as jack pine, ponderosa pine, and lodgepole pine, rely on fire for their reproduction. The cones of these trees contain a resin that is highly flammable and must be melted to open, thus dispersing the seeds. The trees' needles drop to the forest floor and dry out, providing a ready source of fuel for fires. After the Yellowstone fires of 1988 the park saw an explosion of young lodgepole pines. Other evergreens, such as the Douglas fir, only grow well in spaces that have been cleared by fire.

Another example of fire-dependent plants is the chaparral of Southern California, a dense growth of tangled, thorny shrubs and small trees. The seeds of chaparral plants can only be forced open by the heat of a fire. Unfortunately, the chaparral itself is a primary source of fuel for those fires, thus making ideal conditions for another fire in the future.

Only in recent decades have foresters recognized that fire plays a vital role in forest ecology (the science of the relationship between living organisms and their environment). Accordingly, they have changed

their policy from fighting every forest fire to allowing some—specifically those that started naturally and do not threaten populated areas—to burn until they put themselves out. Of course, they keep a careful eye on such fires to make sure they do not become dangerous.

A possible connection between wildfires and global warming

Wildfire activity in the forests of the western United States forest has increased dramatically in recent decades. According to an article in the August 18, 2006, issue of Science, much of the increase may be due to climate change. Researchers examined records of wildfires in the Northern Rockies since 1970, and concluded that the greatest increase had occurred at mid-elevations, where human land-use patterns would have had little effect on fire risks.

The researchers proposed instead that the sharp increase in wildfire frequency, size, and duration, and the lengthening of the wildfire season since the 1980s, has been due to increased spring and summer temperatures and an earlier spring snowmelt. Both of these regional climate changes may be related to global climate change. The researchers concluded that higher average temperatures played the biggest role in recent wildfire increases. Wildfire frequency was most closely connected to the

timing of the spring snowmelt. An earlier snowmelt resulted in a drier, longer fire season. The year-round fire season of 2005–2006 could become the norm rather than the exception.

The Yellowstone fires

Yellowstone National Park, located in the northwest corner of Wyoming and extending into small portions of Montana and Idaho, was the scene of huge forest fires in the summer of 1988. In all, eight fires devastated 793,000 acres (317,000 hectares) in the park, about one-third of the parkland, and 600,000 acres (240,000 hectares) near the park. Twenty-four buildings inside the park burned, as did another forty outside the park. There were two deaths due to the blaze. The stage for fire had been set by the extreme drought throughout the western United States that summer. All but one of the fires are believed to have been started by lightning; the other was caused by human carelessness.

Some days the fires advanced as far as 14 miles (23 kilometers). The largest burn in a single day occurred on August 20, a day nicknamed Black Saturday. On that day, 165,000 acres (66,000 hectares), an area twice the size of Chicago, was burning. This acreage was greater than the total of all Yellowstone parkland that had burned since the park was first opened in 1872.

At first Yellowstone officials allowed the natural fires to burn, convinced that fire was needed to restore the park's ecological balance. Due to high winds, however, the fires spread rapidly. The flames grew to 200 feet (61 meters) and were hot enough to melt steel. By mid-July, 16,600 acres (6,640 hectares) had burned, new fires had been sparked by lightning, and no rain was in the forecast. As the blaze threatened to travel beyond the bounds of the park, officials decided it was time to step in. By then, however, the fire had become impossible to control.

Approximately twenty-five thousand firefighters and four thousand military personnel from across the nation were enlisted in one of the largest battles against fire in history. The blaze was fought using hand tools, fire trucks, bulldozers, dozens of helicopters, and airplanes. Aircraft dropped some 10 million gallons (45 million liters) of water and more than 1 million gallons (4.5 million liters) of fire retardant on the flames. Around $120 million was spent fighting the Yellowstone fires, making it the highest firefighting bill in the history of the United States to that time. Despite these monumental efforts, it took two months and September rains to stop the burning. Some areas continued to smolder into November.

Many people were upset about the extent of the damage caused by the Yellowstone fires and blamed the park management for initially letting the blaze burn. Yellowstone Superintendent Robert Barbee defended the park management's actions. "Yellowstone is still the magnificent place it always has been," stated Barbee in a published report. "Fires are a part of the life processes here, and the park will heal and regenerate its natural scars as it has countless times before. I'm excited about that process."

This conclusion is important to forest managers because the currently proposed solutions to the problem of increased fire risk, ecological restoration, and fuels management are based on the assumption that increased risk is primarily due to prior land use. If increased risks are largely due to recent climate change, then ecological restoration and fuels management may be ineffective.

It may also be possible that increased wildfires are contributing to global warming. Forests, such as the boreal forests of Alaska, are in carbon balance. The amount of carbon stored in the vegetation of forestland remains relatively constant over long periods. Trees die and decay or portions of the forest burn, but these dead trees and burned portions are replaced by new growth. However, if a significant portion of the forest burns, or losses due to fire exceeds the forest's ability to regenerate, then there could be an overall increase in greenhouse gases in Earth's atmosphere.

Fighting fires: The technology connection

Many types of technology are employed in both the detection and the fighting of fires. Today, the forest ranger in a lookout tower plays a minor role in finding wildfires. While the U.S. Forest Service still owns more than five thousand towers, only a few are in use. Helicopters and airplanes are now on the front lines of both finding fires and putting them out.

Lookout aircraft are outfitted with infrared cameras that "see" heat, even at night. Infrared is a type of electromagnetic radiation that has a wavelength longer than visible light. Infrared cameras create color-coded maps of an area, with each color representing a specific temperature range. These maps pinpoint "hot spots," places where fires are beginning. Infrared cameras can identify a hot spot 6 inches (15 centimeters) wide from a height of 8,000 feet (2,400 meters). Some aircraft are also equipped with special cameras that detect water in vegetation. Those cameras identify the driest areas so rangers and pilots can keep a close eye there for fire.

Aircraft also monitor sites reported by weather stations to have been struck by lightning. Lightning strikes are observed by sensors mounted on poles throughout the western United States. The sensors transfer information about strikes to weather stations, and the stations forward this data to fire-detection aircraft.

Once a blaze has been identified, aircraft pilots transmit the fire's location by radio. Firefighters are then notified and transported to the scene quickly.

Methods of fighting fires

Firefighters employ numerous methods in the fight against wildfires. Each of those methods is aimed at depriving a fire of one or more of its essential ingredients: fuel, oxygen, and heat.

One method of fighting a fire is creating a fire line, or control line. A fire line is a cleared strip of ground constructed in the path of the approaching fire. Firefighters chop away brush using a tool called a Pulaski, which is a combination of an ax and a hoe. If possible, chainsaws and bulldozers are brought in to help do the job. The purpose of the fire line is to keep the fire from spreading by creating an area without any fuel. Firefighters sometimes also use natural fire lines such as bodies of water, rocky cliffs, bare mountain-tops, or clearings in the woods.

Another way to stop a wildfire is to set a small fire, called a backfire, in the path of the oncoming blaze. The backfire is a controlled fire that consumes the vegetation in a given area, so that fuel will be unavailable once the main wildfire arrives. To start a backfire, firefighters shoot flares into the desired area or spread flaming gasoline or diesel fuel using a drip torch. Backfires must be treated with extreme caution to make sure they do not get out of control and become wildfires themselves.

Firefighters also try to stop the fire with water sprayed from hoses or dropped from the air by helicopters. When water hits the flames it creates steam. The steam creates an insulating layer around the flames that deprives the flames of oxygen. Water also lowers the temperature of the fuel, thereby reducing the heat. Many fire vehicles, both on the ground and in the air, use a water-foam mixture instead of just water. This foam penetrates and coats fuels more effectively to reduce the heat and extinguish the fire.

Another way to slow the spread of wildfires is to apply fire retardant on the fuel ahead of the flames. The mixture of water, chemicals (which encourage new plant growth), and sticky gum is dropped on fires from aircraft. The retardant works against fires by coating the vegetation and thereby restricting the fire's supply of fuel. Fire retardants, often colored red so aircraft pilots can see which areas they have covered, are dropped on vegetation ahead of a fire to create a zone of nonflammable material.

Fire-fighting planes

Water or fire retardant is often dropped by a C-130 cargo aircraft carrying a specially designed five-tank system called a Modular Airborne FireFighting System (MAFFS). These carry 3,000 gallons (11,370 liters) of fire retardant. The MAFFS is a freestanding unit and can be loaded into any suitable C-130 aircraft. A newer model, the MAFFS II is built of carbon fiber, so it is lighter but can hold more, up to 3600 gallons (14,000 liters), can drop partial loads, and can operate with the cargo bay doors closed.

Classes of firefighters

There are several different groups of firefighters, each of which utilizes their particular skills to battle blazes. The largest groups of firefighters, which may consist of hundreds of professionals and volunteers, are called line crews. These firefighters dig fire lines and spray water on the flames. They also conduct the final stage of the operation: a lengthy process called mopping-up, in which they seek out heat sources that may reignite fires, such as smoldering logs, and cover them with water or dirt. Line crews also dig up patches of underground fires, such as smoldering roots, and put them out.

Some firefighters are part of engine crews. For fires accessible by road, engine crews travel to the scene on specially equipped, four-wheel-drive fire engines. The engines have large water tanks and hoses with high-pressure nozzles.

There are two elite groups of firefighters employed to fight large and small wildfires: hotshots and smokejumpers. Hotshots are teams that venture into very dangerous areas and work long shifts to contain fires; they are often the first firefighters on the scene. In 2007, ninety-two crews were available for the fire season, employed by the USDA Forest Service, Bureau of Land Management, National Park Service, various Native American tribes, and the states of Alaska and Utah.

Smokejumpers are firefighters who parachute from airplanes into remote locations to battle newly identified blazes. They wear padded, fire-resistant jumpsuits and helmets with face masks (for protection from the trees they may encounter on their descent). Once they land, they retrieve their heavy backpacks full of equipment, weighing up to 110 pounds (50 kilograms), that are also sent down by parachute. The primary way they fight the fire is by digging fire lines. When smokejumpers have finished fighting a fire, they typically have to walk several miles (kilometers) with the heavy backpacks to get out of the forest.

Smokejumpers must be in top-notch physical condition, and the training to become a smokejumper is very rigorous. Many smokejumpers, when they decide to stop jumping out of airplanes, become hotshots. In recent years the role of smokejumpers has expanded to include assisting in the management of natural resources.

The perils of firefighting

Putting out a wildfire is difficult, dangerous, exhausting, and dirty work, and many firefighters die each year while fighting fires. There is no safe haven in the vicinity of a wildfire. When the winds shift, a hill that may have seemed out of the way can abruptly be set aflame. A sudden gust of wind can turn a moderate blaze into a firestorm. Wildfires typically occur in remote areas, far from hospitals and fire stations. If a firefighter is injured, help may be hours away.

Firefighters can inhale as much smoke in one day as one would get from four packs of cigarettes. The smoke sometimes makes it difficult to breathe and is irritating to the eyes. In addition, firefighters face dangers from falling trees or limbs (that are sometimes aflame), rock slides, and wild animals. Firefighters work long shifts around the clock, sometimes thirty-six to forty hours straight. They work in all types of weather and in intense heat, and get little sleep.

An August 1910 wildfire in the Idaho panhandle was one of the deadliest for firefighters in U.S. history. The fire, called the Big Burn, consumed 3 million acres (1.2 million hectares). It took the lives of seventy-eight firefighters as well as seven civilians. One firefighter, a ranger named Edward Pulaski, became a hero by saving the lives of thirty-nine of his crew members. Pulaski guided the men into a cave and instructed them to wet themselves down with spring water. Although the men temporarily lost consciousness due to the smoke and gases in the cave, they emerged alive after the fire had passed.

It was after that fire that the U.S. Forest Service instituted its policy of putting out all blazes, no matter what their cause. Specifically, park officials were charged with extinguishing any blaze before ten o'clock the following day. If they could not meet that order, they would have to fill out lengthy reports explaining why they had failed.

More recently, in the summer of 1994, fourteen firefighters, some of them smokejumpers, were killed battling a fast-moving blaze near Glenwood Springs, Colorado. The fire spread from 50 acres (20 hectares) to 2,000 acres (800 hectares) in just a few hours because of wind gusts. Some of the burned area contained areas of highly flammable Gambel oaks. The flames were as tall as a ten-story building.

Wildland firefighters, like other firefighters, must deal with arsonists (people who deliberately set fires). In the late summer and early fall of 2006, an arsonist set a series of fires across Southern California. The last one set, known as the Esperanza fire, was started on October 26, 2006. The fire burned for six days and consumed 40,200 acres (16,300 hectares), cost the lives of five firefighters and destroyed many homes and outbuildings. A suspect was arrested and charged with five counts of murder, multiple counts of arson, and multiple counts of using an incendiary device. He was linked to a total of twenty-three fires.

Prescribed burns prevent fires

One technique used by foresters to prevent wildfires is the prescribed burn, which is a planned, controlled fire that clears flammable debris from the forest floor. The goal of a prescribed burn is to keep dead vegetation from piling up and becoming fuel for a wildfire. Before a prescribed burn is set, fire lines are carefully constructed. The fire is only set when winds are calm and conditions are relatively wet, to prevent the accidental spread of the fire beyond the prescribed area. In Yosemite National Park, for instance, prescribed burns are conducted on hundreds to thousands of acres each year.

A matter of survival

There are several fire safety rules that, if followed, minimize the risk of starting fires. The first is to never play with matches. Matches and lighters should be kept out of reach of children. Adults, too, should be monitored for their use of matches. Matches, cigarettes, and other items that involve fire should be disposed of properly.

Always build campfires away from nearby trees or shrubs. Keep a bucket of water or a fire extinguisher nearby in case the flames suddenly leap beyond the fire enclosure. Do not leave a fire unattended while it is still burning. When the flames have disappeared, pour water on the coals and stir. Repeat this step and feel for heat. Only leave a campfire when the coals are wet and cold.

Fireworks, many of which are illegal, are one of the biggest causes of wildfire. Only use fireworks that are legal, and with adult supervision. Keep fireworks away from flammable materials. Do not set off fireworks in natural areas or during dry periods.

Shelters save firefighters' lives

One essential item that all wildland firefighters carry with them is a portable shelter, which is a thin, lightweight sheet of aluminum-coated fiberglass cloth (aluminum reflects away heat). Shelters are only used when firefighters become trapped in rapidly expanding fires. If a firefighter lies on the ground inside the shelter, he or she can survive temperatures as high as 1,600°F (871°C) for short periods. The shelter, also called a shell, is the size of a newspaper when folded and unfolds to a small tent in a matter of seconds.

Affectionately referred to as "shake and bakes," shelters have saved many lives. In August 1985, for example, seventy-three firefighters took refuge in their shelters during an explosive wildfire in Idaho's Salmon National Forest and came out alive. Unfortunately, shelters are not effective against extremely hot fires, or ones in which the high temperatures last for more than a few minutes. They also do not provide air for breathing, so firefighters must hold their breath or try to keep enough air trapped inside the shelter.

Finally, if you are a visiting a park or forest, learn the fire danger in that area. In national parks, daily fire-danger ratings are issued that describe the likelihood of fires. When there are high ratings, campfires may not be allowed. If you are in a natural area that is experiencing a wildfire, follow the instructions of park officials. Evacuate the area immediately if told to do so. If you observe someone being careless with fire, report that person's actions to park officials or the nearest fire station.

Smokey Bear

Smokey Bear (which is the official name; not Smokey the Bear, as popularly called) is a character that was developed by the U.S. Forest Service in 1945 to help spread the message of fire prevention. The anti-fire campaign, called the Cooperative Forest Fire Prevention (CFFP) program, resulted from a need to protect forests and thereby ensure a supply of lumber for the United States during World War II (1939–45). In the beginning of the campaign, the CFFP used wartime slogans on its posters. In 1944, it made the Disney character, Bambi, its official spokes-animal. The following year Bambi was replaced by Smokey Bear. Smokey was named for "Smokey" Joe Martin, Assistant Chief of the New York City Fire Department from 1919 to 1930.

Smokey became famous on posters and radio announcements with the message: "Only YOU Can Prevent Forest Fires." The campaign educated citizens about how people cause forest fires and ways to prevent their occurrence. Since the beginning of this public education program, the damage caused by forest fires started by people in the United States has been reduced by 80 percent.

In 1950, the name Smokey was given to a real bear cub. That cub, which was found badly burned and clinging to a tree after a forest fire in the Lincoln National Forest in New Mexico, was treated for its wounds and then placed in the National Zoo in Washington, D.C. The real-life Smokey Bear became the official, if unknowing, symbol of the anti-forest fire campaign. Smokey was made an honorary member of the Washington, D.C. Fire Department. He remained at the zoo until his death in 1976.

Rules for residents of fire-prone areas

People who live in or near wilderness areas can take several steps to lessen the danger of fire. If constructing a new property, it is best to locate it at the edge of a forest—as close as possible to fire-protection facilities. Use fire-resistant materials when building or renovating and be sure they are in compliance with local building codes. Avoid the use of untreated wood shingles, for instance, since they burn easily. Stone walls are useful for deflecting heat and flames. Smoke detectors should be installed on every floor and near bedrooms, and the home should have several accessible fire exits.

It is also wise to build patios and swimming pools at wildland homes, since those structures provide a safety zone from encroaching fire. Homeowners with swimming pools can purchase pumps to spray water on flames.

The following steps are also recommended to residents who live in or near wildlands:

  • Keep the chimney clean and cover it with wire mesh. 5 inch (1.3 cm) or smaller to prevent the ejection of fiery debris.
  • Regularly clear the roof and rain gutters of dead limbs, leaves, and needles.
  • Store firewood piles away from the house and clear the area around the house of dead vegetation.
  • Replace native vegetation around the house with fire-resistant plants, such as periwinkle, ice plant, or Texas privet shrub.
  • Keep branches near the house pruned to less than 10 feet (3 meters) and remove dead limbs and moss from nearby trees. Trees near the house should not be closer than 10 feet (3 meters) from one another.
  • Avoid the open burning of leaves or other materials.

[See AlsoClimate; Climate Change and Global Warming; Human Influences on Weather and Climate; Landslide; Thunderstorm ]

For More Information

BOOKS

Gantenbein, Douglas. A Season of Fire: Four Months on the Firelines of America's Forests. New York: Jeremy P. Tarcher/Penguin, 2003.

Johnson, Edward A., and Kiyoko Miyanishi. Forest Fires: Behavior and Ecological Effects. San Diego, California: Academic Press, 2001.

Leschak, Peter M. Ghosts of the Fireground: Echoes of the Great Peshtigo Fire and the Calling of a Wildland Firefighter. San Francisco: Harper, 2003.

Pyne, Stephen J., Patricia L. Andrews, and Richard D. Laven. Wildland Fire. 2nd ed. New York: John Wiley & Sons, 2002.

Thybony, Scott. Wildfire. Tucson, AZ: Western National Parks Association, 2002.

PERIODICALS

Bridge, S. R. J., Kiyoko Miyanishi, and Edward A. Johnson. "A Critical Evaluation of Fire Suppression Effects in the Boreal Forest of Ontario." Forest Science. (February 2005): pp. 41-50.

Hodges, Glenn. "Russian Smokejumpers." National Geographic. (August 2002): pp. 82-100.

Kitzberger, Thomas, Peter M. Brown, Emily K. Heyerdahl, Thomas W. Swetnam, and Thomas T. Veblen. "Contingent Pacific-Atlantic Ocean Influence on Multicentury Wildfire Synchrony over Western North America." Proceedings of the National Academy of Sciences. (January 9, 2007): pp. 543-548.

WEB SITES

"Natural Hazards—Wildfires." United States Geological Survey. 〈http://www.usgs.gov/hazards/wildfires/〉 (accessed March 25, 2007).

"Wildfire." Federal Emergency Management Administration. 〈http://www.fema.gov/hazard/wildfire/index.shtm〉 (accessed March 24, 2007).

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Wildfire

Wildfire

Wildfire is a periodic ecological disturbance, associated with the rapid combustion of much of the biomass 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 savannah. 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 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

Barbour, M.G., et al. Terrestrial Plant Ecology, 2nd ed. Don Mills, Ont.: Benjamin/Cummings Pub. Co, 1987.

periodicals

Christensen, N.L., et al. "Interpreting the Yellowstone Fires of 1988." BioScience 39 (1989): 678-685.


Bill Freedman

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.

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