Increasing CO from subambient to superambient concentrations alters species composition and increases above-ground biomass in a C /C grassland.

•  The glacial-to-present increase in atmospheric CO concentration is likely to have stimulated plant production, but experimental tests in natural ecosystems are lacking. •  We measured above-ground biomass production, plant nitrogen (N) accumulation, and species dynamics in a C /C grassland exposed for 4 yr (1997-2000) to a continuous gradient in CO from 200-560 mol mol . •  Biomass increased with CO concentration in 1997-99. Biomass increases ranged between 121 and 161 g m per 100 mol mol rise in CO and were similar at subambient and superambient concentrations. Biomass responses to CO were determined by different species or functional groups of species during different years. Increasing CO accelerated a successional shift initiated by release from grazing in which C forbs increased at the expense of a C grass. Effects of CO on tissue N concentration varied among species and functional groups, but CO did not alter total N in above-ground tissues. •  Results imply that rising CO has stimulated plant production and accelerated successional change and that grasslands will remain sensitive to rising CO for several decades.