REDWOODS: How tall can they grow?

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No existing species of tree can grow higher than 130 meters (427 feet).

So declares a Nature article by plant physiologist-ecologist George W. Koch of Northern Arizona University and three colleagues from Humboldt State University in Arcata and Pepperdine University in Malibu.

The world's tallest trees are California redwoods, technically known as Sequoia sempervirens. Until recently, some tree experts had theorized that the redwoods' couldn't exceed 120 meters (394 feet). But the theory clashed with historical legends of taller trees.

"There have been accounts of a Douglas fir tree that was measured early in last century and was 125 meters (410 feet) tall. It's no longer standing," Koch said in a phone interview. Also, folklore records redwoods and a tall- growing form of eucalyptus that approached 130 meters, supposedly in the Pacific Northwest and Australia, "but those are less trustworthy records," he notes.

To reach the treetops at Humboldt Redwoods State Park, team member Stephen C. Sillett, a botanist at Humboldt State, shot arrows toward sturdy- looking branches about two-thirds of the way up a typical redwood tree. Attached to the arrows were ropes. Then the scientists climbed the ropes.

On their way up, they collected leaf samples. They also used instruments to measure height-related changes in water pressure and solar-driven photosynthetic activity.

As one ascends the redwoods, one ascends into another ecological world. Leaf sizes get smaller and smaller, as a result of the ever-greater difficulty faced by water as it squeezes through capillaries toward the treetops.

Several years ago, in independent research, Sillett and colleagues measured the tallest coastal redwood: 112.7 meters (370 feet) high. His measurement tools vary; they include a laser range finder and a weighted fiberglass tape that he drops from the treetop.

What's it like to climb the world's tallest trees? "To be honest, above 350 feet or so they all seem very tall," Koch said with a laugh. Way up there, life abounds: "Squirrels. Ravens. Two bald eagles flew by once and landed on a nearby branch. Not many bugs. The northern spotted owl is not uncommon, and the marble murrelet."

Koch and his colleagues studied the correlations among the tree's height, its internal water pressure, leaf size, photosynthesis and other factors. Based on that, they concluded the tallest possible redwood is somewhere between 122 and 130 meters, at least under "current environmental conditions" in the coastal redwoods.

Their "breathtaking field study," as scientist Ian Woodward calls it in an accompanying Nature editorial, reveals the primary factor in determining the maximum height among redwoods: water pressure.

These are thirsty trees. Even a comparatively short 45-meter-high (148 feet) redwood uses 600 kilograms (1,323 pounds, or about two-thirds of a ton) of water daily.

As water rises in the trees, it escapes through the leaves into the atmosphere. As it escapes, more water rises to replace it, as if the tree were a giant soda straw on which someone is sucking. But the water is fighting an uphill battle against gravity. Toward the top of the tree it's "like trying to suck water through a very thin straw," Koch says.

Why does tree growth stop past a certain point? Above a certain height, water pressure drops so far that air bubbles begin to form within the tree's water-conveying capillaries. These bubbles, or "embolisms," tend to block water flow, thus the surrounding leaves get smaller. Leaves make photosynthesis possible; smaller leaves means less photosynthesis. Result: the tree's growth slows or stops.

Besides Sillett, Koch's co-authors are botanist Gregory M. Jennings of Humboldt State and plant ecologist Stephen D. Davis of Pepperdine. Their research funds came from Global Forest Science in Banff, Alberta, Canada, and the Save the Redwoods League of San Francisco.

The Koch team's research was driven mainly by scientific curiosity. But they also hope their findings will contribute to scientists' ability to monitor the health of the redwood ecosystem.

"Knowing that water stress increases with height in these trees tells us that if the climate dries out, that should lower the height of these trees. We'd expect their tops to start 'dying back,' " Koch says. Hence, it's possible that "monitoring the redwoods would be a good indicator of the impact of climate change on these magnificent temperate rain forests."

Of the coastal redwoods that were standing 200 years ago, "95 percent of them are no longer standing. They were cut down for nice houses in San Francisco, shingles and -- recently -- hot tubs," he said. "We wander through (the redwoods) and wonder: 'Gosh, what were the trees like 250 years ago?' "