Background Vehicular emissions significantly contribute to urban air pollution, threatening both human health and vegetation. Roadside trees act as bioindicators, exhibiting morphological and anatomical changes under pollutant stress. In developing regions such as Ghana, studies linking air pollutants and morpho-anatomical traits of roadside vegetation remains limited. This study therefore assessed the responses of four abundant roadside tree species Albizia lebbeck (L.) Benth., Azadirachta indica A. Juss, Khaya senegalensis (Desr.) A. Juss, and Senna siamea (Lam.) H.S. Irwin & Barneby along three major arterial roads in Winneba Municipality with varying traffic densities. Methods Ambient PM2.5 and PM10 were measured using EPAM-7500 particle monitors, while CO and NOx were determined with Aeroqual Series 500 gas monitors (Auckland, New Zealand) through purposive rotation along selected roads. Leaf samples were collected in triplicate at 1.9 m height from the third phyllotaxis position per site for morphometric analyses. Leaf length and width were measured with a ruler, and surface area calculated using graph paper. For micromorphology, epidermal peels were prepared via the lasting impressions method, photographed with a Leica DM750 microscope, and analyzed in ImageJ v12 to estimate stomatal count, area, and size. Tukey-b was used to test variations in leaf morphology among roads, while multiple regression with VIF diagnostics analyzed their association with air quality. Results Vehicle emissions, particularly, NO2 and PM2.5, had the most pronounced effects on macro- and micro-morphological traits. High-traffic sites recorded significant reductions in leaf length, width, and area, especially in A. indica and S. siamea. In contrast, A. lebbeck and K. senegalensis showed relatively stable macro traits. Regression and multicollinearity analyses indicated varied correlations between pollutants and changes in leaf micro-morphology. Micro-morphologically, K. senegalensis exhibited strong associations with NO2 (VIF = 31.077) and PM2.5 (VIF = 25.016), while A. lebbeck, A. indica, and S. siamea showed moderate associations. PM10 and CO had minimal effects on micro traits across all species. Conclusions K. senegalensis, A. lebbeck, A. indica, and S. siamea are suitable candidates for urban greening and roadside planting to enhance ecological buffering capacity and mitigate air pollutants, particularly NO2 and PM2.5, in Winneba and similar developing urban environments.

Air pollution and plant responses: a study on morphological and anatomical changes in roadside trees along traffic-dense roads

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2025年9月1日

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Francis Kwaku Nkansah

摘要

Background

Vehicular emissions significantly contribute to urban air pollution, threatening both human health and vegetation. Roadside trees act as bioindicators, exhibiting morphological and anatomical changes under pollutant stress. In developing regions such as Ghana, studies linking air pollutants and morpho-anatomical traits of roadside vegetation remains limited. This study therefore assessed the responses of four abundant roadside tree species Albizia lebbeck (L.) Benth., Azadirachta indica A. Juss, Khaya senegalensis (Desr.) A. Juss, and Senna siamea (Lam.) H.S. Irwin & Barneby along three major arterial roads in Winneba Municipality with varying traffic densities.

Methods

Ambient PM_2.5 and PM₁₀ were measured using EPAM-7500 particle monitors, while CO and NO_x were determined with Aeroqual Series 500 gas monitors (Auckland, New Zealand) through purposive rotation along selected roads. Leaf samples were collected in triplicate at 1.9 m height from the third phyllotaxis position per site for morphometric analyses. Leaf length and width were measured with a ruler, and surface area calculated using graph paper. For micromorphology, epidermal peels were prepared via the lasting impressions method, photographed with a Leica DM750 microscope, and analyzed in ImageJ v12 to estimate stomatal count, area, and size. Tukey-b was used to test variations in leaf morphology among roads, while multiple regression with VIF diagnostics analyzed their association with air quality.

Results

Vehicle emissions, particularly, NO₂ and PM_2.5, had the most pronounced effects on macro- and micro-morphological traits. High-traffic sites recorded significant reductions in leaf length, width, and area, especially in A. indica and S. siamea. In contrast, A. lebbeck and K. senegalensis showed relatively stable macro traits. Regression and multicollinearity analyses indicated varied correlations between pollutants and changes in leaf micro-morphology. Micro-morphologically, K. senegalensis exhibited strong associations with NO₂ (VIF = 31.077) and PM_2.5 (VIF = 25.016), while A. lebbeck, A. indica, and S. siamea showed moderate associations. PM₁₀ and CO had minimal effects on micro traits across all species.

Conclusions

K. senegalensis, A. lebbeck, A. indica, and S. siamea are suitable candidates for urban greening and roadside planting to enhance ecological buffering capacity and mitigate air pollutants, particularly NO₂ and PM_2.5, in Winneba and similar developing urban environments.