Photograph by Michael Nichols, National Geographic Stock

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Fog shrouds redwoods in California's Humboldt Redwoods State Park.

Photograph by Michael Nichols, National Geographic Stock

Giant Redwoods May Dry Out; Warming to Blame?

Some of the planet's tallest and longest-lived trees may be harmed by declining fog cover on California's coast, a new study suggests.

Declining fog cover on California's coast could leave the state's famous redwoods high and dry, a new study says.

Among the tallest and longest-lived trees on Earth, redwoods depend on summertime's moisture-rich fog to replenish their water reserves.

But climate change may be reducing this crucial fog cover. Though still poorly understood, climate change may be contributing to a decline in a high-pressure climatic system that usually "pinches itself" against the coast, creating fog, said study co-author James Johnstone, an environmental scientist at the University of California, Berkeley.

For the new study, scientists measured the cloud ceiling—the height of the lowest level of Earth's cloud layer—at two area airports, as well as examined a long-term record of daily maximum temperatures in the region. The research revealed that fog was 33 percent more common a century ago than it is today.

Because redwoods are limited to a narrow, humid band along California's coastline, the trees aren't adapted to long, dry spells, which could kill them, scientists say.

(Related: "Ancient Ginkgoes, Redwoods Threatened in China.")

Most other California tree species that are adapted to drought are efficient at holding onto water,

"So when they get into a dry summer, they're pretty good at closing down their [leaf] pores and restricting water loss," Johnstone said.

On the other hand, "Redwoods are not terribly good at that. They can't survive a warm, dry climate."

Murky Prognosis

Studies haven't yet studied the effects of less fog on redwoods, Johnstone added.

(Related pictures: "Redwoods: The Super Trees.")

But prior tree-ring studies have found a connection between less foggy summers and slower growth in other coastal California tree species.

Though fog may continue to decline, the prognosis for redwoods is still murky, Johnstone said.

"Is global warming going to kill the redwoods?" he said. "That's ... something that needs further analysis."

Research appears this week in the journal Proceedings of the National Academy of Sciences.