fire ecology

 

Fire ecology is concerned with the processes linking fire behavior and ecological effect. Campaigns such as “Smokey Bear” in the USA have molded public opinion to believe that wildfires are always harmful to nature. This view is based on the outdated belief that ecosystems progress toward an equilibrium and that disturbance (such as fire) disrupts the harmony of nature. More recent ecological research has shown, however, that fire is an integral component to the function and biodiversity of many communities, and that the organisms within those communities have adapted to withstand and even exploit it. Fire suppression, in combination with other human-caused environmental changes, has resulted in unforeseen changes to ecosystem dynamics and species composition and has backfired to create some of the largest, most intense wildfires yet. Land managers are faced with tough questions about where it is appropriate to restore a fire regime and how to do it. These questions are crucial today as we see the consequences of years of fire suppression and the continued expansion of people into fire-adapted ecosystems.

Components

A fire regime describes the pattern that fire follows in a particular ecosystem. It consists of the following components (Bond and Keeley 2005):

1. Fuel Consumption and Spread Patterns

Fire can burn at three levels. Ground fires burn through soil that is rich in organic matter. Surface fires burn through dead plant material that is on the ground. Crown fires burn in the tops of shrubs and trees. Ecosystems may experience mostly one level of fire or a mix of the three.

2. Intensity

Defined as the energy release per unit length of fireline (kW m^-1). Can be estimated 1) as the product of linear spread rate (m s^-1), low heat of combustion (kJ kg^-1), and combusted fuel mass per unit area, or 2) via flame length correlation (Byram 1959).

3. Severity

This is a term ecologists used to refer to the impact that a fire has on an ecosystem. Ecologists can define it in many ways, but one way is through an estimate of plant mortality.

4. Frequency

This is a measure of how common fires are in a given ecosystem. It is either defined as the interval between fires at a given site, or the amount of time it takes to burn the equivalent of a specified area.

5. Seasonality

This refers to the time of year during which fires are most common. They often occur during the dry season, and in some areas also co-occur with the time of year when lightning is present for fire ignition.

Impacts

The following is a brief description of the immediate impacts of fire on abiotic and biotic components of ecosystems. Longer-term dynamics and whole-community changes are discussed in Section 3.

Abiotic responses to fire

Fire has important effects on the abiotic (non-living) components of an ecosystem, particularly the soil, through both direct contact with the soil and its effects on the plant community using the soil (Hart et al. 2005).

A. Temperature:

By removing overhead vegetation, fire opens soil up to increased solar radiation and warming during the day. Alternately, the loss of vegetation also allows soils to become cooler and do so more quickly at night.

B. Moisture:

Soil moisture does not change predictably with fire, and is a function of fire intensity and soil properties. Fewer leaves left to intercept rain allows more rain to reach the soil’s surface and decreased transpiration (the process by which water travels through plants and evaporates through pores in the leaves) because of the smaller leaves of post-fire plants allows the soil to retain more moisture. This overall positive effect on moisture can be counteracted when fires increase the ground’s exposure to sunlight and evaporation, and/or when fire creates water-repellent soils. Water-repellent soils may form when fire heats organic matter on the ground into a waxy covering. This can lead to increased erosion.

C. Physical and Chemical Properties:

Fire causes nutrient loss through a variety of mechanisms, including oxidation, volatilization, and increased erosion and leeching by water. Temperatures must be very high, however, to cause a significant loss of nutrients, and these nutrients are often quickly replaced by dead organic matter left behind in the fire. Charcoal is able to counteract some nutrient and water loss because of its absorptive properties.

Overall, soils become more basic (lower pH) following fires because of acid combustion. By driving novel chemical reactions at high temperatures, fire can even alter the texture and structure of soils by affecting the clay content and the ability of soil to form aggregates (clumps of soil that increase the ground’s porosity to water).

Biotic adaptations and responses

A. Plants:

Plants have many adaptations to fire. In chaparral communities in Southern California, some plants have leaves coated in flammable oils that foster an intense fire. The heat will cause their fire-activated seeds to germinate and capitalize on the lack of competition in the burnt landscape. Other plants have smoke-activated seeds and/or fire-activated buds. Lodgepole pine (Pinus contorta) cones are sealed with resin until fire melts it away and releases the seeds (USDA Forest Service). Many plant species, including shade-intolerant giant sequoia (Sequoiadendron giganteum), require fire to make light gaps in the vegetation canopy. This allows their new seedlings to compete with more shade-tolerant seedlings of other species and establish themselves in a process known as “recruitment” (US National Park Service).

Because their stationary nature precludes fire avoidance, plants span the range from fire-intolerant species to fire-tolerant to fire-resistant species (Kramp et al. 1986):

i. Fire-Intolerant Plants: Fire-intolerant species tend to be highly flammable and completely destroyed by fire. Some of these plants and their seeds may simply fade from the community after a fire and not return, yet others have adapted to ensure that their offspring survive in the next generation. “Obligate seeders” are plants with large, fire-activated seed banks that germinate, grow, and mature rapidly following a fire in order to reproduce and renew the seed bank before the next fire (Kramp et al. 1986, Knox and Clark 2005).

ii. Fire-Tolerant Plants: Fire-tolerant species, on the other hand, are able to withstand some forms of fire and grow despite some damage. These plants are sometimes referred to as “resprouters.” Ecologists have shown that some species of resprouters store extra energy in their roots for recovery and re-growth following a fire (Kramp et al. 1986, Knox and Clarke 2005).

iii. Fire Resistant Plants: Fire-resistance refers to plants that suffer little damage during a characteristic fire regime. This mostly applies to large trees whose flammable parts are high above surface fires. Mature ponderosa pine (Prosopis glandulosa) is an example of one such tree that suffers virtually no crown damage under its naturally mild fire regime because it sheds its lower, vulnerable branches as it matures (Kramp et al. 1986, Pyne 2002).

2. Animals and Microbes

Like plants, animals display a range of post-fire responses, but they differ from plants in that they must avoid the actual fire to survive. Though birds are vulnerable when nesting, they are generally able to escape the fire. They often profit off of prey items fleeing from the fire and recolonize burned areas quickly because of their high mobility. Mammals are also often capable of either fleeing the fire or seeking cover while it passes and then recolonizing quickly. Amphibians and reptiles may avoid flames by burrowing into the ground or using the burrows of other animals. Amphibians in particular are able to take refuge in water or very wet mud (Kramp et al. 1986). Some arthropods may also take shelter during a fire, though the heat and smoke actually attracts some of them to their deaths (DeBano et al. 1998). Microbial organisms in the soil vary in their heat tolerance but are more likely to survive the deeper they are in the soil, the lower the fire intensity and residence time, and the drier the soil. A post-fire increase in nutrients may result in larger microbial communities than before the fire (Hart et al. 2005).

Long term impacts

Fire behavior is different in every ecosystem and the organisms in those ecosystems have adapted accordingly. One sweeping generality is that in all ecosystems fire creates a mosaic of different habitat patches, with sites ranging from just burned to untouched by fire for years, through a process known as succession. Succession is the progress of site through continuous and directional phases of colonization by and extinction of species populations after a disturbance, such as fire (Begon et al. 1996, pg. 692). Ecologists usually characterize succession through vegetation. After a fire, the first species to colonize are those whose seeds are already present or those whose seeds disperse to the burned area quickly. These are generally fast-growing herbaceous plants that need lots of light and are poor competitors in crowded areas. As time passes, more slowly growing, shade-tolerant, and competitive, woody species crowd out the herbaceous plants. These woody plants may be shrubs or trees (Begon et al. 1996, pg 700). Different species of plants, animals, and microbes specialize in exploiting different successional stages, and by creating these different types of patches, fire allows a greater number of species to exist within a landscape. Below are some characteristics of soils and the three main types of fire-adapted ecosystems. Specific examples are given to illustrate ecosystem-level responses to fire.

Forests

Mild to moderate fires burn in the forest understory, removing small trees and herbaceous groundcover. Only high-intensity fires burn into the crowns of the tallest trees. Crown fires can either be passive crown fires or active crown fires. Passive crown fires require support from ground fuels to maintain the fire in the forest canopy whereas active crown fires can burn in the canopy independent of ground fuel support. [1] Prescribed fires typically aim for low to moderate intensity, whereas wildfires can evolve into crown fires. When a forest burns frequently and thus has less plant litter build-up, below-ground soil temperatures rise only slightly and thus are not lethal to roots deep in the soil (DeBano et al. 1998). Though there are some characteristics inherent to forests that influence fire, Beaty and Taylor (2001) point out that extrinsic factors such as climate and topography also play an important role in determining fire severity and fire extent. They found that fires spread most widely during drought years, are most severe on upper slopes, and are influenced by species composition.

Forests in British Columbia:

In Canada, forests cover 10% of the land yet harbor 70% of the country’s bird and terrestrial mammal species. Bunnell (1995) found that natural fire regimes were important in maintaining a diverse assemblage of vertebrate species in twelve different forest types in British Columbia. Different species have adapted to exploit different stages of succession and habitat created by fire (such as downed trees and debris). He also found that characteristics of the fire, such as its size and intensity, caused the habitat to change differentially and thus influenced how vertebrate species were able to use burned areas.

Shrublands

Shrub fires typically concentrate in the canopy and spread continuously if the shrubs are close enough together. Shrublands are typically dry and are prone to accumulations of highly volatile fuels, especially on hillsides. Burns follow the path of least moisture and greatest amount of dead fuel material. Surface and below-ground soil temperatures during a burn are generally higher than those of forest fires because heat is concentrated lower to the ground, though they can vary greatly (DeBano 1998).

South African Fynbos Shrublands:

Fynbos shrublands occur in a small belt across South Africa. The plant species in this ecosystem are highly diverse, yet the majority of these species are obligate seeders (see above, Biotic Adaptations and Responses to Fire: Plants). Wisheu et al. (2000) hypothesize that plants evolved into obligate seeders as a response to fire and nutrient-poor soils. Because fire is common in this ecosystem and the soil has limited nutrients, it is most efficient for plants to produce many seeds and then die in the next fire. Investing a lot of energy in roots to survive the next fire when those roots will be able to extract little extra benefit from the nutrient-poor soil would be less efficient. They also hypothesize that the rapid generation time of these obligate seeders has led to more rapid evolution and speciation in this ecosystem resulting in its highly diverse plant community.

Grasslands

Grasslands burn more readily than forest and shrub ecosystems, with fire moving through the stems and leaves of herbaceous plants and only lightly heating the underlying soil even in cases of high intensity. In most grassland ecosystems, fire is the primary mode of decomposition, making it crucial in nutrient cycling (DeBano et al. 1998).

South African Savanna:

In the South African savanna, recently burned areas have new growth that provides palatable and nutritious forage compared to older, tougher grasses. This new forage draws large herbivores off unburned grazed lawns, which are areas kept short by constant grazing. On these lawns, only species adapted to grazing are able to persist. The distraction provided by the newly-burned areas allows grazing-intolerant grass species to grow in the abandoned lawns and thus persist within the ecosystem (Archibald et al. 2005).

Fire suppression

As alluded to above, fire serves many important functions within fire-adapted ecosystems. Fire plays an important role in nutrient cycling, diversity maintenance, community composition, and habitat structure. Suppression of fire has led to unforeseen changes in ecosystems that often adversely affect plants, animals, and humans. Below are examples of some of the consequences of fire suppression. Wildfires that deviate from the historical fire regime because of fire suppression are called “uncharacteristic fires.”

Chaparral communities in southern California

In 2003, southern California witnessed powerful chaparral wildfires. Hundreds of homes and hundreds of thousands of acres of land went up in flames. Extreme fire weather (low humidity, low fuel moisture, and high winds) and the accumulation of dead plant material from 8 years of drought contributed to the catastrophic outcome. Although some have maintained that fire suppression contributed to unnatural fuel loads (Minnich 1983), detailed analysis of historical fire data (Keeley et al. 1999) has shown that fire suppression activities have failed to exclude fire from southern California chaparral. Research showing differences in fire size and frequency between southern California and Baja has been used to imply larger fires north of the border are the result of fire suppression, but this opinion has been seriously challenged by numerous investigators and is no longer supported by the majority of fire ecologists.

One consequence of the 2003 fires has been the increased density of invasive and non-native plant species that have quickly colonized burned areas, especially those that have been burned in the previous 15 years. Because shrubs in these communities are adapted to a particular historical fire regime, altered fire regimes may change the selective pressures on plants and favor invasive and non-native species that are better able to exploit the novel post-fire conditions (Keeley et al. 2005).

Boise National Forest

Following several uncharacteristically large wildfires, Burton (2005) found that the high-severity fires had an immediate negative impact on fish populations, posing particular danger to small and isolated fish populations. Burton also found, however, that in the long term fire appears to rejuvenate fish habitat by causing hydraulic changes that increase flooding and lead to silt removal and deposition of favorable habitat substrate. This led to larger post-fire populations of the fish that were able to recolonize these improved areas. He concludes that though fire generally appears favorable for fish in these ecosystems, the more intense immediate detriments of uncharacteristic wildfires, in combination with fragmentation of populations by human barriers to dispersal (such as dams) pose a threat to fish populations.

Ponderosa pine forests of the southwest United States

Ponderosa pine forests are now facing severe damage under harsher fire regimes brought on by fire suppression and aggravated by natural drought cycles (Savage and Mast 2005). Fires in these forests now result in crown fires that cause extensive mortality, whereas these forests historically suffered mild to moderate fires that generally did not reach the crown and left most of the trees alive. McCullough et al. (1998) also notes that fire suppression leads to increased defoliation of the trees by herbivorous insects whose populations might otherwise be moderated by fire.

As a management tool

As a discipline,“restoration ecology” is currently receiving attention as a way to potentially reverse or mitigate some of the changes that humans have caused in ecosystems. Fire is one tool that is currently receiving considerable attention as a tool of restoration and management. Applying fire may create habitat for species negatively impacted by fire suppression, or managers may use it as a novel tool, such as in the control of invasive species without resorting to herbicides or pesticides. What state managers should restore an ecosystem to is a matter in and of itself. Does “natural” mean pre-human? Pre-European? MacDougall et al. (2004) point out that fires set by indigenous people, not natural fires, historically maintained the diversity of the oak grasslands of Canada. When, how, and where managers should use fire as a management tool is subject to debate. Below are some case studies of fire as a restoration tool.

The Florida everglades

The Florida everglades is one example of an ecosystem with a historical regime of frequent fires. Currently, the everglades are undergoing long-term and large-scale restoration. Beckage et al. (2005) suggest that ecologists and managers look to the climate to answer questions about how frequently to prescribe burns, pointing out that there is a strong relationship between climate and fire in Florida. The El Niño Southern Oscillation increases the frequency of lighting strikes, opening up a window for fire before there is too much precipitation. They do warn, however, that human-induced climate change may result in a perpetual El Niño that never allows conditions dry enough for fire and thus thwarts management efforts for fire-dependent species.

The Great Plains shortgrass prairie

For more details on this topic, see Shortgrass prairie.

The combination of heavy livestock grazing and fire-suppression has drastically altered the structure, composition, and diversity of the shortgrass prairie ecosystem, allowing woody species to dominate many areas and promoting fire-intolerant invasive species. In semi-arid ecosystems where decomposition is slow, fire is crucial for returning nutrients to the soil and allowing the grasslands to sustain their high productivity. Though fire historically occurred during growing and dormant seasons, Brockway et al. (2002) found prescribed fire during the dormant season was most effective at increasing grass and forb cover, biodiversity, and plant nutrient uptake in shortgrass prairie. Managers must also take into account, however, how invasive and non-native species respond to fire if they also want to restore native ecosystem integrity. For example, Emery and Gross (2005) found that fire could only control the invasive spotted knapweed (Centaurea maculosa) on the Michigan prairie grasslands in the summer because that is the time in knapweed’s life cycle that is most important to population growth.

Mixed conifer forests in the Sierra Nevada

Mixed conifer forests in the Sierra Nevada had fire return intervals that ranged from 5 years up to 300 years depending on the local climate. [2] Lower elevations had frequent fire return intervals and higher and wetter elevations had less frequest natural fires. In areas with frequent fire return intervals, fire suppression has modified the natural fire regime resulting in the heavy accumulation of forest fuels such as downed course woody debris and ladder fuels composed of shade tolerant species such as white fir (Abies concolor). [3] Forests stand are typically treated to modify ground fuels and the forest structure so that wildland fire burning in the stand will burn within the historic range of variability. These treatments are completed with either hand crews, mechanical equipment or a combination of the two. Hand thinning has the advantage of being very "light on the land" but has the disadvantages of high cost and ineffectiveness where the ladder fuels are above 14 inches in diameter measured at breast height. Mechanical treatment methods have the advantage of being able to remove valuable small diameter logs and biomass, but can not treat steep slopes inexpensively and can compact soils if great care is not taken. Both hand thinning and mechanical treatments can retain large downed woody materials and snags important for wildlife habitat. Slash from both hand thinning and mechanical treatments must be processed before prescribed or natural wildland fires can be reintroduced into the stand. [4] With hand thinning, piles are created and burned. Mechanical treatments can either remove the slash where it can be processed at the landing or masticated or burned as slash mats. Regardless of the methodology, once the forest has been treated, fire can be reintroduced. [5]

Current policies

United States

Fire policy involves the federal government, individual state governments, tribal governments, interest groups, and the general public. The new federal outlook on fire policy parallels advances in ecology and moves toward the view that many ecosystems depend on disturbance for diversity and process maintenance. Though human safety is still the number one priority in fire management, new government goals include long-term consideration of ecosystem function. The newest policy allows managers to gauge the relative values of private property and resources in particular situations and set their priorities to maximize economic efficiency (USDA Forest Service). Advances in policy technique, such as sophisticated risk assessment strategies that integrate the latest in ecological research with the social and economic consequences of a particular outcome, are one way to make the most informed fire policy decisions based on the interests of many stakeholders (Dellasala et al. 2004, Fairbrother and Turnley 2005). The government now recognizes that the longer fuel accumulates in fire suppressed areas, the greater the damage will be when an unexpected fire burns out of control (USDA Forest Service). One of the primary goals in fire management is increased public education in order to deprogram some of the “Smokey the Bear” fire suppression mentality and introduce the public to the benefits of regular natural fire. Some Impediments to fire reintroduction include funding, regulations set by the Clean Air Act and the Environmental Protection Agency concerning wildfire emissions, limited fire professionals, potential property damage from escaped fire and complaints about smoke and destruction of scenic views (USDA Forest Service).

References

Bibliography

• Beaty, M.R. and A.H. Taylor. 2001. Spatial and temporal variation of fire regimes in a mixed conifer forest landscape, Southern Cascades, California, USA. Journal of Biogeography 28: 955-966.

• Begon, M., J.L. Harper and C.R. Townsend. 1996. Ecology: individuals, populations, and communities, Third Edition. Blackwell Science Ltd., Cambridge, Massachusetts, USA.

• Bond, W. J., and J. E. Keeley. 2005. Fire as a global 'herbivore': the ecology and evolution of flammable ecosystems. Trends in Ecology & Evolution 20: 387-394.

• Brockway, D.G., R.G. Gatewood, and R.B. Paris. 2002. Restoring fire as an ecological process in shortgrass prairie ecosystems: initial effects of prescribed burning during the dormant and growing seasons. Journal of Environmental Management 65:135-152.

• Bunnell, F.L. 1995. Forest-dwelling vertebrate faunas and natural fire regimes in British Columbia: patterns and implications for conservation. Conservation Biology 9: 636-644.

• DeBano, L.F., D.G. Neary, P.F. Ffolliot. 1998. Fire’s Effects on Ecosystems. John Wiley & Sons, Inc., New York, New York, USA.

• Dellasala, D.A., J.E. Williams, C.D. Williams, and J.F. Franklin. 2004. Beyond smoke and mirrors: a synthesis of fire policy and science. Conservation Biology 18:976-986.

• Emery, S.M., and K.L. Gross. 2005. Effects of timing of prescribed fire on the demography of an invasive plant, spotted knapweed Centaurea maculosa. Journal of Applied Ecology 42:60-69.

• Fairbrother, A., and J. G. Turnley. 2005. Predicting risks of uncharacteristic wildfires: application of the risk assessment process. Forest Ecology and Management 211:28-35.

• Hart, S. C., T. H. DeLuca, G. S. Newman, M. D. MacKenzie, and S. I. Boyle. 2005. Post-fire vegetative dynamics as drivers of microbial community structure and function in forest soils. Forest Ecology and Management 220: 166-184.

• Keeley, J.E., M.B. Keeley., and C. J. Fotheringhamb. 2005. Alien plant dynamics following fire in Mediterranean-climate California shrublands. Ecological Applications 15:2109-2125.

• Keeley, J.E., Fotheringham, C.J., Morais, M. 1999. Reexamining fire suppression impacts on brushland fire regimes. Science Vol. 284. Pg. 1829-1832.

• Knox, K.J.E. and P. Clarke. 2005. Nutrient availability induces contrasting allocation and starch formation in resprouting and obligate seeding shrubs. Functional Ecology 19: 690-698.

• Kramp, B.A., D.R. Patton, and W.W. Brady. 1986. Run wild: wildlife/habitat relationships. U.S. Forest Service, Southwestern Region.

• MacDougall, A.S., B.R. Beckwith, and C.Y. Maslovat. 2004. Defining conservation strategies with historical perspectives: a case study from a degraded oak grassland ecosystem. Conservation Biology 18:455-465.

• McCullough, D.G., R.A. Werner, and D. Neumann. 1998. Fire and insects in northern and boreal forest ecosystems of North America. Annual Review of Entomology 43: 107-127.

• Minnich, R.A. 1983. Fire mosaics in Southern California and Northern Baja California. Science 219:1287-1294

• Pyne, S.J. “How Plants Use Fire (And Are Used By It).” 2002. PBS NOVA Online. 1 January, 2006. http://www.pbs.org/wgbh/nova/fire/plants.html.

• Savage, M. and J.N. Mast. 2005. How resilient are southwestern ponderosa pine forests after crown fires? Canadian Journal of Forest Research 35: 967-977.

• Stephens, S. L., and J. J. Moghaddas. 2005. Fuel treatment effects on snags and coarse woody debris in a Sierra Nevada mixed conifer forest. Forest Ecology and Management 214:53-64.

• United States Department of Fish and Agriculture (USDA) Forest Service. www.fs.fed.us.

Federal Wildland Fire Management Policy and Program Review (FWFMP).

http://www.fs.fed.us/land/wdfire.htm.

• United States National Park Service (USNPS). www.nps.gov.

Sequoia and King’s Canyon National Parks. 13 February 2006. “Giant Sequoias and Fire.”

http://www.nps.gov/seki/fire/segi.htm

• Wisheu, I.C., M.L. Rosenzweig, L. Olsvig-Whittaker, A. Shmida. 2000. What makes nutrient-poor Mediterranean heathlands so rich in plant diversity? Evolutionary Ecology Research 2: 935-955.

External links

• USGS Western Ecological Research Center- Fire Ecology

• Yellowstone National Park- Fire Ecology

• Tall Timbers Research Station and Land Conservancy- Fire Ecology

• The Nature Conservancy's web site for fire practitioners- Fire Ecology

• The Nature Conservancy's Global Fire Initiative- Fire Ecology

• The International Journal of Wildland Fire

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