updated 4/21/2004 5:45:54 PM ET 2004-04-21T21:45:54

Redwood trees are the tallest living things on Earth, but they are stretching up against a growth limit that likely cannot be overcome no matter how ideal the conditions, according to a new study.

The results suggest the tallest a redwood might ever grow is about 420 feet -- a height reached in the past but not in evidence today after decades of logging, said George Koch of Northern Arizona University.

After climbing a towering grove in Humboldt Redwoods State Park in northern California, Koch and a team of researchers took careful measurements that showed four major factors in tree growth all hit their limit at the top.

Water flow, leaf density, photosynthesis and carbon dioxide concentration all appear to converge at their minimum levels of efficiency at the heights reached by the California redwoods.

"We were quite surprised by that," Koch said. "If you only look for one factor, it wouldn't impress you very much. But when they converge in a very narrow range, that did impress us."

The study published Thursday in the journal Nature marked the first time a direct test has been carried out at the top of some of the world's tallest trees.

The five trees in the study included the tallest known tree -- which stands about 370 feet, the equivalent of a 30-story building. The others were the second-, fourth-, sixth- and eighth-tallest known trees in the world, all in the same park.

The stratospheric growth of redwoods is driven by competition for sunlight among trees that have everything else they need in abundance -- so the only way to survive is to grow taller than your neighbor, he said.

The trees also help insulate each other from the wind and thrive in the moist, mild climate of northern California and southern Oregon.

Other scientists said the study pulls together bits of data from other research to provide the first strong statistical evidence that tree growth does have upper limits.

"There are huge mechanical problems of maintaining massive amounts of biomass so far above ground," said Barbara Bond, an Oregon State University professor who studies tree physiology.

Researchers estimate it can take up to 24 days for water entering the base of a redwood trunk to reach the treetop.

"It creates an environment in a big tree analogous to what smaller tree in the desert would face," Bond said. "Even if there's a lot of water in the soil it's really, really hard to get that water to the top of the tree."

Copyright 2004 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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