Consistent responses of vegetation gas exchange to elevated atmospheric CO₂emerge from heuristic and optimization models

Abstract. Elevated atmospheric CO2 concentration is expected to increase leaf assimilation rates, thus promoting plant growth and increasing area. It also decreases stomatal conductance, allowing water savings, which have been hypothesized drive large-scale greening, in particular arid semiarid climates. However, the area could reduce benefits of elevated through soil depletion. The net effect on leaf- canopy-level gas exchange remains uncertain. To address this question, we compare outcomes a heuristic model based Partitioning Equilibrium Transpiration Assimilation (PETA) hypothesis three variants optimization theory. Predicted relative changes rates are used as metric responses concentration. Both approaches predict reductions leaf-level transpiration rate due decreased conductance under CO2, but negligible or no (optimization) compensatory increased Leaf- predicted increase, with an amplification fertilization at canopy level enhanced vapour pressure deficit (VPD) warmer conditions generally decrease sensitivity both models. consistent predictions by different models that varies little combined reduction highlight coordination physiological morphological characteristics vegetation maximize resource use (here water) altered climatic conditions.