Do species and functional groups differ in acquisition and use of C, N and water under varying atmospheric CO₂ and N availability regimes? A field test with 16 grassland species

Summary To evaluate whether functional groups have a similar response to global change, the responses to CO 2 concentration and N availability of grassland species from several functional groups are reported here. Sixteen perennial grassland species from four trait‐based functional groups (C 3 grasses, C 4 grasses, non‐leguminous forbs, legumes) were grown in field monocultures under ambient or elevated (560 µmol mol −1 ) CO 2 using free‐air CO 2 enrichment (FACE), in low N (unamended field soil) or high N (field soil +4 g N m −2 years −1 ) treatments. There were no CO 2 × N interactions. Functional groups responded differently to CO 2 and N in terms of biomass, tissue N concentration and soil solution N. Under elevated CO 2 , forbs, legumes and C 3 grasses increased total biomass by 31%, 18%, and 9%, respectively, whereas biomass was reduced in C 4 ‐grass monocultures. Two of the four legume species increased biomass and total plant N pools under elevated CO 2 , probably due to stimulated N‐fixation. Only one species markedly shifted the proportional distribution of below‐ vs aboveground biomass in response to CO 2 or N. Although functional groups varied in responses to CO 2 and N, there was also substantial variation in responses among species within groups. These results suggest that current trait‐based functional classifications might be useful, but not sufficient, for understanding plant and ecosystem responses to elevated CO 2 and N availability.