Modelling a spring wheat crop under elevated CO₂ and drought

Summary The simulation model DEMETER was used here to investigate which mechanisms led to a larger CO 2 effect on biomass production and yield of a spring wheat crop under drought compared with unlimited water supply. Field data of the free‐air CO 2 enrichment (FACE) wheat experiments in Arizona (1993–94) were used to test the model. The influence of a particular mechanism leading to a higher CO 2 effect under drought was investigated by eliminating the influence of the other causes on the simulation results on selected days during the growing seasons. A larger CO 2 effect under drought was caused in the model by the lower potential transpiration rate, higher root biomass and the nonlinear functional dependence of net assimilation rate on leaf internal CO 2 concentration. The contribution of the different mechanisms changed in significance during the growing season depending on the degree of soil water limitation. The model successfully described the qualitative and quantitative behaviour of the crop under elevated CO 2 . A well‐tested simulation model can be a useful tool in understanding the complex interactions underlying observed ecosystem responses to stress under elevated CO 2 .