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

Fire and watersheds

Two people in a stream below raise a bucket up to a person on the bridge above them.
Collection of sediments deposited within a catchment basin to measure sedimentation rates within the Teakettle Creek drainage of the Kings River Experimental Watersheds, Sierra National Forest. U.S. Forest Service photo by Amber Olsson.

A watershed—an area of land from which water drains into a larger water source—provides a science-based unit for the study of ecosystem processes related to water. The condition or health of a watershed is a function of climate, topography, soils, nutrients, vegetation, and historical land use. These watershed characteristics also affect the amount, the movement and timing, and the quality of water, which  influence the health of aquatic ecosystems. Fire can have positive and negative effects on soil, nutrients, vegetation, and water quantity and quality depending on its extent and intensity.

Water is stored in only a few places in the ecosystem: the atmosphere; deep groundwater; surface water, such as streams, lakes, and reservoirs; and soils. Soil is the lifeblood of nearly all terrestrial ecosystem functions. It stores and releases water and nutrients to support the growth of vegetation and development of habitat for animal and human communities. Since forest vegetation uses and transfers water to the atmosphere from the soil and surface water (a process known as transpiration), vegetation type and amount determine how much water is available for desired forest vegetation communities, aquatic ecosystems, and human use. Extensive areas of high-severity fire pose risks to long-term soil quality by altering its water-holding capacity and nutrient content, affecting vegetation growth and reducing soil resilience to other stressors.

Both fire and tree thinning remove carbon and nitrogen, essential nutrients for forest vegetation. When the forest floor’s litter and duff layers burn, most of the nitrogen they contain is lost to the atmosphere in gaseous form; carbon loss is similar with proportionately more carbon lost as more forest floor is combusted. Despite large changes in the forest floor, total carbon and nitrogen pools in mineral soil often remain unchanged after prescribed fire. However, high intensity wildfires can alter the nutrient content of mineral soil, sometimes to the point of sterilization, and cause soils to repel water (become hydrophobic).

Some forests in California are under significant stress from air pollution (ozone and nitrogen). Excess nitrogen in the atmosphere and long-term fire suppression have increased nitrogen concentrations in forest litter and water that passes through it. Fire in these landscapes can help with removal of this excess nitrogen and reduce the risk of damage to aquatic ecosystems.

In the future, soil erosion and sedimentation from large, uncharacteristic and/or more frequent wildfires may pose the greatest risk to aquatic ecosystems and human water supplies. Therefore, forest restoration and/or fuel reduction treatments across the landscape can be designed to both reduce the likelihood of wildfires with substantial negative effects and improve conditions for nutrient balance, water storage and yield, and vegetation communities. Ongoing experiments like the Kings River Experimental Watersheds and observational studies like the Sierra Nevada Adaptive Management Project are providing new research findings on this topic.

Science synthesis:

Several synthesis and review publications address fire with respect to effects on watersheds, water, and aquatic ecosystems:

Publications and references: