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. © 2025 Elsevier B.V., All rights reserved.

Cited by: 0