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

Abstract The magnitude of changes in carboxylation capacity in dominant plant species under long‐term elevated CO 2 exposure (elevated pC a ) directly impacts ecosystem CO 2 assimilation from the atmosphere. We analyzed field CO 2 response curves of 16 C 3 species of different plant growth forms in favorable growth conditions in four free‐air CO 2 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 CO 2 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 CO 2 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 CO 2 in different species and plant growth forms.