Group of Trees in Wis. Forced Into the Future

Experiment Bathes Aspens, Birches and Maples In Ozone and Carbon Dioxide to Study Changes

Trees in 12 rings are subjected to elevated levels of CO2, ozone or combined ozone and CO2. (David Karnosky)

RHINELANDER, Wis. -- There is something slightly eerie about the rings of white pipes and wooden crosses that loom up suddenly here in the lush northern forest.

So it seems only natural to hear the scientists who work here describe the rings, each more than 30 yards across, as "time capsules." They are using them to try to mimic the conditions scientists expect to see in nearly 50 years, when the atmosphere is likely to be filled with higher levels of carbon dioxide and ozone.

From May to October, pumps blow carbon dioxide, ozone and normal air from the 30-foot-high notched pipes that form the perimeter of each of the 12 rings, bathing the stands of young aspen, birch and sugar maples inside each ring in varying mixtures of the gases.

Three of the rings have elevated levels of ozone, three have elevated carbon dioxide, or CO _2, and three have high levels of both. The other three serve as controls, blowing normal air onto the trees, which were planted at the start of the experiment in 1997.

Most scientists say it is inevitable that levels of carbon dioxide and ozone will rise because of the burning of fossil fuels and vehicle emissions. What is not well known is how these changes will affect the ecosystem and the climate. This experiment, known as Aspen FACE, is trying to come up with an answer.

It aims to measure the various effects of elevated CO ₂ and ozone on northern forest ecosystems and, by extension, other ecosystems with similar vegetation. The project costs about $2 million a year to run and will likely continue for about a decade.

The Department of Energy, one of the sponsors of the project, runs a number of FACE (Free-Air Carbon Dioxide Enrichment) experiments around the country, but Aspen FACE is the only large-scale outdoor experiment, measuring the combined effects of ozone and CO ₂ . The experiment is also one of the few that operates outdoors in a realistic ecosystem, not indoors or in a covered chamber.

"As scientists, we tend to want to look at one thing at a time," said Art Chappelka, a professor of forest biology at Auburn University. "But every breath you take contains lots of things. These multiple-factor studies in a real world environment are very important."

Nancy Grulke, a plant physiologist with the Forest Service Pacific Southwest Research Station near Riverside, Calif., agreed.

"Chamber environments always seem to modify the environment of the plant and thus alter the plant's response," she said.

To keep the experiment going, huge canisters of liquid CO ₂ are delivered twice daily, and vaporizers on the premises heat the liquid until it turns to gas. An ozone generator fires powerful sparks -- which David Karnosky, the Michigan Technological University's Ecosystem Science Center professor spearheading the experiment, compares to lightning strikes -- through a supply of pure oxygen to rearrange the usual two-atom molecules into a three-atom form: ozone. At ground level, ozone is a pollutant that forms by the interaction of nitrogen oxides and hydrocarbon emissions with sunlight.

Stainless steel and copper pipes carry the gases to the designated rings to raise the concentration of carbon dioxide there to 560 parts per million -- 200 parts per million higher than the natural atmosphere here -- and ozone to 60 parts per billion, 1.5 times the usual level.

On the ground, wire baskets collect the "leaf litter" for analysis. Some branches are encased in black bags for studies of insect development. Other ground-level plants are draped in white netting knotted on top, making them look like squadrons of little ghosts. Aluminum foil covers electrodes on trunks that measure sap flow. White tubes pounded into the ground have miniature cameras that snap time-lapse photos of root growth and turnover.

After seven years, the rings of trees exposed to elevated CO ₂ are noticeably taller and leafier than the other groups. The ground is also relatively barren in these plots, because little light breaks through the rich leafy canopies. It is not surprising that CO ₂ increases plant growth, because it is a crucial component of plant respiration.