The role of stomatal acclimation in modelling tree adaptation to high CO2

Carbon dioxide enrichment changes the balance of photosynthetic limitations due to water, nitrogen, and light. This paper examines the role of stomata in these changes by comparing enrichment responses predicted by an optimality-based tree growth model, DESPOT, using three alternative 'setpoints' for stomatal acclimation: leaf water potential (psi(1)-setpoint), the ratio of intercellular to ambient CO2 mole fraction (c(i)/c(a)-setpoint), and the parameters in a simple model in which stomata are controlled by H2O and CO2 supply and demand (linked feedback). In each scenario, stomatal conductance (g(s)) and photosynthetic capacity (V-m) declined, productivity and leaf area index (LAI) increased, and c(i)/c(a) remained within 5% of its pre-enrichment value. Height growth preceded the LAI response in the psi(1)-setpoint and linked feedback scenarios, but not in the c(i)/c(a)-setpoint scenario. These trends were explained in terms of photosynthetic resource substitution using the equimarginal principle of production theory, which controls carbon allocation in DESPOT: enrichment initially increased the marginal product for light, driving substitution towards light; height growth also drove substitution towards N in the psi and feedback scenarios, but the inflexibility of c(i)/c(a) prevented that substitution in the c(i)/c(a) scenario, explaining the lack of height response. Each scenario, however, predicted similar behaviour for c(i)/c(a) and carbon and water flux. These results suggest that 'setpoints' may be robust tools for linking and constraining carbon and water fluxes, but that they should be used more cautiously in predicting or interpreting how those fluxes arise from changes in tree structure and physiology.