©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(6): November 2023 331 woody species in Iran (e.g., Pourhashemi et al. 2012; Panahi et al. 2013; Nowghani et al. 2016), no prior research has evaluated their relationship with air pol- lutants. Although some studies have investigated the relationship between air pollutants and tree leaf vari- ables in Tehran (e.g., Rashidi et al. 2017; Abbasi et al. 2021) and other cities of Iran (e.g., Rafiee et al. 2014; Omidi et al. 2018), they were limited in scope and did not include the Caucasian hackberry (Celtis cauca- sica Willd.) as a subject of investigation. Since the climate of Tehran is arid and semi-arid and C. cauca- sica is a species adapted to such conditions, it is always used as one of the important species for plant- ing in Tehran’s urban green spaces. The present study aimed to fill this gap by examining a broader range of urban forests and a new species. MATERIALS AND METHODS Study Areas This study was carried out on P. orientalis and C. caucasica, 2 main species of Tehran urban forests, in 10 urban forests (Table 1, Figures 1 and 2). Tehran, with an area of 740 km2, is located at 51°17ʹ to 51°33ʹE and 35°36ʹ to 35°42ʹN. According to the synoptic sta- tion of Mehrabad airport, Tehran has a dry and cold climate, with an average annual temperature of 17.2 °C. The range of temperature is 43 °C in July and at least −15 °C in December. The average annual rainfall is 230.5 mm with the maximum in March (9.99 mm) and minimum in September (1.1 mm). The number of freezing days is 48 days throughout the year, the maximum of which is 18 days in January. The lowest and highest relative humidity values are between 25% and 26% in June and January, respectively. The study urban forests were selected in such a way that they (1) have both species in the desired diameter at breast height (DBH), (2) have a variety of pollution levels, and (3) have the shortest distance to the pollution measuring station of that region. The information of the closest air pollution measuring sta- tions (7 stations) to each study urban forest was used to investigate the relationship between leaf morpho- logical variables and pollutants. The location of the pollution measuring stations are presented in Figure 1. In this study, 4 pollutants (CO, NO2, SO2, and O3) were used. Pollution reduces the levels of chlorophyll a, chlo- rophyll b, carotenoids, ascorbic acid, relative leaf water content, and the contamination tolerance index (Chukwu and Adams 2016). One of the most com- mon impacts of air pollution is the gradual disappear- ance of chlorophyll, leading to the yellowing of leaves and a consequent decrease in the capacity for photosynthesis (Joshi and Swami 2007). Chlorophyll is the primary photoreceptor in photosynthesis, the light-driven process in which carbon dioxide is “fixed” to yield carbohydrates and oxygen. When plants are exposed to environmental pollution above the normal physiologically acceptable range, photo- synthesis gets inactivated. The pollutants cause severe damage to leaves of Joannesia princeps Vell. (Euphor- biaceae), a native species of the Atlantic Forest in Minas Gerais, Brazil (da Silva et al. 2023). Results of a study on the particulate matter (PM) deposition on Quercus ilex L. leaves in 4 districts of the City of Terni (Italy) for 3 periods of the year showed that variations in PM deposition were correlated with dis- tance to main roads and downwind position relative to the industrial area (Sgrigna et al. 2015). Reductions in leaf area and leaf number may be a defense mechanism to limit the level of exposure to airborne contamination (Jochner et al. 2015). Reduc- tion in leaf area due to air pollution can affect the plant’s ability to photosynthesize and its ability to adapt to the stress of air pollution (Tiwari et al. 2006). The reduction of leaf area in leaves of Albizia lebbek (L.) Benth. was reported under the stress of air pollu- tion (Seyyednejad et al. 2009). The leaf area of plants that had the furthest distance from the source of con- tamination was higher than those close to the source of contamination (Chukwu and Adams 2016). The study conducted in Quetta, Pakistan aimed to investi- gate the impact of air pollution on the morphological characteristics of the leaves of 13 common plant spe- cies. The findings of the study indicated that, when compared with their counterparts from non-polluted sites, all plant species in polluted areas displayed a significant reduction (p < 0.05) in leaf length, width, area, and petiole length (Leghari and Zaidi 2013). The primary objective of this research was to examine the effect of air pollutants in Tehran on leaf area and specific leaf area (SLA)(which is associated with leaf area and leaf humidity) and explore the rela- tionship between them. While some previous studies have investigated the leaf area and SLA of several
November 2023
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