Carbon Studies Explain Role Forests Play

Kate Ramsayer from The (Bend) Bulletin
SISTERS, Ore.

In the ponderosa pine forest near Black Butte, a skinny scaffolding tower rises above the trees.From the top of the 100-foot-tall tower, the panoramic views of the butte and Three Sisters in the distance are some of the best in the area. But the instruments attached along the span of the tower are providing scientists with information about an unseen side of nature.

The equipment measures how much carbon dioxide is passing back and forth between the atmosphere and the trees, shrubs, soils and dead vegetation in the forest. It's one station in a nationwide network that, when the data is compiled, helps scientists investigate how much carbon dioxide forests across the continent remove from the atmosphere every year.And in this era of concern about global warming, it's information that interests researchers and forest managers alike.

For instance, the data from stations in Oregon has shown that forests take in the equivalent of between 30 percent and 50 percent of carbon released statewide annually in fossil fuel emissions.And through measuring this carbon exchange in the Metolius Basin and other types of forests, Bev Law and others hope to understand more about their role."We call it 'the breathing of the biosphere,'" said Law, a professor at Oregon State University in Corvallis and the science chairwoman for AmeriFlux, the network of more than 100 such carbon measuring sites across the United States and other countries.

Through their studies, the researchers can figure out when forests start acting as storage sites for carbon dioxide, a greenhouse gas, and how much of that gas is released into the atmosphere when forests burn or are cut down."The goal of the program is to be able to quantify and understand the role of terrestrial ecosystems in the global carbon balance," Law said.The Metolius Basin was the site of one of the first stations in the AmeriFlux program, which started in 1996. Law, who had previously done work across Oregon, chose the site because of its ponderosa pines."We were intrigued by ponderosa pine because it's very widely distributed globally, so it's an important species," she said.And by placing these towers and instruments near the Metolius, Law and her colleagues have been able to determine differences between young and old forests, between wet and dry years, and also contribute to regional studies of the movements of carbon dioxide.

The instruments collect information about the precise levels of carbon dioxide in the air, recording measurements as many as 20 times a second. But the equipment on the tower also gathers other kinds of information.The researchers are studying water vapor as well, to see how it relates to carbon dioxide levels, Law said, and are looking at different layers of wind speeds and temperatures through the forest canopy.

Wind speed is also measured because it can affect how much carbon dioxide can be picked up by the instruments, said Christoph Thomas, a research associate with OSU who works on the project.From all this information, researchers can get results about how much carbon the forest is absorbing or releasing.

Trees absorb carbon through photosynthesis, Law said, but they also release it through respiration.And then there's the rest of the organic matter in the forest, like soil, roots and vegetative litter being chewed up by microbes, which people sometimes forget about when they think of carbon cycles in forests, she said.

One question that scientists had been considering, she said, was how long it takes a new forest to go from being a carbon source, where it releases more carbon to the atmosphere than it takes in, to being a carbon sink, where the ecosystem absorbs more carbon than it releases.Through studies at the Metolius sites, Law and her colleagues have found that in a ponderosa pine forest that has been disturbed by fire or harvest, the length between those phases can be considerable."

It took those sites somewhere between 15 and 20 years before they became carbon neutral again," she said.From there, forests continue taking in increasing amounts of carbon until they're between about 70 and 150 years old, Thomas said. At that point, they still store more carbon than they release.But that doesn't mean older forests are no longer useful for storing carbon."People need to understand that these old-growth forests are very important," Thomas said.Studies in both Western and Eastern Oregon have shown that after a major disturbance in a mature forest, like a catastrophic fire or a clearcut, it could take between 100 and 200 years to get back the amount of carbon that had previously been stored in the ecosystem, Law said.

Information like this can be useful on multiple levels, Thomas said. It answers some scientific curiosity about such things as what happens in forests of different ages.But when information from across the AmeriFlux network is compiled, it can also be used to help answer larger-scale questions, he said, such as how much carbon dioxide forests across the continent remove from the atmosphere every year.

Determining when carbon is stored in trees and how much is stored there is a complex question, said Brian Tandy, silviculturist with the Sisters Ranger District.But it's something that people could start thinking about, he said, adding that he recently had a conversation about the possibility of setting up a carbon credit program, where people could reforest areas and get a credit for the carbon that would be absorbed by the trees in future years."It's a huge thing that I don't know anybody has a handle on," he said, "but there's lots of stuff to consider."