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Disappearing Fog: Clear skies put coastal redwoods at risk

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The thick blanket of fog that often swirls along the northern California coast envelops beaches, forests, and the streets of San Francisco in its moist grasp. Those fog-shrouded days, however, are growing fewer and farther between, according to a study by postdoctoral scholar James A. Johnstone and Todd Dawson, a professor of environmental science, policy and management. If the trend continues, the scientists say, the continued survival of another regional icon—the towering coastal redwoods known as Sequoia sempervirens—will be at risk.

“Fog prevents water loss from redwoods in summer, and is really important for both the tree and the forest. If the fog is gone, we might not have the redwood forests we do now,” Dawson says. Coastal redwoods grow from Big Sur to Oregon, along a 30-mile-wide strip that enjoys year-round cool temperatures and high humidity.

The researchers analyzed hourly weather reports recorded at two California airports, in Monterey and Arcata. When the researchers compared this data to temperatures collected all along the Pacific Coast, they found that coastal fog tends to occur when the coast is very cool while inland areas are baking. This temperature differential has decreased along the Pacific coastline from Seattle to San Diego. Theresearchers calculate that since 1901, the number of hours of fog along the coast in summer has dropped by about three hours per day.

The pattern appears linked to an ocean cycle called the Pacific Decadal Oscillation that affects water temperatures in the northern Pacific Ocean. It is unclear whether these shifts are part of a natural cycle or the result of human activity. Whatever its cause, the change could impose drought stress on redwoods, which depend on fog for 25 to 40 percent of their water. “As fog decreases, the mature

redwoods along the coast are not likely to die outright, but there may be less recruitment of new trees; they will look elsewhere for water, high humidity, and cooler temperatures,” Dawson said.

Eventually, Dawson and Johnstone hope to correlate fog frequency with climate data stored in redwood tree rings. Dawson has established a method to identify what percentage of the oxygen stored in tree ring tissues comes from rainwater versus fog. The new fog data will allow Dawson and Johnstone to calibrate their tree ring isotope data with

actual coastal fog conditions in the past century, and then extrapolate to yield coastal climate conditions going back 1,000 years or more.

-Robert Sanders

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