Trees in California are dying at the highest rate in at least 15 years, raising the risk of faster-moving and more-intense forest fires, according to the U.S. Forest Service.
Aerial surveys conducted by the service last year estimated a tenfold jump in tree mortality since 2014. According to results released last month, an estimated 27.6 million dead trees were found in the forest landscape last year. In 2014, an estimated 3.3 million dead trees were identified. The statewide aerial surveys date back to 2001.
The species most affected are low-elevation pines, especially Ponderosa, Pinyon and sugar pines, said Jeffrey Moore, forest health protection surveyor with the Forest Service.
The southern Sierra has been the area hardest hit – from the Eldorado National Forest south, Moore said.
About 20.8 million of the 27.6 million trees believed to have died are in the southern Sierra region, but elevated tree mortality has also been seen in northern forests.
“We’re getting reports from our foresters about elevated mortality in the Tahoe and Lassen areas,” said Moore. “They’re reporting a lot of mortality in lower-elevation pine stands.”
The dead trees are causing fires to burn faster and hotter, said Daniel Berlant, spokesman with the California Department of Forestry and Fire Protection.
The Valley Fire in Lake County, which burned 76,067 acres and caused four fatalities last year, is symptomatic of the danger of dead trees.
“The Valley Fire … burned at a significantly fast rate and a lot of that had to do with the big patch of dead trees there,” Berlant said.
Drought and the spread of the bark beetle are spiking tree deaths. The beetles are the size of a rice kernel and can tunnel under bark to cut off a tree’s water and food supply.
A warmer climate allows bark beetles to live in high-elevation pines, with temperatures needing to drop below minus-20 for at least a week to kill a beetle brood, depending on the species, according to Beverly Bulaon, forest entomologist with the Forest Service.
Tree mortality numbers could rise even higher because the trees often take awhile to reveal they’re dying, Moore said.
“It takes a year or more for a tree to dry out and change color so we can see it from a distance,” he said.
The annual surveys are conducted from a Cessna aircraft flying in 3-mile grid patterns. The annual surveys demand 150 flights over 45 million acres.
“A lot of the trees killed last year still look green and healthy. I wouldn’t be surprised if we see double the mortality this year,” Moore said.
In the Tahoe Basin, mortality doubled in 2015. The rate also spiked in hot spots such as the Crystal Bay region, said Patricia Maloney, a UC Davis forest ecologist.
She said an estimated 35,000 dead trees were found in the basin last year.
Tree mortality has spiked in the past, but only in regional concentrations, as happened in the early 2000s in the Lake Arrowhead region of Southern California, said Jeannie Wade Evans, deputy regional forester with the Forest Service.
“In the last couple of years there has been tree mortality in Colorado and the West, but in California the mortality has been more wide-ranging than we’ve ever seen or recorded,” Evans said.