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One problem of accounting for carbon is trying to figure out how much carbon goes into forests and comes out again. That's important because forests could become carbon banks, as they're called, storing carbon and keep it out of the atmosphere. The legislation pending in Congress would pay people to grow trees. But as NPR's Christopher Joyce reports, carbon banking is a tricky business.

CHRISTOPHER JOYCE: The idea is simple: Grow more forests or save existing ones, and you curb climate change. Growing trees suck carbon dioxide out of the atmosphere and store it in wood and roots. Dead or burned trees release CO2 into the air.

Forests have become a great green hope for curbing global warming. For one thing, growing them is usually cheaper than reducing emissions from factories and power plants.

The rub is in the accounting. How much carbon will forests store and for how long? Let's say you let a forest grow unchecked. That puts a lot of carbon in the bank. But Western U.S. forests can become tinderboxes.

Here's forest ecologist Matthew Hurteau from Northern Arizona University.

Mr. MATTHEW HURTEAU (Northern Arizona University): Without regular surface fire, the litter, branches, coarse woody debris build up over time, and then when fire does occur, end up releasing more energy and carbon.

JOYCE: Frequent, small fires keep the woody debris down. That's why foresters regularly start them on purpose — to keep those huge fires from happening later on.

But any fire puts CO2 into the air. So do frequent small fires put more or less carbon into the atmosphere than the occasional massive fire?

Mr. HURTEAU: You know, that is really a question that we're looking for the answer to right now.

JOYCE: Hurteau's research in the Sierra Nevada Mountains suggests that one way to keep fires to a minimum, and still store lots of carbon, is to return those forests to what they were 150 years ago: large trees growing in patches, lots of space between them, and fewer small trees. But no one knows for sure.

Forests may also grow differently in the future. It's already warmer, and climate scientists say it will keep getting warmer.

Gregory Latta is a forest economist at Oregon State University who studied how Northwestern forests might be affected by warming. In some places, warming could slow growth, and that means less carbon in the bank.

Mr. GREGORY LATTA (Forest Economist): At the lower elevations, they have moisture. And as you crank up the temperature in the growing season, that moisture evaporates. And so it's kind of taking away from them a little bit.

JOYCE: Latta adds that a warmer climate would probably have the opposite effect on higher-altitude forests — they'd grow faster. In the end, Northwestern forests might see overall increases, but that may not apply elsewhere.

Mr. LATTA: So you can't really put this broad brush across the whole area and say, here's the impact of climate change. It's going to really impact different areas depending on what their starting point was.

JOYCE: Then there's another trend to watch: drought. Climate models predict the West will get drier. David Breshears, at the University of Arizona, says scientists are seeing more dead and dying trees already.

Mr. DAVID BRESHEARS (University of Arizona): There's a potential for kind of a double-whammy between having warmer temperature and drought, which could trigger fire or tree die-off, and that'll pose a challenge in terms of how much carbon can you store in a system.

JOYCE: Breshears says scientists don't really know yet how forests will store carbon for a potential climate bank as well as deliver timber, provide a home for wildlife, and not burn up too often.

In fact, one of the first experiments to manage a forest carbon bank has already run into trouble. Twelve years ago, the Nature Conservancy and the Bolivian government set aside a forest in Bolivia called Noel Kempff Mercado. They closed down saw mills and stopped cutting timber. Experts predicted that over 30 years, they would reduce CO2 from deforestation by 55 million tons. Now, they say, it's more like 5 1/2 million tons.

That's not a failure, though, say experts at the conservancy. They simply know more now about storing carbon in trees — knowledge they can share with others planning similar carbon banks.

Christopher Joyce, NPR News.

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