Abstract: [Objective]The pollution of heavy metals emitted from vehicles can be absorbed by ornamental trees along highway. In this paper, the variation of biological accumulation of heavy metals by ornamental trees with different species at different vertical distance to highway was studied to provide reference for the construction of tree belts with phytoremediation opinion.[Method] Four ornamental tree species of Eucalyptus robusta, Nerium indicum, Ligustrum lucidum, and Cinnamomum camphora were chosen as object of study, and they were grown at 0-5 m and 10-60 m away from the Yong-Tai-Wen highway. The leaves and twigs of trees were sampled for determining contents of Pb, Cd and Cr. The measurement data were analyzed with box-whisker scatter distribution and principal components.[Result] No interactive effects between distance and tree-species were detected on any heavy metal contents. However, contents of Pb and Cr from trees located at 0-5 m away from the highway were higher than those at 10-60 m, with more than 160% higher on average. Heavy metal contents were different among tree species. E. robusta and C. camphora had higher content of Pb in leaves than the other two species (P=0.0007), and twig Pb content in E. robusta was 92% higher than that in L. lucidum (P=0.0004), but the content was 80% lower than that in N. indicum twig (P=0.0371). The accumulated contribution of the first three principle components (PCs) to variation ranged from 71%-75% in terms of the factor of distance, while that ranged from 92%~96% in terms of the factor of species. As for the distance factor, the 1^st PC was characterized by Cd and Cr accumulations in twigs and branches, but the 2^nd and 3^rd PCs were both characterized by Pb accumulation, while the eigenvalue of 3^rd PC was negatively correlated with Cd content (R=-0.954; P=0.046). For the species factor, the 1^st PC was characterized by Pb accumulation in leaves, twigs, and branches, whose eigenvalue was positively correlated with Cr content (R=0.682; P=0.039). The absolute values of eigenvalues for Pb content in E. robusta and C. camphora were higher when composing the analysis between the 1^st and 2^nd PCs, while those for Cr and Pb were evenly distributed with no conclusive data trait.[Conclusion]According to differences between distances away from the highway and the scattered results in tree species comparisons, E. robusta and N. indicum are suggested to be proper for the ornamental tree belt construction along the highways in the study area, for the purpose of potential phytoremediation of heavy metals.

Key words: bioaccumulation, phytoremediation, box-whisker plots, principal component analysis