Sustain Our Nation's Forests and Grasslands

Understanding burn windows to inform prescribed fire plans

Helicopterengged in burn operations; smoke billowd from the mountainside in the background
Helicopter engaged in a prescribed burn operation in 2018. USDA Forest Service photo by Pacific Northwest Research Station.

OREGON – Few if any treatments can compete with prescribed fire for its combination of economy and effectiveness in maintaining healthy fire-adapted forests. Prescribed burns are planned meticulously to ensure that they are safe and effective. When the environmental variables such as fuel moisture and weather conditions are balanced so that the fire will accomplish its objectives (reducing fuels, modifying wildlife habitat, restoring ecological function) while remaining under control, fire managers refer to this as the burn window of opportunity.

Many land managers would like to increase use of prescribed fire treatments, but they often cite a major impediment: the seasonal limitations to acceptable burning conditions. In other words, the burn window is too short, narrow or unpredictable.

Pacific Northwest Research Station fire ecologist Morgan Varner and climate scientist Sim Larkin along with University of Washington climate scientist Andy Chiodi sought to improve understanding of the burn weather window in the Southeast United States, where 70 percent of the country’s prescribed burns take place. They wanted to know how the burn window varies with location and season, and why. They used a 30-year record of fire weather (daily precipitation, wind speeds, relative humidity, atmospheric mixing heights, and transport winds) to illustrate climatological variation of the preferred weather window by month across the Southeast United States. They focused on conditions critical to balancing the need for good smoke dispersion and low risk of erratic fire behavior.

They found that over most of the Southeast, the winter and spring months showed high availability of acceptable weather, whereas the summer season had tight limitations. Varner, Larkin and Chiodi confirmed that much more burning took place across the Southeast in the winter and spring than summer. Another wide window of acceptability opened in the fall and was identified as a currently overlooked opportunity for prescribed burning by land managers. The implication here is that fall is a potential time where prescribed fire activities could be increased to meet the regional needs for fuels treatment application.

Varner, Larkin and Chiodi will be expanding this research to the Pacific Northwest this summer. Preliminary work has been completed, including the collection of weather information to create a tool that will be used to understand how burn windows change with the seasons, and across the region. By analyzing the potential burn windows, the researchers hope to identify increased opportunities for prescribed burning. This work will also inform ongoing discussions about smoke management in the Pacific Northwest.

Fire has a natural role in forest renewal. Yet after decades of fire exclusion, large areas of fire-adapted forests have become susceptible to uncharacteristic wildfire: fires burning so hot or over so large an area that most or all the trees in the overstory are killed. Fuel treatments including thinning, prescribed fire and managed wildfire, can help create fire-resilient forests, but measuring their effectiveness is inherently complex. Forest Service scientists are studying fuel treatment effectiveness using a variety of approaches. Their research provides land managers with information on the extent to which treatments change fire behavior.