Tree Mortality In Sierra Nevada Doubles In 20 Years, Likely Due To Climate

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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.

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