Abstract: Tropospheric ozone (O₃) is a harmful air pollutant negatively impacting forest health, causing O₃-specific visible foliar injury (O₃ VFI). Ozone monitoring in forests has usually implemented by passive samplers, although they cannot detect the diurnal peak when a significant part of stomatal O₃ uptake occurs. This results into uncertainties for the calculation of stomatal O₃ uptake. This study compares the stomatal-flux-based POD₁ (phytotoxic ozone dose above a threshold of 1 nmol m⁻²s⁻¹) for forest trees/shrubs estimated from data collected by either passive samplers or active O₃ monitors to evaluate O₃ damage to plants in terms of O₃ VFI in the Southern Alps. The study was conducted over two years (2018–2019) in a mountainous Alpine area (Valle Stura, Italy). An integrative monitoring station for active O₃ monitoring, as well as passive O₃ monitors, were installed in an open field area (OFD). The O₃ VFI was investigated in woody species in the light exposed sampling Site (LESS—Betula pendula, Fagus sylvatica, Larix decidua, Populus tremula, Salix caprea, Rubus sp. and Vaccinium myrtillus) in late summer according to the international co-operative programme on assessment and monitoring of air pollution effects on forests (ICP Forests) manual. The results confirmed that Fagus sylvatica and Rubus sp. are O₃-sensitive species showing relatively high POD₁ (> 20 mmol m^⁻2), while Larix decidua is O₃-tolerant. We derived flux-based critical levels (CL) corresponding to the presence of O₃ VFI (5, 25, and 50% of symptomatic plants along the LESS) from flux-effect relationships for forest protection against O₃ VFI. The results support the hypothesis that passive samplers cannot detect episodic high stomatal O₃ fluxes (> 1 nmol m⁻² s⁻¹). According to the active monitoring, the CL for O₃ VFI occurrence was estimated to be 17.1 mmol m^⁻2 POD₁ for 25% presence and 34.3 mmol m^⁻2 POD₁ for 50% presence of symptomatic plants, while passive samplers underestimated POD₁ values for CL calculations by 17% on average, with underestimation increasing at higher CL thresholds. The findings demonstrate that active monitoring refines CLs towards a proper quantitative assessment of O₃ impact, particularly in capturing peak flux events that are crucial for evaluating plant damage and emphasizes the importance of active O₃ monitoring for reliable forest health assessments.

Key words: Stomatal conductance model, Leaf symptoms, Forest, Phytotoxic ozone dose