Trace metal elements accumulation in roadside ecosystems using Fraxinus excelsior L. as a bioindicator species

Air pollution from road traffic is a major source of trace metal elements (TMEs) in urban areas worldwide, affecting both developing and developed countries. This study investigated Cd, Cu, Cr, Pb and Zn in surface soils, leaves and bark of Fraxinus excelsior L. along roadsides in Constantine city (NE Algeria), to assess the potential of this species as a biomonitor of metallic pollution. Samples were collected from 20 distinct traffic-exposed sites along the national road N°05 and compared with 3 control sites located in the Chettaba forest. The studied soils exhibited significant TME enrichment, with Pb and Cd concentrations exceeding background values in 70% and 55% of the studied sites, respectively. The bio-concentration factor (BCF) values indicated that bark accumulates more TMEs than leaves, with mean values following Cu > Cd > Zn > Cr > Pb for leaves, and Cd > Cu > Pb > Zn ≈ Cr for bark. The comprehensive bio-concentration index (CBCI) ranged from 0.03 to 0.72 for leaves and 0.06 to 0.74 for bark. The metal accumulation index (MAI) confirmed these findings, with sub-indices ranking Cr > Zn > Cu > Pb ≈ Cd for leaves, and Cu > Zn > Cd > Cr > Pb for bark. Bark showed higher MAI values than leaves (bark/leaf ratio of 1.61), except for Pb, which accumulated more in the leaves. The results of this research demonstrate that F. excelsior exhibits strong TMEs accumulation, particularly in bark, making it a promising species for biomonitoring, phytoremediation, and use as a biological barrier in urban roadside environments.