Green giants: tree scientists dare breathtaking heights to solve a mystery surrounding one of nature's tallest skyscrapers

Science World, Jan 24, 2005 by Jeanna Bryner

Walking on the moist, spongy carpeting of the forest floor, Stephen Sillett squeezes through thick patches of trees. Their brown trunks climb high into the sky like magic beanstalks--topping out at a dizzying 30 stories tall. Sillett is exploring Humboldt Redwoods State sunlight, keeping the air cool and the surroundings dim. But as Sillett hoists himself up the rope, sparse rays hit his face. "Sometimes you get these incredible sunbeams that cruise through the forest," he says.

Good thing: Sunlight is vital for tree growth. To get the most sunlight, this redwood has to grow tall enough to poke its top above the shade of neighboring Park--a protected area for coast redwoods, Sequoia sempervirens (SUH-coy-uh SEM-purVIE-renz). His goal: find out what keeps a coast redwood, or even the giant sequoia (shown, far right), from spindling upward forever.

In his quest to solve the mystery, Sillett, a botanist (plant scientist) at Humboldt State University in California, winds deeper into the park. He halts beneath a towering conifer (type of tree that bears cones). Nicknamed "Stratosphere Giant," it is the world's tallest living tree, with a height of 112 meters (370 feet).

Undaunted by its heights, Sillett has no intention of remaining at the tree's base. That's because he knows that the answer to his question lies among the topmost branches. Joined by George Koch, a botanist from Northern Arizona University, Sillett clips on his beltlike harness and a safety helmet. He spies his lifeline--a heavy rope that's looped over a clump of sturdy branches about 60 m (200 ft) off the ground. Holding tightly to the rope, Sillett moves his feet step by step up the tree trunk.

SUN CATCHERS

Near the forest floor, tightly packed trees squeeze out trees. "[Stratosphere Giant] has been competing with its neighbors for light, and it's been racing to the sky," Sillett explains.

The tree's needlelike leaves are its most powerful sun-grabbers. And Sillett is determined to get a sample. He tip-toes out onto a giant tree branch as if he's on a balance beam. "All the action is out toward the edge of the branches where the light is," says Sillett. That's where he snips off a stem of leaves and seals it in a plastic bag. The waxy leaves contain chlorophyll, or the emerald pigments that capture the sun's energy to power photosynthesis. This process tunas carbon dioxide (C[O.sub.2]) and water into food (see Nuts & Bolts, p. 18). Sillett wraps his hands around the rope and swings over to another branch for more leaf collecting.

BY THE BUCKETFUL

Inching upward by leaping from branch to branch takes a toll on Sillett. "Climbing trees is a very physical thing--especially if you're climbing the world's tallest trees," he says. While his ascent is demanding, there's a trickier climb taking place beneath the redwood's bark. Water that's in the soil gets soaked up by the redwood's roots. Then, hair-strand-thick tubes called xylem tissue carry roughly 500 gallons of water daily from the roots to its top leaves.

Making that journey possible: teeny pores, or stomata (STO-mah-tuh), that dot the redwood's leaves. During the day, the pinhole pores open. Heat from sunrays causes water to evaporate (turn from a liquid to a gas) out of the open pores--a process called transpiration. And water molecules tend to stick together. So as water exits needles on the tree's upper branches, more water gets tugged upward through the xylem.

STRETCH AND SNAP

When Sillett reaches the treetop, he looks out over a field of rolling, green hills. "When you get up there, the wind is blowing, and each tree is moving a little bit independently. It's an impressive sight," says Sillett.

Another observation: "At the bottom of these redwood trees the leaves are long, extended needles. At the top, they become tiny scales," Sillett explains (see photo, middle left). Why the change? Leaves swell and grow larger when they are full of water. But leaves at the tops of coast redwoods can get parched. That's because as water molecules inch upward, gravity (force that pulls two objects together) pulls them downward. That causes the water stream to stretch and get thinner. "The water column is like a rubber band being stretched tighter and tighter," says Koch.

And for the "Mount Everest" of the forest world, it takes a huge upward tug to out-compete gravity. Sometimes, that pull power can be greater than water's sticky forces. When that happens--snap!--the water band breaks apart and an air bubble, or embolism (EM-buh-LIH-zum), fills the void. "When air bubbles form inside (the xylem], the tree can't conduct water to its top," says Sillett. So the leaves collapse like popped water balloons.

STILL GROWING

Turns out, water stress keeps redwoods from reaching cloud-brushing heights. "As a tree gets taller, and farther from the soft, it's harder to pull up water," says Koch. Is there a maximum redwood height? To find out, Sillett and Koch calculated the height at which the tree's water band would snap. Answer: 122 m (400 ft). So even this redwood has growing room.