Study shows how pollution affects tree growth

As trees leafed out this spring, an international group of researchers is headed to northern Wisconsin to continue a long-term study that is revealing how air pollution affects northern forests.

The study also is providing insights into the role forests may play in global climate change.

The scientists are finding that carbon dioxide and ozone dramatically alter tree growth, says Eric Kruger, a UW–Madison forest ecologist participating in the project. The gases also may change forest ecology and diversity in the long term.

Carbon dioxide is increasing around the world. It is the gas primarily responsible for the “greenhouse” effect and global warming. Ozone levels also are rising. Elevated ozone levels are now common across much of the eastern United States. Increases in both gases can be traced to our reliance on fossil fuels.

Experts predict that concentrations of these gases will double in the next 100 years, with high ozone levels spreading over much of Wisconsin. The forest scientists are studying how quaking aspen, paper birch and sugar maple — major components of the forests that blanket almost half of Wisconsin — will respond to the levels of carbon dioxide and ozone expected in the north by 2050.

Scientists already know that carbon dioxide acts like a fertilizer, stimulating plant growth, while ozone harms plants. But they didn’t know what would happen with both gases in a realistic, long-term study.

“Our results have been remarkably consistent,” says Kruger. “They show that high carbon dioxide increases the growth of young aspen and birch, high ozone decreases their growth, and the gas’s effects on growth cancel each other out when both are elevated.”

“Before this study, our understanding of how carbon dioxide and ozone affected trees was based largely on exposing small plants to one or another of the gases for a short time in a greenhouse or growth chamber,” Kruger says. “No one had conducted long-term experiments on trees in natural settings.”

UW–Madison scientists have joined more than 40 university and government researchers who are testing how carbon dioxide and ozone — acting alone and in combination — affect forests. The study now taking place on 80 acres of U.S. Forest Service land near Rhinelander includes more than 10 scientists from three departments in the university’s College of Agricultural and Life Sciences.

The effects on tree growth are particularly important because some people argue that forests will lessen the threat of global warming by soaking up carbon dioxide like a sponge and holding it in trees and soils. However, that view overlooks the impact of rising ozone concentrations.

“There are about a dozen major studies around the world looking at how an increase in carbon dioxide will affect different ecosystems,” says Richard Lindroth, a UW–Madison insect ecologist. “But this study is the only one in which plants are exposed to both carbon dioxide and ozone.”

The Rhinelander site features 12 widely spaced rings, each nearly 100 feet in diameter. After preparing the entire site in 1997, researchers planted seedling aspen, birch and maple in identical patterns in each ring. Sensors measure the carbon dioxide and ozone concentrations in each ring and computers control the levels by releasing more or less of the gases from vertical pipes that encircle each ring.

The carbon dioxide concentration of the air in three of the 12 rings is maintained at 560 parts per million — about 55 percent higher than the background level. Three rings have an ozone concentration 50 percent higher than the background ozone level. And three rings have elevated levels of both carbon dioxide and ozone. There is no carbon dioxide or ozone added to the air in the final three rings, which serve as a reference for comparisons.

The initial results show that aspen and birch grew 20 percent to 28 percent faster with elevated carbon dioxide than they did in the reference area. The trees also produced more leaves and more fine roots, both of which they shed in fall, according to Kruger.

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.

“We’re finding that one species responds in a certain way to elevated levels of these gases and a second species responds in a different way,” Lindroth says. According to Kruger, certain aspen clones also tolerate high levels of the gases better than others. The researchers say the results suggest that increasing levels of ozone and carbon dioxide could change the makeup and biological diversity of northern forests. Only more research can provide definitive answers.

“We hope to continue the experiment for at least 15 years to determine the impact the gases have in the long term on the trees themselves and the larger ecological community,” Lindroth says.

Although many agencies support research at the Rhinelander site, the UW–Madison scientists there are supported by state funding to the College of Agricultural and Life Sciences, and by grants from the Department of Energy and the National Science Foundation.