Parameterization of the Stomatal Component of the DO(3)SE Model for Mediterranean Evergreen Broadleaf Species

An ozone (O(3)) deposition model (DO(3)SE) is currently used in Europe to define the areas where O(3) concentrations lead to absorbed O(3) doses that exceed the flux-based critical levels above which phytotoxic effects would be likely recorded. This mapping exercise relies mostly on the accurate estimation of O(3) flux through plant stomata. However, the present parameterization of the modulation of stomatal conductance (g(s)) behavior by different environmental variables needs further adjustment if O(3) phytotoxicity is to be assessed accurately at regional or continental scales. A new parameterization of the model is proposed for Holm oak (Quercus ilex), a tree species that has been selected as a surrogate for all Mediterranean evergreen broadleaf species. This parameterization was based on a literature review, and was calibrated and validated using experimentally measured data of g(s) and several atmospheric and soil parameters recorded at three sites of the Iberian Peninsula experiencing long summer drought, and very cold and dry winter air (El Pardo and Miraflores) or milder conditions (Tietar). A fairly good agreement was found between modeled and measured data (R(2) = 0.64) at Tietar. However, a reasonable performance (R(2) = 0.47–0.62) of the model was only achieved at the most continental sites when g(s) and soil moisture deficit relationships were considered. The influence of root depth on g(s) estimation is discussed and recommendations are made to build up separate parameterizations for continental and marine-influenced Holm oak sites in the future.