The inability of long-rooted trees to reach their subsurface water supply in the Sierra Nevada mountain range led to widespread forest die-offs following the drought of 2012-2015. A new study provides a better understanding of the climatic and biological mechanisms in play.

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.

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