Different treatments were applied to different plots. Some were given a single application, such as excess carbon dioxide gas, while others received various combinations of elevated carbon dioxide, heat, water and/or nitrogen fertilizer.

Initially, each plot contained between five and 20 varieties of grasses and wildflowers. The goal of the experiment was to see how different combinations of treatments would affect species diversity over a three-year period.

Diversity loss

The results were dramatic. Plots that received all four treatments lost more than one-fourth of their wildflower species, while those given elevated nitrogen or carbon dioxide suffered a 10 to 20 percent decline. However, plots treated with excess water experienced a 10 percent increase in wildflower diversity and a 3 percent gain in the number of annual grass species.

“We found that elevated carbon dioxide caused a loss in species, while added precipitation caused an increase. We were surprised they had such opposite effects,” said study co-author Christopher B. Field, a professor by courtesy of biological sciences at Stanford and director of the Carnegie Institution’s Stanford-based Department of Global Ecology. “One hypothesis is that elevated carbon dioxide added moisture to the soil, which tended to extend the growing season of the dominant plants, leaving less room for other species to grow.”

On the other hand, he noted, increasing precipitation by 50 percent may have encouraged growth in late-season plants that normally stop growing during the dry California summer: “We think the effects of elevated carbon dioxide and increased precipitation were more or less the same, but because they were separated in time by a couple of weeks, they actually produced opposite results. In our ecosystem here, things that happen at different times in the season are really important.”

The study also revealed that heat, in the absence of other treatments, had no significant impact on diversity. However, when experimental plots were exposed to higher temperatures along with excess nitrogen, carbon dioxide and water, the number of wildflower species plummeted.

“One take-home message of our study is that certain kinds of species are much more sensitive to climate and atmospheric changes than others,” Zavaleta observed.

“It turned out that wildflowers were much more sensitive to the treatments than grasses were, no matter what combination of treatments we tried,” she added, noting that a large-scale change in diversity could diminish the ability of grasslands to support birds, deer, butterflies and other wildlife — as well as commercial grazing.

Additive response

The researchers discovered that they could make remarkably accurate predictions of species diversity in plots where multiple treatments had been applied simply by adding up losses and gains observed under single treatments. For example, in quadrants receiving excess nitrogen, heat and carbon dioxide, wildflower diversity decreased by about 27 percent — almost exactly what would be expected if one added up the percentages of loss in quadrants given single treatments of carbon dioxide (18 percent), nitrogen (8 percent) and heat (2 percent).

“One possible reason we see this overall additive response is that the mechanisms that are driving the changes are not interacting,” Field said — a finding that could prove beneficial in forecasting how global environmental changes will affect plant diversity in other ecosystems.

“We hope to move into the domain where we can predict responses rather than just record them and report them,” he added.

Other coauthors of the PNAS study are Harold A. Mooney, the Paul S. Achilles Professor of Environmental Biology at Stanford; Nona R. Chiariello, research coordinator of the Jasper Ridge Biological Preserve; and M. Rebecca Shaw of the Nature Conservancy.

The study was supported by the National Science Foundation, the David and Lucile Packard Foundation, the Morgan Family Foundation, Jasper Ridge Biological Preserve, the Carnegie Institution of Washington, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the Switzer Foundation, the A.W. Mellon Foundation and the Nature Conservancy.