Dry heat this summer that has all of the Central Sierra Nevada range in extreme drought is stressing the region’s mountain forests and improving conditions for bark beetles, which could lead to another outbreak of bug-kill tree mortality later this year, U.S. Forest Service researchers said Friday.
“It’s pretty bad, and if we don’t get a real winter it’s going to be real bad,” Chris Fettig, a Davis-based, doctorate-level research entomologist who specializes in insect studies for the forest service, said Friday in a phone interview.
The most recent severe tree mortality episode in the Central and Southern Sierra came in 2015 and 2016 on the heels of several consecutive drought years. Scientists estimated that statewide 150 million trees have died since 2010.
“To put the drought in perspective, there’s a lot of evidence now that we are in the midst of a 20-year drought,” Fettig said. “We had 2012 to 2015, drought years. 2014 was the hottest and driest year on record. 2015 was the third driest and second warmest. That 2012 to 2015 drought, it looks like it was the most significant drought event in a 1,200-year period, and that resulted in a lot of that tree mortality, mainly 2015, 2016 and 2017, five or six years overall.”
Fettig said he expects to see some increase in tree mortality this year, though it should not be as severe as it was in 2015 and 2016.
“We’ve already lost so many trees that competition for scarce water has been reduced in a lot of areas,” he said. “It remains to be seen how severe this summer’s dry conditions and high temperatures will be in terms of bark beetle reproduction, tree resistance, and eventual infestations. The real unknown is if we don’t get a normal winter in terms of precipitation, we could see a significant increase in tree mortality in 2022.”
Scientists say bark beetles are a normal part of the ecosystem and are present every year in Central Sierra mountain forests. When conditions get drier and hotter than normal, trees dry out and can’t produce enough resin to keep the insects at bay. Beetles can then overwhelm defense mechanisms of trees and then kill them.
Fettig has been studying forest health and disturbance ecology, including insects, drought, fires, and climate change, for the forest service’s Pacific Southwest Research Station in Albany since July 2001.
Fires burning up and down the Central Sierra have prevented pilots and researchers from doing tree mortality surveys so far this summer, Jeff Moore, the Pacific Southwest Region’s forest health aviation team leader and aerial survey program manager, said Friday in a phone interview.
Moore spoke from Sonoma during a break between aerial survey flights.
“We’re doing the North Bay and the Coast Range, near the Mendocino National Forest,” he said. “We’ve done Southern California, too. We’ve been nowhere in the Sierra Nevada.”
The North Coast has seen a slight uptick in tree mortality, in Douglas fir and the evergreens called tanoak, Moore said. He and his team have also noticed slightly higher levels of tree mortality in the Angeles, San Bernardino, Cleveland, and Los Padres national forests in southern parts of the state.
“It was a little elevated,” Moore said of bug-kill evident during recent survey flights. “Nothing too major over the past two weeks.”
Forecasting tree mortality is always an open question, Moore said, and fires are slowing the start of aerial surveys on the state’s most crucial mountain watershed forests. Temporary flight restrictions on airspace over the multiple fires burning in the Sierra Nevada range mean no one can fly in those areas except firefighting aircraft, he said.
Triple-digit heat waves in the Mother Lode foothills often mean it’s also drier and hotter than normal at higher elevations in the Stanislaus National Forest. A five-station index for the Central Sierra region that includes Calaveras Big Trees and Hetch Hetchy shows the Stanislaus and Tuolumne watersheds have received 47%t of average precipitation since the current water year started Oct. 1.
The same index also shows the Central Sierra has received 18.3 inches of rain and snow so far in 2020-21, which is less than one of the region’s driest water years, 2014-2015, when the region got 19 inches.
Temperatures in the Central Sierra, including portions of the Stanislaus National Forest in Alpine, Calaveras, and Tuolumne counties, were 4 to 8 degrees hotter than average for 30 days preceding July 20, the California Department of Water Resources said this week.
More than 85 percent of the Golden State was in extreme drought as of Thursday, according to scientists with the U.S. Drought Monitor. One-third of the state — including southeast Tuolumne County and most of the Southern Sierra — were in exceptional drought, the most dire category.
Residents of Sonora and other towns in the Mother Lode foothills can expect highs in the 90s this weekend and approaching triple-digits by the middle of next week. Forecasters also say more mountain thunderstorms and fire weather are possible Sunday to Wednesday.
Anyone who lives in or near the Stanislaus National Forest on land with pine trees and notices signs of drought stress or potential for bark beetle outbreaks should consult with an arborist or other tree expert, Fettig said.
“This is probably a bellwether for the future given climate change,” Fettig said. “The warming temperatures and reductions in precipitation are like a predictor for the future. With climate change, we’re likely to see our climate get increasingly warmer, more variability in precipitation, and more frequent and severe droughts. As a result, a lot of our forests are going to become more stressed and suffer higher levels of tree mortality.”
Contact Guy McCarthy at email@example.com or 770-0405. Follow him on Twitter at @GuyMcCarthy.