On a hill in Oakland, CA, Igor Lacan looks out from under his Stetson hat at the neighborhood below and begins listing trees.

“Maples to birches to plums to liquid amber,” says Lacan, horticulture advisor for the University of California Cooperative Extension. “A cedar. I see some palms, and then you’ve got a monkey puzzle up here!”

In between the trees is a criss-crossing web of power lines, delivering electricity to the houses below. Lacan works as an advisor for California utilities like PG&E, and he says while most of the trees seem to be flourishing, that’s not true for some nearby acacias. He points upwards to a spiral of dead bark hanging off an acacia branch.

“If you can see the wood underneath, which in this case you can, that’s typically a sign that that part of the tree is dead. Which is why we didn’t stand under that branch.”

According to researchers at UC Berkeley, opportunistic fungi are killing these trees. California’s climate change-fueled drought, which has persisted for the better part of two decades, has stressed the trees and made them vulnerable to parasites.

Lacan says of the local acacias, “We have never seen the sort of mass mortality that we’re seeing now.”

Climate change has stoked a host of threats to trees, not just in California but across the country. Extreme storms, droughts, disease and insects are stressing and killing trees, and these trees pose a growing threat of wildfires and to grid reliability, many large utilities say. The Dixie Fire in northern California, which has already burned more than 950,000 acres, was likely sparked by a tree falling onto a power line.

According to more than a dozen of the country’s largest utilities, branches and trees falling on power lines are a leading source of power outages. Some utilities say that because of factors related to climate change, trees are dying faster than they can reach them on their normal trimming cycles.

Outages caused by falling vegetation go beyond inconvenience for customers, says Tremaine Phillips, commissioner on the Michigan Public Service Commission. “We know that there are individuals who rely on medical equipment, and that equipment requires electricity. We know that there are families who have medications that need to be kept refrigerated,” he says, “So these impacts are real and for certain families, very acute and potentially dire.”

Nina Bassuk, professor of urban horticulture at Cornell University, explains that climate change can kill tree cells through a confluence of stressors. “It’s not like an animal which dies when you pierce the heart — trees die cell by cell,” she says.

Shorter and warmer winters can allow insects and diseases to proliferate, she says. Fluctuating temperatures, heatwaves and drought can disrupt growth or health.

“So if you have a bunch of cells that are dead, a branch, for instance, that will be more apt to fall,” she says.

In 2020, Tropical Storm Isaias left 1.2 million Connecticut residents without power for a week in the middle of a summer heatwave. The state’s Public Service Commission recently fined the state’s largest utility, Eversource, the maximum $28.5 million dollars for failing to prepare and respond to that storm.

Diego Cerrai is professor of engineering at University of Connecticut, and he’s been working with Eversource to study why the outages occurred.

“That storm created many more outages than what was expected by anyone,” says Cerrai, who manages the Eversource Energy Center. “We started investigating why. And one of the reasons was that there was a severe drought over the past two years in this area of the United States, and trees were weaker and more likely to fail.”

Connecticut’s drought, severe storms and winds have weakened trees’ defenses, says Sean Redding, the vegetation manager for Eversource Connecticut. That makes trees vulnerable to insects like gypsy moths, emerald ash borer and hemlock woolly adelgid, and diseases like root rot. Now, those dead trees and their branches are falling on power lines.

“When you have that amount of trees succumbing to these bugs and resulting in mortality, you have a large impact to customers,” Redding says, “Because of the cumulative effect of climate change, insect infestation and disease on our forests here in Connecticut, more trees came down, resulting in more outages.”

Redding says these days when it comes to dead trees, “There are more of them than we can manage in a timely manner.”