Validation of ozone response functions for annual Mediterranean pasture species using close-to-field-conditions experiments

Ozone (O₃) critical levels have been established under the Long-Range Transboundary Air Pollution Convention to assess the risk of O₃ effects in European vegetation. A recent review study has led to the development of O₃ critical levels for annual Mediterranean pasture species using plants growing in well-watered pots at a coastal site and under low levels of competition. However, uncertainties remain in the extrapolation of the O₃ sensitivity of these species under natural conditions. The response of two O₃-sensitive annual Mediterranean pasture Trifolium species at the coastal site was compared with the response of the same species growing at a continental site, in natural soil and subject to water-stress and inter-specific competition, representing more closely their natural habitat. The slopes of exposure- and dose-response relationships derived for the two sites showed differences in the response to O₃ between sites attributed to differences in environmental growing conditions, growing medium and the level of inter-specific competition, but the effect of the individual factors could not be assessed separately. Dose-based O₃ indices partially explained differences due to environmental growing conditions between sites. The slopes showed that plants were more sensitive to O₃ at the continental site, but homogeneity of slopes tests revealed that results from both experimental sites may be combined. Although more experimental data considering complex inter-specific competition situations and the effect of important interactive factors such as nitrogen would be needed, these results confirm the validity of applying the current flux-based O₃ critical level under close to natural growing conditions. The AOT40-based O₃ critical level derived at the coastal site was also considered a suitable risk indicator in close to natural growing conditions in the absence of soil moisture limitations on plant growth.