Plants are responding in unexpected ways to increased carbon dioxide in the air, according to a twenty-year study conducted by researchers at the University of Minnesota and published in the journal Science. For the first 12 years, researchers found what they expected regarding how different types of grasses reacted to carbon dioxide. However, researchers' findings took an unanticipated turn during the last eight years of the study.

Researchers planted 88 plots with two different types of grasses, warm-season C 4 grasses and cool-season C 3 grasses, and exposed them to different levels of carbon dioxide, current carbon dioxide levels and the elevated levels the Earth might experience later this century due to human activity.

"Because carbon dioxide is needed by plants to grow, we expected grasses that have the C 3 photosynthetic pathway to grow more under elevated CO 2 , because these plants are known to be able to increase their CO 2 capture as CO 2 levels rise. We also expected that growth of grasses with the C 4 photosynthetic pathway would not be affected by higher CO 2 levels, because these plants are generally less able to capture extra CO 2 as CO 2 levels rise," said University of Minnesota Professor Peter Reich. "While that held true for the first dozen years, that pattern changed."

Researchers found that during the last eight years of the study, C 4 plant species grew more in an elevated CO 2 environment than C 3 plants. While it's uncertain why this shift happened, these findings could have significant implications.

"If mature grasslands worldwide behave like our experiment did, this could have long lasting impacts on how we think about the conservation and restoration of grasslands around the world," Reich said. "Grasslands cover between 30 and 40 percent of land and play a key role in soaking up carbon dioxide released by burning fossil fuels."

Along with impacts on conservation and restoration planning, these data could be used to help computer models better predict how plants will respond to changing CO 2 concentrations in the atmosphere.

"Our results suggest that the predictions made by these models might not be quite right and that we should not be overly confident about our assumptions regarding where, and by how much, land ecosystems will keep absorbing extra CO 2 out of the air," Reich said.

Reich, a professor with the College of Food, Agricultural and Natural Resource Sciences' (CFANS) Department of Forest Resources and Institute on the Environment (IonE) fellow, was the lead researcher on the study. Other study investigators included Professor Sarah Hobbie and graduate student Melissa Pastore, with the Department of Ecology, Evolution and Behavior in the College of Biological Sciences, and Professor Tali Lee from the University of Wisconsin, Eau Claire.