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Yosemite trees in decline; climate change is lead suspect

August 4, 2009 |  4:45 pm

Yosemite

Yosemite National Park has fewer large trees than it did 70 years ago. Researchers believe climate change is behind the decline.

From the 1930s to the 1990s, Yosemite's large-diameter tree density decreased 24%, according to a study by researchers at the U.S. Geological Survey and the University of Washington. Scientists compared the park's earliest records (1932-1936) with records from 1988-99. The study was published in the journal Forest Ecology and Management.

"Climate change is a likely contributor to these events and should be taken into consideration," said USGS scientist emeritus Jan van Wagtendonk. "Warmer conditions increase the length of the summer dry season and decrease the snowpack that provides much of the water for the growing season. A longer summer dry season can also reduce tree growth and vigor, and can reduce trees' ability to resist insects and pathogens."

Scientists also believe Yosemite may now be more vulnerable to major wildfires, since areas that have not experienced fires in almost 100 years have shifted from fire-tolerant ponderosa pines to fire-intolerant trees, such as white fir and incense cedar.

The study comes on the heels of recent findings by a team of UC Davis scientists that a decline in winter chilling hours due to global warming is having a dramatic effect on trees in the Central Valley, where much of the nation's fruit and nut crops grow.

-- Amy Littlefield

Photo: Yosemite National Park's Merced River. Credit: Brian Vander Brug / Los Angeles Times

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