Assessment of past, present, and future impacts of ozone and carbon dioxide on crop yields

To estimate potential O3-induced crop losses during the last nine years (1984–1992) in the state of Hessen, Germany, we calculated exposure statistics from statewide records of surface O3 concentrations and used these data as input for published O3 exposure-response functions of several major crops. Estimated potential yield losses for a “background level” of O3, which roughly reflects pre-industrial conditions, were 0.1% (barley and an O3-tolerant winter wheat cultivar in 1984) in comparison to 32.6% (an O3-sensitive spring wheat cultivar in 1992) under present-day conditions, with an increasing trend in response since 1984. However, available O3 exposure-response functions were derived from experiments with controlled O3 concentrations, but generally under present-day CO2 supply. Since current CO2 concentrations exceed pre-industrial levels by about 26%, a potential CO2 fertilization effect has to be considered, when assessing crop losses to ozone relative to pre-industrial atmospheric conditions. For two spring wheat cultivars a data set from different experiments was available, allowing us to account for CO2 fertilization and O3 effects simultaneously assuming a linear-additive crop response to CO2 and O3. The model equations were subsequently used to describe the time course of relative yield performance of these wheat cultivars since 1900 as a function of different scenarios of CO2 and O3 in the atmosphere. Based on these results, the implications for the design of current experimental studies on crop responses to CO2 and O3 are discussed.