Group of Trees in Wis. Forced Into the Future

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

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 CO ₂ , look fairly similar, with moderate tree size and weed densities.

In a nutshell, the excess CO ₂ "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 CO ₂ 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 CO ₂ 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 CO ₂ 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."