Prescribed burning is the deliberate use of fire in specific areas within prescribed fuel and weather conditions (e.g., fuel moisture content, relative humidity, wind speed). Some call the activity “controlled” burning, but this overstates reality: fire is a self-sustaining chemical reaction, and if conditions change (e.g., wind speed increases), prescribed fires can escape control and cause extensive damage (e.g., the Cerro Grande Fire was a prescribed fire that escaped control and burned 237 homes in Los Alamos, New Mexico, in May 2000). Some observers include as prescribed burning naturally occurring fires that are allowed to burn because they are within acceptable areas and conditions, as prescribed in fire management plans. The agencies use the term wildland fire use for such fires; although they do not include them as prescribed fires, they do include the acres burned in wildland-fire-use fires as acres treated for fuel reduction.
Because of the relatively high moisture levels and low wind speeds, prescribed burning primarily eliminates the fine and small fuels. Indeed, burning and decomposition are the only means for reducing fine and small fuels. Burning converts the biomass to smoke (carbon dioxide, water vapor, fine particulates, and other pollutants) and ashes (minerals from the organic matter, readily available to feed new plant growth).
However, prescribed burning has its limitations. It has little effect on the large diameter fuels. Also, prescribed fire is not a discriminating tool for reducing tree density, crown density, and fuel ladders, since the fire burns whatever biomass is available, depending on a host of site-specific and micro-climatic conditions. Prescribed burning is also risky; several escaped prescribed fires have become notable wildfires, with houses and even lives lost. As a result, fire managers tend to err on the side of caution, with substantial (possibly excessive) personnel and equipment, and thus high implementation costs. Finally, the smoke can be a significant health hazard, especially since fire prescriptions tend toward high relative humidity and low wind speeds, which are often associated with inversions and stagnant air masses.
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Note: The first version of this article was drawn from R40811 Wildfire Fuels and Fuel Reduction by Ross W. Gorte, Congressional Research Service on September 16, 2009.