RALEIGH, N.C. &mdash In a corner of Duke Forest, rings of white towers rise above the green canopy of the pine forest like futuristic monuments.
Day and night, thousands of pounds of odorless carbon-dioxide gas spews from the towers in computer-controlled streams. The goal is to simulate a forest with greenhouse gas levels that approximate the levels predicted by 2050.
For 13 years, researchers have been using the Blackwood section of Duke Forest near Chapel Hill as a laboratory to explore whether forests will grow fast enough in the future to help control predicted increases of carbon dioxide, the primary greenhouse gas. Trees absorb carbon dioxide through tiny pores in their leaves and store carbon in wood, bark, leaves and soil.
The findings from Duke Forest are mixed. Loblolly pines do grow faster with elevated carbon dioxide, and there's potential for forests to soak up more carbon and convert it to wood. But the prospect for extra growth is limited in some regions by poor soil and lack of rainfall, which might constrain the forests' growth.
"If anybody expected forests to resolve our problems with elevated carbon dioxide, they will be disappointed," said Ram Oren, a Duke University professor and principal investigator at the research site. "We make tremendously more carbon dioxide than can be sequestered by natural means."
What makes carbon dioxide a focus of intense study, international treaties and even TV commercials about carbon footprints is its potent ability to trap heat. Scientists think the buildup of carbon dioxide is causing the planet to grow warmer, though the rate at which the temperature will rise remains a topic of debate.
Carbon dioxide is produced by ordinary human activities such as tilling a garden, building a campfire, even breathing. It is also produced by extraordinary natural events such as volcanic eruptions and forest fires. But the main contributors to higher carbon-dioxide levels are the burning of fossil fuels and the clearing of land.
Scientists have chronicled an increase in the concentrations of carbon dioxide and other heat-trapping greenhouse gases since the 1700s, as humans began building machines and factories that burned fossil fuels.
As a result, carbon dioxide levels in the atmosphere today are about 35 percent higher than before the start of the Industrial Revolution, scientists say. Current carbon-dioxide levels are higher than at any time in the past 25 million years and are expected to increase an additional 50 percent by 2050, based on increasing population and emissions.
The Duke researchers chose a loblolly pine forest because the trees thrive throughout the Southeast and grow quickly. After more than a decade, the results from Duke Forest are complex and sometimes surprising.
The trees in the plots with elevated levels of carbon dioxide are about 3 feet taller on average than trees in plots that are not bathed with carbon, said Jeff Pippen, a member of the research staff at Duke University's Nicholas School of the Environment and Earth Sciences. Pippen has tracked the growth of about 1,000 trees outfitted with spring-loaded metal belts that measure the trees' girths. As the trees grow, the belts expand like elastic waistbands.
"They are growing 10 to 15 percent more per year," Pippen said.
But the gains in height and girth vary widely even among trees exposed to more carbon dioxide. Scientists were surprised by that, but suspect it has to do with soil fertility. If so, forests growing in poor soils are unlikely to provide the same level of increased carbon storage. Scientists are studying why.
"The sense is soil fertility controls this variation," Oren said. "But to demonstrate it, you need to manipulate soil fertility with fertilizer and observe the response."
The Duke site, which was built for about $2 million, costs $2.5 million a year to operate. It is one of four research projects funded by the U.S. Department of Energy examining ecosystem response to elevated carbon levels. Other sites are in east Tennessee, Wisconsin and Nevada.
The faster growth might be a mixed blessing. Not only do pine trees grow faster, but plants and noxious weeds such as poison ivy thrive as well.
Shannon LaDeau, a research associate at the Smithsonian Institution who did graduate work at Duke, found that the trees exposed to higher levels of carbon dioxide produced twice as many seed cones during a six-year period and that more trees produced pollen - the fine yellow dust that coats cars and porches each spring.
And that might mean less carbon absorption. Here's why: If trees grow bigger trunks that convert carbon to wood fiber and deeper roots that flush the carbon into the soil, that represents a long-term storage of carbon. But if the extra growth concentrates in more pine needles, cones and fine roots, they simply rot on the forest floor, returning carbon to the air.
Oren said that forests might absorb some carbon dioxide but that reducing emissions would be far more effective. "It's going to be a whole bunch of small solutions that come together to solve a big problem," he said.