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Trees’ growth keeps climbing with age

Oldest specimens pack on weight fastest, making them potentially best carbon collectors

TREE OF LIFE  Big, old trees such as this Shorea smithiana tend to grow faster than smaller, younger ones. Most tree species continuously get bigger as they age.

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As trees grow older and bigger, they bulk up faster and faster, researchers report January 15 in Nature.

The findings revise scientists’ understanding of big trees’ role in stockpiling carbon drawn from the air.

“This will come as a surprise for many people,” says forest ecologist Maurizio Mencuccini of the University of Edinburgh. “The basic perception is that trees are less capable of growing as they age.”

For years, many scientists believed that trees’ growth was quick in the beginning and tapered off in old age. But the evidence for this pattern is mostly indirect, says study coauthor Nathan Stephenson, an ecologist at the U.S. Geological Survey in Three Rivers, Calif.

In a forest with trees of the same age, scientists have found that productivity — the mass of all the limbs, branches and trunks in a forest — tends to decline over time. And the leaves of big, old trees don’t convert sunlight into sugar as well as the leaves of smaller, younger trees do.

“The in-between scale — the individual tree — has tended to be ignored,” Stephenson says.

Though some researchers have suggested that trees’ growth rate increases continuously rather than declining, until now, no one had examined a large collection of individual trees. So Stephenson and colleagues measured the diameters of thousands of trees in California’s Sierra Nevada mountains and teamed up with other scientists who had collected similar measurements on forested continents around the world. The researchers estimated the growth rate of each tree from two measurements, taken on average five to 10 years apart.

All together, Stephenson’s group calculated growth rates for 673,046 trees from 403 species. In 87 percent of the species, bigger trees tended to pack on the pounds more quickly than smaller trees.

Since trees use carbon to grow bigger, more growth probably means more carbon sucked from the atmosphere, Stephenson says. Big, old trees are already known as carbon reservoirs, but they may be active carbon-sucking sponges, too. If forests were sports teams, Stephenson says, “the star players would be the 90-year-olds.”

The new work “might help us to appreciate those bigger trees and to conserve them,” says ecologist Yude Pan of the U.S. Department of Agriculture Forest Service in Newtown Square, Pa.

But a booming growth rate doesn’t necessarily reflect high carbon intake, says forest ecologist Frida Piper of the Center for Ecosystems Research in Patagonia in Coyhaique, Chile. Old trees could be stocking less carbon in their roots. “We need to know what’s happening below ground level,” she says.

The findings might also reveal clues about trees’ longevity. Stephenson thinks that giant sequoias and bristlecone pines — trees that can see thousands of birthdays — might live so long because they don’t wear out and stop growing as humans do.

“They don’t have an internal clock telling them it’s time to die,” he says.

But even ancient trees can succumb to deadly diseases, insect attacks and lightning strikes.

“Trees aren’t immortal,” Stephenson says. “But if a tree is able to avoid accidents, it looks like it can live indefinitely.”

Editor's Note: This story was updated January 14, 2014, to correct the spelling of Maurizio Mencuccini's name.

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