Forest fires release less CO2 than previously thought, challenging some forest management practices, study says

Forest fires don’t release nearly as much carbon into the atmosphere as previously believed, according to a recently published University of Idaho study.

The findings, published last week in the journal Global Change Biology, question one commonly employed rationale for thinning trees in the West: carbon sequestration.

Lead author Jeff Stenzel, a doctoral student, was first interested in the topic last year when he and other classmates noticed overwrought and dramatic descriptions of the aftermath of forest fires in media reports.

Terms like “moonscape” and “post-apocalyptic” recall images of total destruction while the reality is fires rarely incinerate trees and provide a raft of ecological benefits, he said.

At first, Stenzel said, he hoped to write an opinion piece in a scientific journal. As he dug into the question, however, he realized scientists were assuming fires burned anywhere from 30% to 80% of the forest biomass. This assumed percentage was being input into models predicting the amount of carbon dioxide released into the atmosphere.

Instead, Stenzel and other researchers found that less than 5% of mature biomass is consumed in fires. According to the study, some regional forest fire CO2 emission estimates, based on flawed assumptions, are 59% to 83% higher than actual emissions across the western United States.

In the past 15 years, forest fires released 250 million tons of carbon compared to the 4.8 billion tons released by burning fossil fuels, according to the study.

“It’s really negligible in most lower-intensity fires,” Stenzel said.

The study, which has gone through the peer-review process, could have implications for forest management. In many states, including Idaho, carbon sequestration is considered when drafting forest management plans.

In 2018, Ryan Zinke, the former interior secretary, cited carbon release as a result of poor federal forest management.

Thinning trees and turning them into wood products is believed by some to be an effective way to store carbon. At the same time, thinning a forest reduces the risk and severity of a forest fire. And salvaging dead trees postfire allows managers to plant new trees that absorb CO2 from the atmosphere.

According to Stenzel, however, overestimating the amount of CO2 that’s released into the atmosphere makes any management decision “look like it’s going to save carbon.

“A lot of times it’s just better to leave that carbon in the ecosystem,” he said.

This doesn’t mean there aren’t good reasons to remove trees, said Tara Hudiburg, a professor in the University of Idaho’s Department of Forest, Rangeland and Fire Sciences and one of the co-authors of the study.

“There are reasons to remove trees, but it’s hard to do that in the context of carbon emission,” she said.

Idaho State Forester David Groeschl said carbon emissions are considered when drafting the state’s forest management plans. Studies like the University of Idaho’s are taken into account.

It’s unlikely that this study would change management practices in Idaho, he said.

“There are a lot of factors that we take into account when we look at the long-term management of our forest,” he said.

The University of Idaho study was only looking at high-volume carbon release events, he said. When considering overall forest health, IDL considers the economic value of the trees, how much carbon decaying biomass releases over decades, how and where to plant new trees that can actively remove CO2 from the atmosphere and the risk of more forest fires.

“I would be a little hesitant that one study is enough to show, using two data sets from two different fires … that the other models are in error,” he said. “I don’t know. I’m not saying the study is wrong.”

The University of Idaho study used field data from a 2002 wildfire in southern Oregon and a 2013 wildfire in central California.

The study also won’t change how the Washington State Department of Natural Resources manages its forestland, Stevie Mathieu, a spokeswoman with DNR, said in an email.

“Reduced CO2 emissions is certainly a benefit of reducing uncharacteristically severe wildfires in our state,” she said. “But emission estimates were not factored in when choosing which forests to focus on under DNR’s 20-Year Forest Health Strategic Plan.”

Still, Hudiburg, the Idaho professor and co-author, said the study adds to the growing body of research around climate change and carbon. Growing that body of scientific knowledge is important as land managers, governments and individuals make choices about how to mitigate the impacts of climate change.

“We should focus on the real emitter, not the trees,” she said of the 4.8 billion tons of CO2 released by burning fossil fuels each year.