Grassland plants react unexpectedly to high levels of carbon dioxide

Researchers planted 88 plots with two different types of grasses, warm-season C₄ grasses and cool-season C₃ 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 C₃ photosynthetic pathway to grow more under elevated CO₂, because these plants are known to be able to increase their CO₂ capture as CO₂ levels rise. We also expected that growth of grasses with the C₄ photosynthetic pathway would not be affected by higher CO₂ levels, because these plants are generally less able to capture extra CO₂ as CO₂ 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, C₄ plant species grew more in an elevated CO₂ environment than C₃ 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 CO₂ 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 CO₂ 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.