California Forests Are Better Protected With Prescribed Burning and Thinning, Study Confirms

California tops the list of U.S. states with the most wildfires — 9,260 in 2021. That year, the number of acres burned in the state exceeded two million, more than twice as many as second-ranked Texas, the Insurance Information Institute said.

A two decades’ long study in California’s Sierra Nevada mountains has confirmed that forest management strategies like restoration thinning, prescribed burning or a combination of the two are effective at lowering catastrophic wildfire risk, a press release from University of California, Berkeley (UC Berkeley), said.

The methods also improve the health of forests, helping them to withstand bark beetles, drought and other stressors. The researchers found that treatments within individual stands of trees do not have a negative impact on wildlife or plant diversity.

“The research is pretty darn clear that these treatments are effective — very effective,” said Scott Stephens, lead author of the study and a UC Berkeley professor of fire science, in the press release. “I hope this lets people know that there is great hope in doing these treatments at scale, without any negative consequences.”

The findings of the national Fire and Fire Surrogate Study, “Forest restoration and fuels reduction work: Different pathways for achieving success in the Sierra Nevada,” was published in the journal Ecological Applications.

California announced a plan last year to expand prescribed fire use to 400,000 acres a year by 2025, the press release said. Several factors stand in the way of the use of these beneficial fires, however, such as fears of potential risks, the need for certain weather conditions and lack of trained workers.

Another viable forest management option is restoration thinning, which can be used alongside prescribed fires with no harm to biodiversity or forest health.

“Our findings show that there’s not just one solution — there are multiple things that you can do to impact the risk of catastrophic fire,” said Ariel Roughton, co-author of the study and Berkeley Forests’ research station manager, in the press release. “Folks can choose from different combinations of treatments that might fit their needs, and we can show them how those treatments might impact things like wildfire behavior, tree growth and carbon holding in their forests.”

Over the last 20 years, Berkeley Forests researchers like Stephens have used restoration thinning, prescribed burning or a combination to treat areas of the Blodgett Forest Research Station, an experimental forest of 4,000 acres approximately 65 miles from Sacramento on lands belonging to the Nisenan Tribe.

One of 13 U.S. studies, the Fire Surrogate Study first started in 1999 with the goal of investigating whether the treatments would have the same beneficial impacts as Indigenous burning practices and lightning fires on the state’s forests. After a century of fire and logging suppression, California forests had become overgrown and dense.

“Prescribed fire and restoration thinning are both surrogates for wildfire, a key process that happened frequently in California before European colonization. The impetus of this study was: If you’re going to implement these treatments at a large scale, is there anything that’s going to be lost?” Stephens said in the press release.

At Blodgett, the researchers used three control plots and nine experimental plots. The control plots were left alone except for the use of fire suppression. Prescribed burns were used to manage three of the experimental plots; three were thinned, then burned; and restoration thinning was used on the remaining three.

After two decades, the team surveyed each plot’s vegetation and estimated the number of trees likely to survive wildfire using computational modeling. They discovered that all three experimental plots were much more wildfire resilient than the control plots, with an 80 percent probability that a minimum of 80 percent of the trees on them would survive.

The team also measured how much the trees had to compete for resources like water, sunlight and soil nutrients, called the “index of competition.” Burning and thinning removed excess vegetation and trees from the forest, which not only limited competition, but made trees less vulnerable to stressors.

The researchers found that the plots that had been treated with both fire and thinning had the best competition index, which suggested they would be most able to withstand climate change impacts.

“When you combine thinning with fire, you’re able to modify all different levels of the forest structure, and it speeds up the timeline for achieving a more resilient structure,” Roughton said in the press release.

There are also financial benefits that come with restoration thinning. Bigger trees can be sold, providing funds to help offset forest management costs. Timber revenue paid for the treatments at Blodgett during the study.

“When I go to Sacramento and talk about [forest management] with legislators, the first question they always ask is about cost. People in the state government are telling us that they can’t be the sole source support for this work. That’s why the economics are so important,” Stephens explained.

On September 9, 2022, Blodgett’s forests were put to the test by the Mosquito Fire, which burned around 300 acres before being contained two days later.

A study control plot was right in the path of the Mosquito Fire. The wildfire burned more than 60 percent of the plot’s trees, while the prescribed burn experimental plots next door served as “fuel breaks” that acted as cooler staging areas for firefighters.

“We think that, overall, our management actions, coupled with the weather, did have a pretty big impact on the behavior of the fire,” Roughton said.

After the Joint Fire Science Program gave the research team a four-year grant to continue the project, they replaced the burned control plot and have plans to apply another fire on the experimental plots. The team is also helping to reestablish Indigenous cultural burning in collaboration with the United Auburn Indian Community.

“We want to be part of the solution, and that’s part of our mission at Blodgett. We hope that by doing these studies and bringing folks here to see the effects of the different treatments, they will take that back and apply it to the land that they’re going to be managing,” Roughton said.