More than 800 million trees in California's forests are being stressed by the state's ongoing drought, with 58 million on the brink of death from water loss, researchers say.

In addition to the risk of dying, the drought puts the trees, among the world's oldest, tallest and most massive, at risk of fires and infestations with bark beetles, they say.

Scientists from the Carnegie Institution for Science in Washington, D.C. used laser-guided imaging spectroscopy tools aboard an aircraft flying over the state's forests to understand trees' responses to the drought.

The data from the Carnegie Airborne Observatory was combined with more traditional satellite data going back to 2011, the researchers explain in their study published in the Proceedings of the National Academy of Sciences.

The result is a graphic representation of the amount of water content the forest canopy has lost since 2011, they say.

Around 41,000 square miles of forest with 888 million trees had measurable losses of canopy water in the last four years, and if the drought continues, 58 million of the worst-affected trees will likely die, they note.

Another consequence could be an increase in greenhouse gases, as carbon stored in tree trunks would be released back into the atmosphere as trees die, they pointed out.

Just how many emissions would result is still being calculated, but it's going to be substantial, according to the research team.

"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,"  says Carnegie researcher Greg Asner. 

The drought, if it continues, could bring long-term changes to ecosystems important as animal habitats and for biodiversity, he says.

California Gov. Jerry Brown declared a state of emergency in October based on dead and dying trees in forests across the state.

Researchers say continuing monitoring by aircraft and satellites will help in guiding actions on the ground to mitigate and manage negative impacts from drought-caused forest losses.

California officials were quick to praise the contribution made by the Carnegie researchers.

"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," says Ashley Conrad-Saydah, deputy secretary for climate policy with the California Environmental Protection Agency.