Fire as a tool
Prescribed burning, sometimes called controlled burning, means setting fires on purpose under chosen conditions to meet management goals. “Prescribed” refers to the written plan, or prescription, that states where, when and how a fire may be set, and what weather and fuel conditions must be met. It is quite different from an accidental or uncontrolled fire.
The practice has deep roots. Many Indigenous peoples used fire for thousands of years to shape landscapes, open land for travel and hunting, and encourage useful plants, and some do so today. Modern prescribed burning draws on that heritage as well as on fire science, though it is now done with formal planning, permits and equipment.
Why managers burn
Reducing fuel. Where fire has been kept out for a long time, dead leaves, branches, shrubs and small trees can build up. In a hot, dry spell, that fuel can feed a very intense fire that is hard to stop. Burning it away under mild conditions can lower the chance and severity of a later wildfire, though the benefit fades as fuels regrow, so repeat burns are often needed. The wider problem is discussed in the article on forest fires.
Supporting fire-adapted ecosystems. Some forests evolved with frequent low-intensity fire. Open pine woodlands and savannas in various parts of the world are examples. Without fire, shade-tolerant trees and shrubs may take over, changing the habitat and crowding out fire-dependent plants and animals. Fire ecology describes the species that rely on these cycles.
Preparing seedbeds. Fire can clear thick litter, expose mineral soil and release seeds from certain cones. In some forest types, a burn helps natural regeneration of species such as pines.
Controlling competing vegetation. Fire can kill or set back shrubs and weed trees that compete with the desired species, and it can reduce some pests and diseases.
Improving habitat and forage. Burning can open up the ground layer, stimulate fresh growth and create mosaics of different habitat types that suit a range of wildlife.
Disposing of harvest residue. After logging, piles or spreads of branches and tops (slash) are sometimes burned, though other approaches, such as leaving it on site, are also used.
What a burn plan includes
A typical plan, prepared by a trained burn manager, covers:
- Objectives: what the fire is meant to achieve, such as reducing certain fuels or removing litter.
- Unit boundaries and map: the exact area, with roads, streams, buildings and sensitive features marked.
- Weather and fuel conditions: acceptable ranges of wind speed and direction, temperature, humidity and fuel moisture. Burns happen only when conditions are within the range, and are postponed when they are not.
- Ignition method and pattern: how and where fire is lit.
- Control measures: firebreaks, personnel and equipment on hand.
- Smoke management: how smoke will be limited and directed away from homes, roads and sensitive populations.
- Notification and permits: neighbors, fire agencies and regulators are informed and approvals obtained where required.
- Contingency plans: what to do if the fire escapes the boundary or conditions change.
How fires are kept under control
Safety rests on several layers.
Firebreaks. Lines of bare soil, existing roads, streams or previously burned areas hold the fire inside the unit. Crews may prepare a break by scraping or wetting the ground before ignition.
Pattern of ignition. Lighting a fire against the wind, called a backing fire, produces slow, low flames that are easier to manage. Fires lit with the wind (heading fires) move faster and burn hotter, and are used only where they are appropriate. Crews often start at the downwind edge and work upwind, building a blackened buffer.
Weather monitoring. Crews check forecasts and on-site measurements before and throughout the burn. A shift in wind or drop in humidity can lead to a halt.
Trained staff and equipment. Crews usually include experienced burners, hand tools, water supplies and vehicles, and they often stay on site afterward to watch for smoldering material.
Mop-up and patrol. After the main fire, crews extinguish hot spots near the edge and watch the unit for hours or days.
Even with these measures, there is some risk. Prescribed fires have occasionally escaped and caused damage, sometimes after weather changed unexpectedly. Managers generally weigh that risk against the risk of leaving heavy fuels in place, and this is a debated trade-off.
Costs and drawbacks
- Smoke. Burns produce smoke that can affect nearby residents, particularly people with asthma or heart or lung conditions. Planning attempts to limit this, and the topic ties to the discussion of wildfire smoke.
- Narrow windows. Suitable weather may occur on only a limited number of days, so crews cannot always burn when they want.
- Damage to trees and soil. Too hot a fire can kill trees, scorch roots and harm the soil, which is why fuel and moisture conditions are specified.
- Carbon and air emissions. Burning releases carbon dioxide and other gases. Some argue this is offset if the burn prevents a larger wildfire, but the balance depends on the site.
- Not suitable everywhere. Forests not adapted to fire, such as many wet tropical forests, may be damaged by it.
- Public concern and liability. Fear of escaped fires, smoke nuisance and legal responsibility can limit use.
Where it fits
Prescribed burning is typically one tool among several. It is often combined with mechanical thinning, which removes the larger stems and ladders of small trees that carry fire into the canopy, followed by burning to deal with the remaining fuel. Landowners considering fire should work with trained professionals and follow local laws, since regulations, permits and training requirements differ widely.