Relating ambient ozone concentrations to adverse biomass responses of white clover: a case study

A white clover model system (consisting of O-3-sensitive [NC-S] and tolerant [NC-R] clones) offers an opportunity to establish cause (temporally dynamic O-3 exposures) and effect (temporally variable, chronic plant biomass responses) relationships under chamberless, ambient conditions on a regional scale. However, our understanding of the performance of the system (ambient O-3 exposures and the responses of NC-S:NC-R) requires improvement. For example, there are residual carry-over effects in the biomass responses of NC-S:NC-R between successive harvests in a given growth season. However, there were no clear increasing or decreasing trends in these ratios from harvest to harvest during a given growth season. Based on observed differential, perhaps random variations in the biomass responses of NC-S:NC-R, one may conclude that growth regulating factors other than the ambient O-3 were responsible for the observed variability. This is supported by the results of the linear regression analysis between the O-3 exposure potentials (O-3 flux) and the biomass responses of NC-S:NGR. Future research must include not only site specific measurements of ambient O-3 concentrations and the biomass responses of NC-S and NC-R, but also hourly horizontal windspeeds, air temperature, global radiation and relative humidity, to derive more realistic and reliable numerical relationships of cause (O-3 fluxes) and effect (NC-S:NC-R biomass). The resulting predictive models need to be of time-series in their nature to account for the stochasticity of the underlying relationships. (C) 1998 Elsevier Science Ltd. Am rights reserved.