Plant-plant interactions and soil moisture might be important in determining ozone impacts on grasslands

Ozone effects on plant species mixtures could depend on the characteristics of the species involved, their mixing ratio, or on environmental condition s. Predicting long-term effects on the dynamics of plant communities requir es an understanding of the interactions involved. The present experiment wa s designed to determine the effects of ozone on grassland species in relati on to mixing ratio and soil water content (irrigation) using binary mixture s. The grass Trisetum flavescens was grown in potted replacement-series mix tures with Centaurea jacea (Experiment A) or Trifolium pratense (Experiment B). The plants were exposed to three concentrations of ozone in open-top c hambers in two irrigation treatments. Total above-ground dry weight over th ree growth periods was measured. The competitive ability of T. flavescens w as expressed as the competitive ratio (CRT). In Experiment B, total above-g round dry weight was reduced by elevated ozone and by reduced soil moisture , and significant interactions were found for ozone x irrigation and ozone x ratio. In Experiment A these effects were not significant. Under well wat ered conditions, CRT tended to be reduced by elevated ozone in Experiment A , but increased significantly in Experiment B, indicating the importance of the competing species in modifying the ozone effect on T. flavescens. In b oth experiments reduced irrigation decreased the magnitude of ozone effects on biomass production, which could be related to observed reductions in sp ecific leaf conductance. The results suggest that under well watered condit ions the effect of elevated ozone on the competitive balance between specie s depends on the species mixture, but that the mixing ratio is less importa nt.