Forest thinning cuts wildfire severity and saves carbon

Forests in California’s Sierra Nevada are not fragile by nature. A new study reveals that when people thin crowded tree stands and reduce fuels, the most destructive kind of wildfire drops sharply.

High-severity burned area in the Sierra Nevada now exceeds historic patterns. The 2020 to 2021 fire seasons, coupled with a punishing drought, set the stage for a natural experiment.

The research was led by Ethan Yackulic of Northern Arizona University. He worked with partners across several environmental organizations, including the U.S. Forest Service, to track exactly how managed and unmanaged forests fared.

Thinning projects reduced fire severity

The team looked at 216 thinning projects initiated in 2016 across the Central Sierra and followed them through 2023. They paired each treated site with a closely matched untreated area to build a real-world test that would hold up under scrutiny.

The researchers measured changes in live tree carbon using satellite based biomass estimates and tracked fire behavior with the composite burn index (CBI), a standard field calibrated scale of fire severity. They then compared outcomes during years with and without fire.

The team paid close attention to where treatments sat on the landscape. Large projects, especially those near fire edges, can influence how a blaze spreads.

Projects that received follow-up work like prescribed burning or extra fuel removal after thinning were also evaluated. Maintenance is routine in ecological forestry, and the team wanted to see if it measurably improved results.

“Treatments reduced average fire severity by 32 percent and the prevalence of high severity fire by 88 percent,” wrote Yackulic. That reduction did not eliminate fire. It shifted fire toward lower intensities, which keeps more trees alive and keeps more live carbon on site.

Forest thinning projects need upkeep

Treatments larger than about 15 acres showed the strongest reductions in severity inside burn perimeters. Scale helps because bigger patches interrupt fuel continuity and give firefighters safer, wider anchors.

A separate landscape study in the northern Sierra found that strategically placed networks of treatments reduced severity and aided recovery, reinforcing the value of placement and size across a broader area, showing landscape level gains.

Maintenance mattered too. Projects that received a second entry, such as prescribed burning, saw lower severity and held more live carbon after fire than one and done treatments. Follow through amplifies the initial investment.

Thinning forests and carbon storage

The study tracked aboveground live carbon, a proxy for the living forest and its carbon sequestration capacity. Treated stands lost carbon in year one because thinning removes biomass, which is expected.

From there, the signal flipped. By year seven, nearly three quarters of treated forests matched or exceeded their baseline carbon, and the average treated site gained 12.8 metric tons of carbon per hectare relative to its matched control.

During the 2020 to 2021 drought and megafire years, untreated areas lost much more live carbon to high severity fire. Treated areas held steady or declined far less because the fires that did burn there were usually less intense.

Restoring natural fire patterns

Historically, many of these forests burned frequently at low intensities. A U.S. Forest Service report documents that pattern and shows how a century of fire exclusion altered structure, density, and fuels.

Thinning and burning are ways to restore that functional pattern in places where safe, low intensity fire no longer happens on its own. The goal is not to eliminate fire but to return it to a level the forest can handle.

Drought pushes trees to their limits, especially when too many stems compete for the same water. A previous analysis in the Sierra Nevada found that stands thinned before the 2012 to 2015 drought saw significantly lower drought-related mortality.

That pattern matches the new findings. Treated stands in the Central Sierra kept accumulating live carbon even through the 2020 drought period, while many untreated areas stalled or lost ground.

Supporting forests in the long term

Managers face a tough balance. Removing biomass lowers fire risk but can undercut carbon goals. Yet over a full decade in a fire-prone landscape, the evidence suggests the opposite – fuel reduction can actually support carbon objectives.

A widely cited paper argues for “operational resilience,” a practical target that reduces competition and surface fuels enough to handle fire and drought. The new results show that such targets are not only compatible with carbon stability, they can support it under stress.

Planning also matters. Grouping larger units along likely fire edges, then maintaining them with prescribed burns, turns them into semi-permanent speed bumps that slow and shape a blaze when it comes.

The study is published in Frontiers in Forests and Global Change.

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