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Summary • Leaf gas exchange responses to elevated CO2 and N are presented for 13 perennial species, representing four functional groups: C3 grasses, C4 grasses, legumes, and nonleguminous forbs. Understanding how CO2 and N effects interact is important to predict plant community response to global change. • Plants were field-grown in monoculture under current ambient and elevated (560 µmol mol-1 ) CO2 concentrations (free-air CO2 enrichment), in combination with soil N treatments, for two growing seasons. • All species, regardless of functional group, showed pronounced photosynthetic acclimation to elevated CO2 , resulting in minimal stimulation of photosynthesis (A ) averaging +15% in C3 grasses, +8% in forbs, +7% in legumes and -2% in C4 grasses. The effects of CO2 and soil N supply did not interact for any leaf traits measured. Elevated CO2 consistently decreased stomatal conductance (g s ) leading to 40% increase in A /g s . • This substantial acclimation of photosynthesis was greater in magnitude than in most field studies, and was associated with the combined effects of decreased g s and decreased leaf N concentrations in response to growth under elevated CO2 . [ABSTRACT FROM AUTHOR]

Summary • 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 (C3 grasses, C4 grasses, non-leguminous forbs, legumes) were grown in field monocultures under ambient or elevated (560 µmol 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 × 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 C3 grasses increased total biomass by 31%, 18%, and 9%, respectively, whereas biomass was reduced in C4 -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. [ABSTRACT FROM AUTHOR]

Presents results of a grassland field experiment in Minnesota, which tests the hypothesis that plant diversity and composition influence the enhancement of biomass and carbon acquisition in ecosystems subjected to elevated atmospheric carbon dioxide concentrations and nitrogen deposition. Methods; Results which show that the enhanced biomass accumulation in response to these atmospheric changes is less in species-poor than in species-rich assemblages.