Controlled burn
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A controlled burn (or prescribed burn) is the practice of intentionally setting a fire to remove buildup of vegetation and decaying organic material in a landscape.[1] It is used for a variety of purposes, such as forest management, ecological restoration, land clearing or wildfire fuel management. Controlled burning can also be referred to as hazard reduction burning,backfire, swailing or a burn-off.
Controlled burns are usually performed during cooler months to decrease the risk of more dangerous fires by removing build up of fuel. They play a role in ecology, such as stimulating the germination of some trees and revealing soil mineral layers to increase seedling vitality. In grassland, controlled burns shift the species present to mainly native grassland species, removing invasive species.[2]
Controlled burning is also a large part of cultural burning practices, having been used for millennia by indigenous people across the world. Colonial legislation in North America and Australia displaced indigenous natives and prevented the traditional use of cultural burns, but this was reversed in the 20th century, with indigenous burning practices being more incorporated into land management.[3]
Uses
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Forestry
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Controlled burning reduces fuels, improves wildlife habitat,[4] controls competing vegetation,[5] helps control tree disease and pests,[6] and perpetuates fire-dependent species.[7][8] To improve the application of prescribed burns for conservation goals, which may involve mimicking historical or natural fire regimes, scientists assess the impact of variation in fire attributes.[9] Parameters measured are fire frequency, intensity, severity, patchiness, spatial scale and phenology.[10]
Furthermore, controlled fire can be used for site preparation when mechanized treatments are not possible because of terrain that prevents equipment access.[11][12] Species variation and competition can drastically increase a few years after fuel treatments because of the increase in soil nutrients and availability of space and sunlight.[13]
Many trees depend on fire as a way to clear out other plant species and release their seeds. The giant sequoia, among other fire-adapted conifer species, depends on fire to reproduce.[14] The cones are pyriscent so they will only open after exposure to a certain temperature. This reduces competition for the giant sequoia seedlings because the fire has cleared non-fire-adapted, competing species.[15][16] Pyriscent species benefit from moderate-intensity fires in older stands; however, climate change is causing more frequent high intensity fires in North America.[17] Controlled burns can manage the fire cycle and the intensity of regenerate fires in forests with pyriscent species like the boreal forest in Canada.
Eucalyptus regnans or mountain ash of Australia also shows a unique evolution with fire, quickly replacing damaged buds or stems in the case of danger[citation needed]. They also carry their seeds in capsules which can be deposited at any time of the year [citation needed]. During a wildfire, the capsules drop nearly all of their seeds and the fire consumes the eucalypt adults, but most of the seeds survive using the ash as a source of nutrients. At their rate of growth, they quickly dominate the land and a new, like-aged eucalyptus forest grows.[18] Other tree species like poplar can easily regenerate after a fire into a like-aged stand from a vast root system that is protected from fires because it is underground.[19]
Grassland restoration
[edit]Native grassland species in North America and Australia are adapted to survive occasional low intensity fires.[20] Controlled burns in prairie ecosystems mimic low intensity fires that shift the composition of plants from non-native species to native species.[2] These controlled burns occur during the early spring before native plants begin actively growing, when soil moisture is higher and when the fuel load on the ground is low[21] to ensure that the controlled burn remains low intensity.
Wildfire management
[edit]Controlled burns reduce the amount of understory fuel so when a wildfire enters the area, a controlled burn site can reduce the intensity of the fire or prevent the fire from crossing the area entirely.[22] A controlled burn prior to the wildfire season can protect infrastructure and communities or mitigate risks associated with many dead standing trees such as after a pest infestation when forest fuels are high.[23]
Agriculture
[edit]In the developing world, the use of controlled burns in agriculture is often referred to as slash and burn. In industrialized nations, it is seen as one component of shifting cultivation, as a part of field preparation for planting. Often called field burning, this technique is used to clear the land of any existing crop residue as well as kill weeds and weed seeds. Field burning is less expensive than most other methods such as herbicides or tillage, but because it produces smoke and other fire-related pollutants, its use is not popular in agricultural areas bounded by residential housing.[25]
Prescribed fires are broadly used in the context of woody plant encroachment, with the aim of improving the balance of woody plants and grasses in shrublands and grasslands.[26][27][28][29]
In Northern-India, especially in Punjab, Haryana, and Uttar Pradesh, unregulated burning of agricultural waste is a major problem. Smoke from these fires leads to degradation in environmental quality in these states and the surrounded area.[30]
In East Africa, bird densities increased months after controlled burning had occurred.[31]
Greenhouse gas abatement
[edit]Controlled burns on Australian savannas can result in a long-term cumulative reduction in greenhouse gas emissions. One working example is the West Arnhem Fire Management Agreement, started to bring "strategic fire management across 28,000 square kilometres (11,000 sq mi) of Western Arnhem Land" to partially offset greenhouse gas emissions from a liquefied natural gas plant in Darwin, Australia. Deliberately starting controlled burns early in the dry season results in a mosaic of burnt and unburnt country which reduces the area of stronger, late dry season fires;[32][33] it is also known as "patch burning".
Procedure
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Health and safety, protecting personnel, preventing the fire from escaping and reducing the impact of smoke are the most important considerations when planning a controlled burn.[34] While the most common driver of fuel treatment is the prevention of loss of human life and structures, certain parameters can also be changed to promote biodiversity and to rearrange the age of a stand or the assemblage of species.
To minimize the impact of smoke, burning should be restricted to daylight hours whenever possible.[35] Furthermore, in temperate climates, it is important to burn grasslands and prairies before native species begin growing for the season so that only non-native species, which send up shoots earlier in the spring, are affected by the fire.[2]
Ground ignition
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Back burning or a back fire is the term given to the process of lighting vegetation in such a way that it has to burn against the prevailing wind. This produces a slower moving and more controllable fire. Controlled burns utilize back burning during planned fire events to create a "black line" where fire cannot burn through. Back burning or backfiring is also done to stop a wildfire that is already in progress. Firebreaks are also used as an anchor point to start a line of fires along natural or man-made features such as a river, road or a bulldozed clearing.[36]
Head fires, that burn with the prevailing wind, are used between two firebreaks because head fires will burn more intensely and move faster than a back burn. Head fires are used when a back burn would move too slowly through the fuel either because the fuel moisture is high or the wind speed is low.[34] Another method to increase the speed of a back burn is to use a flank fire which is lit at right angles to the prevailing wind and spreads in the same direction.[34]
Grassland or prairie burning
[edit]In Ontario, Canada, controlled burns are regulated by the Ministry of Natural Resources and only trained personnel can plan and ignite controlled burns within Ontario's fire regions or if the Ministry of Natural Resources in involved in any aspect of planning a controlled burn.[37] The team performing the prescribed burn is divided into several roles; the Burn Boss, Communications, Suppression and Ignition.[34] The planning process begins by submitting an application to a local fire management office and after approval, applicants must submit a burn plan several weeks prior to ignition.[37]
On the day of the controlled burn, personnel meet with the Burn Boss and discuss the tactics being used for ignition and suppression, health and safety precautions, fuel moisture levels and the weather (wind direction, wind speed, temperature and precipitation) for the day. On site, local fire control authorities are notified by telephone about the controlled burn while the rest of the team members fill drip torches with pre-mixed fuel, fill suppression packs with water and put up barricades and signage to prevent pedestrian access to the controlled burn. Driptorches are canisters filled with fuel and a wick at the end that is used to ignite the lines of fire. Safe zones are established to ensure personnel know where the fire cannot cross either because of natural barriers like bodies of water or human-made barriers like tilled earth.[34]
During ignition, the Burn Boss relays information about the fire (flame length, flame height, the percent of ground that has been blackened) to the Communications Officer who documents this information. The Communications Officer relays information about the wind speed and wind direction so the Burn Boss can determine how the direction of both flames and smoke and plan their lines of fire accordingly. Once the ignition phase has ended in a section, the suppression team "mops up" by using suppression packs to suppress smoldering material. Other tools used for suppression are RTVs equipped with a water tank and a pump and hose that is installed in a nearby body of water. Finally, once the mop up has finished, the Burn Boss declares the controlled burn over and local fire authorities are notified.[34]
Slash pile burning
[edit]There are several different methods used to burn piles of slash from forestry operations. Broadcast burning is the burning of scattered slash over a wide area. Pile burning is gathering up the slash into piles before burning. These burning piles may be referred to as bonfires. High temperatures can harm the soil, damaging it physically, chemically or sterilizing it. Broadcast burns tend to have lower temperatures and will not harm the soil as much as pile burning,[38] though steps can be taken to treat the soil after a burn. In lop and scatter burning, slash is left to compact over time, or is compacted with machinery. This produces a lower intensity fire, as long as the slash is not packed too tightly.[38]
The risk of fatal fires that stem from burning slash can also be reduced by proactively reducing ground fuels before they can create a fuel ladder and begin an active crown fire. Predictions show thinned forests lead to a reduction in fire intensity and flame lengths of forest fires compared to untouched or fire-proofed areas.[39]