Using Trees To Curb Climate Change Not So Simple

Forests are fast becoming a great green hope for slowing climate change.

The idea is simple: Forests suck up carbon dioxide and store it in wood and roots. So simply grow more forests, or save existing ones, and you curb climate change.

Climate planners are keen to create forest carbon banks, places designed to store carbon and keep it out of the atmosphere. Legislation pending in Congress would pay people to grow trees — and international negotiators want to do the same for the rest of the world.

But banking carbon in forests turns out to be something of a mystery.

The Effect Of Fire

Forest ecologist Matthew Hurteau of Northern Arizona University points out that if you let forests in the drier western United States grow unchecked, you turn them into tinderboxes.

"Without regular surface fire," he says, "the litter, branches and coarse woody debris build up over time, and then, when fire does occur, ends up releasing more energy and carbon."

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

But any fire puts carbon dioxide into the air. So do frequent small fires put more or less carbon into the atmosphere than the occasional massive fire? "That is really a question that we're looking for the answer to right now," Hurteau says.

Hurteau's research in the Sierra Nevada 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 with lots of space between them and fewer small trees. But no one knows for sure.

The Rate Of Growth

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

Gregory Latta, a forest economist at Oregon State University, says that in some parts of the Northwest, warming could slow growth, and that means less carbon in the bank.

"At the lower elevations ... 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," Latta says.

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

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

The Chance Of Drought

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.

"There's potential for kind of a double-whammy between having warmer temperature and drought, which could trigger fire or tree die-off, and that will pose a challenge for how much carbon you can store in a system," he says.

Breshears says scientists don't really know yet how forests will store carbon for a potential climate bank while also delivering timber, providing a home for wildlife and not burning 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, that would reduce carbon dioxide from deforestation by 55 million tons. Now, they say, it's more like 5 1/2 million tons.

That's not a failure, 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.