Testing visible ozone injury within a Light Exposed Sampling Site as a proxy for ozone risk assessment for European forests

Biologically meaningful and cost-effective indicators are needed for assessing and monitoring the impacts of tropospheric ozone (O-3) on vegetation and are required in Europe by the National Emission Ceilings Directive (2016). However, a clear understanding on the best suited indicators is missing. The MOTTLES (MOnitoring ozone injury for seTTing new critical LEvelS) project set up a new generation network for O-3 monitoring in forest plots in order to: 1) estimate the stomatal O-3 fluxes (Phytotoxic Ozone Dose above a threshold Y of uptake, PODY); and 2) collect visible foliar O-3 injury, both within the forest plot (ITP) and along the Light Exposed Sampling Site (LESS) along the forest edge. Nine forest sites at high O-3 risk were selected across Italy over 2017 - 2019 and significant correlations (p < 0.05) were found between the percentage of symptomatic plant species within the LESS, and POD1 (PODY, with Y = 1 nmol O-3 m(-2) s(-1)) calculated for mixed forest species (r = 0.53) and with the occurrence and severity of visible foliar O-3 injury on the dominant species in the plots (r = 0.65). A generic flux-based critical level for mixed forest species was derived within the LESS and it was recommended using 11 mmol m(-2) POD1 as the critical level for forest protection against O-3 injury, similar to the critical level obtained in the ITP (12 mmol m(-2) POD1). It was concluded that the frequency of symptomatic plant species within a LESS is a suitable and effective plant-response indicator of phytotoxic O-3 levels in forest monitoring. LESS is a non-destructive, less complex and less time-consuming approach compared to the ITP for monitoring foliar O-3 injury in the long term. Assessing visible foliar O-3 injury in the ITP might only underestimate the O-3 risk assessment at individual sites. These results are biologically meaningful and useful to monitoring experts and environmental policy makers.