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.