CORVALLIS, Ore. -- At a forest research site in the Coast Range, Beverly Law is studying the breathing patterns of a fast-growing Douglas fir stand.

A tower shooting up from the forest floor is rigged to measure the carbon dioxide concentration of the air 20 times per second.

The water vapor being released from the trees, the sap moving through their trunks, and the growth and decomposition of their roots are being carefully measured.

"This forest is wired," said Law, stepping over a bundle of cords tethered to the ground.

A forest science professor at Oregon State University, Law is the director of the AmeriFlux Network, which is measuring the carbon intake and output of 90 forests across the continent.

Measuring how much carbon forests can store will be key to determining their value in carbon offset markets as governments move toward policies to combat global climate change.

The trees in Law's Coast Range research site are around 40 years old -- close to their maximum growth rate. But she is finding that the trees won't stop taking up and storing carbon later.

"When we figured out that older trees store more carbon, the question became how can you make more?" she said.

By taking up carbon dioxide and releasing water vapor, trees offset some of the greenhouse gas emissions and provide a much-needed carbon sink for the planet.

Through the Western Climate Initiative, Oregon is looking at launching a cap-and-trade system to regulate greenhouse gases. Forest management that increases carbon storage could have a cash value on a carbon offset market.

A forest carbon offset market would allow companies exceeding the regulatory cap on carbon dioxide emissions to buy an equivalent carbon offset from forest land owners based on increases in carbon sequestered in trees and wood products.

As the specifics get hashed out, Law's research suggests that the best thing is to leave the trees alone.

Law's studies show it can take up to 20 years after a harvest for a forest to return to "carbon sink" status -- absorbing more carbon than it releases.

Studies have shown the amount of carbon retained in wood products is only 30 percent of the total storage capacity. The rest is lost through the logging and milling processes, or it ends up in a landfill.

"When you cut an old stand, you're automatically saying you're going to release about 70 percent of that carbon back into the atmosphere," said Law.

Some conservation groups argue carbon offset credits should only be given to forestland owners for leaving forests intact, but timber companies and small woodlands owners want to sell credits for turning trees into wood products, converting woody debris into biofuels and for using wood to replace energy-intensive building materials like concrete and steel.

The Western Climate Initiative includes the Carbon Forest Working Group, which is looking at all the possibilities and debating their virtues.

Greg Miller, public affairs manager for Weyerhaeuser Co., has been taking part and says he hopes the new market will include credits for a variety of carbon storage and emissions reduction efforts.

Making wood products doesn't store as much carbon as leaving trees in the forest, he said, but the right system could reward both actions appropriately.

Traditional forestry wisdom suggests after 150 years forests become "carbon neutral," giving off as much carbon dioxide through decay as they take up.

But Law says that is based on outdated information and doesn't apply everywhere.

Her analysis shows old forests will serve as carbon sinks if they are left alone.

"Young, fast growing trees don't capture more carbon than older ones," she said. "Older forests store huge quantities, and they continue to absorb through age 80."

Even as the absorption of carbon in older trees peaks, she said, "the trees continue to form a bank of stored carbon that cannot be equaled by a newly sprouted stand."

Law says measuring the carbon storage potential in forests isn't as simple as tracking tree growth.

"There's more to the story than growing trees and aboveground production," she said. There's soil respiration, foliage, underground root systems..."

An evaluation of logged forests showed carbon emissions from the soil increase for years as the leftover material decomposes.

She found 70 percent of respired carbon dioxide in the forest comes from the soil surface.

Law found Oregon's forests absorbed 50 percent of the state's fossil fuel emissions in 2000. In 2003, a record fire year, the number was still 30 percent.

One way to increase that percentage, she said, would be to leave more trees on the ground longer to increase carbon uptake and storage -- at least until renewable energy sources are developed.

Even though forests may be replanted after clear-cuts, she said, the microbes in the soil continue to emit more carbon than the young trees for years.

"Microbes in soil dominate the carbon balance after logging - not trees," she said.

On the Coast Range, where the Douglas fir and hemlock thrive in the temperate rainforest, the carbon storage rate is about twice the global average.

Law looked at what would happen if the trees were allowed to grow for another 50 years without harvest. For the Coast Range, she found the amount of carbon storage on the ground could increase by 15 percent.

Mike Cloughesy, forestry director for the Oregon Forest Resources Institute and a member of the Carbon Forest Working Group, said there are two schools of thought: One believes the best way to store carbon is to grow trees as long as possible and store the maximum amount of carbon in the forest. Another argues forest management can maximize the increase in carbon storage by growing trees for 50 or 60 years, turn them into wood products and repeat that process.

On one hand, not cutting trees poses the risk of fire and creates the possibility that timber and wood products will have to come from a place with fewer environmental regulations. On the other, offering rewards to companies cutting trees for wood products risks a diminishing return of net carbon storage in the forests.

"My view, shared by a lot of people in forestry, is there's kind of room for both," Cloughesy said. "Both can work, but they have to be done carefully, with measuring and monitoring so you can show how much carbon we're storing."

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