Developing ozone critical levels for multi-species canopies of Mediterranean annual pastures

Ozone (O₃) critical levels (CLe) are still poorly developed for herbaceous vegetation. They are currently based on single species responses which do not reflect the multi-species nature of semi-natural vegetation communities. Also, the potential effects of other factors like the nitrogen (N) input are not considered in their derivation, making their use uncertain under natural conditions. Exposure- and dose-response relationships were derived from two open-top chamber experiments exposing a mixture of 6 representative annual Mediterranean pasture species growing in natural soil to 4 O₃ fumigation levels and 3 N inputs. The Deposition of O₃ and Stomatal Exchange model (DO₃SE) was modified to account for the multi-species nature of the canopy following a big-leaf approach. This new approach was used for estimating a multi-species phytotoxic O₃ dose (POD_y-MS). Response relationships were derived based on O₃ exposure (AOT40) and flux (POD_y-MS) indices. The treatment effects were similar in the two seasons: O₃ reduced the aboveground biomass growth and N modulated this response. Gas exchange rates presented a high inter-specific variability and important inter-annual fluctuations as a result of varying growing conditions during the two years. The AOT40-based relationships were not statistically significant except when the highest N input was considered alone. In contrast, POD_y-MS relationships were all significant but for the lowest N input level. The influence of the N input on the exposure- and dose-response relationships implies that N can modify the O₃ CLe. However, this is an aspect that has not been considered so far in the methodologies for establishing O₃ CLe. Averaging across N input levels, a multi-species O₃ CLe (CLe_f-MS) is proposed POD_1-MS = 7.9 mmol m^-2, accumulated over 1.5 month with a 95% confidence interval of (5.9, 9.8). Further efforts will be needed for comparing the CLe_f-MS with current O₃ CLe_f based on single species responses.