Photosynthetic responses of 13 grassland species across 11 years of free‐air CO 2 enrichment is modest, consistent and independent of N supply

If long‐term responses of photosynthesis and leaf diffusive conductance to rising atmospheric carbon dioxide (CO 2 ) levels are similar or predictably different among species, functional types, and ecosystem types, general global models of elevated CO 2 effects can effectively be developed. To address this issue we measured gas exchange rates of 13 perennial grassland species from four functional groups across 11 years of long‐term free‐air CO 2 enrichment ( e CO 2 , +180 ppm above ambient CO 2 ) in the BioCON experiment in Minnesota, USA. Eleven years of e CO 2 produced consistent but modest increases in leaf net photosynthetic rates of 10% on average compared with plants grown at ambient CO 2 concentrations across the 13 species. This e CO 2 ‐induced enhancement did not depend on soil N treatment, is much less than the average across other longer‐term studies, and represents strong acclimation (i.e. downregulation) as it is also much less than the instantaneous response to e CO 2 . The legume and C3 nonlegume forb species were the most responsive among the functional groups (+13% in each), the C4 grasses the least responsive (+4%), and C3 grasses intermediate in their photosynthetic response to e CO 2 across years (+9%). Leaf stomatal conductance and nitrogen content declined comparably across species in e CO 2 compared with ambient CO 2 and to degrees corresponding to results from other studies. The significant acclimation of photosynthesis is explained in part by those e CO 2 ‐induced decreases in leaf N content and stomatal conductance that reduce leaf photosynthetic capacity in plants grown under elevated compared with ambient CO 2 concentrations. Results of this study, probably the longest‐term with the most species, suggest that carbon cycle models that assume and thereby simulate long‐lived strong e CO 2 stimulation of photosynthesis (e.g.> 25%) for all of Earth's terrestrial ecosystems should be viewed with a great deal of caution.