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A wildfire, also known as a forest fire, vegetation fire, grass fire, brush fire, or bushfire (in Australasia), is an uncontrolled fire often occurring in wildland areas, but which can also consume houses or agricultural resources. Common causes include lightning, human carelessness and arson.

Drought and the prevention of small forest fires are major contributors to extreme forest fires.

The word "wildfire" originated as a synonym for Greek fire, a napalm-like substance used in medieval Europe as a naval weapon; the word attained its present meaning by a common misunderstanding of the expression "spread like wildfire".

Background

Wildfires are common in many places around the world, including much of the vegetated areas of Australia, 'veld' in the interior and 'fynbos' in the Western Cape of South Africa, forest areas of the United States and Canada, where the climates are sufficiently moist to allow the growth of trees, but feature extended dry, hot periods when fallen branches, leaves, and other material can dry out and become highly flammable. Wildfires are also common in grasslands and scrublands. Wildfires tend to be most common and severe during years of drought and occur on days of strong winds. With extensive urbanization of wildlands, these fires often involve destruction of suburban homes located in the wildland urban interface, a zone of transition between developed areas and undeveloped wildland.

Fires tend to happen during the Summer in times of drought and the Fall in times of winds.

Today it is accepted that wildfires are a natural part of the ecosystem of wildlands, where, at the least, plants have evolved to survive fires by a variety of strategies (from possessing reserve shoots that sprout after a fire, to fire-resistant seeds), or even encourage fire (for example eucalypts contain flammable oils in the leaves) as a way to eliminate competition from less fire-tolerant species. In 2004, researchers discovered that exposure to smoke from burning plants actually promotes germination in other types of plants by inducing the production of the chemical butenolide. Most native animals, too, are adept at surviving wildfires.

On occasions, wildfires have caused large-scale damage to private or public property, destroying many homes and causing deaths, particularly when they have reached urban-fringe communities.

Slash (small, rotten, misshapen, or otherwise undesirable wood discarded during logging) has historically provided the fuel for devastating fires such as the fires in Michigan in the 19th century.

The aftermath of a wildfire can be as disastrous if not more so than the actual fire itself. A particularly destructive fire burns away plants and trees that prevent erosion. If heavy rains occur after such a fire, landslides, ash flows, and flash floods can occur. This can result in property damage outside the immediate fire area, and can affect the water quality of streams, rivers and lakes.

Wildfires burned long before humans evolved. One main component of Carboniferous north hemisphere coal is charcoal left over by forest fires. The earliest known evidence of a wildfire dates back to Late Devonian period (about 365 millions of years ago) .

Behavior

When the water reserves in the soil are between 100% and 30%, the evaporation of water in plants is balanced by water absorbed from the soil. Below this threshold, the plants dry out and under stress release the flammable gas ethene (ethylene). A consequence of a long hot and dry period is therefore that the air contains flammable essences and plants are drier and highly flammable.

The propagation of the fire has three mechanisms:

• "crawling" fire: the fire spreads via low level vegetation (e.g., bushes)
• "crown" fire: a fire that "crowns" (spreads to the top branches of trees) can spread at an incredible pace through the top of a forest. Running crown fires can be extremely dangerous to all inhabitants underneath, for all the oxygen is sucked out to feed the fire above. Asphyixiation can occur.
• "jumping" or "spotting" fire: burning branches and leaves are carried by the wind and start distant fires; the fire can thus "jump" over a road, river, or even a firebreak.

The Nevada Bureau of Land Management identifies several different wildfire behaviors. For example, extreme fire behavior includes wide rates of spread, prolific crowning and/or spotting, the presence of fire whirls, or a strong convection column. Extreme wildfires behave erratically and unpredictably.

In southern California, under the influence of Santa Ana winds, wildfires can move at tremendous speeds, up to 40 miles (60 km) in a single day, consuming up to 1,000 acres (4 km²) per hour. Dense clouds of burning embers push relentlessly ahead of the flames crossing firebreaks without pause.

The powerful updraft caused by a large wildfire will draw in air from surrounding areas. These self-generated winds can lead to a phenomenon known as a firestorm.

French models of wildfires dictate that a fire's front line will take on the characteristic shape of a pear; the major axis being aligned with the wind. In the case of the fires in southeastern France, the speed of the fire is estimated to be 3% to 8% of the speed of the wind, depending on the conditions (density and type of vegetation, slope). Other models predict an elliptical shape when the ground is flat and the vegetation is homogeneous.

Prevention

For many decades the policy of the United States Forest Service was to suppress all fires, and this policy was epitomized by the mascot Smokey Bear and was also the basis of parts of the movie Bambi. The policy began to be questioned in the 1960s, when it was realized that no new sequoias had been grown in the redwood forests of California, because fire is an essential part of their life cycle. This produced the policy of controlled burns to reduce underbrush. This clears much of the undergrowth through forest and woodland areas, making travel and hunting much easier while reducing the risk of dangerous high-intensity fires caused by many years of fuel buildup.

The previous policy of absolute fire suppression in the United States has resulted in the buildup of fuel in some ecosystems such as dry ponderosa pine forests. However, this concept has been misapplied in a "one-size-fits-all" application to other ecosystems such as California chaparral. Fire suppression in southern California has had very little impact over the past century. The amount of land burned in 6 southern California counties has been relatively unchanged. In fact, fire frequency has been increasing dramatically over the past century in lock step with population growth. Urbanization can also result in fuel buildup and devastating fires, such as those in Los Alamos, New Mexico, East Bay Hills, within the California cities of Oakland and Berkeley between October 19 and 22, 1991, all over Colorado in 2002, and throughout southern California in October 2003. Homes designed without considering the fire prone environment in which they are built have been the primary reason for the catastrophic losses experienced in wildfires. Ī On average, wildfires burn 4.3 million acres (17,000 km²) in the United States£ annually. In recent years the federal government has spent $1 billion a year on fire suppression. 2002 was a record year for fires with major fires in Arizona, California, Colorado, and Oregon.

The risk of major wildfires can be achieved partly by a reduction of the amount of fuel present. In wildland, this can be accomplished by either conducting controlled burns, deliberately setting areas ablaze under less dangerous weather when conditions are less volatile or physical fuel removal by removing some trees as is conducted in many American forests. Such techniques are best used within the wildland/urban interface where communities connect with wild open space. Prescribed burns in the backcountry, away from human habitations, are not particularly effective in preventing large fires. All the large catastrophic fires in the United States have been wind driven events where the amount of fuel (trees, shrubs, etc.) has not been the most important factor in fire spread.

People living in fire-prone areas typically take a variety of precautions, including building their homes out of flame-resistant materials, reducing the amount of fuel near the home or property (including firebreaks, their own miniature control lines, in effect), and investing in their own firefighting equipment.

Rural farming communities are rarely threatened directly by wildfire. These types of communities are usually located in large areas of cleared, usually grazed, land, and in the drought conditions present in wildfire years there is often very little grass left on such grazed areas. Hence the risk is minimized. However, urban fringes have spread into forested areas, for example in Sydney and Melbourne, and communities have literally built themselves in the middle of highly flammable forests. In Cape Town, the city lies on the fringe of the Table Mountain National Park. These communities are at high risk of destruction in bushfires, and should take extra precautions.

Fire suppression

Most fire-prone areas have large firefighter services to help control bushfires. As well as the water-spraying firetrucks most commonly used in urban firefighting, bushfire services use a variety of alternative techniques. Typically, forest fire fighting organizations will use large crews of 20 or more people who travel in trucks to the fire. These crews use heavier equipment to construct firebreaks, and are the mainstay of most firefighting efforts. Other personnel are organized into fast attack teams typically consisting of 5–8 people. These fast attack teams are helicoptered into smaller fires or hard to reach areas as a preemptive strike force. They use portable pumps to douse small fires and chainsaws to construct firebreaks or helicopter landing pads if more resources are required. Hand tools are commonly used to construct firebreaks and remove fuels around the perimeter of the fire to halt its spread, including shovels, rakes, and the pulaski, a tool unique to wildland firefighting. In the eastern United States, portable leaf blowers are sometimes used. In the western United States, large fires often become extended campaigns, and temporary fire camps are constructed to provide food, showers, and rest to fire crews. These large fires are often handled by 20 person hand crews, sometimes known as "hotshot" crews, specially organized to travel to large fires.

Fast attack teams are often considered the elite of firefighting forces, as they sometimes deploy in unusual ways. If the fire is on a particularly steep hill or in a densely wooded area, they may rappel or fast-rope down from helicopters. If the fire is extremely remote, firefighters known as smokejumpers may parachute into site from fixed-wing aircraft. In addition to the aircraft used for deploying ground personnel, firefighting outfits often possess helicopters and water bombers specially equipped for use in aerial firefighting. These aircraft can douse areas that are inaccessible to ground crews and deliver greater quantities of water and/or flame retardant chemicals. Managing all of these various resources over such a large area in often very rugged terrain is extremely challenging, and often the Incident Command System is used. As such, each fire will have a designated Incident Commander who oversees and coordinates all the operations on the fire. This Incident Commander is ultimately responsible for the safety of the firefighters and for the success of firefighting efforts.

Large fires are of such a size that no conceivable firefighting service could attempt to douse the whole fire directly, and so alternative techniques are used. In alternative approaches, firefighters attempt to control the fire by controlling the area that it can spread to, by creating "control lines", which are areas that contain no combustible material. These control lines can be produced by physically removing fuel (for instance, with a bulldozer), or by "backfiring", in which small, low-intensity fires are started, using a device such as the driptorch, or pyrotechnic flares known as "fusees", to burn the flammable material in a (hopefully) controlled way. These may then be extinguished by firefighters or, ideally, directed in such a way that they meet the main fire front, at which point both fires run out of flammable material and are thus extinguished.

Unfortunately, such methods can fail in the face of wind shifts causing fires to miss control lines or to jump straight over them (for instance, because a burning tree falls across a line, burning embers are carried by the wind over the line, or burning tumbleweeds cross the line).

The actual goals of firefighters vary. Protection of life (those of both the firefighters and "civilians") is given top priority, then private property according to economic and social value and also to its "defendibility" (for example, more effort will be expended on saving a house with a tile roof than one with a wooden-shake roof). In very severe, large fires, this is sometimes the only possible action. Protecting houses is regarded as more important than, say, farming machinery sheds, although firefighters, if possible, try to keep fires off farmland to protect stock and fences (steel fences are destroyed by the passage of fire, as the wire is irreversibly stretched and weakened by it). Preventing the burning of publicly owned forested areas is generally of least priority, and, indeed, it is quite common (in Australia, at least) for firefighters to simply observe a fire burn towards control lines through forest rather than attempt to put it out more quickly; it is, after all, a natural process. On any incident, ensuring the safety of firefighters takes priority over fire suppression. When arriving on a scene a fire crew will establish a safety zone(s), escape routes, and designate lookouts (known by the acronym LCES, for lookouts, communications, escape routes, safety zones). This allows the firefighters to engage a fire with options for a retreat should their current situation become unsafe. In addition all fire suppression activities are based from an "anchor point" (such as lake, rock slide or road). From an anchor point firefighter can work to contain a wildland fire without the fire outflanking them. As last resort, all wildland firefighter carry a fire shelter. In a unescapable burnover situation the shelter will provide limited protection from radiant and convective heat, as well as superheated air. As such a greater emphasis is placed on safety and preventing entrapment, and is reinforced with a list of 10 fire orders and 18 "watch out situations" for firefighters to be aware of, which warn of potentially dangerous conditions.

In North America, the belief that fire suppression has substantially reduced the average annual area burned is widely held by resource managers and is often thought to be self-evident. However, this belief has been the focus of vocal debate in the scientific literature recently. Direct empirical evidence of the effects of fire suppression in boreal forest ecosystems is essentially limited to just two studies by Stocks (1991) and Ward and Tithecott (1993), that use Ontario government fire records to make comparisons of average annual area burned between areas with and without aggressive fire suppression policies. While numerous subsequent studies have been done, they often present the same information (Martell 1994, Martell 1996, Weber & Stocks 1998, Li 2000, Ward & Mawdsley 2000). The proponents of these studies argue that areas without aggressive fire suppression policies have larger average fire sizes and greater average annual area burned and a longer interval between fires and that this is evidence of the effect of fire suppression.

Several recent papers have argued against this idea (Johnson et al. 2001, Miyanishi & Johnson 2001, Miyanishi et al. 2002, Bridge et al. 2005). These papers claim that statistically rigorous techniques for estimating the average annual area burned, called the fire cycle, do not show changes in the fire cycle associated with fire suppression and that the evidence used to support the effect of fire suppression is biased and has been presented in a way that is flawed. Note that none of these papers criticize fire management agencies for being anything less than completely committed to their mandate. Nor do they suggest that fire personnel are not well trained, efficiently deployed or well managed. Instead, these papers simply suggest that despite the resources employed, fire management agencies are simply unable to effectively reduce the average annual are burned.

Atmospheric effects

Most of the Earth's weather and air pollution reside in the troposphere, the part of the atmosphere that extends from the surface of the planet to a height of between 8 and 13 kilometers. A severe thunderstorm in the area of a large wildfire can have its vertical lift enhanced to boost smoke, soot and other particles as high as the lower stratosphere (Wang, 2003).

Previously, it was thought that most particles in the stratosphere came from volcanoes or were generated by high-flying aircraft. Collection of air samples from the stratosphere in 2003 led to detection of carbon monoxide and other gasses related to combustion at a level 30 times higher than can be accounted for by commercial aircraft.

Satellite observation of smoke plumes from wildfires revealed that the plumes could be traced intact for distances exceeding 5,000 kilometers. This observation suggests that the plumes were in the stratosphere above weather conditions that would have brought the plume back to earth.

Atmospheric models suggest that these concentrations of sooty particles could increase absorption of incoming solar radiation during winter months by as much as 15% (Baumgardner, et al., 2003).

Statistics

Every year, the burnt surface represents about:

• France: 300 km², 12,140 acres, 0.04% of the territory
• Portugal:
  • 1991 : 1,820 km², 449,732 acres, i.e. 2% of the territory
  • 2003 : 4,249 km², 1.05 million acres, i.e. 4.6% of the territory; 20 deaths ;
  • 2004 : 1,205 km², 297,836 acres, i.e. 1.3% of the territory
  • 2005 : 2,864 km², 707,668 acres, i.e. 3.1% of the territory; 17 deaths;
• United States: 17,400 km², 4.3 million acres i.e. 0.18% of the territory

See also

• List of forest fires
• Aerial firefighting
• Defensible space
• Glossary of wildland fire terms
• Controlled burn, a tool to help prevent wildfire

• Country Fire Service (South Australia)
• Country Fire Authority (Victoria, Australia)
• New South Wales Rural Fire Service (Australia)

References

• Baumgardner, D., et al. 2003. Warming of the Arctic lower stratosphere by light absorbing particle. American Geophysical Union fall meeting. Dec. 8-12. San Francisco.
• Bridge, S.R.J, K. Miyanishi and E.A. Johnson. 2005. A Critical Evaluation of Fire Suppression Effects in the Boreal Forest of Ontario. Forest Science 51:41-50.
• Fromm, M., et al. 2003. Stratospheric smoke down under: Injection from Australian fires/convection in January 2003. American Geophysical Union fall meeting. Dec. 8-12. San Francisco.
• Johnson, E.A. and Miyanishi K. (Eds.) 2001. Forest Fires - Behavior and Ecological Effects. Academic Press, San Diego.
• Johnson, E.A., K. Miyanishi, and S.R.J. Bridge. 2001. Wildfire regime in the boreal forest and the idea of suppression and fuel buildup. Conserv. Biol. 15:1554-1557.
• Li, C. 2000. Fire regimes and their simulation with reference to Ontario. P. 115-140 in Ecology of a managed terrestrial landscape: patterns and processes of forest landscapes in Ontario, Perera, A.H., D.L. Euler, and I.D. Thompson (eds.). UBC Press, Vancouver, BC.
• Martell, D.L. 1994. The impact of fire on timber supply in Ontario. For. Chron. 70:164-173.
• Martell, D.L. 1996. Old-growth, disturbance, and ecosystem management: commentary. Can. J. Bot. 74:509-510.
• Miyanishi, K., and E.A. Johnson. 2001. A re-examination of the effects of fire suppression in the boreal forest. Can. J. For. Res. 31:1462-1466.
• Miyanishi, K., S.R.J. Bridge, AND E.A. Johnson. 2002. Wildfire regime in the boreal forest. Conserv. Biol. 16:1177-1178.
• Pyne, S.J. et al. 1996. Introduction to Wildland Fire. Wiley, New York.
• Stocks, B.J. 1991. The extent and impact of forest fires in northern circumpolar countries. P. 197-202 in Global biomass burning: atmospheric, climatic and biospheric implications, Levine, J.S. (ed.). MIT Press, Cambridge, MA.
• Wang, P.K. 2003. The physical mechanism of injecting biomass burning materials into the stratosphere during fire-induced thunderstorms. American Geophysical Union fall meeting. Dec. 8-12. San Francisco.
• Ward, P.C., and W. Mawdsley. 2000. Fire management in the boreal forests of Canada. P. 274-288 In Fire, climate change, and carbon cycling in the boreal forest, Kasischke, E.S., and B.J. Stocks (eds.). Springer, New York, NY.
• Ward, P.C., and A.G. Tithecott. 1993. The impact of fire management on the boreal landscape of Ontario. Aviation, Flood and Fire Management Branch Publication No. 305. Ont. Min. Nat. Res., Queens Printer for Ontario, Toronto, ON.
• Weber, M.G., and B.J. Stocks. 1998. Forest fires in the boreal forests of Canada. P. 215-233 in Large forest fires, Moreno, J.M. (ed.). Backhuys Publishers, Leiden, The

External links

• The International Journal of Wildland Fire
• The International Association of Wildland Fire
• Current Wildland Fire Information in the United States
• FEMA report on the East Bay Hills Fire
• Global Fire Monitoring Center (GFMC)
• Interesting Forest Fire Policy
• The 10 standard fire orders and 18 watch out situations
• SIADEX: A decision support system for fighting forest fires in Andalusia
• GeoMAC: Wildfire information and internet map viewer of current wildfires in the United States
• Locations of large fires in the US and Canada based upon MODIS satellite imagery
• Fire in the East, Fire in the West An overview of conditions and agents of wildfire in different U.S. ecosystems
• Volunteer Wildfire Services, Cape Town, South Africa
• Natural Disasters - Forest Fire Great research site for kids.
• Forest Fire Warnings - Broadcast emergency warnings to communities adjacent to a forest fire.

• Wildfires
• Natural hazards
• Climate forcing agents