RENEE MONTAGNE, HOST:

Like other animals, we humans grow when we're young and then stop growing once we mature. But this turns out not to be a universal rule of nature. Scientists have discovered that trees keep growing faster the older they get. NPR's Richard Harris reports on this unexpected twist of nature.

RICHARD HARRIS, BYLINE: Talk to any forester and you'll hear that trees seem to be a lot like people. They grow to a certain height, and then they stop getting taller. So it's easy to assume that their overall growth stops when trees reach that stage of maturity.

NATE STEPHENSON: A very common assumption has been their growth rate declines with increasing size.

HARRIS: Nate Stephenson, a forest ecologist with the U.S. Geological Survey, decided to challenge that common assumption. He rounded up dozens of colleagues from 16 countries and asked them to look back at the growth rate of nearly 700,000 trees that have been the subject of long-term studies. And according to results published in Nature magazine, the growth didn't slow down as these big trees got old and mighty.

STEPHENSON: What we found was the exact opposite, that tree growth rate increases continuously as trees get bigger and bigger.

HARRIS: Trees eventually stop getting taller, but they keep getting wider. And when scientists measured that extra girth, they discovered that these trees packed on more and more mass the older they got.

STEPHENSON: It's as if on your favorite sports team you find out that the star players are a bunch of 90-year-olds. They're the most active. They're the ones scoring the most points. That's an important thing to know.

HARRIS: Because in the world of trees, that means the oldest trees in the forest are doing the most to pull carbon dioxide out of the air and store it as carbon in their wood. Stephenson says that's another argument for preserving old-growth forests.

STEPHENSON: Not only do they hold a lot carbon, but they're adding carbon at a tremendous rate. And that's going to be really important to understand when we're trying to predict how forests will change in the future, in the face of a changing climate or other environmental changes.

HARRIS: Some ecologists have argued that young forests are more important than old forests for combating climate change. That's because the thousands of small trees that replace the few big ones collectively do pull more carbon dioxide out of the air than the mature forest does. But Stevenson says that doesn't give full credit to the importance of old, mature trees.

Previous studies have suggested that old trees grow faster than young ones, but this huge study really makes that point. And Nathan Phillips, at Boston University, says the study's results have implications that go beyond conservation strategies. The findings challenge an assumption that seemingly applied to all of biology.

NATHAN PHILLIPS: We didn't think that things could have unlimited growth potential at some level, at the organismal level. So there's been a long history of that kind of thinking.

HARRIS: But the new study shows that when it comes to growth in trees, well, the sky's the limit. And this leaves Phillips wondering whether trees might in fact have the potential to live forever. He tries to imagine what would happen to a tree if you could prevent it from being blown down or succumbing to drought or disease.

PHILLIPS: How long could it go? And I think that it could go for a long, long time - basically indefinitely.

HARRIS: He's seen 500-year-old Douglas fir trees that are still producing scads of pinecones, which means they're still reproducing. So when it comes to aging, trees have something very special going on. Richard Harris, NPR News.

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