Pilots and scientists who did an aerial survey of dead trees up and down California last year found elevated levels of tree mortality, with an estimated 9.5 million dead trees on 1.3 million acres, according to a 2021 Forest Service report, summaries, and a graphic released last week.
“Most of the mortality can likely be attributed to the lingering effects of the 2012-2016 exceptional drought and subsequent successful bark and engraver beetle attacks” that have now killed more than 172 million trees since 2010, Forest Service researchers said in a January report, “2021 Aerial Detection Survey Results: California.”
For the Stanislaus National Forest, the findings showed an estimated 587,000 dead trees on 95,000 acres. Separate estimates for counties that comprise the federal forest, from north to south, showed 332,000 dead trees on 44,000 acres in Alpine County; 168,000 dead trees on 24,000 acres in Calaveras County; 300,000 dead trees on 61,000 acres in Tuolumne County; and 211,000 dead trees on 30,000 acres in Mariposa County.
“The most recent aerial survey confirms what many of us living and recreating in the Sierra Nevada and on the Stanislaus National Forest feared — conditions are challenging, trees are becoming more stressed due to drought and climate change, leaving them vulnerable to predation by bark beetles and other insects,” Benjamin Cossel, spokesman for the Stanislaus National Forest, said Tuesday.
“This also sets up a situation where there will be more dead fuel on the forest floor susceptible to fire,” Cossel said. “This is why forest efforts like SERAL and other fuels-reduction work is so critical — to remove that dead biomass and, where needed, thin the forest to give the surviving trees the best chance to withstand all that is being thrown at them.”
Cossel was referring to a massive, multi-year project to reduce fuels and fire threats on 118,808 acres of public and private lands that include 94,823 acres in Forest Service jurisdiction in the South Fork and Middle Fork Stanislaus watersheds, called Social and Ecological Resilience Across the Landscape.
The aerial survey results from last year are slightly below those recorded in surveys two years earlier. No aerial survey was conducted in 2020 due to COVID-19 concerns.
Although the results from last year’s survey were below the 2019 results, they’re still well above what scientists consider normal tree mortality in California. The aerial survey’s summary report is based on data recorded and shared by the Forest Service, the USFS Region 5 office of State & Private Forestry, and the USFS Aerial Detection Survey program.
More than 60% of tree mortality in the Golden State was noted in stands of fir, followed by ponderosa pine. Severe levels of mortality in California red fir were detected in many areas, particularly in the Central Sierra, including high-elevation areas of the Stanislaus National Forest.
Statewide, whitebark pine mortality increased to 270,000 trees across 31,000 acres last year, with mortality common from the Stanislaus National Forest south to Sequoia and Kings Canyon national parks. Also statewide, western white pine mortality decreased from about 88,000 trees across 12,000 acres in 2019 to 190 trees across 210 acres in 2021. Most of this occurred east of Dardanelle in the Stanislaus National Forest.
Pine mortality attributed to western pine beetles increased statewide last year to more than 2 million dead trees estimated across 300,000 acres. Mortality was most widespread in the Klamath and Shasta-Trinity National Forests. It was most intense in the Tahoe and Stanislaus National Forests.
Mortality caused by pine beetles made up 27% of all detected tree mortality in California last year.
“This is really hard to project given we are not yet through the wet season,” Christopher Fettig, a research entomologist and team leader who studies insects that infest California forests, said Tuesday of what the recent survey results mean for the future. “Last week’s U.S. Drought Monitor indicates the region is in severe drought and if that continues, or intensifies, then I expect to see some uptick in levels of tree mortality attributed to bark beetles in areas where sufficient host trees still remain.”
Drought is a key factor in tree stress that can weaken trees and make them more susceptible and inviting to infestation that precedes mortality. The state has been in drought for at least 10 of the past 15 years, with 2010, 2011, 2016, 2017 and 2019 being the exceptions, according to a May water conditions report from the state Department of Water Resources.
“This prolonged water deficit has had profound impacts on California forests, killing trees outright and predisposing them to insect outbreaks and other damage agents,” Forest Service researchers said. “California has had significantly elevated amounts of forest tree mortality since 2015.”
Also, according to state DWR scientists and federal hydrologists with U.S. Geological Survey, the 2020-2021 water year was the third driest on record statewide in California since January 1895.
In 2020, central and northern California were mostly in moderate to severe drought situations. Last year, the state experienced the most intense exceptional drought on record, with central portions of the state being the most severely impacted.
Overall statewide precipitation last year was 50% of average.
Several parts of the state were not covered by normal aerial survey operations last year due to multiple large wildfires that burned for weeks to months. In addition, areas that burned in the 2019 and 2020 fire seasons were largely not surveyed because recent insect and disease activity is difficult to detect in those areas.
News of the recent aerial survey tree mortality findings comes on the heels of the driest January-February combination in recorded history in the Central Sierra and up and down the parched Golden State (see related story on front page about latest snow survey).
Contact Guy McCarthy at firstname.lastname@example.org or (209) 770-0405. Follow him on Twitter at @GuyMcCarthy.