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

A white clover model system (consisting of O3-sensitive [NC-S] and tolerant [NC-R] clones) offers an opportunity to establish cause (temporally dynamic O3 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 O3 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 O3 were responsible for the observed variability. This is supported by the results of the linear regression analysis between the O3 exposure potentials (O3 flux) and the biomass responses of NC-S:NC-R. Future research must include not only site specific measurements of ambient O3 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 (O3 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.