Tropospheric ozone (O 3) is one of the most toxic compounds for plants in the atmosphere. The large amount of an-thropogenic O 3 precursors in the urban areas promote O 3 formation, thus making Mediterranean forests located in periurban areas particularly vulnerable to this pollutant. O 3 fl ux measurements have been carried out using the Eddy Covariance technique over a Holm oak forest located 25 Km from Rome downtown, inside the Presidential Estate of Castelporziano (Italy).Two transition

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... ods-early Spring and late Fall-in two consecutive years were examined. The uncommon low precipitation recorded in both transition periods in 2012 allowed to evaluate the infl uence of water availability on O 3 fl uxes during seasons which are not commonly affected by drought stress. Overall, the forest canopy showed to be a net sink of O 3 , with peak values of mean daily O 3 fl uxes of-8.9 nmol m-2 s-1 at the beginning of fl owering season and-4.6 nmol m-2 s-1 at the end of Fall. O 3 fl uxes were partitioned between stomatal and non stomatal sinks using the evaporative/resistive method based on canopy transpiration in analogy with an Ohm circuit. By comparison of the two years, water availability showed to be an important limiting factor during Spring, since in this season plants are more photosynthetically active and more sensitive to water availability, while in Fall, under conditions of low stomatal conductance, the dependence on water availability was less appreciated.