Seasonal variations of metal (Cd, Pb, Mn, Cu, Zn) accumulation in a voluntary species, Salix subfragilis, in unpolluted wetlands

Abstract

Willow trees are generally valuable bioindicators. Flooding during the rainy season affects the life history of Salix subfragilis, a voluntary willow species that grows in submerged wetlands, as it causes senescence of old leaves and emergence of new shoots. In this study, we assessed the potential of S. subfragilis, a Northeast Asian willow species, as a bioindicator. We also investigated seasonal variations in the concentrations of Cd, Pb, Mn, Cu, and Zn in the leaves and stems of willows growing in submerged wetlands containing low concentrations of metals, over the growing season after flooding caused by heavy rain. A significant difference in the concentrations of metals in the sediments was reflected in the leaves for Cd and Pb and in the stems for Cd, Pb, Mn, and Zn. Higher bioconcentration factors in the samples collected in spring suggest that the best monitoring time for S. subfragilis is from April to May. Whereas the concentrations of Cd, Pb, and Mn appeared to increase, especially in the leaves, the concentrations of Cu and Zn in the leaves and stems decreased toward the end of the growing season, regardless of the emergence period. Higher concentrations of Cd, Pb, and Zn were generally noted for the stems than for the leaves whereas Mn and Cu were consistently present at higher concentrations in the leaves than in the stems. These results suggest that S. subfragilis can be a useful bioindicator for Pb, as well as for Cd and Zn. Particularly, samples of new stems collected in spring could provide a good index for environmental pollution.