Modelling a spring wheat crop under elevated CO2 and drought

The simulation model DEMETER was used here to investigate which mechanisms led to a larger CO2 effect on biomass production and yield of a spring whea t crop under drought compared with unlimited water supply. Field data of the free-air CO2 enrichment (FACE) wheat experiments in Arizo na (1993-94) were used to test the model. The influence of a particular mec hanism leading to a higher CO2 effect under drought was investigated by eli minating the influence of the other causes on the simulation results on sel ected days during the growing seasons. A larger CO2 effect under drought was caused in the model by the lower pote ntial transpiration rate, higher root biomass and the nonlinear functional dependence of net assimilation rate on leaf internal CO2 concentration. The contribution of the different mechanisms changed in significance during th e growl ng season depending on the degree of soil water limitation. The mod el successfully described the qualitative and quantitative behaviour of the crop under elevated CO2. A well-tested simulation model can be a useful tool in understanding the co mplex interactions underlying observed ecosystem responses to stress under elevated CO2.