California relies on its forests to help reduce Earth-warming carbon dioxide in the atmosphere. But that element of the state’s arsenal of climate change solutions could be in jeopardy, as a new UC Irvine study reports that trees in California’s mountains and open spaces are dying from wildfires and other stressors — and new Trees fill the gaps less and less.
“Forests can’t keep up with these fires,” said study co-author James Landerson, the Ralph J. and Carol M. Cicerone Professor of Earth System Science at UCI. Statewide tree cover has declined 6.7 percent since 1985. “These are huge changes in less than 40 years,” he said.
This is the first time researchers have been able to measure the decline in California’s tree population and attribute the changes to pressures such as wildfires, drought stress and logging.
In this study, the UCI-led team used satellite data from the USGS and NASA’s Landsat mission to study vegetation changes between 1985 and 2021. They found that one of the areas with the greatest decline in tree cover was Southern California, where 14 percent of the tree population of the local mountain range disappeared, possibly permanently.
“The ability of southern forests to recover from fire appears to be diminishing,” said Jonathan Wang, a postdoctoral researcher in Randerson’s research group who led the study published in AGU progress“Meanwhile, shrub and grass cover is on the rise in the state, which may herald more lasting ecosystem changes.”
The speed and size of the decline varies from state to state. For example, tree cover in the Sierra Nevada remained relatively stable until around 2010, when it began to decline sharply. Sierra’s 8.8 percent death toll coincided with a severe drought from 2012 to 2015 that was followed by some of the worst wildfires in state history, including the 2020 Creek Fire.
Fortunately, “in the north, there’s a lot of recovery after the fire,” Wang said, possibly because of higher rainfall and cooler temperatures in the area. But even so, the high fire years of 2018, 2020 and 2021 took their toll.
The loss of trees also affects the state’s carbon storage capacity, Landerson said, adding that the next step is to precisely quantify the impact on the forest’s ability to absorb anthropogenic carbon dioxide. Co-author Michael Gulden, UCI professor of Earth system science and director of the Center for Ecosystem Climate Solutions, is using the data to understand how changes in forest cover affect water resources, carbon storage and fire behavior across the state.
“This threat to California’s climate solutions is not going away anytime soon,” Wang said. “We may be entering a new era of fires and fragile forests.”
Collaborators include Clarke Knight and John Battles of the University of California, Berkeley. The research was supported by the California Strategic Growth Council and the UC National Laboratory Grant Program, as well as long-term funding from NASA and the U.S. Department of Energy’s Office of Science.