Thick fog through the redwoods is a hallmark tableau of the northern California coast, but the decline of one now threatens the other. Summertime fog has decreased in the last century, stressing out some of the tallest and longest-lived trees in the world, a new University of California, Berkeley, study shows in today’s PNAS.

Redwoods are dependent on cool, humid summers, and without enough fog, the heat becomes too intense for growth, says co-author James Johnstone (San Francisco Chronicle). “Foggy nights are needed to rehydrate the trees that can’t tolerate long droughts,” adds co-author Todd Dawson.

Fog is also a reason trees are able to grow so tall. Without the ability to absorb nearly 40 percent of their water directly from the mist, the trees wouldn’t be able to move water from their roots all the way to their tops (Wired).

Looking at cloud measurements taken hourly since 1951 and extrapolating back to 1901 using long-term land temperature data, Johnstone and Dawson find that fog frequency in summers has declined by about one-third since the early 20th century. When the coast heats up faster than inland, the temperature difference between the two goes down, reducing the force pulling fog ashore. Warmer seawater also causes less fog to form in the first place. All these factors combine to decrease fog by about 3.5 hours a day over the last century.

In other water shortage experiments, Dawson has seen die-back at the very tops of the biggest trees. “At the other end of the spectrum, you can also imagine that if the soil gets dryer and dryer because there’s less and less fog," he says (Wired). "Then the newest trees — the little seedlings that are germinating — they may not survive through the dry summer months.”

Nor would the plants and animals that live beneath their canopy. "Salamanders, worms and many plants like the giant sword ferns that are abundant in the shaded ground beneath the trees all depend on fog drip for their water,” Dawson says (SF Chronicle).

The cause of the fog decrease appears to be natural oceanic and atmospheric variations. As for climate warming, “there’s no clear evidence for that at this point,” Johnstone says (USA Today).

Image: Marin County redwoods in fog by Tom Spaulding via Flickr under Creative Commons.