Impact of ground-level ozone on Mediterranean forest ecosystems health

Given the high ozone concentrations observed in the Mediterranean region during summer, it is crucial to extend our knowledge on the potential ozone impacts on forest health with in situ studies, especially to protect typical endemic forests of the Mediterranean basin. This study is focused on ozone measurements and exposures over the Eastern Adriatic coast and on the calculation of different O₃ metrics, i.e., accumulated exposure AOT40 (AOT40_dir, AOT40_ICP, AOT40_pheno) and stomatal O₃ fluxes with an hourly threshold of uptake (Y) to represent the detoxification capacity of trees (PODY, with Y = 0, 1, 2 nmol O₃ m^-2 s^-1) used for forest protection. Finally, we provide an assessment of the relationships between the forest response indicators and environmental variables. Passive ozone measurements and monitoring of forest health indicators, namely growth and crown defoliation, were performed for Quercus ilex, Quercus pubescens, Pinus halepensis, and Pinus nigra forests. Results showed that, for all the analysed species, ozone levels were close to reached the upper plausibility limits for passive monitoring of air quality at forest sites (100 ppb), with the highest values found on P. halepensis in the summer period. O₃ metrics based on exposure were found to be higher in pine plots than in oak plots, while the highest values of uptake-based metrics were found on P. nigra. Regarding relationships between environmental variables and forest-health response indicators, the crown defoliation was significantly correlated with the soil water content at various depth while the tree growth was correlated with the different O₃ metrics. The most important predictors affecting tree growth of Q. pubescens and Q. ilex were AOT40_pheno and AOT40_dir and POD0 for P. nigra.