Prescribed burns help forests survive megafires

A view near Loup Loup Pass showing the impact of the 2014 Carlton Complex wildfire in north central Washington. (Credit: Susan Prichard/U. Washington)

Previous tree thinning and prescribed burns helped forests survive the 2014 Carlton Complex wildfire, researchers report.

The fire was the largest contiguous fire in Washington state history. In just a single day, flames spread over 160,000 acres of forest and rangeland and ultimately burned more than 250,000 acres in the midst of a particularly hot, dry summer.

The wildfire, driven by strong winds and explosive growth, was unprecedented in how it burned the landscape, destroying more than 300 homes. But “megafires” like this have become more common in western US forests as the climate warms and trees crowd forests after years of fire exclusion.

The new research shows that that even in extreme wildfires, reducing built-up fuels such as small trees and shrubs pays off.

“Our study suggests that the fuel treatments were worth the investment, yielding a more desirable post-fire outcome than if they hadn’t been implemented,” says lead author Susan Prichard, a research scientist at the University of Washington’s School of Environmental and Forest Sciences.

“There are a lot of benefits to creating more resilient landscapes, and this study suggests that even in the worst-case scenario wildfires, it can be worth it.”

How do prescribed burns help?

On July 17, 2014, sustained winds of 35 mph raced through Washington state’s Methow Valley, blasting oxygen into several fires that had started earlier that week. Before long, the fires joined and spread. A column of smoke stretched 30,000 feet high, dropping embers and igniting dry grasses, trees, and other fuels on the ground over thousands of acres.

Propelled by the fast-moving column of smoke and wind, the flames traveled all the way to the Columbia River.

“It was the kind of fire that even experts who have studied them for decades had not seen before,” Prichard says. “It was a devastating fire for our community. I truly thought people would be killed by that fire, and it still feels miraculous that everyone survived.”

In the aftermath, Prichard and collaborators wondered if actions such as controlled burns and thinning help in the case of megafires like the Carlton Complex. State and federal agencies sink ample resources into these efforts to increase the resiliency of forests to wildfires by removing small- and medium-sized trees, leaving larger, mature trees more likely to survive future drought and wildfires.

After fire managers thin a forest, they use prescribed burning to reduce leftover woody debris on logging sites or across landscapes that have built up downed trees, pine needles, grasses, and shrubs.

Protection from the next fire

The Carlton Complex fire burned across hundreds of sites that were previously thinned or burned, offering a testbed for the researchers to analyze whether the work helped reduce fire impacts in those areas during the megafire. The researchers used satellite images of burn severity to examine how past fuel treatments performed in the context of this extreme wildfire event.

They found that even during the first explosive days of the Carlton Complex, areas that managers thinned and prescribed burned areas had more trees survive than areas that didn’t receive those fuel treatments.

“Some of the treatments measurably reduced fire impacts even under very hot, dry, and windy conditions,” says coauthor David W. Peterson, a research scientist at the US Forest Service Wenatchee Forestry Sciences Lab.

“Our results suggest that as we increase our ‘restoration footprint’—the proportion of forest area treated to reduce fuels—forests may become increasingly resilient to wildfires under a broad range of conditions.”

Researchers also found that thinning and burning on slopes protected from prevailing winds worked more effectively to reduce the wildfire’s impacts on the landscape than similar treatments in areas directly exposed to the wind.

Because winds often move through the Methow Valley in a predictable pattern, fire managers could thin and burn strategically in areas where these activities would be most effective.

Those efforts helped mature, naturally fire-resistant ponderosa pines survive. These mature trees, in turn, will provide seeds that produce younger trees across the landscape.

While the study focused on the Carlton Complex fire, its results have broader implications for forests around the world that are prone to wildfires. The work adds to the growing library of studies showing how thinning trees and controlled burns can reduce the severity of the next fire on the landscape.

“I’m hopeful our study will encourage policymakers as well as managers to invest in restoration,” Prichard says. “It’s our best hope for protecting our streams, rivers, and landscapes from catastrophic fires.”

The paper appears in Ecological Applications. Additional coauthors are from the University of Washington and the US Forest Service Wenatchee Forestry Sciences Lab. The US Forest Service’s Western Wildland Environmental Threat Assessment Center, the Joint Fire Science Program, and the National Fire Plan funded the work.

Source: University of Washington