controlled burn
A controlled or prescribed (Rx) burn, which can include hazard reduction burning, backfire, swailing or a burn-off, is a fire set intentionally for purposes of forest management, fire suppression, farming, prairie restoration or greenhouse gas abatement. A controlled burn may also refer to the intentional burning of slash and fuels through burn piles. Fire is a natural part of both forest and grassland ecology and controlled fire can be a tool for foresters.
Hazard reduction or controlled burning is conducted during the cooler months to reduce fuel buildup and decrease the likelihood of serious hotter fires. Controlled burning stimulates the germination of some desirable forest trees, and reveals soil mineral layers which increases seedling vitality, thus renewing the forest. Some cones, such as those of lodgepole pine, sequoia and many chaparral shrubs are pyriscent, meaning heat from fire opens cones to disperse seeds.
In industrialized countries, controlled burning is usually overseen by fire control authorities for regulations and permits.

History
There are two basic causes of wildfires. One is natural, mainly through lightning, and the other is human activity. Controlled burns have a long history in wildland management. Pre-agricultural societies used fire to regulate both plant and animal life. Fire history studies have documented periodic wildland fires ignited by indigenous peoples in North America and Australia. Native Americans frequently used fire to manage natural environments in a way that benefited humans and wildlife, starting low-intensity fires that released nutrients for plants, reduced competition, and consumed excess flammable material that otherwise would eventually fuel high-intensity, catastrophic fires.Fires, both naturally caused and prescribed, were once part of natural landscapes in many areas. In the US, these practices ended in the early 20th century, when federal fire policies were enacted with the goal of suppressing all fires. Since 1995, the US Forest Service has slowly incorporated burning practices into its forest management policies.Fire suppression has changed the composition and ecology of North American habitats, including highly fire-dependent ecosystems such as oak savannas and canebrakes, which are now critically endangered habitats on the brink of extinction. In the Eastern United States, fire-sensitive trees such as the red maple are increasing in number, at the expense of fire-tolerant ones like oaks.

Forest use
Controlled burning reduces fuels, may improve wildlife habitat, controls competing vegetation, improves short term forage for grazing, improves accessibility, helps control tree disease, and perpetuates fire dependent species. 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. Fire frequency is the most discussed fire attribute in the scientific literature, likely because it is considered the most critical fire regime aspect. Scientists less often report data concerning the effects of variation in other fire attributes (i.e., intensity, severity, patchiness, spatial scale, or phenology), even though these also likely impact conservation goals.Furthermore, low-intensity fire treatments can be administered in places where mechanized treatments such as disc harrowing cannot. In some areas where grasses and herbaceous plants thrive, species variation and cover can drastically increase a few years after fuel treatments.Many trees depend on fire as a successful way to clear out the competition and release their seeds. In particular, the giant sequoia depends on fire to reproduce: the cones of the tree open after a fire releases their seeds, the fire having cleared all competing vegetation.Eucalyptus regnans or mountain ash of Australia also depends on fire but in a different fashion. The plant structure shows a unique evolution with fire, quickly replacing damaged buds or stems in the case of danger. They carry their seeds in capsules which can be deposited at any time of the year. 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 eucalyptus forest grows.

Agricultural use
In addition to forest management, controlled burning is also used in agriculture. In the developing world, this 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.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.In Northern-India, especially, In Punjab, Haryana, and Uttar Pradesh, crop residue burning is a major problem. CRB leads to degradation in environmental quality in these and neighboring states including in the capital of India, New Delhi.In East Africa, bird densities increased months after controlled burning had occurred.

Controversies
The Oregon Department of Environmental Quality began requiring a permit for farmers to burn their fields in 1981, but the requirements became stricter in 1988 following a multi-car collision in which smoke from field burning near Albany, Oregon, obscured the vision of drivers on Interstate 5, leading to a 23-car collision in which 7 people died and 37 were injured. This resulted in more scrutiny of field burning and proposals to ban field burning in the state altogether.In the European Union, burning crop stubble after harvest is used by farmers for plant health reasons under several restrictions in cross-compliance regulations.With controlled burns, there is also a risk that the fires get out of control. For example, the Calf Canyon/Hermits Peak Fire, the largest wildfire in the history of New Mexico, was started by two distinct instances of controlled burns, which had both been set by the US Forest Service, getting out of control and merging.
In the north of Great Britain, large areas of grouse moors are managed by burning in a practice known as muirburn. This kills trees and grasses, preventing natural succession, and generates the mosaic of ling (heather) of different ages which allows very large populations of red grouse to be reared for shooting. The peat-lands are some of the largest carbon sinks in the UK, providing an immensely important ecological service. The governments has restricted burning to the area but hunters have been continuing to set the moors ablaze, releasing a large amount of carbon into the atmosphere and destroying native habitat.

Political history
The conflict of controlled burn policy in the United States has roots in historical campaigns to combat wildfires and to the eventual acceptance of fire as a necessary ecological phenomenon. Following colonization of North America, the US used fire suppression laws to eradicate the indigenous practice of prescribed fire. This was done against scientific evidence that supported prescribed burns as a natural process. At the loss to the local environment, colonies utilized fire suppression in order to benefit the logging industry. The notion of fire as a tool had somewhat evolved by the late 1970s as the National Park Service authorized and administered controlled burns. Following prescribed fire reintroduction, the Yellowstone fires of 1988 occurred, which significantly politicized fire management. The ensuing media coverage was a spectacle that was vulnerable to misinformation. Reports drastically inflated the scale of the fires which disposed politicians in Wyoming, Idaho, and Montana to believe that all fires represented a loss of revenue from tourism.  Paramount to the new action plans is the suppression of fires that threaten the loss of human life with leniency toward areas of historic, scientific, or special ecological interest.There is still a debate amongst policy makers about how to deal with wildfires. Senators Ron Wyden and Mike Crapo of Oregon and Idaho have been moving to reduce the shifting of capital from fire prevention to fire suppression following the harsh fires of 2017 in both states.Tensions around fire prevention continue to rise due to the increasing prevalence of climate change. As drought conditions worsen, North America has been facing an abundance of destructive wildfires. Since 1988, many states have made progress toward controlled burns. In 2021, California increased the amount of trained personnel to perform controlled burns and created more accessibility for landowners.

Procedure
Depending on the context and goals of a prescribed fire, additional planning may be necessary. While the most common driver of fuel treatment is the prevention of loss of human life, certain parameters can also be changed to promote biodiversity and to rearrange stand ages appropriately.
For the burning of slash, waste materials left over from logging, there are several types of controlled burns. 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, 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.The risk of fatal fires can also be reduced proactively by reducing ground fuels before they can create a fuel ladder and begin an active crown fire. Predictions show thinned forests lead to mitigated fire intensity and flame length compared to untouched or fire-proofed areas.Back burning 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. While controlled burns utilize back burning during planned fire events to create a "black line", back burning or backfiring is also done to stop a wildfire that is already in progress. Firebreaks are also often 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.To minimise the impact of smoke, burning should be restricted to daylight hours whenever possible.

Greenhouse gas abatement
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; it is also known as "patch burning".

See also
Agroecology
Fire ecology
Fire-stick farming
Native American use of fire in ecosystems
Wildfire suppression

References
Further reading
Beese, W.J., Blackwell, B.A., Green, R.N. & Hawkes, B.C. (2006). "Prescribed burning impacts on some coastal British Columbia ecosystems." Information Report BC-X-403. Victoria B.C.: Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Retrieved from: http://hdl.handle.net/10613/2740
Casals P, Valor T, Besalú A, Molina-Terrén D. Understory fuel load and structure eight to nine years after prescribed burning in Mediterranean pine forests. DOI: 10.1016/j.foreco.2015.11.050
Valor T, González-Olabarria JR, Piqué M. Assessing the impact of prescribed burning on the growth of European pines. DOI: 10.1016/j.foreco.2015.02.002.

External links

U.S. National Park Service Prescribed Fire Policy
Savanna Oak Foundation article on controlled burns
The Nature Conservancy's Global Fire Initiative