Flux-Based Ozone Risk Assessment for a Plant Injury Index (PII) in Three European Cool-Temperate Deciduous Tree Species

• Main Authors: Yasutomo Hoshika, Elisa Carrari, Barbara Mariotti, Sofia Martini, Alessandra De Marco, Pierre Sicard, Elena Paoletti
• Format: Article
• Language: English
• Published: MDPI AG 2020-01-01
• Series: Forests
• Subjects: visible foliar ozone injury; european alder; mountain ash; european blueberry; stomatal ozone flux; free-air ozone exposure
• Online Access: https://www.mdpi.com/1999-4907/11/1/82

This study investigated visible foliar ozone (O₃) injury in three deciduous tree species with different growth patterns (indeterminate, <i>Alnus glutinosa</i> (L.) Gaertn.; intermediate, <i>Sorbus aucuparia</i> L.; and determinate, <i>Vaccinium myrtillus</i> L.) from May to August 2018. Ozone effects on the timing of injury onset and a plant injury index (PII) were investigated using two O₃ indices, i.e., AOT40 (accumulative O₃ exposure over 40 ppb during daylight hours) and POD_Y (phytotoxic O₃ dose above a flux threshold of Y nmol m^&#8722;2 s^&#8722;1). A new parameterization for POD_Y estimation was developed for each species. Measurements were carried out in an O₃ free-air controlled exposure (FACE) experiment with three levels of O₃ treatment (ambient, AA; 1.5 &#215; AA; and 2.0 &#215; AA). Injury onset was found in May at 2.0 &#215; AA in all three species and the timing of the onset was determined by the amount of stomatal O₃ uptake. It required 4.0 mmol m^&#8722;2 POD₀ and 5.5 to 9.0 ppm&#183;h AOT40. As a result, <i>A. glutinosa</i> with high stomatal conductance (<i>g</i>_s) showed the earliest emergence of O₃ visible injury among the three species. After the onset, O₃ visible injury expanded to the plant level as confirmed by increased PII values. In <i>A. glutinosa</i> with indeterminate growth pattern, a new leaf formation alleviated the expansion of O₃ visible injury at the plant level. <i>V. myrtillus</i> showed a dramatic increase of PII from June to July due to higher sensitivity to O₃ in its flowering and fruiting stage. Ozone impacts on PII were better explained by the flux-based index, POD_Y, as compared with the exposure-based index, AOT40. The critical levels (CLs) corresponding to PII = 5 were 8.1 mmol m^&#8722;2 POD₇ in <i>A. glutinosa</i>, 22 mmol m^&#8722;2 POD₀ in <i>S. aucuparia</i>, and 5.8 mmol m^&#8722;2 POD₁ in <i>V. myrtillus</i>. The results highlight that the CLs for PII are species-specific. Establishing species-specific O₃ flux-effect relationships should be key for a quantitative O₃ risk assessment.