California has more than 100 million dead trees in its forests — and no easy way to deal with them.
Based on aerial surveys, the U.S. Forest Service concluded that more than 102 million trees have died across the state's forests since 2010. More than half — 62 million — died this year alone. There are 4.1 billion live trees in California forests, according to U.S. Forest Service data. So the dead trees represent about 2.5% of that population.
The die-off raise many ecological questions that officials are still trying to grapple with. The dead trees represent a major fire threat. But removing them poses logistical, financial and public safety risks.
So what happens to all those dead trees?
There are not nearly enough resources available to effectively manage 102 million dead trees. So officials have formed a task force to try to fell the dead trees in what they call "high-hazard areas" — before the trees snap on their own.
That means targeting for removal dead trees that are close to critical power lines, roads and areas where people live, play and work.
"Right now we're just focused on trees that pose a health-and-safety hazard," said Stephanie Gomes, a U.S. Forest Service spokeswoman. "It's kind of a triage approach. First and foremost we have to protect people and property."
Officials have already spent more than $200 million in state and federal funds to cope with the problem, said Janet Upton, a deputy director for the California Department of Forestry and Fire Protection. Agencies have collectively removed more than 423,000 dead trees within the last year.
They have a few ways to get rid of the trees. Crews can cut them down, take them to the side of the road and organize them into log "decks." Then mills can purchase and pick up the logs and use them for lumber.
The trees can also be taken to a facility where the dead timber gets turned into biomass. This option burns the wood waste in a high-pressure boiler to create steam, which turns turbines and produces renewable energy.
These options are preferable, but Gomes said that in general, mills are not taking dead wood from California. Further complicating matters, most wood-processing facilities are far from where the tree mortality has been the heaviest, and the cost to transport trees across that distance is high.
In addition, some of the biomass facilities' power-purchase agreements are expiring.
"There's not the infrastructure in place to take this on this quantity of trees," Gomes said.
If there is no place to take the dead trees, crews can burn them, but that creates air-quality problems for the forest.
What is the fire risk?
When a lot of dead fuel remains on the ground, fires burn hotter and damage the soil, Cal Fire officials say. Once fires burn through the fuel on the ground, they can climb up a "ladder" of dry branches and timber until they get into the crown of the tree. And once a fire gets to the top of a tree, fire authorities say it can spread quickly — hopping from tree to tree rather than winding slowly across the ground.
But a number of scientific studies knock down the idea that dead trees — and the beetle infestations that kill them — increase wildfire risk.
Water-starved trees are unable to produce their usual pitch or sap, the secretions that tend to protect them from fungi and insects. So bark beetles can burrow destructive paths into the limbs and trunks of trees, where they mate and lay eggs.
A 2015 study of areas infested by mountain pine beetle outbreaks across the western United States found that the outbreaks had "no effect" on areas burned by wildfire — results which the authors said "refute the assumption that increased bark beetle activity has increased area burned."
Another 2011 study that examined the lodgepole pine forests of the greater Yellowstone region concluded that rather than increasing the wildfire risk, beetle attacks reduce it by thinning tree crowns.
What is the long-term effect of this die-off?
Because California hasn't experienced this level of mortality, no one knows for certain.
Researchers are still grappling with how fire affects the forests differently than drought, the mixed findings on fire danger and how tree mortality affects the supply of water and snow.
Adrian Das, an ecologist with the U.S. Geological Survey, said he is somewhat torn.
"I work with trees because I like being in the forest, and seeing large swaths of the forest dying is challenging," he said. "The one silver lining as a scientist is, in a sense, we're having a real dramatic natural experiment. … It's tragic, but it's been a tremendous opportunity to learn."
What caused the die-off?
Scientists say five years of drought are to blame for much of the destruction. The lack of rain has put California's trees under considerable stress, making them more susceptible to the organisms, such as beetles, that can kill them. Unusually high temperatures have added to the trees' demand for water, exacerbating an already grim situation.
How much is the rain helping?
Experts say rain helps a lot, and, thankfully, certain parts of Northern California have enjoyed a wet start to the rainy season. One important index in the northern Sierra has already measured 16.2 inches of rain — more than twice the normal amount.
Central and Southern California have been less fortunate.
Officials say the majority of the dead trees are located in the southern and central Sierra Nevada, where precipitation levels have been average or below average. But officials have also warned that the death march is moving north into counties such as Siskiyou, Modoc, Plumas and Lassen.
Officials have said the parts of the forest at lower elevations — about 5,000 to 6,000 feet — continue to get hit the hardest. In the higher elevations, it can sometimes appear as if there is no drought and the trees are much healthier.
And experts say that even if a deluge somehow managed to end the drought this year, trees would continue to die for at least another year or two.
There are still many trees "that are dead that don't know they're dead," said Randy Moore of the U.S. Forest Service.