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MELISSA BLOCK, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

(Soundbite of music)

BLOCK: And it's time now for Climate Connections, a yearlong series with National Geographic. We're exploring how people are changing climate and how climate changes people. And today's topic is our breathing forests.

The world's forests are getting more attention these days from scientists and climate negotiators alike. That's because they store carbon, the prime agent of global warming. You might call a forest the kind of carbon bank. Scientists who monitor the balance sheets of this carbon banks have been surprised again and again by what they've found.

NPR's Dan Charles visited Harvard University's forest in Central Massachusetts, which is exceeding expectations.

DAN CHARLES: The director of the Harvard Forest, David Foster, looks like a park ranger and talks like a historian.

Mr. DAVID FOSTER (Director, Harvard Forest): I grew up in Southern New England in a farm landscape and became fascinated with nature and recognized that in order to understand nature, you have to understand the processes that shaped it and to view that, you have to understand history.

CHARLES: There's evidence of that history all around us. The lines of piled-up rocks, for instance. A hundred and fifty years ago, there were fences around pastures where cattle grazed.

Today, the Harvard Forest is a 3,000-acre laboratory. Hundreds of trees wear shiny metal belts to measure their growth. Buckets collect falling leaves. Bit by bit, the forest is giving up its secret.

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CHARLES: One of the most recent revelations came from an experiment just above the forest on top of a steel tower.

Researcher Julian Hadley leads the way up the steps. Eighty feet above the forest floor, we climb into sunlight. The tops of hemlock trees form a blanket of green around us.

Mr. JULIAN HADLEY (Researcher): It's at the very top here. You can't actually see the ground very well because of the dense foliage around us, which is good for people with acrophobia.

CHARLES: Above us, suspended on a steel rod like a weather vane, there's a sensor that detects the slightest air current and an intake valve that samples the air going by. This equipment is watching the forest breathe.

Mr. HADLEY: You know, it's taking in carbon dioxide when it's growing and releasing carbon dioxide at night, for instance, when there's no photosynthesis.

CHARLES: This part of Harvard Forest was never completely cleared. Some of the trees are 300 years old. Until recently, scientists thought that mature forests released about as much carbon as they took in. Any carbon captured by new growth they figured was canceled out by carbon dioxide released from decaying dead trees.

But careful measurements deliver a surprise. This old stand of hemlocks captures a lot more carbon from the air than it gives up: about a ton more, per acre, per year. And it's not just going into trees.

Mr. HADLEY: Only about half of the carbon that we see coming out of the atmosphere and being stored in the forest is going into wood. And so the remainder of that carbon must be going into - in increasing soil materials or organic matters, so carbon.

CHARLES: Other places in this forest, where the trees are younger, have been capturing even more carbon. And Steven Wofsy, the scientist at Harvard who pioneered the monitoring techniques used here, says at the site he's watched the longest, 17 years. The forest has been soaking up carbon faster and faster.

Professor STEVEN WOFSY (Atmospheric Chemistry, Harvard University): And that is a real mystery. We know where it's going, but we don't know why it's doing that. We don't know whether that's kind of normal for this forest. We're trying to unravel that because it's a big question. It's one of the big questions in climate change science.

CHARLES: Wofsy would love to know how long this forest can keep it up.

Prof. WOFSY: The forests of the world are currently taking up something like 20 to 25 percent of all the CO2 that's emitted by fossil fuel use, and we would very much like to know whether that's going to continue, whether it would accelerate or whether it might turn around, and the forests put that carbon that they've stored over the last decades back into the atmosphere.

CHARLES: But trees certainly can't capture carbon if they're cut down. Harvard Forest director David Foster worries they might be. We visit the forest museum where a diorama shows how 19th-century farmers cleared 80 percent of the Massachusetts landscape to make way for fields and pastures.

Then factories replaced farms. People abandoned the land and the forests gradually came back. They now cover most of New England, but once again, those forests are shrinking. This time, Foster says, buildings, lawns and parking lots are taking over.

Mr. FOSTER: This time, we're deforesting it for good. So this is what I like to call a hard deforestation, a permanent deforestation, as opposed to the original deforestation, which is a soft deforestation.

CHARLES: Foster believes passionately in preserving the forests, but that doesn't mean leaving them all untouched. New Englanders consume lots of imported wood. Foster says they should buy that wood locally. Then if landowners can make money from their forests by harvesting timber occasionally, they're more likely to agree to keep trees on the land.

Mr. FOSTER: It means retain the forested landscape, retain some infrastructure that produces clean air, clean water and benefits the globe by storing carbon. It's a fabulous situation to be in.

CHARLES: It's not just fabulous for New England. A third of the United States right now, about 750 million acres is covered by trees. The scientists who study those forests are increasingly convinced they are a treasure for the nation and the planet.

Dan Charles, NPR News.

BLOCK: You can find the narrated photo tour of Harvard Forest's history from the settlers who first cleared the forest to the modern-day scientists studying it. That's at npr.org/climateconnections.

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