Earlier this year, the USDA/USFS Aerial Detection Monitoring sector released the R5 ADS Final Report (aka the 2023 Aerial Detection Survey Results for Region 5) highlighting the tree mortality rate in California.

Since insects and disease play a critical role in shaping forest ecosystems, the USFS annually performs aerial surveys over tree-dense lands to create maps that track areas having recent defoliation, conifer/hardwood mortality, and other damage.

In its 2023 Report, they found an overall increase in mortality in Eldorado and Tahoe National Forests and in the area managed by Lake Tahoe Basin Management Unit.

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R5 Aerial Detection Survey Program Manager Jeffrey Moore says that the USFS has done aerial forest health surveys and supporting reporting since the 1950s to give forest managers a pulse of what’s going on with tree mortality in the forest nationwide.

He explains that from 2010-2016, a huge drought in Central California created a mass die off in trees, and mortality rates in the Southern Sierra Nevada Range were particularly widespread and severe. Then in 2020-2022 there was a three-year drought in the northern Sierra Nevada and northern interior California regions where recent mortality has been concentrated.

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When asked if disease and bark beetles tend to infiltrate wildfire-affected areas more prominently, Moore replies, “The bark beetle was already there; the drought provides a smorgasbord for them.” The tree’s primary defense for the bark beetle is to “pitch them out” but the trees need moisture to create enough resin/sap to fend them off. And when there are large populations of beetles that attack a tree, with high success rates causing whole tree mortality.

The mortality of the trees is very weather dependent, and Moore says that it’s helped that we’ve had two normal and above-average water years (in 2022/23 and 2023/24). Therefore, the surviving trees have less competition and can better defend themselves.

“I expect ongoing elevated levels of mortality because the beetle populations are still greatly elevated a lot but not on the scale of the 2021-2023 era,” Moore says.

He adds that part of what affects tree mortality is fire exclusion. “California and western forests have been adapted with regular reoccurring fires, but we’ve been putting them out. What’s happened with the fir trees is that the little ones have survived, so even in a good year they’re all still competing for the same resources. California fires are hotter, and those little trees little trees are now much bigger and go up like roman candles,” he adds.

The branches of fir trees hang low to the ground and there are so many of them that they choke the bigger sturdier ones out or inhibit their chance for survival when a wildfire rips through.

“Sugar and yellow pines are a lot like the big pine in that they are adapted to fires. They have that thick bark and strong root systems,” Moore says.

Sugar Pine Foundation Executive Director Maria Mircheva reiterates the difference in tree structure, saying that the sugar pines’ thick trunks and self-pruning lower branches help protect them.

Mircheva has seen that the number of sugar pines in the Tahoe Forest has been on the decline in recent years mainly because of wildfire, logging, and disease.

“There’s a known native white pine blister rust disease that’s decimating the forest,” she says. “Sugar pines made up a quarter of the Tahoe National Forest before the Comstock era, but now they make up for less than five percent, a lot of it due to logging and fire suppression.

The white pine blister rust is also an issue… it’s been in the Tahoe region for the last 20-30 years and it thrives in wetter conditions.

When asked whether lack of rain or drought years has helped kill the fungus, Mircheva replies, “One hot season is not enough, and multiple drought years is not good for the fungus but it’s also not good for the trees. Water builds immunity for the trees.”

“In general, the mortality of trees in the Tahoe basin is not bad compared to places like Yosemite. I think a lot of that is due to tree thinning that’s been going on for many years. There are a lot of great projects in the works but especially in terms of forest thinning we’re pretty far advanced in dealing with that. Land managers, parks, conservancies, have all been focused on this for a while,” Mircheva says.

Going back to tree thinning, Moore says that firs have branches down to the ground, thin bark, and they stress out the big pines with not enough water to go around. “We’re better off without so many firs. A lot of the logging going on is likely for the fir to favor the big pines which will be left with more resources,” he adds.

Kevin Leary, CEO of Tahoe Forest Products/Hallador, says they absolutely track the type of trees processed in the new lumber mill.

“Page Four [of the R5 ADS Report] is pretty darn scary. Our first year—2022– was all high elevation red fir, which is uncommon below about 7,500 feet in the Basin. Most of what you see in the Basin is Jeffrey Pine, Ponderosa Pine, White Fir, smaller amounts of other firs (red, Douglas), and a smattering of other lumber logs like cedar, sugar pine, and lodgepole. But the lion’s share of what’s in the basin, and what we received in 2023 and beyond is Jeffrey/Ponderosa pine and white/red fir,” Leary says.

“Fir is the most worrisome Basin species. Much of the die-off that is plainly visible above Meeks/Emerald Bay is fir. We can still make use of these trees until they lose their needles, which is within about 12 months of the tree dying. After the needles are gone, so is any financial recovery and the tree will rot and become fuel for the next wildfire,” he adds.

However, Moore believes there is hope for the mortality of Tahoe forests. “2023 and 2024 have been good precipitation years statewide and ongoing mortality will likely decrease with the surviving trees being healthier due to decreased competition,” he says.

Lake Tahoe Basin Management Unit Vegetation Management Staff Officer Victor Lyon also added, “We anticipate that tree mortality in the Tahoe Basin will continue to be an issue. However, the slightly above normal precipitation this year, following the exceptionally wet winter of 2022-23, should increase tree resilience to forest pests and help taper the pace of tree mortality we’ve been observing in recent years.”

Here are some key findings from the 2023 report:

Pg. 5- Overview

– Elevated levels of tree mortality (i.e., more than 1% of forested area affected) were recorded on more than 2.4 million acres, totaling an estimated 28.8 million dead trees. Most of the trees killed were recorded as fir (Abies spp.), followed by ponderosa pine (Pinus ponderosa), and Douglas fir (Pseudotsuga menziesii).

– Mortality was particularly severe and widespread in the central Sierra Nevada Range and northern interior California (Figure 1).

– Elevated levels of tree mortality can be attributed in part to the ongoing effects of long-term, intermittent, exceptional drought conditions and subsequent successful bark and engraver beetle attacks that have resulted in an estimated 239 million trees killed since 2010.

– In 2023, the number of trees killed, acreage with mortality, and average severity of that mortality all decreased compared to 2022. Both years, however, were greatly elevated both in intensity and range compared to non-drought conditions. Tree mortality was common throughout the Sierra Nevada Range but was particularly concentrated in the central Sierra Nevada Range and northern interior where the recent drought was most intense.

Pg. 11- The Eldorado Forest

– Mortality increased to an estimated 2.2 million dead trees across 150,000 acres in 2023 from ~1.3 million dead trees across 91,000 acres in 2022.

– Mortality was widespread throughout much of the Forest and was particularly severe at higher elevations.

– California red fir mortality increased to an estimated 2.1 million dead trees across 120,000 acres in 2023 from ~1 million dead trees across 66,000 acres in 2022 and was located primarily in the Pacific Ranger District.

– White fir mortality decreased to approximately 66,000 dead trees but increased in spread over 23,000 acres in 2023 from ~180,000 dead trees across 20,000 acres in 2022.

– Ponderosa pine mortality decreased to an estimated 24,000 dead trees across 2,900 acres in 2023 from ~42,000 dead trees across 3,000 acres in 2022.

Pg. 12- Lake Tahoe Basin

– Mortality increased to an estimated 1.7 million dead trees across 66,000 acres in 2023 from ~1.4 million dead trees across 58,000 acres in 2022.

– Although mortality was widespread in eastern and northern areas it was most intense west of Lake Tahoe.

– California red fir mortality increased to approximately 1.6 million dead trees across 63,000 acres in 2023 from ~1.3 million dead trees across 54,000 acres in 2022.

– In 2023, white fir mortality decreased to 6 dead trees from an estimated 14,000 dead trees across 1,500 acres in 2022.

Pg. 18- Tahoe National Forest

– Mortality increased to an estimated 6.5 million dead trees across 330,000 acres in 2023 from ~5 million dead trees across 260,000 acres in 2022. Mortality occurred at moderate to severe levels over many areas, particularly along central, high elevation areas of the Forest.

– California red fir mortality increased to approximately 6 million dead trees across 290,000 acres in 2023 from ~3.8 million dead trees across 170,000 acres in 2022.

– White fir mortality decreased to approximately 160,000 dead trees across 22,000 acres in 2023 from ~890,000 dead trees across 74,000 acres in 2022.