Abstract
This study investigated visible foliar ozone (O3) injury in three deciduous tree species with different growth patterns (indeterminate, Alnus glutinosa (L.) Gaertn.; intermediate, Sorbus aucuparia L.; and determinate, Vaccinium myrtillus 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 PODY (phytotoxic O3 dose above a flux threshold of Y nmol m−2 s−1). A new parameterization for PODY 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 × AA; and 2.0 × AA). Injury onset was found in May at 2.0 × 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−2 POD0 and 5.5 to 9.0 ppm·h AOT40. As a result, A. glutinosa with high stomatal conductance (gs) 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 A. glutinosa with indeterminate growth pattern, a new leaf formation alleviated the expansion of O3 visible injury at the plant level. V. myrtillus 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, PODY, as compared with the exposure-based index, AOT40. The critical levels (CLs) corresponding to PII = 5 were 8.1 mmol m−2 POD7 in A. glutinosa, 22 mmol m−2 POD0 in S. aucuparia, and 5.8 mmol m−2 POD1 in V. myrtillus. 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.
Highlights
Tropospheric ozone (O3 ) is one of the major concerns for forest health due to its phytotoxicity [1].Despite the fact that peak O3 concentrations have tended to decrease in the eastern part of UnitedStates and some European countries due to precursor emission controls [2], the global background O3 concentration still remains high enough to cause negative impacts on tree physiology [3].Forests 2020, 11, 82; doi:10.3390/f11010082 www.mdpi.com/journal/forestsVisible foliar injury by O3 (O3 visible injury) is the first unequivocal visually detectable sign ofO3 damage and indicates an impairment of leaf physiological functions [4]
After we found the first symptom in any O3 treatment, the percentage of symptomatic leaves per plant (LA) and the percentage injured area in the symptomatic leaves (AA) were scored (A. glutinosa on 31 May, 21 June, 10 and 21 July, and 8 and 20 August; S. aucuparia on 31 May, 21 June, 10 July, and 8 and 20 August; V. myrtillus on 31 May, 21 June, 4, and 11 and 19 July) with a ×10 hand lens and the help of photoguides (Innes et al 2001 and Paoletti et al 2009)
O3 visible injury in A. glutinosa was related to high gs ; plant injury index (PII) was affected by stomatal
Summary
Tropospheric ozone (O3 ) is one of the major concerns for forest health due to its phytotoxicity [1].Despite the fact that peak O3 concentrations have tended to decrease in the eastern part of UnitedStates and some European countries due to precursor emission controls [2], the global background O3 concentration still remains high enough to cause negative impacts on tree physiology [3].Forests 2020, 11, 82; doi:10.3390/f11010082 www.mdpi.com/journal/forestsVisible foliar injury by O3 (O3 visible injury) is the first unequivocal visually detectable sign ofO3 damage and indicates an impairment of leaf physiological functions [4]. States and some European countries due to precursor emission controls [2], the global background O3 concentration still remains high enough to cause negative impacts on tree physiology [3]. Ozone visible injury has been broadly investigated in native and exotic trees, shrubs, and herbs in Asia, Europe, and North America, and partly validated under controlled conditions [6,7,8]. An exposure-based index such as AOT40 (accumulated exposure over a threshold of 40 ppb) is used to assess O3 risks to European forest trees [9,10]. Previous studies have reported an AOT40-based assessment of the first symptom onset of O3 visible injury in field [11,12] or open-top chambers [13,14]
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