Dying trees in Sierra likely due to warming

By Alex Breitler
Stockton, CA (August 8, 2007)- Trees in the Sierra Nevada are dying at a rate nearly double that of two decades ago, and scientists say global warming is likely to blame. A federal study released this week says that one-time evergreens have turned brown and brittle because of drought, a condition that may become more frequent and more intense as the climate changes.

While small trees are most at risk, the death rates have increased for a range of species at just about any elevation, the study by the U.S. Geological Survey says. Environmentalists said the news should reinforce the serious consequences of global warming by providing visual proof for the thousands who visit Lake Tahoe or Yosemite National Park each year or, closer to home, drive through the Stanislaus National Forest.
“It’s not as simple as saying temperatures will be a little bit warmer and you won’t have to put on a sweater. The impacts reach much further than that,” said Jason Barbose, an advocate with the global warming watchdog group Environment California.
USGS scientists in 1983 began monitoring more than 21,000 trees in 21 locations at Yosemite and Sequoia national parks. Annual visits to each tree showed that death rates were climbing an average of 3 percent each year, while the rate of new growth didn’t change.
The die-offs seemed to coincide with periods of drought, which typically make trees more susceptible to bug infestations or pathogens. Many fir and pine trees appeared to be most at risk; giant sequoias were too sparsely located to detect any trends.
Climate change can bring about both drought and drenching rain, but in the arid Sierra Nevada, temperatures are warming without a marked increase in precipitation. What’s more, the moisture that until now has fallen as snow may turn to rain. This is less beneficial for trees, since the water immediately drains downstream rather than slowly melting over a period of months.
“I think it’s pretty obvious that long, dry summer periods and reduced snowpack result in an overstressed forest,” said John Buckley of the Central Sierra Environmental Resource Center in Twain Harte. So far, the tree die-offs have been “gradual and subtle,” says the study headed by USGS scientist Phil van Mantgem. But if the forests are as sensitive as the study suggests, they may be subject to greater declines in the future.
Stanislaus National Forest officials said Tuesday that they’re attempting to remove some trees and vegetation to create more room in the forest and reduce demand for a limited amount of moisture. They have not studied whether global warming may be causing tree deaths, a spokesman said.
There are many factors that can lead to bug infestations, and it’s difficult to predict when and where tree deaths might occur as a result, said forest entomologist Beverly Bulaon. “It’s too short of a time span right now to say that a lot of the insect distribution activity is attributable to global warming,” she said.
Buckley of the environmental resource center said that global warming could worsen many human-caused problems- such as pesticides that drift from the Central Valley into the mountains and air pollution. “We have all these factors working together,” he said. “Global warming is just literally heating them in a test tube experiment that is coming to fruition as we watch.”
For the full article, visit the Stockton Record.