Wildfires and other stressors are killing trees in California’s mountains and open spaces — with fewer new trees filling the void, reports a University of California, Irvine (UCI) study. “Forests can’t keep up with these fires,” said study co-author Professor James Randerson of UCI Earth System Science. Since 1985, tree cover across the state has fallen by 6.7 percent. “It’s a huge change 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 believe that stressors such as wildfires, drought and logging are responsible for the changes. In this study, the UCI-led team used satellite data from the USGS and NASA’s Landsat mission to study changes in vegetation from 1985 to 2021. They found that Southern California had one of the worst tree-cover declines, with 14 percent of the trees in the local mountains gone, possibly permanently. The speed and magnitude of tree declines in the state varied from place to place. Tree cover in the Sierra Nevada, for example, remained relatively stable until around 2010, then began to decline sharply. The 8.8% tree death in the Sierra Nevada coincided with a severe drought from 2012 to 2015, which followed some of the worst wildfires in the state’s history, including the 2020 Creek Fire. Jonathan Wang, a postdoctoral researcher in Randerson’s research group, led the study, published in the journal AGU Advances . Fortunately, he said, “a lot of vegetation has recovered after the northern fires,” possibly because of higher rainfall and cooler temperatures in the area. But even in these places, the fire-prone years of 2018, 2020 and 2021 took their toll. The loss of trees has also affected the state’s carbon storage capacity, Randerson said, adding that the next step is to precisely quantify the impact of these changes on the forest’s ability to absorb carbon dioxide.
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