Photosynthetic responses of 13 grassland species across 11 years of free-air CO₂ enrichment is modest, consistent and independent of N supply

If long-term responses of photosynthesis and leaf diffusive conductance to rising atmospheric carbon dioxide (CO₂) levels are similar or predictably different among species, functional types, and ecosystem types, general global models of elevated CO₂ 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 CO₂ enrichment (eCO₂, +180ppm above ambient CO₂) in the BioCON experiment in Minnesota, USA. Eleven years of eCO₂ produced consistent but modest increases in leaf net photosynthetic rates of 10% on average compared with plants grown at ambient CO₂ concentrations across the 13 species. This eCO₂-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 eCO₂. 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 eCO₂ across years (+9%). Leaf stomatal conductance and nitrogen content declined comparably across species in eCO₂ compared with ambient CO₂ and to degrees corresponding to results from other studies. The significant acclimation of photosynthesis is explained in part by those eCO₂-induced decreases in leaf N content and stomatal conductance that reduce leaf photosynthetic capacity in plants grown under elevated compared with ambient CO₂ 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 eCO₂ stimulation of photosynthesis (e.g.> 25%) for all of Earth's terrestrial ecosystems should be viewed with a great deal of caution.