The ecology of fire is a fascinating and important subject that we need to clearly understand. Fires are powerful forces of nature that have the ability to destroy, define, and shape ecosystems. While fires can be extremely destructive, and devastating to the environment, they are natural processes that are an essential part of ecology, and are widely used as a land management tool. Fire is a reaction between various components of an ecosystem – a fire needs three basic elements to sustain it: fuel (vegetation or combustible material/liquid/gas), oxygen (air/wind), and an ignition source (spark).
Fires can be started by natural elements, such as lightning, or when moving boulders strike each other (e.g. earthquakes or rockfalls). More commonly, they are started by man, or man-made sources, either accidentally, or on purpose. Some common ignition sources responsible for accidental fires include sparks from power lines or machinery, cigarettes being tossed out of car windows, or as a result of flying sparks from campfires or barbecues that have been left unattended, or which were not extinguished properly. Fires can be started intentionally as a management tool: to burn firebreaks, to reduce fire risk by removing deadwood, or to encourage regrowth of young shoots. Sometimes fires are lit intentionally by arsonists for all the wrong reasons, with little thought for the environment, welfare of animals, or the economical implications of their irresponsible actions.
Types of Fire
Fires can be characterized into several different types:
- Surface Fires: Where combustible material, including grass, shrubs, bushes, leaf litter, dead twigs and branches, burns on the surface of the ground.
- Crown/Canopy Fires: These can develop in forested areas with tall trees, and are often a result of surface fires that have spread upwards into the tree canopy. In highly forested areas, particularly forests consisting of trees with a high oil content, such as eucalyptus and pines, these fires can spread very quickly, and burn with a fierce intensity.
- Ground Fires: Where sub-surface combustible material in organic, or peat soils, is ignited – often by surface fires – and smolders underground.
Fire regimes refer to the pattern of wildfire occurrence in an ecosystem with regards to factors such as the frequency, intensity, extent, type of fire, and season of occurrence. Because ecosystems differ in terms of their ecological makeup, climatic conditions, ignition sources, vegetation type and fuel load, fire regimes differ widely between ecosystems.
Natural fire regimes have, in many cases, been changed by humans, and consequently we have been able to manipulate (intentionally or not) the evolution of ecosystems to a large degree. Humans have the ability to start fires; and to prevent, or control fires that would otherwise form part of a natural fire regime. Man has used fire to clear large tracts of forests, savannah, or grassland for agriculture or urban development. Natural fuel loads of combustible material have been reduced by logging, and by the introduction of non-indigenous species such as cattle and goats, which graze heavily on vegetation, keeping the combustible fuel load low.
An ecosystem may be subjected to one or more types of fire that may occur seasonally, or over a number of years, which may vary in magnitude, frequency and intensity. This geographical fire regime is similar to geographical climate, in that as weather conditions, storm frequency, and intensity, may differ from day to day, or month to month, weather patterns remain fairly constant over time. The same is true for patterns exhibited by fire – while fire behavior, type, frequency and intensity may differ over seasons and years, this typically follows a distinct trend, that is referred to as the fire regime.
Geographical climate – particularly rainfall patterns – play an important role in determining the fire regime of an ecosystem. Rainfall not only reduces the immediate fire risk by preventing vegetation from drying out, but it also encourages growth of vegetation, which consequently increases the combustible fuel load when conditions get drier. Fuel load and fuel type are both important factors in determining fire type, fire intensity, and behavior. Wind, temperature, and humidity are other weather conditions that affect fire.
The topography, particularly slope, affects the speed at which a fire advances. A fire will move quickly up a slope, for two reasons: increased radiant heat, and increased convective heat. Combustible material ahead of the fire is pre-heated by radiant heat from the flames, which are much closer to the fuel source on a slope. The rising heat from the fire not only dries out the vegetation ahead of the flames, it also causes a draft that draws the fire up the slope, causing it to advance at a much quicker rate than it would on a flat surface.
Effects of Fire on Plant and Animals
Plants and animals adapt to fire regimes, rather than to fire itself. They develop mechanisms to survive the pattern of burning that occurs in their ecosystem. However, should an unseasonal or irregular type of fire occur, organisms may not be suitably adapted to recover readily. Some ecosystems are more adapted to fire than others, for example savannah and fynbos regions recover very quickly following a fire, whereas rainforest vegetation is not adapted to fire, and does not recover as readily.
Unlike animals, plants can’t move away from a fire, so they are doomed to their fate if a fire sweeps through an area. However, many plants have evolved mechanisms that enable them to survive a fire at individual, population, or community level. Individual survival mechanisms include the protection of vulnerable tissue from heat by having: underground roots and stems, thick insulating bark, or the ability to regenerate through rapid re-sprouting either above-ground, or from underground stems and rhizomes.
Other plants are stimulated to reproduce after a fire, where nutrient rich ash and bush clearance following a fire, provides optimal conditions for seed germination: an increase in the availability of space, light, water, and nutrients; with a reduction in competition. In fact, some plants are dependent on fire for germination, and seeds are only released when stimulated by fire related cues (serotiny). Heat and smoke can germinate seeds contained in soil seed banks, or can stimulate buds of some plants to fruit. Hard exterior shells may be penetrated by heat, allowing seeds to escape, or water to enter to encourage seed germination. Chemical cues in smoke can trigger some plant species to flower, while chemicals and nutrients released by fire can enhance and enrich soils, allowing the germinated seeds to flourish.
Animals that inhabit areas that are prone to wildfires are generally relatively fire tolerant, having developed a number of strategies to survive fires. Some animals may avoid a fire by dispersing away from the area, or they may move onto unburned areas within the fire zone. Other animals seek refuge from the heat of a fire in burrows under the soil, or in rock crevices. Yet other species actively make use of fire for feeding, and in some cases even for breeding – some birds nest on burnt ground, and have eggs that are camouflaged to blend with ash.
Unless animals become trapped by fences or other barriers, the number of animals killed by fires is typically rather low in large motile animals. Mortality may tend to be higher in slower moving insects and invertebrates, or if the fire is unseasonal, it may kill chicks in the nest or result in clutch failure. However, fires tend to affect animals more severely through indirect effects, such as changes in habitat, food availability, and increased vulnerability to predation. As wildfires drastically alter the structure of a habitat, including the vegetation, micro-habitat, and availability of food resources, they generally tend to change the composition of animals found in an area, as many choose to move away to areas where food, shelter, or camouflage is more plentiful. Conversely, other animals may be attracted to the area to take advantage of new food resources, such as dead insects, fresh new buds, or succulent new grass shoots. While the lack of vegetative cover may expose some animals to a greater risk of predation, yet others may benefit as prey is more visible. The change in species composition of both plant and animal communities following a fire can be affected by the characteristics of the fire, and the fire regime.
Effects of Fire on the Environment
The burning of woody vegetation changes the cycling of nutrients between soil, plants, and the atmosphere, and releases the carbon stored within plants, together with other nutrients, such as nitrogen, potassium, magnesium, phosphorous, and sulphur, to the atmosphere in the form of gases, fine particles, and charred organic matter. Carbon and nutrients are also returned to the soil through deposition of ash – both on site, and further afield once airborne particles and debris settle.
Frequent wildfires can reduce the soil organic matter, which can in turn limit the ability of the soil to absorb and hold water, and reduces the availability of nutrients and minerals – notably nitrogen – through the mineralization of organic matter. The loss of nitrogen, a key nutrient that is essential for production, can have the biggest impact on ecosystem dynamics.
Wildfires not only contribute to air pollution, they are important emitters of greenhouse gases – the most important being carbon dioxide, methane (21 x more potent than CO2), and nitrous oxide (310 x more potent than CO2). An increase in greenhouse gases in the atmosphere increases global warming, which in turn provides feedback that can increase the frequency and intensity of fires through climate change (e.g. increased temperatures, and/or decreased rainfall). According to the CSIRO, bush fires in Australian savannahs contribute as much as one third of anthropogenic carbon dioxide emitted by Australia. However, because vegetation removes equivalent amounts of carbon dioxide from the atmosphere during the recovery and regrowth phase following a bushfire, these figures are not included in their inventory of greenhouse gas emissions.
The amount of carbon dioxide released into the atmosphere is largely dependent on the intensity of a fire. As fires are likely to be more intense towards the end of a dry season, when fuel loads are high, and dry, the timing of burning is an important factor in greenhouse gas emissions, as well as carbon storage and cycling. Using fire as a management tool should involve careful consideration and planning, as fires not only contribute heavily to atmospheric carbon dioxide and other greenhouse gases, but the removal of vegetation also reduces the potential for carbon dioxide to be removed from the atmosphere through photosynthesis.
The Human Factor
By manipulating fire and suppressing natural fire regimes, humans have effectively interfered with the ecology of fire adapted ecosystems. By suppressing natural fire regimes, natural ecological processes are interrupted, and fuel loads are allowed to build up to levels that could be disastrous to the ecosystem in the event of a runaway fire. Similarly, if fires occur too frequently, minerals, nutrients, and organic matter that are essential for healthy ecosystem functioning, are lost, and plants and animals that are adapted to a different pattern of fire may have difficulty recovering. This type of disturbance often allows non-indigenous invader species to take hold, changing the ecosystem completely. According to the US Fish and Wildlife Service, the change in intensity and seasonality of natural fire regimes poses a global threat to biological diversity.
The behavior of fire is complex, and is dependent on multiple factors; so too are the ecological effects of fire. Consequently, much research and study is required to gain a better understanding of the ecology of fire, and the environmental impacts of burning (or not burning), and suppressing natural fires, before land management decisions are undertaken.
Featured Image by John McColgan – Edited by Fir0002 [Public domain]