EL DORADO COUNTY, Calif. (KTXL) — In forests throughout the Golden State, trees are turning a dark shade of rust, succumbing to the impacts of the drought in a well-documented phenomenon known to forest scientists as tree mortality. 

The problem first peaked in 2016 when the U.S. Forest Service released images from a statewide aerial survey, estimating 62 million trees died that year.

Heavy rain and snowfall in 2017 gave the forests a new lease on life. But the climate keeps changing the terms of that lease. According to the Forest Service, 9.5 million trees died last year in California, mostly fir and pine. And scientists are concerned one more year of drought could lead to another mass die-off.

“It’s on a trajectory to be even worse than when the last one peaked in 2016,” said Curtis Ewing, a senior environmental scientist with Cal Fire.

As an entomologist, Ewing is an expert in the study of insects.

“The beetles are attacking the weakest trees,” Ewing said.

A healthy tree produces sap, or a thicker compound called pitch, that pushes pests out. But trees weakened by years of drought are left defenseless against invasive beetles that bore into them like drills.

“They have a very low water pressure inside them,” Ewing explained. “Their roots are compromised. Everything is compromised. So they just cannot fight them off.”

Bark beetles and similar insects thrive and reproduce inside California trees that are pining for water. They take over and kill the trees.

“In a normal year, there would be lots of rain over the winter, it would be cooler,” Ewing said. “They would stop developing. They wouldn’t be able to make any new attacks. They wouldn’t be hatching new adults. But in 2021-2022, the winter was very unusual. They just kept developing. They kept emerging and they kept attacking all the way through. It didn’t rain from January to March. That, to them, was just an extended fall.”

FOX40 asked Ewing if there’s anything that can be done.

“You need to thin the trees,” Ewing said.

The federal and state governments are promising to fund more forest-thinning projects. The U.S. Forest Service escorted FOX40 to a forest-thinning work site near Echo Lake in El Dorado County on a recent October day. Some of the timber in that forest was touched by the Caldor Fire in 2021. Even though other areas of the forest did not burn, they are still in trouble due to too many trees, too close together, competing for limited amounts of water in a time of drought.

“That one is very much on its way out,” observed U.S. Forest Service silviculturist Rita Mustatia as she touched the dry branches of a fir tree. “Brittle and the needles are dying. And then that’s just going to continue to spread.”

Mustatia manages forests in the Lake Tahoe Basin.

“A lot of people are calling because they’re concerned about how much mortality they’re seeing on our forests,” Mustatia said.

Gray trunks and branches indicate a tree died in a previous season. The reddish trees are ones that just recently died. The Forest Service identified 70,000 of the latter in the Tahoe Basin last year, according to the most recent survey results online.

As if bark beetles and competition for water weren’t enough, many of Tahoe’s pine trees are receiving a kiss of death from mistletoe.

“This parasitic plant basically will take the water and nutrients that the tree is bringing in for itself and it will basically take it from it,” Mustatia explained.

Dead trees pose dangers to homes and hikers when the limbs fall off and the rotting trees topple.

“Once the tree does fall, it’s also a hazard because now it’s dry fuels on the ground,” said Mustatia.

The threat of catastrophic fire is perhaps the biggest concern of all when talking about tree mortality. Historically, forest fires eliminated overgrowth and dead trees. Now that many people have made their homes in the woods, fire suppression is more important than ever to saving lives and property. But when a forest is untouched by flames for many decades, and then dies off due to drought, the result is a volatile combination of dense and dry fuel. In Lake and Napa counties, the problem is so pervasive the county governments declared tree mortality emergencies this year.

“In my mind, the best way to deal with dead and dying trees is prevention and making sure you do everything you can to have a healthy forest, a healthy tree to begin with, by limiting competition,” Mustatia said. “It’s why we’re so busy at work trying to thin the forest and reduce the number of trees that you see out there on the ground. It’s about making it easier to suppress fires for sure, but it’s also about reducing that competition for a limited amount of water.”

“If you can thin out the forest, increase the species heterogeneity, so there’s more species in the stand, and there’s more space in between them, they have better light,” Ewing explained. “They’re getting more water which is the critical resource they need to fight these beetles off.”

“If it continues to get dryer, we want to have fewer trees in order to have some trees,” Mustatia concluded.

Thinning forests is not always a popular proposal. But everyone can agree an abundance of rain and snow would go a long way toward mitigating a problem rooted in years of drought.

“I do have hope that we get enough moisture and precipitation in the future that we can do some planting in areas that make sense, and at least get a new generation of trees growing,” Mustatia said, keeping hope alive that California forests can survive beyond the shadow of a drought.