Evaluating the Bioindicator Potential of Tree Species in Response to Traffic-Related Air Pollution in Urmia, Iran

Abstract

This study evaluates the bioindicator potential of tree species in response to traffic-related air pollution in Urmia, Iran. Nine intersections with varying traffic congestion (low, moderate, and high) were selected, along with the Nazlu campus of Urmia University as a control site. Leaf and soil samples of Cupressus arizonica Greene. (Cupressaceae), Pinus nigra Arnold. (Pinaceae), and Robinia pseudoacacia “Umbraculifera” DC. (Fabaceae) were collected and immediately transferred to the laboratory for morphological, physiological, and biochemical analysis. Results indicated that traffic-induced air pollution reduced leaf area, stomatal density, cuticle layer thickness, and total chlorophyll content, while electrolyte leakage increased across all investigated tree species. The concentration of heavy metals, particularly Pb, in soil and leaf samples increased in correlation with rising traffic congestion levels. Overall, all the studied tree species responded to traffic-related pollution. However, the intensity of their responses varied, indicating that species-specific traits influence both leaf and soil properties under pollution stress. These results confirm the potential of C. arizonica, P. nigra, and R. pseudoacacia as good bioindicators of urban air pollution. The Metal Accumulation Index (MAI) indicated that C. arizonica had the highest capacity to absorb and accumulate heavy metals. This finding highlights C. arizonica as the most tolerant and resilient species to traffic-related pollution among the studied trees, making it an excellent candidate for planting in high-traffic areas in Urmia. These findings offer practical guidance for sustainable urban planning, urban greening, and air quality monitoring, contributing to effective pollution mitigation strategies.