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.

5 Comments

1. 1. lakota2012 06:50 PM 12/9/10

   I lived in Boulder Creek for awhile, just down the road from Big Basin Redwoods State Park, and if the climatologists are saying "the region's fog has decreased by 30 percent in the past six decades," there's no doubt in my mind we'll be seeing a decline in the redwoods.

   Reply | Report Abuse | Link to this

2. 2. timbo555 12:06 PM 12/19/10

   The life span of the redwoods is 500 to 700 years. Is it remotely possible that global temperatures have been more than one degree (if that) colder or warmer in that amount of time? Is it even remotely possible that the concentrations of fog have fluctuated back and forth over that period?
   
   The Giant redwoods live over 3200 years. It is ridiculous to assume that a temperature increase of a degree (or less) will destroy the ecosystem. But it sure makes for great headlines, doesn't it?
   
   Yes sir! Scare the pants off the ingorant yokels and they'll do anything you say. Nothing is happening now, Of course, But if we keep projecting all the gloom and doom just far enough into the future, we'll never have to take responsibilty for the lies we spread today.

   Reply | Report Abuse | Link to this

3. 3. chicagoman58 in reply to timbo555 09:22 AM 12/23/10

   The article states that edges of the forest are already under stress, so it is not true that "nothing is happening now". Scientists just haven't yet proven that the stress is due to the decline in fog. It is also irresponsible for you to say that article is a lie. You don't have any evidence that the science in the article is a lie.

   Reply | Report Abuse | Link to this

4. 4. timbo555 01:19 PM 12/23/10

   "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."
   
   "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."
   
   Observable damage to the "edge" of the ecosystem does not translate into destruction of the ecosystem. This is my point. The article starts by blaming climate change for a current non problem that may or may not become a catastrophe sometime in the future event though there is no evidence that current trends will continue, or that they're even caused by climate "change".
   
   We go from science (Hey! there is damage happening to a tiny portion of of the California redwood population) to blame (it's probably caused by "Climate change", but we don't know that yet) to worry: (this could kill all the trees. probably. Someday. Or. not.)
   
   It's not the observations (science)that are a problem in this case, it's the inference that, as I said in my previous comment, up until humans started burning fossil fuels, the climate was unchanged and unchangeable. Because of human activity we should worry that "some time in the unknowable future", all the redwoods will be dead.
   
   The environmental sciences have created an entire cottage industry out of scaring the pants of the population in this way over the past forty years. "If there's no problem, there's no grant money."

   Reply | Report Abuse | Link to this

5. 5. neeld in reply to chicagoman58 12:41 AM 3/23/12

   Plants at the edge of their range are usually a bit stressed. That's why it's the edge of their range. The conditions are such that they can't survive further, in this case, inland or south. Those populations that are occupying the edge of the range are occupying what is marginal habitat at the best of times.

   Reply | Report Abuse | Link to this