Drought’s harm to forests more severe than feared, study finds

Worsening drought conditions may be doing more damage to forests in California and throughout the West than their ecosystems can handle, causing a spiral of death that could have a devastating impact, a U.S. Forest Service study concluded Monday.

The 300-page report, “Effects of Drought on Forests and Rangelands in the United States,” outlines how hotter, drier and more extreme weather will spark massive insect outbreaks, tree and plant die-offs, bigger and more costly wildfires, and economic impacts to timber and rangeland habitat.

“There are growing concerns that extreme precipitation events, droughts and warmer temperatures will accelerate tree and shrub death,” said report co-author Toral Patel-Weynand, the Forest Service’s director of sustainable forest research. “In addition to that, we obviously have impacts on timber, seed production, water and recreational activities.”

The study — by 77 scientists from the Forest Service, universities, non-governmental organizations and national labs — seeks to bring together years of peer-reviewed research and provide the best science to forest and rangeland managers as they grapple with the effects of climate change on the 193 million acres of national forest. There are 21 million acres in California’s 18 national forests.

“Our forests and rangelands are national treasures, and because they are threatened, we are threatened,” said U.S. Secretary of Agriculture Tom Vilsack. “Every region of the country is impacted by the direct and indirect effects of drought conditions and volatile weather patterns.”

Although drought has caused some problems in the Southeast, the brunt of the damage has occurred in the West, according to the report. Among the problems are larger, more volatile wildfires, vast stretches of dead and dying trees ravaged by insects, and increases in invasive plants.

A study released in December by the Carnegie Institution for Science counted as many as 58 million trees from the redwood forests of the North Coast to the pine forests of the southern Sierra suffering from severe water loss — far more than previously thought.

An earlier study showing that 12 million trees had been killed by drought prompted Gov. Jerry Brown to declare a state of emergency in the fall. He called it the state’s “worst epidemic of tree mortality in modern history.” Many of the weakened trees are being attacked by bark beetles, which have been multiplying in the warm weather.

Even native ferns, normally well adapted to dry summers and periodic droughts, have been affected, according to a study published Monday in the journal New Phytologist.

The UC Santa Cruz study found that the drought hampered the ability of ferns, which form the lush understory of California’s redwood forests, to photosynthesize and store energy. It could mean fewer new leaves in the spring and a higher vulnerability to insects and disease, according to the study.

“These understory species ... serve as indicators of how climate change may affect our redwood forests,” said Jarmila Pittermann, an associate professor of ecology and evolutionary biology at UC Santa Cruz. “Considering that these plants are adapted to persist through a typical summer dry season, the die-back emphasizes just how unprecedented this drought has been over the past three years.”

James Vose, a Forest Service research ecologist, said droughts have always shaped Western forests and rangelands, but contended, “There is something quite large-scale that is happening in North America and across the globe.”

He said California, Arizona, Texas and Canada, among other places, have all been profoundly impacted by severe drought.

“These large-scale mortality events indicate that the forest’s capacity to tolerate is being exceeded,” Vose said. “Often it’s called a tipping point, where large-scale changes occur from these external stresses.”

California wildfires were decidedly larger and more widespread last summer, fed by exceptionally dry brush and chaparral.

Blazes in Lake County burned so hot that the flames generated their own wind, exacerbating dangerous conditions for residents and firefighters. Smoke plumes rose 35,000 feet into the atmosphere, and when they collapsed they blew out embers as if from a cannon, fire officials said.

Winters are shorter and fire season now averages 78 days longer than in the 1970s, Forest Service officials said. Since 2000, at least 10 states have experienced their biggest fires on record.

These drought-driven events could cause rivers and lakes that are naturally cleaned by forest ecosystems to become degraded. That would mean less carbon dioxide soaked up by trees, exacerbating global warming, according to the research.

The report may help the Forest Service as well as timber companies and other landowners find ways to improve ecosystem health, be it through reducing tree densities, planting drought-tolerant vegetation, or adding shade trees and plants near creeks and rivers.

“It’s really incredibly critical,” Vose said. “This is and will continue to be a major challenge for forest and rangeland managers — certainly if the current drought continues or gets worse.”

Vilsack said 60 million Americans rely on drinking water that originates in national forest and grasslands. These areas, he said, support 200,000 jobs and contribute over $13 billion to local economies every year.