Assessing the Air Pollution Mitigation Potential of Urban Trees in Ghana's Central Region

Air pollution is a major environmental concern in urban areas, necessitating the identification of tree species capable of mitigating its effects. This study assessed the Air Pollution Tolerance Index (APTI) and Anticipated Performance Index (API) of four common urban tree species (Albizia lebbeck (L.) Benth., Azadirachta indica A. Juss, Khaya senegalensis (Desr.) A. Juss, and Senna siamea (Lam.) H.S. Irwin & Barneby) to evaluate their potential for air pollution control. Leaf extract pH was determined by homogenizing 5 g of leaf tissue in 10 mL deionized water and measuring with a calibrated digital pH meter. Relative leaf water content (RWC) was calculated using fresh, turgid, and dry weights of leaf samples. The ascorbic acid (AA), total chlorophyll (TC), and carotenoid contents were determined spectrophotometrically using 721 Vis spectrophotometer. API was determined by integrating APTI with morphological and socioeconomic traits. One-way ANOVA and correlation matrices were employed to analyze variations across roads and the relationships among parameters, respectively. The findings revealed significant variation in biochemical responses among the studied species. Principal component analysis (PCA) identified AA, RWC, and TC as consistent key drivers of variation across all roads, while leaf pH exhibited site-specific influence. The mean APTI values of the four tree species varied from 5.39 to 8.96. Based on the APTI, the tree species were classified as either tolerant (> 7.5), intermediate (6.5-7.5), or sensitive (APTI < 6.5). A. indica exhibited consistent RWC and chlorophyll levels, earning the highest API score (81.25%) and an "Excellent" performance rating, making it a potential candidate for urban greenbelt development. K. senegalensis can be used as sentinel species in urban greening because of its general sensitivity to the pollutants. A. lebbeck and S. siamea showed strong potential as bioindicators because of their physiological responses to pollution stress. This study introduced a new classification range for using roadside trees as bioindicators of vehicular pollution, particularly under conditions of low APTI.