Â鶹ÒùÔº

The odds of high-severity wildfire were nearly one-and-a-half times higher on industrial private land than on publicly owned forests, a new study found. Forests managed by timber companies were more likely to exhibit the conditions that megafires love—dense stands of regularly spaced trees with continuous vegetation connecting the understory to the canopy.

The research, led by the University of Utah, University of California, Berkeley, and the United States Forest Service, is the first to identify how extreme weather conditions and forest management practices jointly impact fire severity. Leveraging a unique lidar dataset, the authors created three-dimensional maps of public and private forests before five wildfires burned 1.1 million acres in the northern Sierra Nevada, California.

The study is published in the journal Global Change Biology.

In periods of extreme weather, stem density—the number of trees per acre—became the most important predictor of a high-severity fire. Even in the face of accelerating climate change, how we manage the land will make a difference.

"That's a really hopeful finding because it means that we can adjust how we manage these landscapes to impact the way fires move through them," said Jacob Levine, postdoctoral researcher at the U and lead author of the study.

"Strategies that reduce density by thinning out both small and mature trees will make forests more robust and resilient to fire in the future."

In a 2022 study, Levine and collaborators found that fire severity was typically higher on privately managed forests. They also discovered the risks extended to areas near to, but not owned by, private industry, threatening the wilderness, small landowners and urban areas in their shadow.

This new study is the first to identify the underlying forest structures that make high-severity fires more likely in some areas than in others.

Lidar unlocks forest structure secrets

Plumas National Forest, the study area in California's northern Sierra Nevada, is emblematic of the wider trend of wildfire occurrence and severity. The region's mixed conifer forests are adapted to periodic, low- to medium-severity fires that cleared vegetation, creating large spaces between clumps of trees.

Efforts to increase timber resources led the U.S. government to implement fire suppression policies in the 1800s, including a ban on controlled burns that Indigenous People practiced for millennia. In the absence of natural fire cycles and Indigenous burning, modern forests have more fodder to fuel high-severity fires, defined as a fire that kills more than 95% of overstory trees.

Plumas National Forest is a mosaic of private industrial and public ownership, and 70% of the study area was burned in five massive wildfires between 2019 and 2021, including the largest single fire in California's recorded history, the Dixie Fire. Serendipitously, a unique dataset had been collected a year before the region burned.

In 2018, the U.S. Forest Service, Geological Survey and National Aeronautics and Space Administration surveyed the Plumas National Forest and surrounding private land using airborne light detection and ranging (lidar) flights. The lidar sensors shoot billions of lasers at the landscape below, which bounce off the grass, shrubs, saplings, tree canopies and other structures in the forest with high precision.

"We have a really detailed picture of what the forest looked like immediately before these massive fires. It's an unbelievably valuable thing to have," Levine said. "Understanding the forest structures that lead to high-severity fire allows us to target mitigation strategies to get ahead of this massive fire problem while still producing enough timber to meet market demand."

Private vs. public management strategies

Timber companies are focused on maximizing profits and providing a sustainable source of wood, a valuable resource for society and an economic engine for rural communities. Most practice plantation forestry—clear-cutting an area and replanting the trees in a tightly packed grid. After 80 to 100 years, they do it all again, leaving a patchwork of dense stands of trees of similar age and size.

"You can think about stacking a bunch of matches together in a grid—that's going to burn a lot better than if you have those matches dispersed as smaller clumps," Levine explained.

"A bigger fire can easily reach the canopy in dense forests. Then it's ripping through one tree after another, tossing out chunks of burning material miles in advance. It's a different story."

The objectives of public lands are more varied, requiring management for grazing, recreation, restoration, timber production and wildlife corridors. They're also beholden to the public, which stymies their ability to do active management. Environmental organizations often sue to stop proposed projects that would remove trees to thin down density.

Although the study demonstrates that private industrial lands fare worse, both private and public agencies have much room for improvement to protect our nation's forests. Most Sierra Nevada trees lack adaptations to recover from high-severity fires, leading to more and more of our forests turning into shrub and grasslands.

"This has major implications for timber, but also for carbon sequestration, water quality, wildlife habitat and recreation," Levine said.

"Shrub and grasslands can be beautiful, but when we think of the Sierra Nevada we picture majestic forests. Without major changes in forest management, future generations could inherit a landscape that looks very different than the one we cherish today."