Historic fire regimes lay groundwork for future forest management in western US

The Creek Fire burns in California’s Sierra National Forest in 2020. The megafire burned more than 379,000 acres. Credit: Ryan WaughAll Rights Reserved.

UNIVERSITY PARK, Pa. — Fires in semi-arid forests in the western United States tended to burn periodically and at low severity until the policy of fire suppression put an end to these low-intensity events and created the conditions for the destructive fires seen today. Understanding the benefits of these periodic fires and the forest structure that they maintained may help land managers and communities avert megafires in the future, according to researchers.

A team of scientists from leading research universities, conservation organizations and government laboratories studied and reviewed more than 1,000 peer-reviewed papers on climate change, wildfire and forest management to provide land managers and decision makers in the West a resource that summarizes the best-available science to aid land management decisions. They reported their findings in an invited three-paper feature today (Aug. 2) in the journal Ecological Applications.

“The pattern of megafire severity, which is troubling in terms of killing the forest overstory and threatening communities, is largely a fuel problem,” said Alan Taylor, professor of geography and associate of the Earth and Environmental Systems Institute, Penn State. “The fuel problem is the result of fire exclusion over the last 120 years. So, now we have overly dense forests, and on top of that, in the last 30 to 40 years, the climate has gotten much hotter and drier. Those fuels — the twigs, branches and leaves that fall from trees and cover the ground, and smaller trees whose crowns touch the canopies above them — are ripe for burning.”

For their part of the feature, Taylor and his colleagues looked at how fire influenced forest structure and composition over the last 500 years. They also looked at climate’s effects on forests.

They found that in the past, low-intensity fires used to burn every 10 to 20 years. Forests at this time were “open and clumpy,” according to Taylor, and the openness created holes in the forest canopy that reduced wildfire spread and canopy mortality across entire landscapes. The fires also favored the growth of fire-resistant tree species with thick bark over tree species that are more susceptible to fire.

Fire suppression, on the other hand, allowed fuels to accumulate for more than 100 years. The changing climate made it possible for less fire-tolerant tree species to spread across the landscape and create the denser forests seen today.

Forest change caused by fire suppression in Lassen Volcanic National Park in northeastern California, shown during four different years: 1923, 1993, 2010 and 2013. Historically, these forests burned about every 10 years at low to moderate severity until fire suppression was implemented in 1905. High surface fuel loads and development of a dense forest understory due to fire exclusion created conditions for the high severity Reading Fire in 2012. Credit: A.E. Weislander, U.S. Forest Service / Alan H. Taylor, Penn StateAll Rights Reserved.

“Based on our extensive review of the literature and the strength of the evidence, the science of adaptive management is strong and justifies a time-tested range of approaches to adapt forests to climate change and wildfires,” said Susan Prichard, a research scientist in the University of Washington’s School of Environmental and Forest Sciences.

The adaptive management approaches identified by the researchers include thinning some dense forests in fire-excluded areas, using prescribed burns, allowing some wildfires to burn in backcountry settings under favorable fuel and weather conditions, and revitalizing Indigenous fire stewardship practices.

“This collection represents a blending of scientific voices across the entire disciplinary domain,” said Paul Hessburg, a research ecologist with the U.S. Forest Service and affiliate professor at the University of Washington. “After reviewing the evidence, it is clear that the changes to forest conditions and fire regimes across the West are significant. The opportunity ahead is to adapt forests to rapidly changing climatic and wildfire regimes using a wide range of available, time-tested management tools.”

The report also addresses ongoing uncertainty over fire management approaches. As a result of fire suppression and a warming climate, drought and massive wildfires have become the driving forces of change in semi-arid forests. The uncertainty over management approaches has stifled time-sensitive projects that aim to create more fire-safe conditions and has led to situations where the management community has become paralyzed, said Taylor.

Active wildfire during the 2020 August Complex in northern California. The fires within the complex burned more than 1 million acres. Credit: Mike McMillan, U.S. Forest ServiceAll Rights Reserved.

“My colleagues and I are hoping that through wide dissemination of this three-paper report, managers will now have solid documentation of what the body of science suggests are the best approaches,” he said.

Clearly presenting the strength of the science on wildfires and management tactics is one purpose of this special report. In one of the papers, the authors address 10 common questions, including whether management is needed after a wildfire, or whether fuel treatments like thinning and prescribed burning work under extreme fire weather. They also discuss how to begin integrating western science with traditional ecological knowledge and Indigenous fire uses that managed western landscapes for thousands of years.

“Fire is going to happen,” Taylor said. “It’s the nature and character of the fires that are really troubling for society. We’re going to have to use wildfires under the right conditions and with these adaptive management and Indigenous burning practices to let fire begin to interact in the ways that it used to with the landscape. We want more good fire.”

The U.S. Fish and Wildlife Service, The Wilderness Society, The Nature Conservancy of Oregon, Conservation Northwest, The Ecological Restoration Institute, Washington State Department of Natural Resources, U.S. Forest Service (Pacific Northwest and Pacific Southwest Research Stations) and the California Department of Forestry and Fire Protection funded this research.

Last Updated October 19, 2021