By Jonathan Amos

BBC News Online science staff



Even the biggest things have their limits

George Koch and colleagues climbed five of the eight tallest trees in the world - including the biggest at 112.7m - and examined their physiology in detail.

The researchers found these massive Californian redwoods pushed the limits to which water could be raised from the ground to support further growth.

The team tells the journal Nature that under present conditions, the trees are unlikely to gain more than a further 5-15m in height.

"It's quite an amazing thing to climb these awesome trees and do something that's useful scientifically," Professor Koch, from Northern Arizona University, at Flagstaff, told BBC News Online.

Bows and arrows

No living thing on Earth currently grows taller than the coastal redwoods (Sequoia sempervirens) in the US northwest.

The trees, which can live for more than 2,000 years, have a voracious appetite for water.

Professor Koch and his team fired arrows trailing wires over five giants in Humboldt Redwoods State Park. They then hauled ropes through the upper branches and climbed into the treetops using mechanical ascenders and harnesses.

The scientists examined the specimens' water flow, leaf structure and density, photosynthesis capability and carbon dioxide concentration. All appear to converge at their minimum levels of efficiency at the heights reached by the Humboldt redwoods.

Changing conditions

"The leaves at the top, because they are water-stressed, are not doing as much photosynthesis per unit mass," said Professor Koch, who is a physiological ecologist.

"In essence, the plant is investing a certain amount into those tissues but they're not providing as much return on that investment because of the water stress."

Conditions at the redwoods' tops are similar to those of a desert, the researchers found. Although the trees are still growing at about 0.25m a year, the team predicts they will not exceed 122-130m.

These heights are comparable with historical records of past record-breakers of Douglas fir trees in British Columbia (125m) and mountain ash trees in Australia.

"We are now monitoring the redwoods on a regular basis," Professor Koch said.

"We will be able to see if, as they keep growing, their growth slows as they approach our predicted maximum height. We will also be able to relate the growth they do each year to variation in climate.

"Some years are wetter than others, and another prediction would be that if the climate becomes drier, their growth slows and the maximum height would be lower."

Big and small

Professor Ian Woodward, a plant scientist at Sheffield University, UK, said the work was a fascinating insight into the extreme capabilities of the redwoods.

"When a tree pulls water up through its roots and through its water vessels, the xylem, it has to overcome gravity and friction in the system," he told BBC News Online.

"Eventually, like all continuous tubes of water, when you pull on them hard enough and far enough, they break. Suddenly bubbles of air develop and the whole thing stops.

"Plants have become adept at having very fine xylem that don't break easily and the redwoods have done a great job at this - but they reach their limit at about 120-130m."

He continued: "George Koch used to work on radishes and the knowledge that the physiology that applies to tiny plants also applies to the tallest trees is kind of comforting."