If long-term responses of plant photosynthesis to rising atmospheric carbon dioxide (CO₂) levels are similar or predictably different among species, functional types, and ecosystem types, general global models of CO₂ fertilization effects can effectively be developed. To address this issue we measured gas exchange rates of 13 perennial grassland species from four functional groups exposed to eleven years of long-term free-air CO₂ enrichment (eCO₂, +180 ppm above ambient CO₂, BioCON). Eleven years of CO₂ enrichment produced consistent but modest increases in leaf net photosynthetic rates of 10% on average compared to plants grown at ambient CO₂ concentrations across the 13 species. This eCO₂-induced enhancement did not depend on soil N treatment and is much less than the average across other longer term studies. Species responded similarly to CO₂ concentration across most parameters and when responses to eCO₂ by species did vary, it was not explained by functional groupings. Related parameters such as leaf stomatal conductance and nitrogen content declined comparably across species in elevated compared to ambient CO₂ and to degrees corresponding to results from other studies. This significant acclimation of photosynthesis is explained in part by declines in leaf N and stomatal conductance. eCO₂-induced changes in photosynthesis were documented over a longer term and for more species than in most other studies, thus providing novel insights into important long-term ecosystem responses.