A catastrophic forest die-off in California's Sierra Nevada mountain range in 2015-2016 was caused by the inability of trees to reach diminishing supplies of subsurface water following years of severe drought and abnormally warm temperatures. That's the conclusion by researchers from the University of California, Irvine and UC Merced outlined in a study published today in Nature Geoscience.
"In California's mixed-conifer mountain forests, roots extend from five to 15 meters deep, giving trees access to deep-soil water," said co-author Michael Goulden, UCI professor of Earth system science. "This is what has historically protected trees against even the worst multi-year droughts."
But Goulden said the severity of California's 2012-2015 dry-spell exceeded this safety margin. Many forest stands exhausted accessible subsurface moisture, leading to widespread tree death.
From 2012 to 2015 the entire state of California experienced a crippling drought, and it was especially severe in the southern Sierra Nevada. With a multi-year combination of below-average precipitation and above-average warmth, the resulting drought was considered to be the most extreme in hundreds of years.
More details can be found here