Using deciduous trees as bioindicators of trace element deposition in a small urban watershed, Indianapolis, IN, USA

Annual and multiyear records of trace element deposition are difficult to develop using monitoring systems but have proven feasible using plant material in several settings. Here, we used material from several tree species (Populus deltoides W. Bartram ex Marshall, Platanus occidentalis L., and Ginkgo biloba L.) to detect atmospheric deposition of trace elements (Cd, Cu, Pb, and Zn) in six localities along a transect from near-urban to far-urban in southeastern Indianapolis, IN, and one control site. We captured soil (legacy footprint), bark (multiannual record), and leaves (seasonal record) across a broad swath of the urban landscape and using a multi-metal approach. Tree bark, leaf, and proximal soil samples were collected and analyzed for their trace element content. The highest trace metal concentrations occurred at the near-urban sites, with particularly high Cu and Pb values. The highest Zn values were found at one of the far-urban sites, which is located near a large brownfield that was a former coal and coke storage and processing facility. No correlation was found between soil trace element composition and that of bark and leaves, perhaps indicating a disconnect between legacy inputs recorded in soils and current inputs recorded in the biological materials. Overall, the tree species analyzed served well as trace element bioindicators, although less so for G. biloba, and thus this approach is promising for further understanding the role that airborne pollution and deposition play in urban watersheds.