Global warming blamed for early deaths of trees

San Francisco Chronicle

SAN FRANCISCO — Trees are dying faster than ever in the old-growth forests of California and the mountains of the West, a phenomenon scientists say is linked to rising regional temperatures and the destructive forces of early snow melt, drought, forest fires and deadly insect infestations brought on by global warming.

Over the past 17 years in some regions — and 25 to 37 years in others — the death rates of mature trees have doubled, the scientists said, raising concerns that the problem goes well beyond the death of trees alone.

“The ultimate implications for our forests and the environment are huge,” said Mark Harmon of Oregon State University, a member of the team that helped write a report that appeared Friday in the journal Science.

As the forests shrink, their capacity to absorb carbon dioxide from industrial lowlands diminishes, which means that more greenhouse gases would be added to the atmosphere, resulting in the acceleration of global warming, the researchers said.

While no trees are immune from the accelerating death rates, the victims are primarily the conifers whose abundance throughout California’s Sierra Nevada makes the mountain forests famed throughout the world. Varied species of pines, firs and hemlocks are most at risk, the scientists said.

The research involved nearly a dozen leading forest ecologists who studied mortality rates of individual trees in 76 plots of unmanaged forests, situated primarily in California, Oregon, Washington and southwestern British Columbia. They also looked at trees in a few interior states: Idaho, Montana, Utah and Arizona.

The increase in death rates for the trees has been “pervasive,” said Phillip J. van Mantgem, a forest expert with the U.S. Geological Survey’s Western Ecological Research Center in Arcata (Humboldt County) and a leader of the research team.

“If current trends continue, our forests will eventually become sparser, the ages of our trees will decrease by half and they will be able to sequester less carbon — further speeding up the pace of global warming,” van Mantgem said during a teleconference sponsored by the American Association for the Advancement of Science.

The most likely cause of the increasing deaths, van Mantgem said, is the widespread rise in average temperatures throughout the study regions over the past three decades — an increase of a full degree Fahrenheit and an amount consistent with the global warming measurements and models reported by the world’s experts of the Intergovernmental Panel on Climate Change.

Van Mantgem led his own California team tracking the fates of 20,000 trees in Yosemite and Sequoia National parks, and found that their death rates had doubled in 25 years. Colleagues did a similar job for the study of old-growth forests throughout the Pacific Northwest. In those forests the deaths were particularly striking among younger and smaller trees, although all ages and sizes were among the dead.

A powerful influence on the rising tree mortality rates has been increasingly frequent droughts in the High Sierra, where more rain instead of snow has been recorded in recent years and the snow pack has melted earlier, said Nathan L. Stevenson of the USGS forest ecology center, the co-leader of the research group.