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.

评估和监测对流层臭氧 (O ₃ ) 对植被的影响需要具有生物学意义和成本效益的指标，欧洲国家排放上限指令 (2016) 也要求提供这些指标。然而，缺乏对最适合指标的清晰理解。MOTTLES（监测臭氧损伤以设置新的临界水平）项目为林地中的O ₃监测建立了新一代网络，以便：1）估计气孔 O ₃通量（植物毒性臭氧剂量高于吸收阈值Y，PODY ); 和 2) 收集可见的叶面 O ₃伤害，无论是在森林地块 (ITP) 内还是沿着森林边缘的光暴露采样点 (LESS)。在 2017 - 2019 年间，意大利选择了9 个高 O ₃风险的森林地点， 发现 LESS 内有症状的植物物种百分比与 POD1（PODY，Y  = 1 nmol O ₃ m ）之间存在显着相关性（p < 0.05）⁻²  s ⁻¹ ) 针对混交林物种 ( r  = 0.53) 以及地块中优势物种的可见叶面 O ₃损伤的发生率和严重程度( r = 0.65）。在 LESS 中导出了一个基于通量的通用临界水平，建议使用 11 mmol m ^-2 POD1 作为森林保护免受 O ₃伤害的临界水平，类似于 ITP 中获得的临界水平（12 mmol m ^-2 POD1)。得出的结论是，LESS 中出现症状的植物物种的频率是森林监测中植物毒性 O ₃水平的合适且有效的植物响应指标。与 ITP 相比，LESS 是一种非破坏性、更简单、更省时的方法，用于长期监测叶面 O ₃损伤。评估ITP 中可见的叶面 O ₃损伤可能只会低估 O ₃个别地点的风险评估。这些结果具有生物学意义，对监测专家和环境政策制定者有用。