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The magnitude of changes in carboxylation capacity in dominant plant species under long-term elevated CO2 exposure (elevated pC(a)) directly impacts ecosystem CO2 assimilation from the atmosphere. We analyzed field CO2 response curves of 16 C-3 species of different plant growth forms in favorable growth conditions in four free-air CO2 enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO2 assimilation (A) by +40+/-5% in elevated pC(a) (49.5-57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC(a) in some species. Photosynthesis at a common pC(a) (A(a)) was significantly reduced in five species growing under elevated pC(a), while leaf carboxylation capacity (V-cmax) was significantly reduced by elevated pC(a) in seven species (change of -19+/-3% among these species) across different growth forms and FACE sites. Adjustments in V-cmax with elevated pC(a) were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC(a) treatment did not affect the mass-based relationships between A or V-cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC(a) on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC(a) effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC(a) at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO2 responses can only be measured experimentally on a small number of species, understanding elevated CO2 effects on canopy N-m and N-a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO2 in different species and plant growth forms.