Towards long-term sustainability of stomatal ozone flux monitoring at forest sites

Stomatal O₃ flux accumulated over the growing season (or phytotoxic ozone dose above a y threshold of uptake, PODy) is nowadays considered as the best biologically based metric to assess O₃ injury to vegetation and establishing critical levels (CLs). So far, CLs have used biomass losses as forest-health indicator in experimental research. Ozone-induced biomass losses of adult forests, however, are difficult to assess. We stress a need to reconcile present CLs and dose-response functions to estimate O₃-induced biomass losses. In fact, a clear discrepancy emerges when comparing CL exceedances calculated from WRF-Chem results and biomass losses estimated based on the presently available dose-response functions for different forest types. We synthesize recent evidence that visible foliar O₃ injury (VFI) recorded annually in large-scale forest surveys may be used as forest-health indicator and coupled with active O₃ monitoring in setting epidemiology-based CLs for forest protection. Bridging experimental and monitoring research is needed for validation and parameterization of field results and models, with the O₃ free-air controlled exposure (FACE) facilities as a reliable experimental tool. In fact, VFI is well correlated to O₃-induced biomass loss at O₃ FACE experiments, and POD1 and O₃ tolerance (when characterized as leaf mass per area (LMA) directly affect VFI under field conditions. POD1 shows a much stronger causality than the O₃ exposure index AOT40 in affecting VFI. Accurate calculation of PODy at field sites is now possible, thanks to recent technological advances that support low-cost and reliable monitoring of PODy at forest sites and thus ensure long-term sustainability of such laborious monitoring. We summarise evidence that active O₃ monitoring is a cost-effective approach for estimating PODy and its economic effects on forests, as the initial investment is compensated in few years by less travels to the field sites than passive O₃ monitoring. These results respond to raising legislative interest into PODy, and support long-term sustainability of PODy monitoring at forest sites.