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 PODY, respectively) are employed to define dose-response relationships. Currently, PODY 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 PODY (PODY_Sap) was thus tested in this study. The aims were: (i) to assess if PODY_Sap is better correlated with O3_VFI than the standard DO3SE model parameters-derived PODY 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.