Hotter and drier conditions limit forest recovery from wildfires

UNIVERSITY PARK, Pa. — Warmer and drier climate conditions in western U.S. forests are making it less likely that trees can regenerate after wildfires, according to a new study published in the Proceedings of the National Academy of Sciences, which includes researchers from Penn State.

Importantly, the research shows that ecologically based forest management can partially offset climate-driven declines in tree regeneration by limiting fire-caused tree death, but only if action is taken quickly. This study provides key information for sustainably managing forests across millions of acres of Western forests in the face of climate change, according to Alan Taylor, professor of geography and ecology at Penn State and co-author on the paper.

Forests are adapted to different types of fire across the West, but hotter and drier conditions in recent decades have increased fire severity which kills more trees. Fewer trees mean fewer seeds available for forests to regenerate after wildfires. Even when seeds are available, a warming climate is reducing the chances that seedlings can establish and grow, according to the researchers.

“We have a chance to buffer these climate effects,” said Taylor. “By reducing fire severity across the landscape and reducing the potential for large, high-severity fires, we can better equip our forests for the future.”

The research examined how the severity of a fire — the number of trees it kills — in combination with the climate conditions after the fire affected the chances of tree seedlings regenerating to establish a new forest. It is the most extensive study to date, assessing regeneration of eight major conifer tree species after 334 wildfires across the West, using information from over 10,000 field plots collected by more than 50 research teams.

Researchers found that warmer, drier conditions over the past four decades have led to a decline in tree regeneration after wildfires and this trend is expected to accelerate in the future. For example, from 1981 to 2000, 95% of the areas studied had climate conditions suitable for tree regeneration after wildfires, but this is projected to decrease to only three-quarters of the West by 2050 under future climate scenarios.

“Fire suppression policy over the last century has increased forest density and fuel, and is a major reason fire severity has increased,” said Taylor. “High severity burn areas are most vulnerable to tree regeneration failure with a warmer and drier climate. Reducing the severity of wildfires can lessen the impact of these conditions on forest recovery.”

The most vulnerable forests were in drier regions in the Southwest and California, while forests in the wetter and cooler regions of the northern Rocky Mountains and Pacific Northwest are still expected to support conifer regeneration in the near-term future, according to the researchers.

“The impacts of climate change and wildfires vary across the West, and the large scope of this study allowed us to highlight where these changes are most concentrated and happening first,” said Philip Higuera, a co-author and professor of fire ecology at the University of Montana.

The study also found that ecological forest management in vulnerable dry forests could offset climate-driven changes by reducing the number of trees killed in wildfires. Specifically, in almost half of the study region, regeneration after wildfires is projected to be likely only if future fires burn at lower severities because fewer trees that produce seeds needed for forest recovery are killed in these fires.

Previous work by Taylor has shown that low severity wildfire and prescribed fire can reduce the chance of high severity burning in large wildfires like the 2021 Dixie Fire in California.

“Research has clearly demonstrated that forest thinning, controlled burns and wildfires burning under moderate weather conditions can effectively reduce fire severity,” said Taylor. “This points to the need to greatly expand the use of these types of management approaches to safeguard forests against fire severity and climate change.”

Two other Penn State researchers who received their doctoral degrees in geography contributed to the study. Lucas Harris, now a postdoctoral scholar at the Rubenstein School of Environment and Natural Resources at the University of Vermont, and Jamie Peeler, now a postdoctoral scholar at the University of Montana.

Funding for this project was provided by The Nature Conservancy. Additional support was provided by the Department of the Interior North Central Climate Adaptation Science Center and from the United States Geological Survey.