It’s the kind of question 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.

“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?

Through 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,” noted 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 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 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 contain the key to water movement within a tree. Others later filled in the rest of the scientific story.

Put to the test

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.

Sillett, now 36, has conducted pioneering research on the creatures and plants living in a redwood’s upper reaches since 1988, 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.

Water the answer

In the fall 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, leaf density. Sometimes they even slept in the musky redwood boughs overnight to make 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. Since water also plays a key role in photosynthesis, the treetops were generating less energy for growth.

In the Nature article, Ian Woodward, an ecologist at the University of Sheffield in England writes that Koch and Sillett’s “breathtaking field study” shows that the “fundamental control of maximum tree height is water supply to the treetop.”

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

Mystery solved

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 build-up of carbon dioxide in the atmosphere.

And, adds Ryan, “it’s always nice to be able to answer your kids.”