The Earth's tallest trees, California redwoods, rely on characteristic coastal fog to reach their towering heights—and that fog may be diminishing, according to new research.



A study by climatologist James Johnstone and biologist Todd Dawson of the University of California, Berkeley, looked at a combination of weather station and airport data along the northern California coast where massive coastal redwood trees thrive. Because fog appears as a cloud that moves off the ocean and sits on the ground, airport monitors of the ceiling height of the clouds were particularly illuminating. The results, published in Proceedings of the National Academy of Sciences March 9, 2010, showed that the region's fog has decreased by 30 percent in the past six decades.



"The airports registered the height of the cloud layer every hour for 60 years. It was an incredible source of data," Johnstone says. What they were really measuring was the fog, which is different from clouds in altitude and meteorology. Those records showed that the length of time the fog hung around at ground level during the day had radically diminished. He, Dawson and others have been trying to figure out what this ongoing loss means for the redwood forests ever since.



Coast redwoods (Sequoia sempervirens) are distributed along a narrow band of California's northern coast. During the summer these red giants take advantage of the fog to capture water out of the air—and summer is the critical growing season for the trees, despite being California's dry season.



To obtain sufficient moisture for photosynthesis and growth, redwoods reach into the air with leaves shaped like baseball mitts and capture the fog that rolls in by night and languishes through most mornings. "From 25 to 40 percent of the moisture in the system comes from fog," says Dawson, who has been studying the relationship between the coastal fog and the redwoods for two decades. Some of the fog simply covers the leaves and prevents evaporation. But some of it also enters the stomata, or tiny pores, on the leaves and is drawn down through the branches to the roots. This is the reverse of transpiration, the normal flow of water from the roots to the leaves that exists in most trees. Redwoods are the first trees found to move water in both directions, though others have been identified.



Fog is not just a vital element for the redwoods—it's also crucial to the entire redwood forest ecosystem. Some of the moisture drips off the redwood leaves, landing on the forest floor to water the trees and young saplings. "It's not just a drip, drip, drip," says ecologist Holly Ewing of Bates College in Lewiston, Maine, who also worked with Dawson. "The moisture can descend into the ground up to 35 centimeters deep, and that's a lot of water."



The fog is a gift of the Pacific Ocean's California Current where winds create upwellings that bring cold, deep, nutrient-rich waters to the surface. Those nutrients incorporated into the fog then become a gift from one of the richest marine currents on Earth to the tallest forest on the planet. Fog rolls in not only bearing moisture but also nitrogen, phosphorus and some minerals. Kathleen Weathers, a biogeochemist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., and co-author of a study on fog and the coast redwoods published February 14, 2009 in Ecosystems with Ewing and others, showed this. Weathers believes that "winds and waves kick the surface scum on the ocean high into the air, where it is incorporated into the fog that moves inland."



But climate change has increased temperatures in the coastal environment, albeit lowering them in the inland environment. The fog rolls in when colder coastal air is drawn inland by greater warmth there. "With the decreasing temperature difference, it is literally turning off the conveyor belt that that moves the fog forward," Dawson says.



The lack of moisture and nutrients during critical dry summers could be fatal for redwood seedlings and saplings, which are vital to the long-term survival of the forest. But it's even worse for mature trees, Dawson says. With a reduction of the fog, a mature tree's great height becomes a liability. Unable to pull as much water from the sky, they are subject to cavitation—a pocket of air that develops in the water being pulled up into the tree, which expands and can block the flow of water in either direction, killing all or part of the tree.



Already, coastal redwoods at the inland edge of the forest as well as the tops of the trees in the southern part of the range are suffering. This may be an effect of fog loss, but Dawson says there is not enough evidence to build a concrete case yet.



Johnstone, now a research associate at the University of Washington in Seattle, thinks that the Pacific Decadal Oscillation—a shift in ocean temperatures that takes place roughly every 30 years—could modulate warming trends along the California coast, providing some temporary relief. Regardless, the iconic California redwood forests may ultimately break up with the evanescing coastal fogs.