To help forests dry out, fire needs to be just the right intensity and happen more than once

Oregon State University research on a fire’s ability to improve the health of a forest has revealed a Goldilocks effect—unless the fire falls within a narrow range of severity, neither too hot nor too cold, it’s not very good at helping forest landscapes to return to their historic, more fire-resistant conditions.

The study, led by Skye Greenler, a graduate researcher in the OSU College of Forestry, and Chris Dunn, an assistant professor in the college, has important implications for land managers charged with restoring ecosystems and reducing fire risk in dry forests. like the east. of the Cascade Range.

The findings, published in PLoS Oneshed light on the situations in which managed fires, as well as post-fire efforts such as thinning and planting, are likely to be most effective in achieving restoration goals.

Wildfires have shaped ecosystems for millennia, the researchers note, but their effects have become a growing social, economic and ecological concern in the western United States. Aggressive fire exclusion policies, forest and resource management practices and climate change have altered the structure and composition of forests – increasing the vulnerability of forests to extreme fires and drought.

“As wildfire activity continues to intensify in the West, it is becoming clear that a variety of management activities are necessary to make ecosystems healthier and reduce fire risk,” Greenler said. “Fuel reduction treatments, such as mechanical thinning and prescribed fire, can reduce community and ecosystem risk, but in most places, the rate and scale of treatments are well below what is needed to substantially change the results and behavior of the fire”.

In an independent project, Greenler and Dunn in collaboration with College of Forestry colleagues James Johnston, Andrew Merschel and John Bailey developed a new way to predict fire severity that is better able to help eastern Oregon forests to return to their historical density, species composition and basal area, a measure of how much land in a given area is occupied by tree trunks.

“We built probabilistic models of tree mortality for 24 species based on their characteristics and remotely sensed fire severity data from a collection of burned areas,” Greenler said. “We then examined unburned stands in the Ochoco, Deschutes, Fremont-Winema, and Malheur National Forests to model post-fire conditions and compared the results to historical conditions. This allows us to determine which fires had the greatest recovery potential.”

The research team, which also included scientists from the University of Washington, the US Forest Service and Applegate Forestry LLC of Corvallis, generally found that core area and density goals could be met by fire within a fairly narrow range of moderate severity.

However, a blaze cannot restore species composition to its historical norm in a forest that evolved amid frequent low-severity fires, the scientists found.

“Landscapes have likely crossed thresholds that preclude the effectiveness of managed fire alone as a restoration tool,” Grinler said. “In a large number of fire-prone western landscapes, forest structure and composition are no longer resilient or resilient to natural disturbance processes such as fire, drought, and endemic insects and pathogens, and the interactions between all of these.”

Although more and more wildfires are burning large areas and at high intensity, the majority of wildfires in the West still burn at low or moderate intensity, the authors note. They cite a recent analysis that found about half of the area burned in Oregon and Washington from 1985 to 2010 did so in low-severity fires—in systems historically characterized by low- and mixed-severity fire regimes.

“Low severity may be ‘too cold’ to achieve restoration goals in areas where significant reductions in tree density or large changes in tree species composition are required,” Greenler said. “To better understand fire intensity that is more restorative, we need empirical modeling that can be applied beyond single fire events and across a wide range of conditions. Our study allows managers and researchers to link forest restoration goals to maps of intended location – fire conditions’.