Forests provide natural climate solutions for sequestering carbon and mitigating climate change, and California is banking on its forests to continue doing just that.
However, trees in California’s mountain ranges and open spaces are dying from wildfires and other pressures, according to researchers at the University of California, Irvine.
“The forests are not keeping up with these large fires,” said study co-author James Randerson, the Ralph J. and Carol M. Cicerone Professor of Earth system science at UCI. Across the entire state, tree cover area has declined 6.7 percent since 1985. “These are big changes in less than four decades,” he said.
The findings show that “The ability of forests to recover from fire appears to be dwindling in the south,” said Jonathan Wang, a postdoctoral researcher in Randerson’s research group, who led the study published in AGU Advances.
Mapping annual disturbances and vegetation change
Wildfires in forests in Russia, Europe, and North America are increasing in intensity and severity – threatening the integrity of forests, and this means it is all the more relevant that areas experiencing rapid climate change, such as California, need close monitoring.
The researchers found there was a lack of digital vegetation maps at a high spatial resolution that can accurately measure changes over a period of decades. So, for the study, the UCI-led team used satellite data from the USGS and NASA’s Landsat mission to study vegetation changes between 1985 and 2021.
The team compiled satellite data and archival records with machine learning to map vegetation type (tree, shrub, and grass) and disturbance agent (fire, harvest, and drought) in California for the time period.
In the 37 years studied, “California lost 4,566 km2 of its tree cover area, equal to 6.7% of its initial tree cover in 1985. Tree cover area initially increased in the 1990s, but rapidly declined after 2000 from larger and more frequent wildfires, resulting in an expansion of shrub and grass cover area,” according to the study.
Fortunately “in the north, there’s plenty of recovery after fire,” said Wang, perhaps because of the region’s higher rainfall and cooler temperatures. But even there, high fire years in 2018, 2020, and 2021 have taken a visible toll.
Because of the decline in the number of trees, there is also concern over the forest’s carbon storage abilities. Co-author Michael Goulden, a 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 are affecting water resources, carbon storage, and fire behavior across the state.
“This threat to California’s climate solutions isn’t going away anytime soon,” Wang said. “We might be entering a new age of intense fire and vulnerable forests.”
Changes in vegetation cover
Even though it has taken 37 years for California to lose 6.7 percent of its forest trees, as Dr. Wang says, “the problem isn’t going away anytime soon.”
Actually, in 2011, researchers at NASA and the California Institute of Technology said the changing climate will also convert 37 percent of the world’s land ecosystems from one type — such as tundra, forest, or grassland — into another by 2100.
Those vegetation shifts are likely to affect animals and insects that have evolved to live among particular plant species and within certain temperature and precipitation ranges, said the study’s lead author, independent scientist Jon Bergengren.
So it will be interesting to see what Dr. Goulden finds as he looks deeper into how changes in forest cover are affecting water resources, carbon storage, and fire behavior across the state. It is going to impact all of us.
