Overlooked water sources aided tree survival during drought, researchers discover

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A less frequently studied layer of weathered rock underlying hillside forests may serve as a major water source for trees during times of drought, according to a study conducted by UC Berkeley and the University of Texas at Austin.

The study, which was published Monday in the scientific journal Proceedings of the National Academy of Sciences, was conducted at the Angelo Coast Range Reserve in Mendocino County. From 2013 to 2016, researchers measured levels of rock moisture in the forested hillsides of the reserve.

The study was conducted during the 2014 peak of California’s recent drought. Its findings may hold implications for how climate modeling researchers predict future drought severity, though “we don’t know that the drought we thought we were done with has come back,” said William Dietrich, study co-author and UC Berkeley earth and planetary science professor.

Though groundwater and soil moisture have always been considered important water sources in hydrologic systems, the transitional zone of weathered bedrock connecting the two layers is often overlooked, according to Daniella Rempe, lead study author and assistant professor of geological sciences at the University of Texas at Austin.

“We’re realizing there’s this huge sponge in between,” Rempe said.

In mountainous regions, tectonic collisions push dense bedrock up toward the surface, causing the rock layer to weather or decay, Dietrich said. The porousness produced by the movement has the ability to store significant amounts of water.

Rock moisture at the study site measured 10 inches — more than twice the amount held in soil, which stored about 2 inches comprising as much as 30 percent of the annual precipitation, Dietrich said.

The trees at the Northern California site, which stand up to 200 feet tall, endure an annual six-month dry period, Dietrich said. Despite their size and the effects of drought, however, trees in the area saw little effect.

The study notes that although more than 100 million trees died throughout California during the 2010-15 drought period, the coastal reserve where the researchers conducted the study demonstrated no tree mortality.

Dietrich said it is likely the trees were able to survive despite low levels of rain by drawing from rock moisture.

“We’re trying to understand (the) implications on (the) carbon cycle, rock weathering, how the ecosystem responds to stress — all inspired by measurements indicating that the trees are using very deep water,” Rempe said.

The research necessitates a rethinking of hydrologic processes across different ecosystems, Dietrich said. He added that the discovery of rock moisture has uncovered a formerly hidden component of Earth’s water budget, changing how researchers predict climate warming and tree survival in California.

“Our droughts are going to be only more severe,” Dietrich said. “Rock moisture is going to play a more and more important role in forests surviving, because they’re going to depend on that more.”

Contact Michele Meltzer at [email protected] and follow her on Twitter at @michele_meltzer.