Leaf gas exchange responses of 13 prairie grassland species to elevated CO₂ and increased nitrogen supply

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.