Trees' upper limit? 425 feet, scientists say

By Michael Stroh
The Baltimore Sun

	BENJAMIN BENSCHNEIDER / THE SEATTLE TIMES, 2003 A platform encircles the trunk of a Douglas fir that attracts visitors along a trail near Lake Quinault on the Olympic Peninsula.
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Now, in a daredevil field study that 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 taller than 425 feet.

Finding the solution, which hinges on a peculiarity of redwood plumbing, demanded Tarzanlike climbing prowess: 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.

"You look up, and it's just a wall of wood," said George Koch, 48, a 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 once had been trees even taller. 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?

Scientists over the years 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," said biologist Michael Ryan of the U.S. Forest Service.

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

In 1994, Ryan and a colleague proposed a third explanation: Growth ultimately was 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 understand fully. For a long time, botanists assumed redwoods and other giants had built-in biological pumps in their roots or trunk tissue.
 
But German botanist Eduard Strasburger in 1893 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 basin 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 pull 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.

Because 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 never had been tested in the world's tallest trees. So Koch and Steve Sillett, a botanist at Humboldt State University, set out to do just that a few years ago.

Since 1988, Sillett, 36, 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.

In the fall of 2000, Sillett, Koch and two colleagues lugged a $30,000 photosynthesis meter and other equipment to the top of Stratosphere Giant and four other redwoods to measure carbon-dioxide levels, water pressure and leaf density. They even slept in the redwood boughs overnight to take measurements at daybreak.

The data confirmed what some scientists had suspected: As Koch and Sillett moved higher, they found the redwoods' hydraulic system was having more difficulty overcoming the downward pull of gravity. Because water also plays a key role in photosynthesis, the treetops were generating less energy for growth.

Others stress that hydraulics probably is not the whole story. Wind, soil conditions and other environmental factors also certainly play a role, the Forest Service's Ryan says.

Knowing how tall a tree can grow, Sillett and others say, could help foresters better manage dwindling stands of giant species and perhaps even help gauge the buildup of carbon dioxide in the atmosphere.

And, Ryan added, "it's always nice to be able to answer your kids."

Copyright © 2004 The Seattle Times Company