LANL study finds wildfire-scorched areas prone to reburning

6 February 2023

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USA – Forests untouched by flames for decades draw scrutiny from fire managers who view the buildup of brush, debris and dense tree cover as ripe for a catastrophic blaze.

Less attention is given to burn areas where wildfires have roared through because the traditional view is the excess flammable material was consumed and the trees thinned, making the forest less prone to another fire.

But as wildfires have grown more frequent — and larger — in the West amid the nearly quarter-century drought magnified by climate change, so, too, have repeat wildfires in areas that were already scorched.

As one might expect, some forested areas are more susceptible to reburning.

In a Los Alamos National Laboratory study, scientists looked at landscapes in 11 Western states that were burned by multiple fires within a 20-year period.

They discovered they could gauge the likelihood of a reburn in an area based on three factors: seasonal temperature, moisture loss from plants and wind speed.

Those are the main drivers that sweep fires across a landscape multiple times and should be considered when deciding where to conduct prescribed burns or mechanical tree thinning — or perhaps try a different treatment method altogether, according to the study.

“We’re trying to understand more about this reburning phenomenon,” said Los Alamos lab scientist Kurt Solander, a hydrologist and the paper’s lead author. “What are the main causes and how much of a problem it is in different areas throughout the West.”

Armed with this research, forest managers could deploy more crews to areas that are warmer, drier and windier because they’re more likely to catch fire again, Solander said.

Managers could also consider the three factors when forming strategies for fire prevention, he said, which might include incorporating this data into computer modeling.

In a sprawling burn area, such as the one left behind by the mammoth Hermits Peak/Calf Canyon Fire in the Sangre de Cristo Mountains, some parts are likely to be more susceptible to a recurring fire than others, depending on how much the three factors are at play, Solander said.

The wildfire, the largest in New Mexico’s recorded history, resulted from two prescribed burns that went awry and merged into an inferno, torching 341,000 acres and destroying hundreds of homes.

The study found reburns occur at about the same rate as first-time wildfires, which means they are increasing as a changing climate makes the West warmer and drier, lengthening and intensifying droughts.

“We are seeing these reburns as part of climate change because the climate is getting drier and that’s leading to the potential for more fires, even in areas that have been treated to prevent or mitigate the occurrence of fires,” Solander said.

In studying the predictive factors for reburning, the researchers focused more on California, which has a large number of neighborhoods abutting forests, known as wildland-urban interfaces. They gave additional attention to where forests and residences converge, partly because of the greater potential for devastating losses.

Logically, the three factors that spur reburning could apply throughout the West in varying degrees, Solander said. But recurring fires in a single area are happening significantly more in California than, say, New Mexico, he added.

Although a reburn isn’t likely to cause more damage to an area because the earlier fires have already consumed much of the trees and vegetation, it does ruin any recovery that’s taken place, Solander said.

“It kind of resets the clock back to zero,” he said.

In general, forested areas that have been thinned or where planned burns have been conducted are not as fireproof as they once were after such treatments. “If this problem becomes severe enough, there might have to be new methods that are developed to treat the forest,” Solander said.

That might entail more frequent prescribed burns or using more intense burn methods, he added.

A U.S. Forest Service spokeswoman noted much of the study is centered on a different ecosystem in California than what’s found in the Southwest.

The forests and grasslands in the Southwest must undergo fire disturbance regularly to be resilient within this region, agency spokeswoman Punky Moore wrote in an email.

The tried-and-true method of conducting prescribed fires in areas that wildfires have burned remains the preferred technique, she added.

“This means for us in the Southwest, after what we call an initial entry — first-time burn — we would like to maintain that area with subsequent burns on a regular basis,” Moore wrote. “Experience has shown that after a prescribed fire is completed, if a future wildfire reaches the area, the fire behavior will usually be modified to a less intense, more manageable surface fire, making it safer for firefighters to engage.”

Scientists used an artificial intelligence system to aid in this project, enabling them to broaden the research and cover a more massive amount of data than would be possible through a traditional, statistical analysis, Solander said.

They could explore more variables, he said. For instance, with winds they could study not only the speed, but how it varies during different times of the year.

Getting a fuller picture also helped researchers to avoid biases, such as where they might assume reburns would occur and how, Solander said.

“This way we can look at everything and remove ourselves and our preconceived biases,” he said.

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