Impacts of Fire
Understanding fire behavior is important for managing fire and for predicting and interpreting the effects of fire. Fire behavior is the manner in which a fire reacts to the influences of fuel, weather, and topography.
Individual fires vary in
- what they burn (fuel consumption)
- where they burn (spread pattern)
- when they burn (seasonally and diurnally)
- how intensely they burn (heat released)
- how long they burn (residency time)
Fire behavior is described with terms like “creeping,” “running,” “smoldering,” “spotting,” “torching,” and “crowning.” Fires are also described by the spatial distribution of burned materials after the fire has passed with terms such as “complete” or “patchy” (mosaic) burns.
Burn severity describes the degree to which a site is altered or disrupted by fire, and is the product of fire intensity (heat) and residency (duration). Fires burning within the parameters of an ecosystem’s natural fire regime are typically ecologically beneficial.
The effects of fire are difficult to predict due to the multitude of variables surrounding the characteristics of a single fire, fire regimes, biological communities, and environmental conditions across time and space. All components of an ecosystem may be affected either by direct exposure to a fire’s flames or through interactions with the changed environment as a site recovers from fire.
Current knowledge of the effects of fire on flora (Brown and Smith 2000), fauna (Smith 2000), soils and water (Neary et al. 2005), and air quality (Sandberg et al. 2002) is synthesized in a series of technical reports published by the USDA Forest Service. The Fire Effects Information System (See Review and Resources section) provides detailed descriptions of fire effects on more than 100 North American animal species and nearly 1,000 plant species.
An obvious, immediate effect of fire on vegetation is plant mortality. After the fire has passed, secondary fire effects further shape and change plant communities and ecological processes.
Survival requires that a plant’s adaptive traits be compatible with characteristics of the fire. Other factors, especially weather and animal impacts (such as pathogens, insects, and grazing herbivores), can greatly affect whether a plant species can reproduce and persist following a fire.
High temperatures during fires can directly damage plant tissue and propagules through combustion, and indirectly damage plant physiological processes through radiant heating (Levitt 1972). The likelihood of plant and seed mortality by fire depends on the duration of exposure to lethal temperatures. Mortality can occur with short exposure to high temperatures (Martin 1963), whereas death at lower temperatures often requires a longer exposure (Ursic 1961).
A plant’s survival is influenced by its growth stage at the time of the fire. Plant tissues, especially growing points (meristematic tissue), are most susceptible when actively growing (Wright and Bailey 1982). Mortality depends on the amount of meristematic tissue killed. Plant mortality is further influenced by pre-fire conditions (such as drought) that may weaken plants, and post-fire stressors such as herbivores, parasites, and pathogens.
Plant species exhibit a variety of traits and mechanisms by which they are able to survive and recover from fire. These traits and mechanisms are common to species found in nearly all terrestrial North American ecosystems (Miller 2000).
Some plants are able to survive a fire because their growing points and propagules are structurally, spatially, or temporally protected from exposure to lethal temperatures (Miller 2000). For example, fire-resistant plant species like Ponderosa pine (Pinus ponderosa) suffer little damage from direct exposure to a surface fire due to thick bark that protects living tissue and seeds that reside in the canopy, high above the fire’s flames. Low-growing plants and plants that sprout from roots, soil surface crowns, and rhizomes often survive and resprout because their meristematic tissues and reproductive structures are near the soil surface where temperatures are generally cooler during a fire. Other plants may avoid mortality simply by completing their life cycle before or after a fire occurs. Fire-intolerant species tend to be highly flammable and completely destroyed by fire. Some fire-intolerant species have large, fire-activated seedbanks that germinate, grow, and mature rapidly following a fire.
Diagram illustrating the effects of fire temperature on different plant life forms (adapted from concepts in Raunkiaer 1934, and Ditomaso and Johnson 2006).
Plant Community Response to Fire
Plants recover after a fire by regenerating from surviving growing points, vegetative propagules, or seeds. The degree to which fire destroys regeneration potential from existing species will define the plant community composition immediately following fire (Brown 2000).
In the first few years after a fire, the recovering plant community will generally include
plants that survived the fire intact
sprouts or suckers that grew from the base or buried parts of top-killed plants
plants that established from seed (dispersed from surviving plants, dispersed from plants off of the burned site, germinated from within the soil seedbank, or developed on plants that resprouted after the fire)
Predicting plant population and community response to fire is difficult. An individual plant’s response to fire is a result of the local fire conditions and a plant’s traits and growth stage. Plant population and community response is a product of the responses of all plants in a burned area and their interactions with each other and the changed environment (Miller 2000).
As a general rule, burned areas tend to return to the same flora that existed previously (Brown 2000). Fires of low severity are typically followed by a strong resprouting response from preexisting perennial vegetation, whereas fires of high severity may create opportunities for new plants to establish (Turner et al. 1997). Over time, the post-fire plant community is further influenced by subsequent interactions with other plants, activities of other taxa (i.e., insects, pathogens, grazing ungulates), climatic variables, environmental changes, and other natural or anthropogenic disturbances (Brown 2000).
Fire affects the composition, structure, and pattern of vegetation on the landscape. In most terrestrial North American ecosystems, fire disturbance is necessary to maintain ecological processes.
Fire Effects on Plant Communities and Processes
The habitat changes caused by fire influence animal populations and communities much more profoundly than fire itself. Animals’ immediate responses to fire are influenced by season, intensity, severity, rate of spread, uniformity, and size of the fire. Long-term faunal response to fire is determined by habitat change, which influences feeding, movement, reproduction, and availability of shelter. The immediate and short-term effects of fire on terrestrial birds and mammals include injury, mortality, emigration, and immigration.
Injury and Mortality
Most fires have the potential to injure or kill fauna and large intense fires are certainly dangerous to animals caught in their path. Fires generally kill and injure a relatively small proportion of animal populations because many vertebrate species flee or seek refuge during fires. Animals with limited mobility living above ground appear to be the most vulnerable to fire-caused injury and mortality, but occasionally even large mammals are killed by fire. Nestlings and juveniles of birds and small mammals are vulnerable to fire-caused mortality.
Emigration & Immigration
While non-burrowing mammals and most birds leave their habitat while it is burning, many return within hours or days. Others emigrate because the food and cover they require are not available in the burn. The length of time before these species return depends on how much fire altered their habitat structure and food supply.
Many insects, birds, and mammals are attracted to fire, smoke, and recently burned areas. Most birds and mammals that immigrate in response to fire are attracted by more abundant or more exposed food resources. Intense activity by insects after a fire attracts insect-feeding birds such as woodpeckers. In fact, bird species that feed on aerial, ground, and bark-dwelling insects favor recently burned habitats (Saab and Powell 2005).
There are few reports of fire-caused injury to reptiles and amphibians (Russell et al. 1999) and little is known about emigration and immigration of these taxa after fire.
The vulnerability of insects and other invertebrates to fire depends on their location at the time of fire (Lyon et al. 2000).
The effects of fire on fish and aquatic macroinvertebrates are mostly indirect in nature (Bisson et al. 2003). Fire typically improves habitat conditions for aquatic species over the long-term. However, fires that result in longer-term changes in stream temperature, ash-laden slurry flows, increases in flood peak flows, and sedimentation due to increased soil erosion can have negative impacts on aquatic organisms.
Chemical, biological, and physical properties of soil are all altered from heat and oxidation of fuels. Effects of fire on soil come primarily from high temperatures that affect surface as well as deeper soils (Neary et al. 2005). Low- and moderate-temperature fires generally have long-term benefits for ecosystems that evolved with fire (DeBano et al. 1998).
Fires affect water quality and water cycle processes to a greater or lesser extent depending on fire severity. When a fire occurs, changes in water quality are primarily the result of soil erosion and deposition of soil materials into water (Neary et al. 2005). Fires may cause suspended sediment, elevated streamflow temperatures, increased pH values, and changed chemical concentrations and aquatic organism populations.
Severe wildfire can produce substantial effects on the streamflow regime of small streams and rivers (Neary et al. 2005). The effects of low severity fires on water resources are generally minimal and short-lived.
Fire can also have positive effects on water resources. For example the increased nutrient flow into streams and lakes can rejuvenate fish populations (F Wetzel, personal communication).
In general, the traits and mechanisms for plant survival and response to fire described above apply to all terrestrial plants regardless of their origin or perceived value as a desirable or undesirable species. The direct effect of fire on any individual plant species, including invasive plant species, depends on the compatibility of the plant’s traits with the characteristics of the fire.
Fires Influence Invasive Plants
A literature review by Johnson et al. (2006) synthesizes knowledge of nonnative plant establishment, spread, and persistence following wildfire in natural areas. Though conventional wisdom is that many invasive plant species will dramatically increase following fire, the authors conclude that invasive species may either increase or decrease depending on
- type of habitat
- fire characteristics (frequency, intensity, severity, and departure from natural fire regimes)
- climatic and physical characteristics (precipitation, aspect, slope)
- fire management activities
- other natural and anthropogenic disturbances
Plant community composition immediately following a fire results from the interaction between fire severity and a plant’s regeneration strategies (Brown 2000). Invasive plants may regenerate or germinate from on-site propagules as well as propagule from off-site sources.
LOW FIRE SEVERITY
HIGH FIRE SEVERITY
Exposed ground surfaces, a flush of nutrients, and high light and low shade may favor regrowth and expansion of invasive plants in burned areas. Because of their early germination and rapid growth rates, some invasive plants may quickly capture newly available resources.
Invasive Plants Influence Fires
Invasive plants may create conditions that alter the characteristics of fire regimes such as spread patterns, intensity, frequency, and seasonality (Brooks et al. 2004). This occurs when invading plants are different in their life-form or phenology from the native plants and thus change the continuity, biomass, and vertical distribution of fuels in a community (D’Antonio 2000). Changes in fire regimes have the potential to further impact the remaining plant community by suppressing species that are poorly adapted and promoting species that are well adapted to fire patterns under the new regime.