Climate change threatens fog and redwoods -study

* Fog 33 percent less common than 100 years ago

* Trees need fog to conserve water

WASHINGTON Feb 15 (Reuters) - The coastal fog that gives San Francisco its romantic ambience is thinning out, a boon to drivers but a real threat to the giant redwoods there, researchers reported on Monday.

It in unclear if natural climate variations or human activity is to blame, but the result could be the loss of trees, they reported in the Proceedings of the National Academy of Sciences.

"Fog prevents water loss from redwoods in summer, and is really important for both the tree and the forest," biologist Todd Dawson of the University of California Berkeley said in a statement.

"The coast redwood is the tallest living tree species and notably long-lived, with some individuals exceeding 2,000 years in age," the researchers wrote in their report, available here

"If the fog is gone, we might not have the redwood forests we do now."

Dawson and colleagues estimated the frequency of fog by looking at weather records, especially airport records dating back to 1951.

"Since 1901, the average number of hours of fog along the coast in summer has dropped from 56 percent to 42 percent, which is a loss of about three hours per day," said Berkeley's James Johnstone, who led the study.

The fog is caused by cool surface waters of the Pacific Ocean meeting warm air from the interior of California. It is held in place by an inversion, caused when cooler air is trapped closer to the surface.

"The data support the idea that Northern California coastal fog has decreased in connection with a decline in the coast-inland temperature gradient and weakening of the summer temperature inversion," Johnstone said.

"As fog decreases, the mature redwoods along the coast are not likely to die outright, but there may be less recruitment of new trees," Dawson added. "They will look elsewhere for water, high humidity and cooler temperatures."

The coast redwood, known scientifically as Sequoia sempervirens, is naturally found in a very narrow band along the northeast Pacific coast.

The researchers found changes all the way down the coast from northernmost California to San Diego.

"Fog is clearly a dominant climatic factor on the California coast, and long-term reductions likely have and may continue to impact the water and carbon economy of redwoods and other coastal endemic species," they concluded.

(Reporting by Maggie Fox, Editing by Sandra Maler)