However, adding just ozone decreased aspen and birch growth by 20 percent to 26 percent. The growth of aspen and birch when both gases were elevated was no different than when neither gas was added. Young sugar maple trees grew at the same rate in all the rings.
Although some aspen are now more than 20 feet tall, researchers are cautious about projecting their results to full-grown trees. However, they suspect that the growth-promoting effects of carbon dioxide will lessen and the negative effects of ozone will increase as the trees mature.
Because insects and soil processes affect the growth and survival of forest trees, the researchers are also examining those aspects of the forest community. Lindroth says that both the performance of insects, as indicated by their size, and the decay of leaf litter are influenced by the chemical composition of the leaves. And this, in turn, is affected by the carbon dioxide and ozone treatments.
Lindroth says the findings are complex because different insect and tree species respond differently. For example, forest tent caterpillars, a major outbreak species in northern Wisconsin, grew larger under the high ozone treatment than in the reference areas. However, ozone didn't affect other insect species that same way.
The researchers have found that leaves from birch trees grown under high carbon dioxide decay more slowly after they fall. Trees depend on the availability of nutrients in fallen leaves to be recycled for future growth. In the long-term, slow decay rates may tie up nutrients and slow tree growth, according to Lindroth. Unlike birch leaves, however, aspen leaves decay at the same rate regardless of the level of carbon dioxide and ozone.