Forestry managers face what seems like an unavoidable tradeoff. Controlled burns in Western forests reduce the risk of wildfire but release carbon dioxide, contributing to climate change. But a team of ecologists recently showed that controlled burns can actually benefit the climate in the long term.
Since the 1960s, attempts to suppress natural low-intensity fires led to forests overgrown with small trees and loaded with dead leaves. These act as fuels that easily accelerate a small fire into a forest-clearing inferno, though burning some fuel when winds are low and humidity is high—making a wildfire unlikely—can help prevent future disasters.
In 2012, calculations suggested that restoring low-intensity fire would release more carbon than not burning. However, new computer simulations of wildfire behavior over several decades showed that prescribed burns and vegetation thinning can both reduce the risk of wildfire and trap more carbon in tree growth than an untreated forest.
“When we’re actually doing treatments in the forest, you can’t just light a fire under what would generate wildfire conditions and see how your treatments burn,” said Matthew Hurteau, a University of New Mexico forestry professor and lead author of a forthcoming study in the journal Ecological Applications. “The modeling is really important because then we can look at a range of conditions under which a fire might occur and see how that interacts with the forest structure.”
After surveying a northern Arizona ponderosa pine forest, Hurteau’s group modeled how the forest responded to high and low probabilities of stand-replacing wildfire, the most intense and damaging kind. They ran many simulations with randomized wildfire behavior to get a range of results for different fire prevention treatments.
The simulations showed that when wildfire is added to the system, even the most invasive treatment, thinning and burning, trapped more carbon than it lost—after 40 years for a high probability of wildfire, and after 51 years for low probability of wildfire.
John Campbell, an Oregon State University forestry professor, remains skeptical that the simulations definitively show that thinning and burning has a positive effect on the atmosphere.
“I don’t think there are any studies…that are going to suggest that an actively thinned and prescribed-burn forest actually in the long term maintains higher carbon than one that isn’t,” Campbell said. However, he does not deny the treatments reduce the risk of intense wildfires.
Malcolm North, a Forest Service research scientist who co-wrote the study with Hurteau, said that how a controlled burn affects the climate depends on the region. In the Pacific Northwest, where forests receive lots of moisture and fires are easier to suppress, a treated forest would not trap as much carbon, North said.
“I think that Campbell’s argument is actually a very good argument in systems that are pretty productive and also where fire suppression really does tamp down the fire,” North said. “But if you go to a drier system that’s not so productive… you get a benefit” from controlled burns. In Southwestern forests, where wildfires are hard to prevent, reducing the amount of fuel through controlled burns creates forests better at both trapping carbon and resisting wildfire, he said.
But even in wetter climates, controlled burns help forests by killing small trees, North said. Forests packed with young trees have greater competition for moisture, making all of the trees more vulnerable to drought. “If you don’t treat the forest, you can sometimes lose the very old-growth trees that you want to hold on to—the trees that hold the most carbon,” he said.
By opening gaps in the canopy, controlled fires restore variety to the forest’s structure, making it more resilient to climate change, said Morgan Wiechmann, a researcher at the University of Minnesota and collaborator with North and Hurteau.
“When you put in place this prescribed fire, you’re restoring some of those ecological benefits that naturally occurring fires used to give,” Wiechmann said.
North said forest managers need to use fire as a tool more often. “We’re fighting a battle and we’re tying one of our arms behind our back, and frankly that arm has the Popeye muscles on it,” North said. “We need to work with the fire, and let the fire help us do some of the fuels reduction.”