Stomatal ozone flux estimation in Mediterranean forests through sap flow analysis

Tropospheric ozone (O₃) poses a serious threat to European forest health and O₃-visible foliar injury (O₃_VFI) is a biological indicator of its phytotoxicity. To evaluate the impacts of O₃ on forest vegetation, exposure and flux-based indices (AOT₄₀ and POD_Y, respectively) are employed to define dose-response relationships. Currently, POD_Y calculation is based on the species-specific DO₃SE stomatal model, parameterized for various environmental factors. On-site validation of species-specific stomatal functions should be implemented, and sap-flow derived POD_Y (POD_Y_Sap) was thus tested in this study. The aims were: (i) to assess if POD_Y_Sap is better correlated with O₃_VFI than the standard DO₃SE model parameters-derived POD_Y and (ii) to determine the exposure- or flux-based critical exposure level (CL) corresponding to 5 % O₃_VFI. Hourly O₃ concentrations and environmental parameters were monitored at four broadleaf Italian forest sites and O₃ indices were calculated over the period 2018-2024. TreeTalker devices were used to measure sap-flow while O₃_VFI injuries were annually evaluated at each site on dominant target tree species (i.e., Fagus sylvatica L., Quercus cerris L., and Quercus ilex L.) in late summer seasons. Results showed that POD₁_Sap had the higher positive correlation with O₃_VFI compared to the standard DO₃SE model parameters-derived POD₁ and a CL for the 5 % O₃_VFI equal to 37.93 mmol O₃/m² POD₁ was proposed for F. sylvatica, which is a very sensitive species to O₃. These findings highlighted that on-site sap flow-derived stomatal conductance data can aid to refine the existing O₃ flux metrics allowing to update CLs for O₃_VFI.