A report by the U.S. Forest Service released earlier this year found that as many as 12 million trees have been killed by drought in California, but did not identify how many more trees were vulnerable due to the dry conditions.

Carnegie researchers found that approximately 10.6 million hectares of forest (about 26.2 million acres or 41,000 square miles) containing up to 888 million large trees has experienced measurable loss in canopy water content.

Severe canopy water loss of 30 percent or more occurred across 1 million hectares (about 2.5 million acres), affecting up to 58 million large trees, the researchers found.

Though increased precipitation thanks to the El Niño event heating up Pacific waters has provided some small relief, California is still in the grip of a four-year drought, the impacts of which are making themselves known in many ways.

On April 1 of this year, for instance, snowpack in California’s Sierra Nevada mountain range was the lowest it’s been in recorded history, possibly the lowest it’s been in half a millenium.

The drought has not only left California with severely reduced snowpack, however — soil moisture, groundwater and reservoir stocks are all greatly diminished, which prompted Governor Jerry Brown to establish the first-ever statewide restrictions on water use.

Now a study by researchers at the Washington, D.C.-based Carnegie Institution for Science has found that up to 58 million large trees in California — some of Earth’s oldest and most massive trees — have experienced severe canopy water loss between 2011 and today, putting them at severe risk.

The Carnegie researchers, whose findings were published Monday in the Proceedings of the National Academy of Sciences, write that the impacts of this “millennial-scale event” on the health of forests has been relatively unknown until now.

A report by the U.S. Forest Service released earlier this year found that as many as 12 million trees have been killed by the drought in California, but did not identify how many more trees were vulnerable due to the dry conditions.

Carnegie’s Greg Asner led a team that used data from the laser-guided imaging spectroscopy tools mounted on the Carnegie Airborne Observatory in combination with more traditional satellite data going back to 2011 to measure the full impact of the drought on California’s forests.

“Our maps reveal much wider, much larger-scale impacts on our forest than the dead trees would tell in themselves,” Asner told the San Francisco Chronicle. “We found that there was measurable drought stress even in areas that are known to be more humid.”

The team’s research revealed that approximately 10.6 million hectares of forest (about 26.2 million acres or 41,000 square miles) containing up to 888 million large trees has experienced measurable loss in canopy water content. Severe canopy water loss of 30 percent or more occurred across 1 million hectares (about 2.5 million acres), affecting up to 58 million large trees, the researchers found.

Some of their findings were surprising. Even the iconic redwoods of California’s North Coast and forests in the often foggy Bay Area were found to be marked by brown leaves and dying limbs.

Compounding the threat to California’s drought-stricken forests is a bark beetle infestation that has killed millions of trees already weakened by the lack of water. In response to what he called “the worst epidemic of tree mortality” in modern history, Gov. Brown declared a state of emergency in October, requested federal aid to remove dead trees, and called for more controlled burns to reduce the risk of wildfire.

By mapping the changes in canopy water content, Asner says, scientists can better identify which trees are under drought stress and hence most vulnerable to forest fires and at the greatest risk of death.

“California relies on its forests for water provisioning and carbon storage, as well as timber products, tourism, and recreation, so they are tremendously important ecologically, economically, and culturally,” Asner said in a statement.

“The drought put the forests in tremendous peril, a situation that may cause long-term changes in ecosystems that could impact animal habitats and biodiversity.”

Asner and his co-authors on the PNAS report write that, if drought conditions continue, even after the temporary reprieve provided by El Niño, they predict “substantial future forest change.”

But Asner adds that “Continued airborne and satellite monitoring will enable actions on the ground to mitigate a cascade of negative impacts from forest losses due to drought, as well as aid in monitoring forest recovery if and when the drought subsides.”

According to Ashley Conrad-Saydah, deputy secretary for climate policy at the California Environmental Protection Agency, the team’s research is already providing valuable insights into the severity of drought impacts on California’s iconic forests.

“It will be important to bring their cutting-edge data and expertise to bear as the state seeks to address the effects of this epidemic of dying trees and aid in the recovery of our forests,” she said.