A new study by UC Irvine and University of Utah researchers reveals an alarming increase in California wildfire severity, leading to unprecedented tree mortality rates across the state's forests.
The research, published in Environmental Research Letters, found that wildfire severity increased by 30 percent between the 1980s and 2010s, causing substantially more damage to tree canopies per acre burned compared to previous decades.
"When fire moves over a forest's floor, often the tree canopy will survive and, in some situations, thrive from fire effects on nutrient cycling," explained James Randerson, professor at UC Irvine's Department of Earth System Science. "The new research suggests more fire is jumping into the tree crowns, causing more damage and tree mortality."
The study highlights how climate change has pushed wildfires into previously resistant northern mountain and coastal regions, where forests are now more vulnerable due to warmer, drier conditions. This expansion has led to a 7 percent decrease in California's total tree cover since 1985.
The iconic giant sequoia groves in Sequoia and Kings Canyon National Parks exemplify this growing threat. These ancient forests face increasing vulnerability to extreme fire behavior as temperatures continue to rise and moisture levels drop.
Using satellite imagery from the Landsat Program, researchers determined that the combination of increased fire severity and forest vulnerability has resulted in double the tree cover loss compared to what would have occurred from larger burn areas alone.
Lead author Jon Wang warns that California's forests cannot keep pace with the current rate of destruction. "If fires were not changing, you'd expect our ecosystems to be in a steady state. One area that's burned is counterbalanced by an area that's recovering."
The study suggests that severely burned forests may eventually be replaced by vegetation better suited to hot, dry climates, with shrub and chaparral ecosystems potentially expanding into areas previously dominated by old-growth forests.
This research provides valuable insights for land managers and stakeholders working to address the evolving challenges posed by climate change to California's forest ecosystems.