|Wildfire is one of three classes of wildland fire, which are any non-structure fire that occurs in the wildland. The other classes of wildland fire are prescribed fire and wildland fire use. Wildfire is any unplanned and uncontrolled wildland fire that, regardless of ignition source, may require suppression response or other action according to government policy. Wildfire may be ignited by lightning, volcanic activity, accident, or arson. In contrast, Wildland Fire Use and Prescribed Fire are terms for fires that accomplish specific forest management goals. Prescribed Fires are ignited by managers to meet specific resource management objectives. Wildland Fire Use are naturally-ignited wildland fires that are allowed to burn in order to accomplish specific resource management objectives. Only wildfire will be discussed in the following account but many of the terms and processes apply to all three types of wildland fire.
Fire is a chemically reactive fluid flow involving rapid oxidation (combustion) of fuel, leading to the production of heat, light, and by-product emissions (mostly gases and particulates). The fire triangle has long been used to represent the three factors (oxygen, heat, and fuel) necessary for combustion and flame production; removal of any of the three factors causes flame production to cease. Wildfires result when sufficient fuel accumulates and becomes dry enough that application of an external ignition source initiates flaming or smoldering combustion under the prevailing weather conditions, followed by fire spread. Wildfires spread in a manner that reflects the influences of fuel, weather, and topography.
Wildland fire is the primary disturbance agent affecting the structure and composition of many American forests. A forest type has a characteristic fire regime that is comprised of fire frequency, size, intensity, and seasonality. Fire frequency affects forests because it can initiate a new cycle of stand development and by conveying competitive advantage to some species over others. Fire size determines landscape patchiness and also affects the dispersal or colonization distance for regeneration. Wildfire influences landscape diversity and watershed processes such as energy flows and biogeochemical cycling. Indeed, fire is critical to the existence of some woody and herbaceous species in some fire-affected forest types.
Technical Information Related To Wildfire
Ignition and Combustion.--Wildfires are ignited by a variety of sources including lightning, untended campfires, off road vehicles, cigarettes, and arsonists. Regardless of the ignition source, the physics of combustion are the same. Combustion involves several phases (pre-combustion, flaming, smoldering, and glowing) as a fire advances through fuel, generating heat that produces a convection column that vertically lifts by-product gases, vapors, and particulates (smoke). In the pre-combustion (or pre-heating) phase, fuel elements ahead of the fire are heated, causing fuels to dry. Heat induces thermal decomposition (pyrolysis) of some components of woody fuels, causing release of combustible organic gases and vapors.
During this pre-combustion stage of burning, heat energy is absorbed by the fuel which, in turn, gives off water vapor and flammable tars, pitches, and gases. These ignite when mixed with oxygen to initiate the flaming combustion phase. The flaming combustion phase is the luminous oxidation of gases evolved from the rapid decomposition of fuel. This phase follows the pre-combustion phase and precedes the smoldering combustion phase, which has a much slower combustion rate. Water vapor, soot, and tar comprise the visible smoke. Relatively efficient combustion produces minimal soot and tar, resulting in white smoke; high fuel moisture content also produces white smoke.
Smoldering combustion is the combined processes of dehydration, pyrolysis, solid oxidation, and scattered flaming combustion and glowing combustion, which occurs in the aftermath of flaming combustion. Smoldering combustion often is characterized by large amounts of smoke consisting mainly of tars. Emissions are at twice that of the flaming combustion phase. In some fuels such as large woody debris or deep organic soils, smoldering combustion may last for days or even months.
The glowing combustion phase is the final phase of combustion following flaming and smoldering phases. Glowing combustion is the process whereby solid fuels are oxidized, accompanied by incandescence. All volatiles have already been driven off, oxygen reaches the combustion surfaces, and there is no visible smoke. This phase follows the smoldering combustion phase and continues until the temperature drops below the combustion threshold value, or until only non-combustible ash remains.
Burning continues until fuels are exhausted or weather conditions change. Fires generally advance because of wind or slope; the wind can be external currents or wind produced by the fire itself as convection of the heated gases upwards draws in air from the surroundings. The zone of a moving fire where the combustion is primarily flaming is called the flaming front; behind this flaming zone, combustion is primarily glowing or involves the burning out of larger fuels (greater than about 7 cm in diameter). Light fuels typically have a shallow flaming front, whereas heavy fuels have a deeper front. Under intense fire conditions, burning embers may be lofted in advance of the flaming front and ignite other fires, expanding the burned area.
The advancing front combustion stage usually is accompanied by flaming combustion that releases heat to sustain a convection column, a rising column of gases, smoke, fly ash, particulates, and other debris (together called emissions) produced by a fire. A convection column has a strong vertical component when buoyant forces override the ambient surface wind. During this convective-lift phase of a fire, most of the emissions are entrained into a definite convection column that may reach thousands of meters in height.
When fires reach a certain size and intensity, they are no longer constrained by continuity of fuels and become dominated by convective dynamics of the plume. When violent convection caused by a large continuous area of intense fire occurs, a firestorm may result that is often characterized by destructive violent surface updrafts near or beyond the fire perimeter and sometimes by tornado-like whirls. Such “extreme” fire behavior implies characteristics that preclude methods of direct control. One or more of the following is usually involved: high rate of spread, prolific crowning and/or spotting, presence of fire whirls, and a strong convection column. These MegaFires are controlled by changes in weather conditions, not by fire suppression. They can behave erratically, sometimes dangerously.
Fuels.--Fuels are vegetation: live, dead, or decomposed (or occasionally trash left or dumped in the woods). Fuels may be dead plant material on the ground, standing dead material such as tree trunks, live material, and dead material draped over live material such as pine needles hanging on mid-story tree branches. Characteristics important for predicting fire behavior include compactness, loading, horizontal continuity, vertical arrangement, chemical content, size and shape, and moisture content. A fuel type has distinctive characteristics that cause a predictable rate of fire spread or resistance to control under specified weather conditions.
Fuel size classes describe the diameter of down dead woody fuels. Fuels within the same size class have similar wetting and drying properties, and pre-heat and ignite at similar rates. The size distribution of fuel can affect the likelihood of initial ignition; fine fuels such as pine needles are easier to ignite than larger woody material. The chemical composition of fuel influences flammability; resins and volatile oils increase flammability (e.g., chaparral plants) and high mineral content decreases flammability.
The continuity of fuel particles affects a fire's ability to sustain combustion and spread. This applies to aerial fuels as well as surface fuels. The vertical arrangement and continuity of fuels above ground affects spread into tree crowns. Patchy ground fuels can limit fire spread.
Fire Weather.--Fire weather refers to meteorological conditions that influence fire ignition, behavior, and suppression. Climate determines the nature and productivity of vegetative communities and thus the nature and rates of accumulation of fuels. Fuel accumulation, likelihood of ignition, drought and other severe weather conditions can be related to climatic patterns such as ENSO (El Niño Southern Oscillation which has a period of approximately 3 to 7 years).
Weather patterns during the fire season affect atmospheric stability; thunderstorms and active fire conditions are common in unstable atmospheric conditions. Rainfall and humidity before and during a fire affect fuel moisture. Increased fuel moisture combined with high relative humidity decreases the likelihood of ignition and rates of combustion and spread. Wind pre-heats and ignites fuels in advance of the flaming front and increases the oxygen available for combustion. Blown embers causes spotting, ignitions far ahead (up to several km) of the active front. Wind direction changes can increase the fire front, endanger suppression crews, and send smoke toward sensitive populations.
Topography.--Topography affects fire behavior by affecting type and distribution of plant communities and by aiding or deterring fire spread. Relief and landform cause variation in local climate, partially determining the distribution of plant communities with different flammability. Topography also provides variation in local climate that affects fuel moisture, relative humidly, and interaction with wind. Topographic features can create firebreaks, thereby limiting or directing fire spread. Slope affects the orientation of the flaming front. A fire ignited in a gully will burn rapidly uphill, in part due to the flames being oriented close to the ground and thereby pre-heating more of the fuel ahead of the flaming front.
Fire Type.--The three types of fire are surface, ground, and crown fire. Surface fires burn the upper litter layer, including small branches that lie on or near the ground and some or all of the live understory vegetation. Surface fires usually move quickly through an area, and do not consume the entire organic layer. Moisture in the organic horizons often prevents ignition of the humus layer, and protects the soil and soil-inhabiting organisms from heat. The heat pulse generated at the burning front of these fast-moving fires does not normally persist long enough to damage tissue beneath the thick bark of large trees. However, it will girdle the root collar of small trees and shrubs, and reduce small-diameter branches and other fine surface fuels.
Ground fires smolder or creep slowly through the litter and humus layers, consuming all or most of the organic cover, and exposing mineral soil or underlying rock. These fires usually occur during periods of protracted drought when the entire soil organic layer dries. They may burn for weeks or months until precipitation extinguishes them or fuel is exhausted.
Crown fires occur when stand structure, weather, and ladder fuels (heavy accumulations of understory material such as slash piles, shrubs, and lower branches of standing trees, often draped with fallen needles) allow surface or ground fires to ignite tree crowns and spread to other crowns. Crown fires occur in forests during periods of drought and low relative humidity, particularly in areas with a dense, volatile understory.
Fire Intensity.--Fire intensity represents the energy released during a fire, expressed as rate of heat production at the flaming front. Fire intensity can vary greatly both between and within fires depending on fuel type and loading, topography and meteorological influences. Intensity is measured in terms of temperature and heat yield. Surface temperatures can range from 50 to greater than 1,500 °C. Heat yields per unit area can be as little as 260 kg-cal m-2 in short, dead grass to as high as 10,000 kg-cal m-2 in heavy logging slash (Pyne and others 1996).
Fire Severity.--Fire severity describes the immediate effects of fire on vegetation, litter, or soils; fires are ranked from low to high severity by the post-fire appearance of the resource of interest. Of necessity, indicators of fire severity are locally determined, not universal. For example, in the western U.S. consumption of surface litter to mineral soil signifies a severe fire but in southern pine forests, consumption of surface litter is one goal of fuel reduction prescribed burning.
Burn severity depends on both fire intensity and residence time (duration) of the burn, which is a function of rate of spread and subsequent smoldering time. Both depend on weather conditions and the nature of the forest fuels. Rate of spread is additionally influenced by topography and wind speed. Rate of spread is an index of fire duration, varying from 0.5 m in a week in smoldering peat fires to as much as 25 km per hr in catastrophic wildfires.
The terms fire intensity and fire severity are often confused and used interchangeably; although often related, they are not synonymous. Whereas a fast-moving, wind-driven fire may be intense, a long-lasting fire that just creeps along in the forest underbrush could transfer more total heat to plant tissue or soil. In this way, a slow-moving, low-intensity fire could have much more severe and complex effects on forest soil than a faster-moving, higher-intensity fire in the same vegetation.
Fire Regime.--Fire Regime describes patterns of fire occurrences, frequency, size, severity, sometimes including fire effects on vegetation and other resources, in a forest type or ecosystem. Fire regime descriptions are by necessity broad and general because of the temporal and spatial variability of fires. A useful classification for understanding fire effects on forest ecosystems is based on fire severity and has four fire regimes (Brown and Smith, 2000):
1. Understory fire regime—Fires are generally non-lethal to the dominant vegetation and do not substantially change the structure of the dominant vegetation. Approximately 80% or more of the aboveground dominant vegetation survives fires.
2. Stand-replacement fire regime—Fires kill aboveground parts of the dominant vegetation, changing the aboveground structure substantially. Approximately 80% of the aboveground dominant vegetation is either consumed or dies as a result of fires.
3. Mixed severity fire regime—Severity of fire either causes selective mortality in the dominant vegetation, depending on different tree species’ susceptibility to fire, or varies between understory and stand-replacement.
4. Non-fire regime—Little or no occurrence of natural fire.
Fire regimes are described as cycles because some parts of the histories usually get repeated, and the repetitions can be counted and measured, thus resulting in a typical fire return interval. Typical fire return intervals are like normal weather, however; although instructive for comparisons, it is the extreme events that are most interesting. The shorter the interval between fires, the more likely that fires kill only small trees or particularly susceptible species, such as thin-barked hardwoods, resulting in an understory fire regime. The high frequency of fires (every 1 to 3 years) prevents accumulations of sufficient fuel in forests to support severe fires. This regime usually perpetuates fire-adapted species. As fire frequency decreases, fuel accumulates, increasing the probability of a fire intense enough to kill nearly all trees. Fires in forests regenerated by a stand-replacement fire regime come at frequencies of 25 to 500 years and probably maintain high levels of diversity in the landscape.
The season of the year in which fire occurs is one determinant of post-fire successional pathways, affects fire intensity through seasonal differences in surface and crown fuel moisture contents, and has a pronounced effect on the structure of post-fire ecosystems and landscapes. Further variability is produced as a fire moves across a landscape and changes fire behavior and effects as it encounters different stand structures, fuels, topography, and weather. Some widely occurring forest types may have two fire regime classes. Shifts in management or climate may significantly change the role of fire and cause a shift in fire regime.
How This Subject Is Applied In the Field Of Forestry in the Americas
Fire as a Conservation Issue.--Wildland fire in the Americas has a long history and was largely unregulated until the beginning of the 20th century when it became the target of increasingly sophisticated prevention and suppression efforts. The notion of protecting forests from wildfire arose in the US and other countries in parallel with active regulation of use of public lands and with the development of professional forestry. Many countries have followed the North American example and developed centralized wildland fire suppression organizations, usually within forestry agencies but not always.
In other countries, however, wildfires and biomass burning go largely unchecked. Thus in many regions of the Americas, widespread changes in vegetation and fire regime have occurred both by excluding wildfire from fire-dependent communities and by introducing or increasing the frequency of burning in fire-sensitive communities. Adding to the problem of changed fire regimes is the increased presence of people. In fire-dependent ecosystems altered by long-term wildfire suppression, people have built homes in increasing numbers, expanding the so-called wildland-urban interface. In fire-sensitive ecosystems, particularly in Tropical America, clearing for agriculture and pasture has resulted in fire escaping into fire-sensitive forests with the resultant destruction of native habitats.
Fire Management.--Increasingly recognition of wildfire as a natural phenomenon is causing fire suppression organizations to consider ecological, social, and economic effects of fire and suppression. Recognition of the ecological role of wildfire has caused a shift in emphasis from suppression to management of wildland fire. Fire management encompasses all activities required for the protection of forests and the use of fire to meet land management goals and objectives. It includes the activities of preventing, detecting, and controlling, containing, manipulating or using fire in a given landscape. In simple terms, fire management can be thought of as a triangle with the sides being prevention, suppression and fire use. In the US, wildfire protection focused on minimizing suppression cost, property loss, and resource damage; first consideration goes to firefighter safety.
Fire management programs aim to modify fire regimes. Prevention measures seek to minimize fires ignited by humans (accident or arson) thus reducing fire frequency. Detection systems are designed to find fires while small and reduce fire size. Detection systems have evolved from remote fire towers manned by a human spotter during the fire season to infrared remote sensing from aircraft. Initial attack systems seek to contain fires while they are small.
The US Forest Service, for example, deals with approximately 10,000 wildfires annually and 99% are contained within a few days (initial or extended initial attack). The 1% of wildfires not contained by initial efforts, however, account for 95% of the area burned and 85% of all suppression costs. Large fire management systems such as the incident command system were developed to minimize infrastructure and resource damage from fires not controlled by initial attack. Restoration of burned areas is undertaken primarily to protect watersheds from severe erosion. Fuel treatments reduce fuels, decrease or increase fire intensity, size, and frequency; they include prescribed burning and mechanical treatments.
Knowledge of fire behavior and the effectiveness of suppression on fire spread underlie predictive and decision support tools for the manager. Many tools have evolved from empirically-based nomograms to sophisticated computer models. The several questions addressed by these tools are: How likely is fire to occur in an area and under what climatic conditions (fire risk, fire season severity)? What is the likely behavior of a wildfire (fuel, weather, behavior models)? What is the appropriate response once a wildfire is ignited (resource positioning and dispatch, values at risk, economic and resource trade-offs)? Should burned areas be restored, salvaged, or both (effectiveness of post-fire restoration treatments, values to be salvaged, effects on timber markets and other resources)? What actions can be taken to minimize damage from future wildfires or to enhance efficacy of suppression and maximize firefighter safety (prescribed burning, non-fire fuel reduction and management treatments, access)?
The preceding reflects North American experience where fire management is a public sector activity and fire management agencies are primarily emergency response organizations. This public sector model may not be appropriate in all societies and indeed, the existing fire management model that gives primacy to suppression is being questioned in North America as well.
A more completely integrated approach combines the technical components of fire management (prevention, suppression and use) with the key ecological attributes of fire, socio-economic effects, and the cultural necessities of using fire. Such an approach focuses on defining and attaining an ecologically appropriate fire regime that minimizes negative impacts on society. Implementing such a model, however, is more than a technological challenge as it will require changes in attitudes towards responsibility for, and levels of, protection.
Fire Ecology.--The complex role that wildfire plays in shaping ecosystems can be described in terms of four vegetation responses: fire-dependent, fire-sensitive, fire-independent, and fire-influenced (Myers 2006). Fire is largely absent from fire-independent ecosystems where conditions are too cold, wet, or dry to burn (e.g., deserts, tundra, and some rainforests). At the other extreme, fire is essential in fire-dependent ecosystems where species have evolved to withstand burning and to facilitate fire spread. Many forest types in the Americas are fire-dependent, notably pines and other conifers of boreal, temperate, and tropical forests; grasslands, savannas, marshes; and palm forests and savannas. Other fire-dependent forests include the Mediterranean-types on the west coasts of the Americas and the oak-dominated forests and woodlands of North America.
Fire-sensitive ecosystems have evolved without fire as a significant process but humans have made them vulnerable to wildfire (fragmented, altered fuels, increased ignitions). Fire-influenced ecosystems generally are adjacent to fire-dependent vegetation where wildfires originate and spread. The response to fire in fire-influenced ecosystems is variable and often subtle (some species are favored and possibly maintained at greater dominance, for example mahogany in Mesoamerica).
Issues and Developments in Fire Science
Fire Behavior Models.--Fire behavior models are based on the Rothermel Spread Model developed in the 1960s. Although still functional, development is underway to overcome several technical limitations of the Rothermel Model and to address combustion environments beyond its design. Specifically, improved capability is needed to predict fire behavior in non-uniform, heterogeneous fuel beds including live and dead fuels and vertically layered fuelbeds.
New understanding and physics-based models are beginning to address very small-scale phenomena such as backing fires in hardwood litter or smoldering in duff and moss layers; moderately large-scale phenomena such as crown fires, burning structures, and spotting; and very large-scale phenomena such as the transition to blow-ups and long-duration phenomena such as residual smoldering combustion in deep organic soils.
Post-Fire Treatments.--What to do after severe fire on public land is being debated; questions include which emergency rehabilitation treatments are effective and whether to salvage dead and at-risk timber. Post-fire emergency rehabilitation treatments are used to minimize the risks of flooding and erosion that threaten life, property, and natural resources after wildfires. These treatments cost agencies millions of dollars annually, for often uncertain benefits.
High severity burned areas have the highest post-fire runoff and erosion rates due to loss of protective ground cover and fire-induced soil water repellency. Post-fire erosion rates are highest in the first post-fire year. Rates decrease by an order of magnitude in each successive year, and typically recover to pre-fire levels within 3 to 6 years. Rainfall intensity is the driving force of post-fire erosion and is directly related to the magnitude of the erosion response.
Post-fire salvage logging seeks to recover the financial value of timber and to avoid build up of insect pest populations that will infest adjacent healthy tree stands. Removing dead and vulnerable trees after fire may help, but is not always necessary. Claims that salvage logging reduces risk of future fires, and rebuttals of these claims, are made with only sparse evidence. There are few data on fuel changes with time after fire, or on how these changes affect and are affected by future fires. Fine fuels are reduced by fire, and then increase as trees or other vegetation die and new growth occurs. Fuel mass can increase on logged sites from left over slash and on unlogged sites from dead branch litter and falling dead trees. The environmental effects of post-fire salvage logging depend on the severity of the burn, slope, soil type, vegetation composition and condition, the presence or building of roads, type of logging system, and post-fire weather conditions. As with most logging, roads are the primary sediment source.
Expanding Wildland-Urban Interface.--Social scientists have increased understanding of the wildland-urban interface where people have moved into and reside in increasing numbers in the interface with fire-prone wildland. Social science research has improved understanding of how people perceive risk and tolerate smoke. Fire managers face novel situations and new questions such as how to recognize the wildland-urban interface, given that it is a gradient of structure and forest density, not a discrete land-use class.
Because the interface is dynamic and generally growing, planning for pre-suppression treatments and suppression activity requires frequent updates on information about structures and populations sensitive to smoke. Remote sensing and geographic information systems combined with land use models should provide cost-effective means for updates. The effect of structures and landscaping on fire behavior are best addressed by new physics-based modeling approaches. Just because an area has converted from forest to residential land use does not mean that fire-prone vegetation has been removed. Practical and affordable fuel reduction and maintenance treatments for the interface have yet to be developed. At the most fundamental level is questions of how much protection should homeowner expect while keeping firefighter safety the highest priority? What is the cost of effective protection? Who should pay?
Global Change.--Climate change, increased human populations, land-use change, and responses to these changes pose the complex challenge of global change. The effects of climate variability are already apparent as summer temperatures increase and many areas experience long-term drought. Weather-related threats to forests such as increased hurricane activity and pest outbreaks increase fuel loads. Changes in climate and land-use at multiple scales have complex effects on weather and fuels and will further challenge our ability to predict fire behavior. Increased frequency of severe fires already affects regional air quality and will affect climate. One large wildfire can easily produce more particulate emissions than a state’s entire yearly industrial emissions.
Mesoscale climate models suggest that precipitation and temperature in distant regions are affected by smoke plumes from severe wildfires. Although it would appear that fire suppression has a positive impact on climate change by sequestering large amounts of carbon in biomass, this ignores the inevitability of wildfire in fire-dependent ecosystems and does not account for the total carbon footprint of transportation fuel use and other factors in suppression.
Suppression Costs and MegaFires.--Fire suppression costs are increasing, with a few large, intense MegaFires accounting for most of the area annually burned by wildfires and for suppression costs. Attempts to contain large or MegaFires under extreme weather conditions are the main reason for escalating suppression costs. MegaFires are driven by weather factors more than by fuel accumulation; they are controlled by changes in weather, not by fire suppression.
The rise in frequency of MegaFires is not because of lack of funding or initial attack effort. For example in 2006, suppression spending on federal lands in the US alone exceeded $US 1.9 billion. MegaFires have substantial financial, human health, and resource costs beyond suppression expenditures. Smoke from MegaFires is routinely transported long distances and can have significant impacts on public health.
The increased frequency of these destructive wildfires in the US (and elsewhere) seems to be the result of three factors converging: drought, accumulation of fuels, and residential growth patterns at the wildland-urban interface. Only one of these factors, fuel accumulation, can potentially be affected by managers. The suggestion that fire managers concentrate on protecting homes and allow “nature to take its course” and let wildfire reduce fuel levels ignores the ecological and resource ramifications of the damage done by such intense fires.
Nevertheless, policy makers in Canada, Australia, and the US are beginning to confront the MegaFire situation and will need credible information on projected costs versus benefits of continuing existing fire management policy or adopting alternative policies given foreseeable trends in climate, fuels, and population growth. Advanced modeling and remote sensing technologies are coming available that can predict when and where MegaFires are most likely to occur and when weather conditions that support MegaFires will abate. Although such techniques have known limitations, they can be deployed to help fire managers reduce suppression costs and credibly communicate risks.
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John Stanturf is a Research Ecologist and Project Leader, Center for Forest Disturbance Science, Southern Research Station, US Forest Service, Athens, Georgia USA.
Posted 18 August 2007
Updated 28 February 2008