Many assert that the threat of severe wildfires and the cost of suppressing fires have grown because many forests have unnaturally high amounts of biomass to fuel the fires, as well as because of climate change and the increasing numbers of homes in and near forests (the wildland-urban interface ). Issues for Congress

Severe wildfires have been burning more structures and more area. The principal tools to reduce biomass fuels are prescribed burning—deliberate use of fire primarily to eliminate fine fuels—and thinning—cutting some vegetation to reduce understory growth, the density of the forest canopy, and the fuel ladders that can allow fires to move from the surface into the canopy. Also, information on funding (from annual appropriations and mandatory accounts) used for thinning and other activities that are substantially intended for wildfire protection, and reporting on results, are inadequate to compare the benefits and costs of these activities and funds. Biomass Fuels for Wildfires

All biomass is potentially fuel for wildfires. Some wildfires are low-intensity, surface fires that burn the surface fuels (e.g., grasses, needles, leaves). Continuity is the other key measure of fuel distribution. Vertical continuity is also important. Fuel Quantity

The quantity of biomass fuels—the fuel load—varies widely. Thus, in many places in the West, these ecosystems currently have fuel loads much higher than was historically natural, with fuel ladders where there were traditionally none. Activities that reduce fuel loads and remove fuel ladders have been documented to reduce wildfire severity (intensity and damage) in surface-fire ecosystems. It is unclear whether fuels are at unnatural levels, sizes, and distributions, and thus it is unclear whether these ecosystems might experience abnormal levels and intensities of crown fires. It is, however, not appropriate for all ecosystems. Some observers have cautioned, however, that such activities do not always reduce the risk of crown fires: 
Thinning and prescribed fires can modify understory microclimate that was previously buffered by overstory vegetation … Thinned stands (open tree canopies) allow solar radiation to penetrate to the forest floor, which then increases surface temperatures, decreases fire fuel moisture, and decreases relative humidity compared to unthinned stands—conditions that can increase surface fire intensity …[and] may increase the likelihood that overstory crowns ignite.… 

		Prescribed Burning

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). Burning and decomposition are the only means for reducing fine and small fuels. This practice is, in effect, allowing a wildfire to burn with monitoring, but not aggressive suppression efforts, when the location and conditions are appropriate. The slash must be treated to contribute to fuel reduction. Thinning With Prescribed Burning

A common proposal is to combine thinning with prescribed burning—thinning first to eliminate ladder fuels and reduce crown density, followed by prescribed burning to eliminate the fine and small fuels in thinning slash. Logging does reduce crown density (completely in clearcutting), but does little for fuel ladders and leaves substantial volumes of slash. The BIA has the most explicit direction; Indian forest land management activities are defined to include
(C) forest land development, including forestation, thinning, tree improvement activities and the use of silvicultural treatments to restore or increase growth and yield to the full productive capacity of the forest environment; [and]
(D) protection against losses from wildfire, including … construction of firebreaks, hazard reduction, prescribed burning ... [emphasis added]
Despite the general, broad, generic authority for biomass fuel reduction activities (prescribed burning, thinning, and more), the agencies have both specific funding programs and the authority to use general management funds for these types of activities. Although this sub-account is the primary source of funds for prescribed burning, it can be used for other fuel reduction activities, such as thinning, as well. The FS has three mandatory spending accounts that also provide funds for activities that can reduce biomass fuels. Other fuel reduction activities, such as thinning, are funded through general land and resource management accounts for each of the DOI agencies. The FS also has other technical and financial assistance programs that might be used to assist in reducing fuels on nonfederal lands.
Severe wildfires have been burning more acres and more structures in recent years. Some assert that climate change is at least partly to blame; others claim that the increasing number of homes in and near the forest (the wildland-urban interface) is a major cause. However, most observers agree that wildfire suppression and historic land management practices have led to unnaturally high accumulations of biomass in many forests, particularly in the intermountain West. While high-intensity conflagrations (wildfires that burn the forest canopy) occur naturally in some ecosystems (called crown-fire or stand-replacement fire ecosystems), abnormally high biomass levels can lead to conflagrations in ecosystems when such crown fires were rare (called frequent-surface-fire ecosystems). Thus, many propose activities to reduce forest biomass fuels.
The characteristics of forest biomass fuels affect the nature, spread, and intensity of the fire. Fuel moisture content is critical, but is generally a function of weather patterns over hours, days, and weeks. Fuel size is also important—fine and small fuels (e.g., needles, grasses, leaves, small twigs) are key to fire spread, while larger fuels (e.g., twigs larger than pencil-diameter, branches, and logs) contribute primarily to fire intensity; both are important to minimizing fire damages. Fuel distribution can also affect damages. Relatively continuous fuels improve burning, and vertically continuous fuels—fuel ladders—can lead a surface fire into the canopy, causing a conflagration. Total fuel accumulations (fuel loads) also contribute to fire intensity and damage. Thus, activities that alter biomass fuels—reducing total loads, reducing small fuels, reducing large fuels, and eliminating fuel ladders—can help reduce wildfire severity and damages.
Several tools can be used to reduce forest biomass fuels. Prescribed burning is the deliberate use of fire in specific areas under specified conditions. It is the only tool that can eliminate fine fuels, but is risky because it burns any fuel available. Wildland fire use is the term used for allowing a wildfire to be used like a prescribed burn (i.e., within specified areas and conditions). Thinning is a broader forestry tool useful for eliminating fuel ladders and total fuels in the crown, but it does not eliminate fine fuels, and it concentrates fuels in a more continuous array on the surface. The combination of thinning with prescribed burning is often proposed to combine the benefits, but it also combines the cost of both. Logging does little to reduce fuel loads.
The federal land management agencies undertake all of these activities under general authorities for wildfire protection and land and resource management. Fuel reduction, primarily via prescribed burning, is funded with direct annual appropriations for wildfire management. Other activities, particularly thinning, are funded through other annual appropriations accounts, such as vegetation management. Also, several mandatory spending accounts provide funds for related activities, such as treating logging and thinning debris. In addition, wildfire assistance funding allows the Forest Service to provide technical and financial aid for reducing forest biomass fuel loads on nonfederal lands, among other things.
The issues for Congress include the appropriate level of funding for prescribed burning and thinning for fuel reduction and the appropriate reporting of accomplishments. Current reporting does not identify ecosystems being treated and the effectiveness of the treatments. Similarly, current appropriations and reporting do not distinguish thinning for fuel reduction from thinning for other purposes, such as enhancing timber productivity. More complete reporting could allow Congress to better target its appropriations for fuel reduction to enhance wildfire protection.