Elevated CO2-induced increase in leaf photosynthesis across 13 grassland species is relatively modest, consistent and does not depend on soil N availability over 11 years of free-air CO2 enrichment
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.