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

To evaluate whether functional groups have a similar response to global change, the responses to CO2 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 mu mol mol(-1)) CO2 using free-air CO2 enrichment (FACE), in low N (unamended field soil) or high N (field soil +4 g N m(-2) years(-1)) treatments. There were no CO2 x N interactions. Functional groups responded differently to CO2 and N in terms of biomass, tissue N concentration and soil solution N. Under elevated CO2, 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 CO2, probably due to stimulated N-fixation. Only one species markedly shifted the proportional distribution of below-vs aboveground biomass in response to CO2 or N. Although functional groups varied in responses to CO2 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 CO2 and N availability.