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 Tree Mortality

Fire management in the face of tree mortality across large landscapes

Close up photo of duff, needles and other organic debris at the base of a tree.
Duff, needles and other organic debris, combined with brush and seedlings, all contribute to the fuel that collects beneath trees. (U.S. Forest Service)

Forests in the western United States have experienced large-scale tree mortality in recent years. How much of a fire danger the dead trees present is complex and is largely dependent on the length of time that has passed since the trees have died and how much of the pine needles, large branches and tree trunks have made it to the forest floor.

In the Sierra Nevada, due to a lack of moisture and a warming climate, fuels that are on the ground tend to stay there for a longer period, and pose a significant wildfire threat. Any increase in tree mortality results in more dead trees, more fuel, and more frequent or intense forest fires.

On the Stanislaus-Tuolumne Experimental Forest, research shows forests that historically experienced frequent fires typically had lower tree density and greater spatial variability than contemporary forests in which fire has been suppressed. These sporadic groups and gaps of trees created natural fuel breaks, reducing wildfire severity and the probability of large-scale crown fire.

Current research: Monitoring fire behavior in California forests with extensive tree mortality

Close up photo of duff, needles and other organic debris at the base of a tree.
Color-coded thermal imagery of the southern portion of the Detwiler Fire, 19 July 2017, near Mariposa, California, as collected by a FireMapper 2.0 thermal-imaging radiometer.

PSW is using airborne thermal-imaging technology to examine the potentially explosive effects of recent tree mortality on wildfire behavior, including rates of fire spread through the canopy of trees with retained dead foliage, fire energy release or intensity, and spot fire generation.

Fire imaging with a FireMapper 2.0 radiometer aboard an aircraft uses multiple bands at thermal infrared wavelengths to measure infrared light emanating from large fire fronts while mapping forest and landscape temperatures, which provide information regarding vegetation types and terrain. Imaging of actively spreading fires is being coordinated with ground observations by a Forest Service Fire Behavior Assessment Team and provided to firefighters.

Measurements of fire behavior and fire severity will be valuable for fuel reduction planning to mitigate fire hazards and will inform firefighters of any elevated hazards that might be expected in fire suppression operations. The FireMapper system has been designed specifically for fire mapping and measurement and to provide fire intelligence needed to understand and safely manage large fires in California’s wildlands.

Additional resources