Water & Drought

Partnership to test whether forest thinning can grow groundwater, snowpack

The purchase of 10,000 acres of watershed land west of Lake Tahoe is slated to launch a living laboratory testing whether the answer to drought lies in fewer trees.

Research by the environmental group the Nature Conservancy has shown that thinning forests of small- to medium-size trees and bushes allows forests to trap more rain because it lets more water run off to streams and into groundwater stores. In winter, thinned areas may also allow more snow to be deposited on the forest floor as snowpack, said Edward Smith, a forest ecologist with the Nature Conservancy.

The Palo Alto-based Northern Sierra Partnership, which includes the conservancy, seeks to put that theory into practice on former logging land it bought earlier this month for $10.1 million about 10 miles west of Lake Tahoe and south of Interstate 80 in Placer County. It’s one of the largest pieces of unprotected land south of Donner Summit and the Tahoe National Forest.

As part of the project, stream gauges at weirs, flow meters in streams and groundwater well monitors will assess whether thinning is having a net effect on water levels above and below ground. The partnership is expecting a modest increase in runoff downstream – between 2 and 10 percent, said Smith.

“We’re excited about this because of the scale of the effort,” he said. “It’s one of the largest living laboratory experiments.”

Some, however, counter that thinning forests to conserve water may offer short-lived benefits at best.

Chad Hanson, principal ecologist with the anti-logging advocacy group the John Muir Project, said regular forest fires already thin trees that soak up water.

“If they’re tied to the thinning concept, they would have to thin and burn every eight to 10 years,” Hanson said. “They should just acquire the land and protect it as forest, and if it burns it will be great post-fire habitat.”

Over 2,100 private donors contributed $9.5 million to purchase the Sierra land, and the state of California contributed $5 million. Management and research of the forest thinning and restoration will be led by the Nature Conservancy. The land will be owned by the American River Conservancy.

The conservancy’s report said thinning dense forest can lead to a 3 percent to 6 percent increase in mean annual stream flow to some watersheds. It was one of the first studies to look into the effects of forest thinning on water yields in the Sierra Nevada. Research conducted in China last year also found significant benefits to water storage there from forest thinning.

Dave Bolland, project manager at the Association of California Water Agencies, said leaving forests alone would not have a positive effect on forest health or public safety.

“There would be catastrophic fires,” said Bolland. “And new trees would come in after the fire and grow back as thickly as before.”

Bolland contends that forests need to be actively managed by thinning to prevent large fires stoked by increasingly thick forests.

He said that healthier, thinned forests could offer better water quality from a watershed. Deeper snowpack in such forests would mean that water would melt more slowly into watersheds, instead of water and sediment running off all at once with intense rain events or little or no snowpack.

Edward Ortiz: 916-321-1071, @edwardortiz