The U.S. Forest Service estimates 147 million trees in California died following the state’s prolonged drought. New research out of UC Merced suggests a culprit: Extremely dry soil. Not all California droughts have led to massive forest die-offs. The difference this time, according to an article published Monday in the peer-reviewed journal Nature Geoscience, was the drought’s intense heat and longevity. Together, those factors led to an overdraft of much-needed soil moisture, even deep below the surface , says Roger Bales, Director of UC Merced’s Sierra Nevada Research Institute and senior author on the study. “We basically reached a tipping point in the Southern Sierra area,” where forests have been most heavily affected by tree mortality, he says. “Every year the trees drew water from a little bit deeper down, and when it rained or snowed and snow melted, it replenished the upper part of the soil but not the deeper part.” Bales and his co-author determined the soil’s moisture
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