Redwoods Siphon Water From the Top and Bottom
JEDEDIAH SMITH REDWOODS STATE PARK, Calif. -- Researchers showed four years ago that California’s coastal redwoods create their own “rain” by condensing heavy fog into drenching showers to nourish their roots during the region’s dry summers.
This summer, they’re finding that the world’s tallest trees’ immense upper stories drink from the sky itself, sucking water directly from the clouds that shroud the coast much of the dry season.
That helps explain how trees 37 stories tall can move enough water from their roots to feed branches and needles nearly twice as high as the Statue of Liberty.
The answer, apparently, is that they don’t have to: the branches and needles get much of their moisture from the air itself.
Researchers are trying to quantify how much moisture the branches and needles absorb. But plant ecologist Todd Dawson of UC Berkeley already knows: “It’s a bunch.”
The ability to siphon water upward against gravity and friction is thought to be one of the most limiting factors in how tall trees can grow.
“Theory says you can’t transport water that high,” Dawson said. “Yet trees do it all the time. We want to understand how.”
Researchers are discovering that the giant trees can alter their environments, both on the ground and in their complex canopies hundreds of feet in the air.
“You essentially have two ends that take in water -- at the top and the bottom,” Dawson said. “That breaks all the rules ... and may explain how they can achieve these great heights.”
Some redwoods have lived since the days of Jesus Christ. With time, their immense, complex canopies trap needles, dust and seeds, creating peaty soil mats a yard thick and as big as a bus that grow plants, sustain animals and absorb water hundreds of feet above the ground.
“Eventually, you get this huge sponge that builds up,” said Steve Sillett, a Humboldt State professor who began studying the phenomena in redwoods in 1996. “During most of the year, it’s an aquatic environment up there” fed by rain and fog.
He’s discovered mollusks, crustaceans and other animals ordinarily found in stream beds -- even the wandering salamander, which lacks lungs and must stay moist to absorb oxygen through its skin.
Like trees in the Pacific Northwest and other temperate rain forests and cloud forests, the redwoods sprout canopy roots from their branches that Sillett believes take in water and nutrients from the hidden gardens.
“It doesn’t have to suck it all the way up from the ground,” Humboldt graduate student Anthony Ambrose said.
Ambrose and other researchers hauled thousands of pounds of solar-powered monitoring equipment, cables and frames into the canopies of 18 redwoods from the Oregon border about 350 miles south to Santa Cruz County.
They inserted probes to measure sap flow speed and direction. Other instruments measure temperature, light, relative humidity, wind and rain.
This month, the researchers will climb the trees again, this time carrying laptop computers to retrieve data collected over the last year.
To get there, they shoot an arrow threaded with fishing line over one of the tree’s highest branches, then haul up stronger ropes. They climb the ropes in ascenders and harnesses, and attach pulleys to pull up the equipment.
Even the tallest redwoods are still growing as much as a foot a year, pushing the limits of height, Sillett said -- but that’s because conditions are good now.
During the next drought, the redwoods’ tops will wither -- only to regenerate when conditions improve once more, a cycle repeated countless times over the trees’ millennial lives.
Researchers are starting to use those cycles to track El Nino, La Nina and other weather patterns -- potentially even global warming -- back hundreds of years.
“What’s locked up in that tree ring could be a key to those environmental changes,” Dawson said.
The giant trees are remnants from the days of the dinosaurs. They grow only along California’s fog belt, a narrow 500-mile band that toes into southern Oregon, where the difference between ocean and air temperatures generates fog much of the summer.
Dawson’s earlier research showed that fog condensation can provide 30% to 40% of a redwood’s water supply -- most of it during the otherwise dry summer months.
The fog and redwoods combine to create a cool, moist environment, like the cloud forests found in scattered areas throughout the world. Where the redwoods are removed, water culled from fog drops by as much as half, Dawson found, and the soil heats up and dries out.
“As you cut the trees, not only are you cutting the fog, you also change the whole microclimate,” he said.
Redwoods are the fastest-growing softwood in North America. They regenerate quickly as shoots sprout off the cut stump, but have difficulty if the entire canopy is removed, Dawson found. He is experimenting with how many large trees it takes to make a forest, research that could discourage commercial clear-cutting.
Logging redwoods is a singularly divisive issue along California’s coast. Dawson’s work has been praised as groundbreaking and criticized for romanticizing redwoods at the expense of less notable trees.
Some coastal residents are using fog-drip research in lawsuits intended to block redwood cutting, arguing that the tree condensation feeds their underground water supply.
The emphasis on redwoods fits the agenda of environmental groups like San Francisco-based Save-the-Redwoods League, critics say. Indeed, Dawson sits on the league’s Board of Councillors, and the league is funding research by both Sillett and Dawson.
The fog drip “happens with any tree that reaches up into the fog,” said Charles Jourdain, vice president of the California Redwood Assn.
“If you had no fog at all, you’d still have redwoods,” said Susan Davis, a park ranger with Redwood National and State Parks. She noted that Humboldt Redwoods State Park is inland from the coastal fog but is home to the world’s tallest tree -- the Stratosphere Giant.
That is the exception that proves the rule, Dawson said.
The biggest trees grow closest to the park’s waterways, as do the southernmost redwoods that depend less on fog than groundwater. Nor do southern redwoods develop the soil mats found in the moister conditions along the northern coast.
But there seems little question that the trees hold the fog, Davis said.
The fog reached much farther inland along Northern California’s Redwood Creek Basin before the redwoods were intensively logged there in the late 1960s and early 1970s, she said. “Today, the fog lifts very quickly. There’s hardly any fog there at all.”