Ozone critical levels for Mediterranean evergreen forests.

Ozone (O3) is the most important and pervasive air pollutant in the Mediterranean region where climatic conditions favor O3 photochemical formation and persistence. O3-induced effects have been reported on the physiology and growth of Mediterranean forest species. However, discrepancies exist between the observed and the predicted O3 effects on Southern European forests based on current O3 critical levels. This discrepancy is likely to be explained by the inadequacy of the critical level and/or the inherent higher O3 tolerance of Mediterranean vegetation. A review of O3 exposure experiments performed with Mediterranean evergreen tree species has been carried out to address this issue. A database of 8 experiments including 6 different tree species and 3 experimental sites was used to derive exposure- and flux-based response functions for evergreen species. The species considered were: one conifer (Pinus halepensis) and 5 broadleaf evergreen species (Quercus ilex ssp ilex, Q. ilex ssp ballota, Q. coccifera, Ceratonia siliqua, Olea europea). All the experiments were performed with seedlings growing in open top chambers exposed to different O3 treatments using charcoal-filtered air as control treatment. Four experiments included water stress treatments. Only those experiments providing information on O3-induced effects on biomass of tree species growing under Mediterranean climate conditions have been considered. The length of the experiments varied between 0.4 to 3 years. Both local and species parameterizations of the Mapping Manual were used to model stomatal conductance. The reduction of biomass (RY) for each O3 treatment (and water treatment if available) was estimated for each experiment relative to the hypothetical biomass at an AOT40 and POD1 equal to zero. Each data point included in the response functions represents a single value for each species, treatment and experiment. Ozone critical levels were calculated based on exposure- and dose-response relationships for a 4% effect. Those effects were calculated starting from a reference biomass estimated for an average POD1 using 10 ppb of O3 as constant concentration (POD1_10). The results indicate that Mediterranean tree species are more tolerant to O3 than species from more humid biomes, yielding higher O3 critical levels than those reported in the Mapping Manual. The results indicate that different O3 critical levels should be used for damage risk assessment in Europe depending on vegetation types.