As climate warms, California wildfires are becoming more severe, killing more trees

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Wildfires continue to damage California's forests as human-driven climate change amplifies their impacts. A new study published in Environmental Research Letters reveals that the severity of the state's wildfires has rapidly increased over the last several decades, contributing to greater forest loss than would have been expected from past increases in burned areas.

"Fire severity increased by 30% between the 1980s and 2010s," said Jon Wang, an assistant professor at the University of Utah School of Biological Sciences and former postdoctoral researcher at the University of California Irvine Department of Earth System Science. This means that for every acre of forest scorched by fire, the damages to mature trees are considerably higher than what occurred in the average fire several decades ago.

"When fire moves through an area on the forest floor, often mature trees survive and, in some situations, they may thrive from fire effects on nutrient cycling," said study co-author James Randerson, professor in the UC Irvine Department of Earth System Science. "The new research suggests more fire is jumping into the tree crowns, causing more damage and tree mortality."

Randerson added that wildfires also have moved into new areas with denser and more vulnerable forests. Those areas include northern mountain and coastal regions that may have been protected in the past by cooler summers and higher levels of surface moisture.

"Forest exposure has increased 41% over the past four decades, suggesting denser forests are now more vulnerable to wildfire," said Wang, who joined the U last year and is the principal investigator for the Dynamic Carbon and Ecosystems lab.

The question Wang and his team wanted to answer was how much rising tree cover loss in California is due to increases in total area burned, how much of the loss is due to increasing wildfire severity, and how much is due to fire moving into new areas with denser forests. Previous research led by Wang revealed that total tree cover in California has decreased by 7% since 1985 from the combined effects of wildfire and drought-induced die-off.

"We found that in many parts of the state, tree cover losses were rising faster than the area burned were," Wang said. "That was implying to us that it's not just that fires are getting larger, they're also burning more intensely, and they're affecting the forests more severely on a per-unit burned area basis."

Using a simple model, the authors found that this increased severity and vulnerability resulted in twice as much tree cover loss as would have occurred from the increased burned area alone.

Wang explained that before California wildfires started growing in severity and size, forests usually had time to regrow and reestablish whatever tree cover was lost. "But all the new recent fires are so big and so intense that our forests can't keep up," Wang said. "If fires were not intensifying, you'd expect our ecosystems to be in a steady state. One area that's burned is counterbalanced by an area recovering."

Destructive fires in the famous groves of giant sequoia redwoods in Sequoia and Kings Canyon national parks are emblematic of fire-driven tree cover loss. Such forests are getting hotter and drier as the climate continues to warm, leaving them more vulnerable to extreme fire behavior when fires strike.

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To conduct the science, Wang and his team analyzed long-term time series of satellite imagery from the U.S. government's Landsat Program. These data, available beginning in the 1980s, allowed the team to quantify fire-driven tree cover losses over the last several decades.

"There's a pretty shocking map of just how much these fires have expanded into northern California forests," Wang said. "There's just a lot more fire in these northern forests than there used to be. Climate change allows severe fires to affect forests that once tolerated milder fires."

Studies like this could help land managers and stakeholders better understand how forests are responding to climate change.

"Severely burned forests could be replaced by something more adapted to the hot, dry climates of Southern California," Wang said. "Shrub and chaparral ecosystems might end up expanding further into areas where big trees and old forests used to thrive."

In future work, Wang aims to collaborate with the U's Wilkes Center for Climate Science and Policy to build off this research to study wildfires and forest conversion across the Western U.S.

More information: Jonathan A Wang et al, Rising forest exposure and fire severity from climate warming amplify tree cover losses from wildfire in California, Environmental Research Letters (2024). DOI: 10.1088/1748-9326/ad86cf

Journal information: Environmental Research Letters

Provided by University of Utah