EFFECTS OF CLIMATE-CHANGE ON PRODUCTIVITY OF CEREALS AND LEGUMES, MODEL EVALUATION OF OBSERVED YEAR-TO-YEAR VARIABILITY OF THE CO2 RESPONSE

The effect of elevated [CO2] on the productivity of spring wheat, wint er wheat and faba bean was studied in experiments in climatized crop e nclosures in the Wageningen Rhizolab in 1991-93. Simulation models for crop growth were used to explore possible causes for the observed dif ferences in the CO2 response. Measurements of the canopy gas exchange (CO2 and water vapour) were made continuously from emergence until har vest. At an external [CO2] of 700 mu mol mol(-1), Maximum Canopy CO2 E xchange Rate (CCFRmax) at canopy closure was stimulated by 51% for spr ing wheat and by 71% for faba bean. At the end of the growing season, above ground biomass increase at 700 mu mol mol(-1) was 58% (faba bean ), 35% (spring wheat) and 19% (winter wheat) and the harvest index did not change. For model exploration, weather data sets for the period 1 975-88 and 1991-93 were used, assuming adequate water supply and [CO2] at 350 and 700 mu mol mol(-1). For spring wheat the simulated respons es (35-50%) were at the upper end of the experimental results. In agre ement with experiments, simulations showed smaller responses for winte r wheat and larger responses for faba bean. Further model explorations showed that this differential effect in the CO2 response may not be p rimarily due to fundamental physiological differences between the crop s, but may be at least partly due to differences in the daily air temp eratures during comparable stages of growth of these crops. Simulation s also showed that variations between years in CO2 response can be lar gely explained by differences in weather conditions (especially temper ature) between growing seasons.