Forests might not be the panacea for global warming that many had hoped, a new study hints. The same power plants and vehicles responsible for rising levels of carbon dioxide contribute to ozone pollution too, and the new research finds that ozone aids insects and diseases that stunt the growth of trees and reduce their ability to soak up carbon dioxide (CO2).
Many scientists have suggested that increasing forest cover might counteract global warming. Because plants bulk up by consuming CO2, they suggest, rising levels of the gas should spur plant growth, leading to bigger plants with even greater appetites for CO2. But that scenario ignores insects and pathogens that might slow plant growth, as well as the effects of other gases that are also on the rise.
A team of more than 50 scientists from several countries has been pursuing these questions at a huge outdoor facility in Wisconsin. Researchers at this "free-air carbon dioxide enrichment" (FACE) site can control the mix of gases pumped over stands of aspens, the most widely distributed trees in North America. The results of 4 years of experiments are reported in the 28 November issue of Nature by Kevin Percy of Natural Resources Canada in Fredericton, New Brunswick, and 13 co-authors.
By pumping air enriched in both carbon dioxide and ozone together (as well as separately) the team was able to study the interactive effects of these gases. Carbon dioxide alone boosted trees' chemical defenses against insects, but when ozone was added to the mix, it had the opposite effect. Ozone also allowed a rust fungus to attack aspen leaves more easily. And it helped enlarge populations of sap-sucking aphids while depressing numbers of insects that prey on or parasitize aphids. Although the impacts on other pests were more complex, the combined effects of ozone and the pests decreased tree growth about as much as carbon dioxide stimulated it.
Tackling interactions between the two gases and looking at insects and their predators at such a large scale in a natural setting is unprecedented, ecologists agree. The study is "an important first step in determining how ecosystems will react to climate change," says entomologist John Trumble of the University of California, Riverside.
The Aspen FACE site in Wisconsin
Other FACE sites
USDA fact sheet on carbon sequestration and agriculture
Third report of the Intergovernmental Panel on Climate Change, which recommended attention to forests as one way of addressing greenhouse gas buildup