USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Fire Science

Managing fuels to modify fire behavior

A forestry technician uses a drip torch to start short flames that burn along the ground in the forest.
A forestry technician uses a drip torch on a prescribed fire on the Stanislaus-Tuolumne Experimental Forest. Prescribed fire is part of the treatments conducted in research to examine forest restoration techniques. U.S. Forest Service photo by Eric Knapp.

Fuel reduction treatments, including fire (either prescribed fire or managed wildfire), mechanical manipulation (e.g., thinning, mastication, chipping), or a combination of the two have been implemented across millions of acres of forestland throughout the western U.S. and are proposed for millions more.

These treatments aim to modify wildfire behavior and effects by reducing surface fuels (dead vegetation on forest floor, such as branches or needles) and ladder fuels (small- to medium-sized trees and shrubs that can carry fire from the ground into the tree canopy), and increasing the spacing between tree crowns. In drier forest types, fuel reduction treatments can often be aligned with other management activities, such as restoring forest structure to increase ecosystem resilience. Common core goals include reducing tree densities, retaining larger, more fire resistant trees, and reducing surface fuel loads.

Several synthesis reports summarize studies on the short-term impacts (<5 years) of these activities in mixed-conifer and ponderosa pine forests. These and other studies demonstrate that properly designed fuel treatments effectively change fire behavior, reducing fire severity and fire-caused tree mortality even under extreme weather conditions. Additionally, such treatments can be conducted in ways that have minimal negative impacts on other forest ecosystem components (e.g., soils, bark beetles, wildlife). Collectively, these studies suggest that treated forests are in a more resilient condition compared to untreated forests in the short term.

A forest in the absence of low to moderate intensity fire or other treatment mimicking fire will likely lose resilience to stressors (drought) or disturbance (high intensity wildfire, insects) over time. Modeling studies at the landscape scale predict much greater forest losses from wildfire in untreated scenarios than in fuels-treated scenarios.

Striving for fire maintenance across large landscapes

Recent research has demonstrated increases in the proportion of high-severity fire, larger fires, and annual areas burned in yellow pine and mixed-conifer forests in the Sierra Nevada region from 1984 – 2010. Researchers point out that these increases coincide with rising regional temperatures and increased long-term precipitation, yet California and the western U.S. as a whole are in a large "fire deficit." These findings, based on research reconstructing fire occurrence over the last 1,500 years using sedimentary charcoal records, show that the current fire deficit is unprecedented throughout the historical record. With temperatures warming over the last several decades, researchers expect to see much higher fire activity, based on historical fire–climate associations and current fire suppression policies.

Given the current state of western U.S. forests adapted to frequent fire and the projected changes in fire occurrence, innovative forest management approaches that focus on large spatial scales will likely be needed to reduce the potential for uncharacteristically large and destructive fires. Informed planning and deployment of fuel reduction treatments can move forests away from fire suppression and into fire maintenance, which is one means of potentially changing current fire patterns.

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