Ozone critical levels for Mediterranean 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 tree species has been carried out to address this issue. Only those experiments estimating O3-induced effects on tree biomass or growth of evergreen tree species growing under Mediterranean climate conditions were considered. A database of 16 experiments including 7 different tree species and 3 experimental sites was used to derive exposure- and flux-based response functions. Among the species considered, one species was a conifer (Pinus halepensis) and 6 were broadleaf evergreen species (Quercus ilex ssp ilex, Q. ilex ssp ballota, Q. coccifera, Ceratonia siliqua, Olea europea, Arbutus unedo). All the experiments were performed with seedlings growing in open top chambers exposed to different O3 treatments. The control treatment in all the experiments was charcoal-filtered air. Four experiments included water stress treatments. The length of the experiments varied between 0.3 to 3 years, with 12 out of 16 experiments considering about one year or longer. Exposure and dose-response function were derived from experiments including at least 60% of the growing period: 12 out of 16 experiments. Ozone-induced effects were better related to stomatal fluxes than to O3 exposure expressed as AOT40, when treatments with drought stress were included. Thus drought stress needs to be considered for ozone flux estimations in water-limited ecosystems. Interestingly, drought stress did not necessarily protect plants from O3 effects. Using a threshold value for estimating ozone stomatal fluxes did not improve the response function. The resulting response functions were compared with published results obtained with 5 deciduous species growing under Mediterranean climate conditions and a conifer species characteristic of Southern-Europe mountain areas. The results indicate that Mediterranean tree species are more tolerant to O3than 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.