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Extremely Dry Soil Connects Forest Die-Offs To Prolonged Drought, Says New UC Study

Kerry Klein
Valley Public Radio
Scores of trees have been removed near the Madera County community of Bass Lake.

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 deficit by pairing satellite imagery of tree canopies, which can be used as a proxy for evapotranspiration rates and the amount of moisture sucked by trees out of soil, with soil measurements throughout the Sierra.

To avoid such devastating tree mortality in the future, he suggests forest thinning, in order to reduce overcrowding and competition for scarce resources during the next big drought. “There’s pretty good consensus among the land managers, the scientific community, and many non-governmental environmental groups” that removal of some trees is necessary for healthy forest growth, Bales says.

Looming large over all of this is climate change. Rising temperatures, the study argues, will continue to exacerbate the imbalance between how much moisture trees need and how much the soil can store.

Kerry Klein is an award-winning reporter whose coverage of public health, air pollution, drinking water access and wildfires in the San Joaquin Valley has been featured on NPR, KQED, Science Friday and Kaiser Health News. Her work has earned numerous regional Edward R. Murrow and Golden Mike Awards and has been recognized by the Association of Health Care Journalists and Society of Environmental Journalists. Her podcast Escape From Mammoth Pool was named a podcast “listeners couldn’t get enough of in 2021” by the radio aggregator NPR One.