Bridging experimental and monitoring research for visible foliar injury as bio-indicator of ozone impacts on forests

Tropospheric ozone (O-3) is a phytotoxic air pollutant and the O-3-induced visible foliar injury (O-3 VFI) is a biomarker. A recently developed Free-air O-3 eXposure (FO3X) is a promising facility to verify field-observed "O-3-like" VFIs and to establish a flux-based threshold for the O-3 VFI onset. The present study compared O-3-like VFI registered in the southern European forest sites with actual O-3 VFI observed in a FO3X experiment. The O-3-like VFIs were evaluated by eye in forests and thus it was subjective. According to the imaging analysis, we firstly demonstrated that major parts of the colors were similar in the field and the FO3X. The color pallets for O-3 VFI was species-specific and considered a advanced tool for the O-3 VFI diagnosis. In addition, we calculated a flux-based threshold for the O-3 VFI onset at the FO3X based on a Phytotoxic Ozone Dose (POD1), which ranged from 4.9 to 18.1 mmol m(-2) POD1. This FO3X-derived threshold partly explained but did not necessarily match with the observation for several tree species in actual forests. The multivariate analysis showed that O-3 VFI was decreased by the presence of various species and suggested the importance of continuous monitoring activities in the field for the further analysis.