After more than a century of fire suppression in California's forests, mounting evidence shows that frequent fire - through practices like prescribed fire or Indigenous cultural burning - can improve forest health, increase biodiversity and reduce the risk of catastrophic wildfire.
But controlled fires can have downsides. In addition to being labor intensive and producing smoke that may harm neighboring communities, burning trees and vegetation releases carbon dioxide into the atmosphere, contributing to global warming.
A new long-term study shows that, while prescribed burning may release carbon dioxide in the short term, the repeated use of controlled fire may boost a forest's productivity, or carbon sequestration capacity, in the long term.
"Over time, we found that the productivity of unmanaged tree stands decreased, likely due to increased competition and climate stress. Meanwhile, prescribed burning helped maintain large, fire-resistant trees, eventually increasing the productivity of these stands," said study lead author Yihong Zhu, a graduate student at the University of California, Berkeley. "We wouldn't be able to detect such a benefit had we not been able to monitor these stands over 20 years and three entries with controlled fire."
The findings provide useful insights for California policymakers and land managers seeking to reduce wildfire hazard while helping the state achieve its goal of net zero carbon pollution by 2045.
"Nature-based climate solutions were a big focus of the 2024 Paris Climate agreement, and either maintaining or increasing forest carbon is one of the most cost-effective strategies," said study senior author John Battles, a professor of forest ecology at UC Berkeley. "We found that, with some management, you may lower the total carbon storage of a forest, but you make it safer from loss from wildfires or pathogen outbreaks. We call it stable carbon."
The paper describes the latest findings from a long-term forest experiment that was first launched at the Blodgett Forest Research Station in the Sierra Nevada in 2000. At the forest, Berkeley researchers have consistently applied different forest management techniques, including prescribed burning and restoration thinning, to individual plots of land over a 20-year period. Other "control" plots have been allowed to grow unhindered.
Through careful field work and lab analysis, the researchers quantified how each treatment impacted the carbon storage and net productivity of each plot. They found that, while the control plots maintained the highest carbon storage, regular prescribed burning significantly enhanced the net productivity of plots that had received three prescribed burning treatments. By the end of the study, the increase in net productivity was nearly enough to make up for the carbon that was released by the fires.
"After the first burn, the net productivity of those plots was really low and the controls looked a lot better," said John Battles, "But by the third burn, the patterns had switched."
As they grow, plants pull carbon dioxide from the atmosphere and convert it into sugars and carbohydrates that become leaves, stems, roots and woody trunks. Carbon can also be stored in the ground as soil organic carbon. This carbon is released back into the atmosphere when plants burn in wildfires, are used for fuel or die and decay.
To quantify the carbon cost of each treatment, Zhu tracked all of the different pools of carbon in the forest, from the carbon stored in decaying pine needles to the trunks of the thickest trees, and also accounted for the different pathways through which carbon could be released back into the environment.
"We looked at big trees, we looked at little trees, we looked at shrubs, we looked at different fuel classes, and then we checked how they changed," said Battles. "It really is just like a massive accounting job, except we're not measuring money, we're measuring carbon."
Fire suppression in the Sierra Nevada has led to the proliferation of smaller, shade-tolerant trees like incense cedar and white fir, whose leafy understory can serve as a "fuel ladder" which can transform small, ground-level fires into massive crown fires. Prescribed burns can help reverse this "fir-ification," supporting the growth of larger, fire-resistant trees like ponderosa pine and sugar pine.
"We've always wondered, if we could restore these ecosystems to a more functional state - lower density and more frequent fire - do we eventually see a bonus? Do we get that golden nugget? And in this work, we were able to actually measure it," said study co-author Scott Stephens, a Berkeley professor of fire science.
In a previous study, the research team found that combining prescribed burning with mechanical thinning was the most effective at reducing wildfire hazard. However, this approach also has the highest carbon cost.
Together, the two studies can be used as a roadmap for communities to make the most informed decisions about forest management. In areas where the highest priority is to prevent fire - such as near inhabited communities, or giant sequoia groves - a combination of prescribed burning and restoration thinning gives managers the highest chance of success. But in deeper wilderness areas, prescribed burning alone may be a better option for maintaining stored carbon while improving forest health.
"We've got to get these treatments out there," Battles said. "Some treatments might be better than others in certain situations, but now we've made the trade-offs explicit so we can pick the right approach."
Additional co-authors of the study include Daniel Foster, Brandon Collins, Robert York, Ariel Roughton and John Sanders of UC Berkeley; and Emily Moghaddas of the U.S. Department of Agriculture Forest Service.
Research Report:Carbon costs of different pathways for reducing fire hazard in the Sierra Nevada
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