What makes certain areas of forests survive wildfires and others burn?
Oregon State University researchers used some new technology and approaches to examine the concept, which has high visibility these days given climate change and the intensity of recent fires.
The research could help forest conservation managers develop strategies for protecting and restoring the most fire-resistant parts of vulnerable forests.
Changing wildfire regimes are affecting forests worldwide, the scientists note, and areas that burn over and over in relatively quick succession may not be able to recover between fires.
Meg Krawchuk of the College of Forestry’s Landscape Fire and Conservation Science lab group, faculty research assistant Will Downing, Matt Gregory of the College of Forestry and Garrett Meigs of the Washington State Department of Natural Resources worked on the report, which was reported in Global Change Biology.
The key piece of the puzzle the scientists studied were fire refugia, areas that burn less frequently and/or less severely than the landscape around them. Such stands are crucial for supporting post-blaze ecosystem recovery, including the resilience of species under pressure.
“Observed and projected forest losses from wildfire tell us that we need to understand where and why refugia persist through multiple fire events,” said Krawchuk.
Understanding the concept is particularly important, Krawchuk said, in the Klamath-Siskiyou ecoregion of southwest Oregon and northwest California. "That area holds some of the most diverse collections of conifers in western North America, and expected increases in fire activity, along with a warming climate, could result in the loss of more than 30% of the region’s conifer forests," she said.
The researchers used recent advances in fire progression mapping and weather analysis — estimating the information between known weather data points — plus a novel application of satellite smoke imagery to build new fire refugia statistical models for the Klamath-Siskiyou region.
The analysis focused on mature, conifer-dominated forests and looked at the key factors behind fire refugia occurrence and persistence through a series of three fire events over 32 years.
“The models suggest hotter-than-average fire weather is associated with lower refugia probability and higher fire severity,” Krawchuk said. “Refugia that persisted through three fire events appeared to benefit from topographic variability — a mix of rocky outcrops and landscape depressions, for example — which means the variability may be an important stabilizing factor as forests experience successive fires.”
Fire refugia are part of a larger category of hardy areas known as disturbance refugia, and comparatively little is known about why certain refugia are able to survive as they pass through successive fires, she said.
Downing, the lead author for the report, noted that “refugia can be transient and survive a single fire because of random weather or fire behavior conditions, or there can be persistent refugia that don’t change very much in the face of multiple fire events.”
“Figuring out which areas are most likely to persist requires using landscape-scale assessments of the factors behind fire behavior and severity: topography, fuels and weather,” Krawchuk said. “Refugia are ecologically important parts of fire severity mosaics, and it appears that the more times a landscape burns, the more important terrain features are for refugia persistence.”
The models analyzed by the researchers also show that smoke density strongly influences the results. Refugia are more likely to occur when smoke is moderate or dense in the morning, a connection the scientists attribute to the shade smoke provides.
“Our hope is that this study can inform management strategies designed to protect fire-resistant portions of biologically and topographically diverse landscapes,” Krawchuk said.
The Klamath-Siskiyou ecoregion is ideal for studying refugia occurrence and resilience, Downing said, because it’s a “biodiversity hotspot” in which fire has been a key ecological component for thousands of years.
“Fire there has contributed to the maintenance of patchy, heterogeneous landscapes of conifer and hardwood forests, shrublands and grasslands,” Downing said. “But a hotter and drier climate and a lack of surviving post-fire seed sources eat away at the ability of conifer forests to recover after a high-severity fire.
“Climate change is expected to increase fire frequency in the region, and repeat burning is projected to convert about a third of the conifer forest to shrublands or hardwood forest by the end of the 21st century.”
The National Fire Plan and the U.S. Forest Service Pacific Northwest Research Station, through agreements between the Aldo Leopold Wilderness Research Institute and OSU, supported the research.