Plant–insect interactions are basic components of biodiversity conservation. To attain the international Sustainable Development Goals (SDGs), the interactions in urban and in suburban systems should be better understood to maintain the health of green infrastructure. The role of ground-level ozone (O3) as an environmental stress disrupting interaction webs is presented. Ozone mixing ratios in suburbs are usually higher than in the center of cities and may reduce photosynthetic productivity at a relatively higher degree. Consequently, carbon-based defense capacities of plants may be suppressed by elevated O3 more in the suburbs. However, contrary to this expectation, grazing damages by leaf beetles have been severe in some urban centers in comparison with the suburbs. To explain differences in grazing damages between urban areas and suburbs, the disruption of atmospheric communication signals by elevated O3 via changes in plant-regulated biogenic volatile organic compounds and long-chain fatty acids are considered. The ecological roles of plant volatiles and the effects of O3 from both a chemical and a biological perspective are presented. Ozone-disrupted plant volatiles should be considered to explain herbivory phenomena in urban and suburban systems.
Biologically meaningful and cost-effective indicators are needed for assessing and monitoring the impacts of tropospheric ozone (O3) 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 O3 monitoring in forest plots in order to: 1) estimate the stomatal O3 fluxes (Phytotoxic Ozone Dose above a threshold Y of uptake, PODY); and 2) collect visible foliar O3 injury, both within the forest plot (ITP) and along the Light Exposed Sampling Site (LESS) along the forest edge. Nine forest sites at high O3 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 O3 m−2 s−1) calculated for mixed forest species (r = 0.53) and with the occurrence and severity of visible foliar O3 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 O3 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 O3 levels in forest monitoring. LESS is a non-destructive, less complex and less time-consuming approach compared to the ITP for monitoring foliar O3 injury in the long term. Assessing visible foliar O3 injury in the ITP might only underestimate the O3 risk assessment at individual sites. These results are biologically meaningful and useful to monitoring experts and environmental policy makers.