Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO₂ across four free-air CO₂ enrichment experiments in forest, grassland and desert

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 C₃ 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 (49.5-57.1Pa), 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 CO ₂ 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.