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Mechanical thinning, prescribed fire or both?

Study shows fuel treatments improve wildfire outcomes

Hilary Clark

Pacific Southwest Research Station

March 13, 2024

A forest fire next to a road, viewed from a car, with towering flames and smoke.
The Antelope Fire strikes northeastern Calif. in 2021 (USDA Forest Service photo by Troy Parrish, Operations Section Chief, Klamath National Forest)


The Antelope Fire strikes northeastern Calif. in 2021 (USDA Forest Service photo by Troy Parrish, Operations Section Chief, Klamath National Forest)

Sometimes out of adversity comes wisdom. That was a lesson Pacific Southwest Research Station Ecologist Eric Knapp learned after the 2021 Antelope Fire burned through long-term research plots in northeastern California.

“It was pretty upsetting to think about those 20 years of research going up in flames,” Knapp said.

Knapp’s colleague, Forester Martin Ritchie, and other scientists, initiated studies at this landscape, known as the Goosenest Adaptive Management Area, in the late 1990s. At that time, white fir and incense cedar filled the site, crowding out pine trees. One of the researchers’ goals was to restore a more open pine-dominated forest, which originally graced this area a century ago.

As part of that objective, they collaborated with staff from the Klamath National Forest to set up different forest and fuel management treatments on 40 to 100-acre plots. Forest staff implemented two methods of fuels treatment - mechanical treatment and prescribed fire – to a portion of the plots. Periodically, they remeasured the plots to evaluate the ecological effects of the treatments.

Burnt forest with charred trees after a wildfire.
Needles blanket the ground in a thick layer of ash at Goosenest Adaptive Management Area (USDA Forest Service photo by Eric Knapp)

Shortly after the fire, Knapp, and fellow researchers convened at the 2,300-acre study area to take stock of the damage. Because the fire burned for multiple days, they could observe how fuel treatments fared under different weather conditions. Wind speeds and humidity levels drove fire behavior, which fluctuated between high intensity to more moderate.

“I realized the fire presented a rare opportunity,” said Knapp. “We could see how fuel treatments performed in real-world conditions, which we highlighted in a recent paper.”

Analysis of the data, led by postdoctoral scholar Emily Brodie, showed areas previously treated with thinning and prescribed burning fared best, with the most living trees. Untreated control areas where no treatments occurred were in the worst shape. At these sites, high-intensity crown fires completely consumed the needles and branches of many trees, leaving bare, blackened stems. Plots treated with either mechanical thinning or prescribed burns, but not both, came out somewhere in the middle, with about half of the trees dying.

A man with a hard hat and vest stands by a tree, possibly measuring it.
Research Ecologist Eric Knapp measures a tree at the Goosenest Adaptive Management Area in northeastern Calif. (USDA Forest Service photo)

“I’ll never forget walking through the untreated control areas not long after the fire went through,” said Knapp. “What were once living green needles of the tree canopy now blanketed the ground in a thick layer of ash. We all remarked how it looked like snow. It was eerie.”

Knapp and others found that under the most extreme conditions, the fire killed some trees, even in plots that had been both thinned and burned. Fuel treatments can only do so much, but they speculated that more recent prescribed burning might have led to more trees surviving the fire.

Forests in this area historically burned about every ten years. Firefighters performed two rounds of prescribed burning at the site – the first around 2001 and another about ten years later. The treated units were due for a third prescribed burn when the Antelope Fire started. Keeping up with prescribed burning to maintain fire resilient forests can be challenging, requiring detailed planning, favorable weather conditions and adequate staffing. Burn units must be carefully prepared. Moisture levels must be low enough for fires to burn, but if they are too low then the prescribed burn cannot proceed. The atmospheric conditions must also support good smoke dispersion.  When there is high wildfire activity, the limited number of firefighters with the technical expertise to perform the burns are sometimes focused on emergency response.

“There’re a lot of factors, unfortunately, that makes it challenging to do prescribed burns at the scale they’re needed,” said Knapp.

Knapp underscored that thinning and prescribed burning play a critical role in preventing wildfires from consuming trees’ canopies, which produce embers that can rapidly spread the flames. Fuel treatments change fire behavior, which can also potentially help reduce risk to nearby communities.

“Mechanical thinning and prescribed fire target different forest fuels. Thinning reduces live tree canopy density and removes smaller trees that can function as ladder fuels, both of which contribute to crown fire behavior,” said Knapp. “Prescribed fire consumes litter, dead and down wood, and knocks back understory shrubs, reducing intensity of any future fire.”

In all the years of the study, Knapp and fellow researchers never imagined a wildfire would burn their plots. In a warming climate, though, wildfires like Antelope are becoming more frequent and severe.

A contrast of burnt and healthy trees in a forest with a car on a dirt road.
The road divides an untreated area on the left, and one that received prescribed burning and mechanical thinning on the right at Goosenest Adaptive Management Area (USDA Forest Service photo by Eric Knapp)

“We also found that if moderating fire behavior is the goal, the benefit of treating fuels was greatest under the most extreme weather conditions,” said Brodie.

She emphasized that the findings make a compelling case for doing both tree thinning and prescribed burning to protect forests in the future.

This suggests that fuel treatments will be increasingly important as climate change contributes to more extreme fire weather. Reducing fuel loads is key for not only protecting communities but sustaining forests and the wildlife that depend upon them.


Klamath National ForestEnvironmentFireFire PreventionForest PlanningForestryHazardous FuelsMonitoringResearch and DevelopmentScienceWildfireWildfire Crisis,  prescribed firetreatment


Reprined from USDA Forest Service U.S. DEPARTMENT OF AGRICULTURE