Ozone: Bad for Trees, Good for What Eats Them

The trees of the future may be much more vulnerable to a variety of pests, say scientists studying greenhouse gases in northern Wisconsin forests. Their work is published in the Nov. 28 edition of the journal Nature.

Researchers in the Aspen FACE (Free-Air Carbon Dioxide Enrichment) Experiment, based in Rhinelander, Wis., have been measuring the effects of elevated levels two greenhouse gases, carbon dioxide and ozone, on aspen forest ecosystems. While the trees, Populous tremuloides (trembling aspen), seem to do relatively well in a carbon dioxide-rich atmosphere, ozone is another story.

Trees growing in an ozone-enriched atmosphere have been hit much harder by their traditional enemies: forest tent caterpillars, aphids and the rust fungus Melampsora.

"This has been a surprise," said Professor David Karnosky of Michigan Technological University's School of Forest Resources and Environmental Science, a principal investigator on the Aspen FACE project. "Our experiment was never meant to look at pest occurrence. But it became obvious that the greenhouse gases were affecting the abundance of pests."

Ozone seems to be a special blessing to aphids. Not only did the tiny insects thrive in high-ozone air, populations of aphids' traditional predators--such as ladybugs and spiders--plummeted. "The aphids had free rein," noted Caroline Awmack, an Aspen FACE researcher from the University of Wisconsin, Madison, Department of Entomology.

Studies have shed some light on why the aspen growing in ozone-rich air were turning into so much bug salad: their leaves seem to be undergoing fundamental changes. "Ozone alters the surface waxes," said Kevin Percy, a research scientist with Natural Resources Canada--Canadian Forest Service who is the lead author of the Nature article, "Altered Performance of Forest Pests under Atmospheres Enriched by C02 and O3."

The number of aphids increased about five-fold in plots with elevated ozone, while the number of aphid predators was cut in half. In plots with elevated levels of both carbon dioxide and ozone, the aphid population tripled, while the number of natural enemies increased slightly, mitigating the aphids' effect on the aspen.

Melampsora infection in the control and CO2-enriched plots was about the same, but increased about 400 percent in the O3 plots and doubled in the plots with extra CO2 and O3. The number of forest tent caterpillars increased by about one-third in the O3 plots and actually decreased slightly in the CO2 plots and the plots with extra CO2 and ozone.

The Aspen FACE Experiment, which involves 11 institutions and 28 researchers, is funded jointly by the Department of Energy's Office of Biological and Environmental Research, the National Science Foundation, the U.S. Forest Service Global Change Program, the U.S. Forest Service North Central Research Station, Michigan Tech, the USDA National Research Initiative Program, Brookhaven National Laboratory and Natural Resources Canada. The FACE system is designed by George Hendrey and his Brookhaven National Lab team.

Professor Kurt Pregitzer, another FACE researcher from Michigan Tech, calls Aspen FACE "a window into the future."

"We're beginning to understand how the changing atmosphere of the Earth is going to impact forests and the interactions that control the growth of trees, the cycling of energy and nutrients, and the movement of water through ecosystems," he said.

"This particular paper points out how important understanding the interactions among plants and insects are in controlling forest growth and forest health.

"We have a lot to learn."