How do thinning, prescribed fire, and wildfire affect future wildfire severity?
High-severity wildfire plays an important role in forests. However, the extent and impacts of high-severity fire have increased over recent decades. As we respond to these changing wildfire patterns, we need a better understanding of how thinning, prescribed burning, and wildfire affect future wildfire severity. The last meta-analysis to describe how forest management affects wildfire severity in the western United States occurred 10 years ago. Since then, wildfires have burned under increasingly severe fire weather conditions and scientists have documented new evidence. In response, we reviewed scientific literature for the western U.S. and completed a meta-analysis that answers three questions: (1) How much do treatments reduce wildfire severity within treated areas? (2) How do the effects vary with treatment type, treatment age, and forest type? (3) How does fire weather moderate the effects of treatments?
We found overwhelming evidence that mechanical thinning with prescribed burning, mechanical thinning with pile burning, and prescribed burning only are all effective at reducing subsequent wildfire severity. These types of treatments resulted in reductions in severity between 62 percent to 72 percent relative to untreated areas. In comparison, thinning only was less effective – underscoring the importance of treating surface fuels if reducing future wildfire severity is the management goal.
Fuel treatments were effective at reducing wildfire severity across the range of forest types and fire weather conditions assessed in this study. Previous wildfire had more nuanced impacts on subsequent wildfire severity, depending on forest type and initial wildfire severity. When the initial wildfire burned at low to moderate severity, it tended to reduce subsequent wildfire severity. For all fuel treatments, we found that effectiveness declined over time. After 10 years, the mean reduction in future wildfire severity decreased more than twofold. Thinning coupled with prescribed fire was the only treatment type that maintained a significant reduction in wildfire severity, 49 percent on average, after 10 years.
Conceptual diagram of how wildfire burns through different forest treatments.