Study of fire in ecosystems
Fire ecology is a scientific discipline concerned with the effects of fire on natural ecosystems. Many ecosystems, particularly prairie, savanna, chaparral and coniferous forests, have evolved with fire as an essential contributor to habitat vitality and renewal. Many plant species in fire-affected environments use fire to germinate, establish, or to reproduce. Wildfire suppression not only endangers these species, but also the animals that depend upon them.
Wildfire suppression campaigns in the United States have historically molded public opinion to believe that wildfires are harmful to nature. Ecological research has shown, however, that fire is an integral component in the function and biodiversity of many natural habitats, and that the organisms within these communities have adapted to withstand, and even to exploit, natural wildfire. More generally, fire is now regarded as a 'natural disturbance', similar to flooding, windstorms, and landslides, that has driven the evolution of species and controls the characteristics of ecosystems.
Fire suppression, in combination with other human-caused environmental changes, may have resulted in unforeseen consequences for natural ecosystems. Some large wildfires in the United States have been blamed on years of fire suppression and the continuing expansion of people into fire-adapted ecosystems as well as climate change. Land managers are faced with tough questions regarding how to restore a natural fire regime, but allowing wildfires to burn is likely the least expensive and most effective method in many situations.
History.
Fire has played a major role in shaping the world's vegetation. The biological process of photosynthesis began to concentrate the atmospheric oxygen needed for combustion during the Devonian approximately 350 million years ago. Then, approximately 125 million years ago, fire began to influence the habitat of land plants.
In the 20th century ecologist Charles Cooper made a plea for fire as an ecosystem process.
Fire components.
A fire regime describes the characteristics of fire and how it interacts with a particular ecosystem. Its "severity" is a term that ecologists use to refer to the impact that a fire has on an ecosystem. It is usually studied using tools such as remote sensing which can detect burned area estimates, severity and fire risk associated with an area. Ecologists can define this in many ways, but one way is through an estimate of plant mortality.
Fires can burn at three elevation levels. Ground fires will burn through soil that is rich in organic matter. Surface fires will burn through living and dead plant material at ground level. Crown fires will burn through the tops of shrubs and trees. Ecosystems generally experience a mix of all three.
Fires will often break out during a dry season, but in some areas wildfires also commonly occur during times of year when lightning is prevalent. The frequency over a span of years at which fire will occur at a particular location is a measure of how common wildfires are in a given ecosystem. It is either defined as the average interval between fires at a given site, or the average interval between fires in an equivalent specified area.
Defined as the energy released per unit length of fireline (kW m−1), wildfire intensity can be estimated either as
Abiotic responses.
Fires can affect soils through heating and combustion processes. Depending on the temperatures of the soils during the combustion process, different effects will happen- from evaporation of water at the lower temperature ranges, to the combustion of soil organic matter and the formation of pyrogenic organic matter, such as charcoal.
Fires can cause changes in soil nutrients through a variety of mechanisms, which include oxidation, volatilization, erosion, and leaching by water, but the event must usually be of high temperatures for significant loss of nutrients to occur. However, the quantity of bioavailable nutrients in the soil usually increases due to the ash that is generated, as compared to the slow release of nutrients by decomposition. Rock spalling (or thermal exfoliation) accelerates weathering of rock and potentially the release of some nutrients.
Increase in the pH of the soil following a fire is commonly observed, most likely due to the formation of calcium carbonate, and the subsequent decomposition of this calcium carbonate to calcium oxide when temperatures get even higher. It could also be due to the increased cation content in the soil due to the ash, which temporarily increases soil pH. Microbial activity in the soil might also increase due to the heating of soil and increased nutrient content in the soil, though studies have also found complete loss of microbes on the top layer of soil after a fire. Overall, soils become more basic (higher 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 soil's porosity.
Removal of vegetation following a fire can cause several effects on the soil, such as increasing the temperatures of the soil during the day due to increased solar radiation on the soil surface, and greater cooling due to loss of radiative heat at night. Less plant matter to intercept rain will allow more to reach the soil surface, and with fewer plants to absorb the water, the amount of water content in the soils might increase. However, ash can be water repellent when dry, and therefore water content and availability might not actually increase.
Biotic responses and adaptations.
Plants.
Plants have evolved many adaptations to cope with fire. Of these adaptations, one of the best-known is likely "pyriscence", where maturation and release of seeds is triggered, in whole or in part, by fire or smoke; this behaviour is often erroneously called "serotiny", although this term truly denotes the much broader category of seed release activated by any stimulus. All pyriscent plants are serotinous, but not all serotinous plants are pyriscent (some are necriscent, hygriscent, xeriscent, soliscent, or some combination thereof). On the other hand, germination of seed activated by trigger is not to be confused with pyriscence; it is known as "physiological dormancy".
In chaparral communities in Southern California, for example, some plants have leaves coated in flammable oils that encourage an intense fire. This heat causes their fire-activated seeds to germinate (an example of dormancy) and the young plants can then capitalize on the lack of competition in a burnt landscape. Other plants have smoke-activated seeds, or fire-activated buds. The cones of the Lodgepole pine ("Pinus contorta") are, conversely, pyriscent: they are sealed with a resin that a fire melts away, releasing the seeds. Many plant species, including the shade-intolerant giant sequoia ("Sequoiadendron giganteum"), require fire to make gaps in the vegetation canopy that will let in light, allowing their seedlings to compete with the more shade-tolerant seedlings of other species, and so establish themselves. Because their stationary nature precludes any fire avoidance, plant species may only be fire-intolerant, fire-tolerant or fire-resistant.
Fire intolerance.
Fire-intolerant plant species tend to be highly flammable and are destroyed completely by fire. Some of these plants and their seeds may simply fade from the community after a fire and not return; others have adapted to ensure that their offspring survives into 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.
Seeds may contain the receptor protein KAI2, that is activated by the growth hormones karrikin released by the fire.
Fire tolerance.
Fire-tolerant species are able to withstand a degree of burning and continue growing despite damage from fire. These plants are sometimes referred to as "resprouters". Ecologists have shown that some species of resprouters store extra energy in their roots to aid recovery and re-growth following a fire. For example, after an Australian bushfire, the Mountain Grey Gum tree ("Eucalyptus cypellocarpa") starts producing a mass of shoots of leaves from the base of the tree all the way up the trunk towards the top, making it look like a black stick completely covered with young, green leaves.
Fire resistance.
Fire-resistant plants suffer little damage during a characteristic fire regime. These include large trees whose flammable parts are high above surface fires. Mature ponderosa pine ("Pinus ponderosa") is an example of a tree species that suffers little to no crown damage during a low severity fire because it sheds its lower, vulnerable branches as it matures.
Animals, birds and microbes.
Like plants, animals display a range of abilities to cope with fire, but they differ from most plants in that they must avoid the actual fire to survive. Although birds may be vulnerable when nesting, they are generally able to escape a fire; indeed they often profit from being able to take prey fleeing from a fire and to recolonize burned areas quickly afterwards. In fact, many wildlife species globally are dependent on recurring fires in fire-dependent ecosystems to create and maintain habitat. Some anthropological and ethno-ornithological evidence suggests that certain species of fire-foraging raptors may engage in intentional fire propagation to flush out prey. Mammals are often capable of fleeing a fire, or seeking cover if they can burrow. 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.
Some arthropods also take shelter during a fire, although the heat and smoke may actually attract some of them, to their peril. Microbial organisms in the soil vary in their heat tolerance but are more likely to be able to survive a fire the deeper they are in the soil. A low fire intensity, a quick passing of the flames and a dry soil will also help. An increase in available nutrients after the fire has passed may result in larger microbial communities than before the fire. The generally greater heat tolerance of bacteria relative to fungi makes it possible for soil microbial population diversity to change following a fire, depending on the severity of the fire, the depth of the microbes in the soil, and the presence of plant cover. Certain species of fungi, such as "Cylindrocarpon destructans" appear to be unaffected by combustion contaminants, which can inhibit re-population of burnt soil by other microorganisms, and therefore have a higher chance of surviving fire disturbance and then recolonizing and out-competing other fungal species afterwards.
Fire and ecological succession.
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 areas ranging from those having just been burned to those that have been untouched by fire for many years. This is a form of ecological succession in which a freshly burned site will progress through continuous and directional phases of colonization following the destruction caused by the fire. Ecologists usually characterize succession through the changes in vegetation that successively arise. After a fire, the first species to re-colonize will be those with seeds are already present in the soil, or those with seeds are able to travel into the burned area quickly. These are generally fast-growing herbaceous plants that require light and are intolerant of shading. As time passes, more slowly growing, shade-tolerant woody species will suppress some of the herbaceous plants. Conifers are often early successional species, while broad leaf trees frequently replace them in the absence of fire. Hence, many conifer forests are themselves dependent upon recurring fire. Both natural and human fires affect all ecosystems from peatlands to shrublands to forests and tropical landscapes. This impacts the way that the ecosystem is structured and functions. Though there have always been wildfires naturally, the frequency of wildfires has increased at a rapid rate in recent years. This is largely due to decreases in precipitation, increases in temperature, and increases in human ignitions.
Different species of plants, animals, and microbes specialize in exploiting different stages in this process of succession, and by creating these different types of patches, fire allows a greater number of species to exist within a landscape. Soil characteristics will be a factor in determining the specific nature of a fire-adapted ecosystem, as will climate and topography. Different frequencies of fire also result in different successional pathways; short intervals between fires often eliminate tree species due to the time required to rebuild a seed bank, resulting in replacement by lighter seeded species like grasses and forbs.
Examples of fire in different ecosystems.
Forests.
Mild to