The hard truth about being a 21st century tree in California

California's expansive forests have experienced a profound tree die-off since 2010, exacerbated by a long drought between 2012 and 2015. These pine trees are tough, though, and have evolved to withstand parched years in the drought-prone Golden State. But not drought like this, which was amplified by the planet's relentless, accelerating warming. 

"The rules are changing," said Nathan Stephenson, a U.S. Geological Survey forest ecologist who monitors trees in California's Sierra Nevada mountain range.

"It wasn't just dry — it was warmer," added Craig Allen, a research ecologist with the U.S. Geological Survey.

New research, published this week in the journal Nature Geoscience, illustrates how trees in the state's sprawling Sierra Nevada mountains hung on to life before succumbing to drought. The great forest die-off is tied to a drying up of the deep, deep soil, up to some 50 feet below ground. California trees withstood a lack of rain for two to three years by drawing on this deep-seated water. But, unfortunately for many trees, those last reserves were eventually exhausted, too.