Exploring a New O-3 Index as a Proxy for the Avoidance/Tolerance Capacity of Forest Species to Tolerate O-3 Injury

Tropospheric ozone (O-3) is a detrimental air pollutant causing phytotoxic effects. Several O-3 indices are used to assess the risk for vegetation, e.g., the exposure-based AOT40 (accumulated ozone exposure over a threshold of 40 ppb) and the stomatal-flux based POD1 (Phytotoxic Ozone Dose above a threshold of 1 nmol m(-2) s(-1)). Leaf Mass per Area (LMA) is recommended as a simple index to explain the plant tolerance capacity to O-3. We therefore tested a new species-specific O-3 index (Leaf Index Flux-LIF: calculated as stomatal O-3 flux/LMA) as a proxy of the avoidance/tolerance capacity against O-3 stress according to datasets of visible foliar injury (VFI) in forest monitoring and a manipulative Free-Air Controlled Exposure (FACE) experiment. For the forest monitoring, AOT40, POD1, and LIF were calculated from hourly O-3, soil moisture, and meteorological measurements at nine Italian forest sites over the period 2018-2022. The results were tested for correlation with the O-3 VFI annually surveyed at the same sites along the forest edge (LESS) or inside the forest (ITP) and expressed as relative frequency of symptomatic species in the LESS (SS_LESS) and Plant Injury Index per tree in the plot (PII_ITP). Based on VFI occurrence at ITP and LESS, Fagus sylvatica was considered the most O-3-sensitive species, whereas conifers (Pinus pinea and Picea abies) and other deciduous/evergreen broadleaf (Quercus petraea, Q. cerris, Q. ilex, and Phyllirea latifolia) showed rare and no O-3 VFI. Shrub species such as Rubus spp. and Vaccinium myrtillus were O-3-sensitive, as they showed VFI along the LESS. AOT40 did not show significant correlations with the VFI parameters, POD1 increased with increasing SS_LESS (p = 0.005, r = 0.37) and PII_ITP (p < 0.001, r = 0.53), and LIF showed an even higher correlation with SS%_LESS (p < 0.001, r = 0.63) and PII_ITP (p < 0.001, r = 0.87). In the FACE experiment, PII was investigated for five deciduous and three evergreen tree species following one growing season of exposure to ambient and above-ambient O-3 levels (PII_FACE). Moreover, PII_FACE resulted better correlated with LIF (r = 0.67, p < 0.001) than with POD1 (r = 0.58, p = 0.003) and AOT40 (r = 0.35, p = 0.09). Therefore, LIF is recommended as a promising index for evaluating O-3 VFI on forest woody species and stresses high O-3 risk potential for forest species with high stomatal conductance and thin leaves.