Exploring a New O3 Index as a Proxy for the Avoidance/Tolerance Capacity of Forest Species to Tolerate O3 Injury

Abstract

Tropospheric ozone (O3) is a detrimental air pollutant causing phytotoxic effects. Several O3 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 O3. We therefore tested a new species-specific O3 index (Leaf Index Flux—LIF: calculated as stomatal O3 flux/LMA) as a proxy of the avoidance/tolerance capacity against O3 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 O3, soil moisture, and meteorological measurements at nine Italian forest sites over the period 2018–2022. The results were tested for correlation with the O3 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 O3-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 O3 VFI. Shrub species such as Rubus spp. and Vaccinium myrtillus were O3-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 O3 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 O3 VFI on forest woody species and stresses high O3 risk potential for forest species with high stomatal conductance and thin leaves.