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

Ozone (O) 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 O fumigation levels and 3 N inputs. The Deposition of O and Stomatal Exchange model (DOSE) 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 O dose (POD). Response relationships were derived based on O exposure (AOT40) and flux (POD) indices. The treatment effects were similar in the two seasons: O 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 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 O CLe. However, this is an aspect that has not been considered so far in the methodologies for establishing O CLe. Averaging across N input levels, a multi-species O CLe (CLe) is proposed POD = 7.9 mmol m, accumulated over 1.5 month with a 95% confidence interval of (5.9, 9.8). Further efforts will be needed for comparing the CLe with current O CLe based on single species responses.