Biologist: At least 100M trees stressed for water

An exclusive look at a preliminary map produced by biologist Greg Asner, of the Carnegie Institution for Science in Palo Alto, shows where trees in Tuolumne County are more starved for water than others due to the four-year drought (blue means more water in the canopies, while brown means less water). Asner believes there are at least 100 million drought-stressed trees throughout the state. Courtesy Greg Asner.

An alarming discovery suggests this year’s explosion of dead trees in the southern and central Sierra Nevada may only be the beginning of a much larger trend for California forests.

While the U.S. Forest Service and Cal Fire estimates some 13 million trees have died due to the four-year drought, biologist Greg Asner, of the Carnegie Institution for Science in Palo Alto, believes there are at least 100 million additional water-starved trees throughout California — as much as 20 percent of the state’s forests.

“You can fly over a tree and determine if it’s dead or not, but it’s really difficult to determine if it’s vulnerable,” Asner said. “We flew over the state of California and believe there’s something in the order of 100 million-plus trees in a chemically vulnerable condition.”

Funded by a $250,000 research grant through the David and Lucile Packard Foundation, Asner conducted his own aerial survey of the state this August in his Dornier 228 that’s equipped with X-ray technology, which allows him to measure the amount of water in tree canopies.

The water content in tree canopies, undetectable by the naked eye or even modern satellite technology, is particularly telling, because that’s where the moisture absorbed from the ground ends up.

“The trunk is just the straw sucking water from the ground to support the foliage up on the canopy,” he said.

Asner has used the technology in the past to measure the effects of drought on Amazon rainforests in Brazil and Peru, but this marks the first time he’s conducted such studies on California forests.

Although repeated surveys are needed to truly assess how close the drought-stressed trees are to dying, Asner said he hopes his work will help state and federal agencies come up with strategies to manage the problem if conditions continue to worsen as expected.

“Our maps are going to provide a more detailed view of tree vulnerability so actions can be taken up to regional scales,” he said. “That could include prescribed fires, selective thinning and fire prevention in areas that need to be protected.”

Despite predictions of a strong El Niño to hit California and bring wetter weather this winter, a growing body of research suggests continually increasing temperatures will cause more frequent and severe droughts in the coming decades.

A recent study published in the scientific journal Geophysical Research Letters found that global warming has worsened the current drought by up to 25 percent.

“The drought and climate warming are now interacting … and making it harder for trees to survive,” Asner said.

Roger Bales, a University of California researcher who has conducted recent studies into the effects of tree density on hydrology in the Sierra Nevada, said the data compiled by Asner will help forest managers and scientists prepare for the warmer future.

Though tree rings dating back 1,500 years show multi-year drought periods have always been a part of the climate, Bales said there’s broad consensus among scientists that the intensity of the dry periods in the future will be made worse by global warming.

“We know that baseball players are good at hitting home runs, but if you give them steroids they’ll do a little better,” he said. “It changes the statistics of the drought a little bit by warming the climate.”

One way to lessen the impact from climate change is to bring the forest to a more sustainable state, Bales said, which means fewer but bigger trees.

Selecting larger trees to remain while thinning out smaller ones will reduce the intensity of wildfires and improve water runoff, Bales said.

“The state and federal agencies that manage these lands could be doing more to adapt to the warming that’s occurring,” he said. “We’re a wealthy state and a wealthy nation, it’s just a matter of how we choose to spend our resources.”

Rapidly spreading tree mortality due to drought-related bark beetle infestation from Sonora to Dardanelle has certainly caught the attention of Tuolumne County leaders, who say the problem has grown beyond their control.

The county Board of Supervisors last month declared a state of emergency in response to the increasing number of dead or dying trees threatening public safety and critical infrastructure, such as power lines, roads and water utilities.

District 2 Supervisor Randy Hanvelt recently received an invitation from Gov. Jerry Brown’s office to discuss tree mortality with some of the governor’s top advisors.

Hanvelt, District 1 Supervisor Sherri Brennan, area logger Mike Albrecht and Deputy County Administrator Tracie Riggs are scheduled to meet with the Governor’s Office on Monday, along with representatives from other counties in the central and southern Sierra foothills.

Hanvelt said he’s not sure what to expect out of the meeting, though he plans to stress the seriousness of the problem for many local residents.

“I don’t think enough attention has been paid to this, and I think everybody’s realizing it now,” he said of tree mortality. “I would like to get at least a commitment to work on planning, and maybe take some initial action.”

By Alex MacLean, The Union Democrat October 24, 2015