The height is in the pipes

April 24, 2004|By Michael Stroh | Michael Stroh,SUN STAFF

It's the kind of stumper a chronically curious kid might pester a parent with: Daddy, how tall can a tree grow?

Now, in a daredevil field study one researcher describes as "breathtaking," a team of U.S. scientists gingerly hauled themselves and more than $30,000 worth of sensitive instruments to the tops of the planet's tallest living organisms - 2,000-year-old California redwoods - and came back with a tentative answer to the height riddle.

Redwoods, and perhaps all trees, can grow no higher than 425 feet into the sky.

Finding the solution, which hinges on a peculiarity of redwood plumbing, demanded Tarzan-like climbing prowess: The researchers scaled five of the world's eight tallest trees, including the 369-foot-4-inch record holder known as Stratosphere Giant in California's Humboldt Redwoods State Park.

If it sprouted in downtown Baltimore, this Guinness-book beast would soar just 30 feet shy of the World Trade Center roofline.

"You look up, and it's just a wall of wood," says George Koch, the 48-year-old Northern Arizona University ecologist who led the study published in the new issue of the journal Nature.

As big as the Stratosphere Giant is, Koch and his team suspected from old logging records that there had once been trees even bigger. In 1895, for example, lumberjacks in British Columbia reportedly felled a 417-foot Douglas fir. The scientists wondered: How big can trees get, and what ultimately limits their growth?

Kicking theories around

Over the years scientists have batted around a number of possibilities. One theory is that tree tissue, like steel or concrete, has natural engineering limits. At some point, a titan becomes too vulnerable to wind and other forces and topples.

"You can only build something so tall," notes biologist Michael Ryan of the U.S. Forest Service, who wasn't involved in the redwood research but has pondered the growth problem in the past.

Another theory centers on genetics. Just as DNA is thought to regulate stature in humans, scientists have speculated that it plays a similar role in plants.

In 1994, Ryan and a colleague proposed a third explanation: growth was ultimately limited by the tree's ability to pipe water to the canopy's upper reaches.

Trees, it turns out, rely on a complex plumbing system that scientists still don't fully understand. For a long time botanists assumed that redwoods and other giants had built-in biological pumps in their roots or trunk tissue.

But in 1893 German botanist Eduard Strasburger devised an ingenious experiment that revealed how a tree's plumbing system really works.

Strasburger sawed down a 70-foot oak tree and plunged the rootless end into a pail of poisonous picric acid. As the acid seeped into the trunk, it began killing tissue. Still, the poison continued to rise.

It was only when the acid invaded and killed the leaves that the liquid's vertical movement stopped. Strasburger correctly concluded that leaves - not roots or trunk tissue - must hold the key to water movement within a tree. Others later filled in the rest of the scientific story.

Trees rely on evaporation to siphon water hundreds of feet into the air. Water enters a tree through root hairs and flows to the leaves via natural pipes in the trunk known as xylem. Upon reaching the leaves, water evaporates into the air through thousands of tiny pores.

Since water molecules are "sticky," the departing molecules tug on one another, drawing fluid up through the tree. This passive pump is powerful, but slow: water can require as much as 24 days to reach the top of a giant redwood, studies show.

The water theory has remained one of the leading contenders to explain what ultimately limits tree height. But the idea had never been tested in the world's tallest trees. So a few years ago Koch and Steve Sillett, a botanist at Humboldt State University, set out to do just that.

Since 1988, the 36-year-old Sillett has conducted pioneering research on the creatures and plants living in a redwood's upper reaches, discovering everything from 30-foot hemlocks to crustaceans in the canopy.

While many scientists who study canopies reach them with a crane, Sillett goes the hard way: by rope, one branch at a time.

Over the years his daring tree-climbing techniques have attracted IMAX film crews and earned him the nickname Tarzan, a label he says really doesn't apply.

"Tarzan kind of swung," says Sillett. "We don't."

Instead, they scale. Sillett loads a rubber-tipped arrow into a powerful hunting bow and aims for a low, sturdy-looking branch. "The first branches that you can trust your life to are often 250 or more feet off the ground, sometimes even higher," he says. The arrow is knotted to a spool of fishing filament. Once the filament is looped over the first branch, Sillett hauls up progressively larger lines -parachute cord, then nylon climbing rope.

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