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 CO2, 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 CO2 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 CO2. 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 CO2 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 CO2 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 CO2 increases plant growth, because it is a crucial component of plant respiration.

The trees exposed to elevated levels of ozone, by contrast, are stubby and sparse, and the ground is covered by a thick mesh of brush several feet tall.

The control rings, and those with both elevated ozone and CO2, look fairly similar, with moderate tree size and weed densities.

In a nutshell, the excess CO2 "fertilizes" the trees and speeds their growth while the ozone stunts and inhibits them.

"Ozone is highly oxidative; it reacts quickly . . . to break down the cell walls and chlorophyll," Karnosky said. "It causes premature leaf drop."

In combination, the two gases cancel each other out to some extent, a finding that has eased fears of massive forest destruction. But sugar maple trees show decreased growth and reduced ozone resistance -- negative effects -- in the excess CO2 rings.

"We're seeing some evidence of the canopy changing," Karnosky said. "Those canopy changes could affect the micro-meteorology of the whole forest."

Besides causing trees to have shorter, narrower trunks and thinner, sparser leaves, the ozone also appears to change the leaf surface, degrading the waxy micro-structures that are the first line of defense against insects and diseases.

Early results also indicate that while elevated CO2 helps the trees' growth in the short term, it might also lead to an increase in pests that could be harmful in the long run. Extra CO2 also seems to be more conducive to a harmful fungus, called venturia, that causes leaves to turn black and drop.

"It's a very complex picture," added Neil D. Nelson, a plant physiologist who oversees the site for the U.S. Forest Service.

Along with studying the effects of climate change on trees and forests, the project aims to explore ways forests can be used to reduce atmospheric carbon dioxide by sequestering carbon.

The researchers are measuring how much carbon is sequestered in the trees' leaves and root systems and whether trees grown under elevated carbon levels are better carbon "sinks."

Though the project has no specific policy aims, the results likely will be used in formulating national environmental policy, including setting emissions standards and clean-air goals.

Some results already are part of federal "ozone criteria documents," a body of peer-reviewed literature used to determine whether current ozone standards are adequately protecting vegetation and human health.

"We're not in the policy game, but this could have important implications for policy," Nelson said. "We let the data speak for itself."

Trees in 12 rings are subjected to elevated levels of CO2, ozone or combined ozone and CO2.Rings in the Aspen FACE experiment include groups of aspen, birch and maple, planted at the start of the experiment in 1997.Student Angela Johansen monitors CO2 efflux from the soil in one of the rings. In a ring with elevated C02, the trees are leafier and the ground is more barren.