GREATER LAKE TAHOE AREA, Calif. – Each year aerial observers for the USDA Forest Service ride in small fixed-wing aircraft 1,000 feet above California forests. Their focus isn’t on the views or the horizon, but rather downward. Their trained eyes are looking for the yellow or red-brown of dried or discolored foliage. It’s their job to observe, survey and report conifer and hardwood mortality, defoliation, and other damage.

In addition to noting the mortality and forest damage locations, they also note several other factors, including the damage type, affected forest area percentage and severity, impacted tree species, as well as the probable damage-causing agent.

“The idea is to map all dead trees once and once only,” Jeffrey Moore, the Forest Service’s aerial survey program manager says.

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“We call it recent mortality,” and he explains, “much of it actually died the previous year.” It often takes several months for the foliage to dry out and turn that red or yellow. The dead foliage eventually grays out or falls off and gets considered older mortality and no longer gets mapped.

Forest land managers use the annual mortality data to plan harvests in order to salvage recently killed trees or trees in beetle-threatened areas before the beetles can get to them.

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It isn’t just land managers who find the data useful. Others use it for research, fire behavior forecasting, invasive insect and disease monitoring and much more.

The reports are split into six different regions across the state. The region incorporating the Lake Tahoe Basin is the northeastern area where they’ve found elevated mortality in recent years. However, this year, observers recorded 439,000 acres of mortality, which is less than the five-year annual average of 730,000 acres.

Within this area, observers found White and California red fir mortality was the most common, mapping approximately 340,000 acres. The report notes the mortality was particularly extensive and intense in the Tahoe National Forest.

Other tree mortality found around Lake Tahoe impacted high-elevation five-needle pine (western white and whitebark pine). Over the entire northeastern survey region, they mapped 7,100 acres of high-elevation five-needle pine mortality.

The local Lake Tahoe Basin Management Unit (LTBMU) of the Forest Service uses these surveys to confirm local observations and recent trends in their data.

“After several years of drought, we’ve seen an increase in forest pathogens, such as fir engraver and bark beetles, especially in years with warm winters,” LTBMU vegetation management staff officer, Victor Lyon reports.

Lyon explains that drought reduces a tree’s ability to pitch out beetles with sap. The warm winters allow additional breeding cycles for beetles. Beetle numbers typically take a couple of years to decline after a wet winter, like that of 2022-2023.

Forest density is also a contributing factor when it comes to increased populations of fir engraver and bark beetles. LTBMU is working to reduce forest density in many areas around the Lake Tahoe Basin, with a focus on areas with elevated tree mortality and in proximity to the wildland-urban interface.

Forest density issues aren’t just a problem for the Lake Tahoe Basin. “Many forests in California and throughout the west are unhealthy due to fire exclusion leading to overcrowded conditions,” Moore says. “Too many trees need more water than is available.”

Thinning, treating, and prescribed burning are current Forest Service objectives around the state.

The aerial survey reports are available publicly on the Forest Service’s website. The service is primarily interested in biotic and other abiotic mortality agents and activity with these reports, so mortality from fires is not included. These are preliminary reports with raw data and are subject to change as they further refine the reports. Moore says this preliminary data may be somewhat inflated. Edited reports will become available early next year.