In the woods

Lake Tahoe’s forests are in increasing trouble as drought and attacking insects kill more and more trees.

Across California, an estimated 156 million trees have died after lengthy drought conditions brought disease and infestations of bark beetles. In places, 80 percent or more of low-elevation pine forests, mostly ponderosa pine, have turned a withered brown.

Lake Tahoe has so far escaped the worst of this, largely due to its higher elevation and a more diverse forest makeup.

“We’ve been fortunate thus far,” said Brian Garrett, Tahoe’s urban forest program manager for the U.S. Forest Service. “But we have seen elevated tree mortality.”

The problem appears to be accelerating. According to the Tahoe Regional Planning Agency, some 168,000 trees died in the Tahoe Basin in 2017, more than twice the 72,000 counted in 2016. In 2015, 35,000 dead trees were counted.

“There were a lot of dead trees last year,” said Christina Restaino, TRPA forest ecosystem health manager. “That’s definitely above the levels we would expect. It’s certainly cause for concern.”

The problem is linked to five years of drought that hit the Sierra beginning in 2012. Deprived of moisture, trees were weakened and made vulnerable to disease and bark beetle infestation. The result is wider swaths of dead trees and heightened risk of major wildfires.

At Tahoe, fir engraver beetles are taking out white and red fir trees. Surveys taken by researchers from University of California, Davis in 2009 and 2017—before and after the drought—show increased mortality among stands of sugar pines, particularly along Tahoe’s north shore. The mountain pine beetle is identified as the culprit there.

It’s not the first time Tahoe’s forests have faced a two-prong threat from drought and insects. Following a seven-year drought in the late 1980s and early ‘90s, one out of three of Tahoe’s trees was dead, with tree mortality measured at up to 90 percent in some locations. And with a warming climate expected to bring more droughts, the problem is likely to recur, experts said.

“I would say [forests] are highly vulnerable from drought, insects and fire,” Restaino said. “With all three of those combined, we are at very high risk of losing trees at Tahoe and all over the Sierra. If we do nothing, tree mortality will increase drastically.”

That’s not to say nothing can be done. Since improving Tahoe’s forest health was identified as a major priority in 1997, land managers have thinned more than 70,000 acres of forest, mostly in areas closest to Tahoe’s communities. Another 50,000 acres are targeted for thinning, and the Forest Service is working to pick up the pace of forest treatment projects in coming years.

Thinned forests have less chance of fueling a major wildfire. And UC Davis surveys also show markedly reduced concentrations of bark beetles in parts of the forest that were previously treated.

“We can’t stop droughts but a healthier, thinner forest is more resilient,” said Geoffrey Schladow, director of UC Davis’ Tahoe Environmental Research Center.

UC Davis researchers are collecting seeds from sugar pines that, for whatever reason, appear to successfully stave off beetle attack. The seeds will be germinated and reared, with some 10,000 available for future revegetation projects around the lake.

And land managers are looking at larger-scale restoration projects, including treatment of an entire 60,000-acre swath of forest west of Lake Tahoe.

“A lot of work is going into trying to find solutions to improve our forest health,” Restaino said. Ω