Warm-weather C4 plant species grew more in an elevated CO2 environment than cool-weather C3 plants.

Plants are responding in unexpected ways to increased carbon dioxide in the air, according to a 20-year study by researchers at the University of Minnesota and published in the journal Science.

The researchers planted 88 plots with two different types of grasses, warm-season C4 grasses and cool-season C3 grasses. They then exposed the grasses to different levels of carbon dioxide, current carbon dioxide levels and the elevated levels the Earth might experience later this century.

"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," says 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, warm-weather C4 plant species grew more in an elevated CO₂ environment than cool-weather C3 plants. While it's uncertain why this shift happened, these findings could have significant implications, the researchers say.

"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 says. "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.