Urban trees for reconstructing historical trends in atmospheric mercury pollution

Tree rings have been used to reconstruct historical atmospheric mercury (Hg) pollution, but the suitability of tree species varies. Species considered suitable, such as European larch and spruce spp., are not always available in urban environments. This study investigated tree species commonly found in and around cities in Central Europe—black locust, Norway maple, cherry and Douglas fir. We compared Hg concentrations in the bole wood and foliage of these species with those of European larch. Samples were collected in Ústí nad Labem, a Czech city with a history of severe Hg pollution. While Hg concentrations in foliage were similar across all species, we observed considerable variation in Hg concentrations in bole wood. On average, the broadleaved species had eight times less Hg in their bole wood than the conifers, likely due to differences in Hg transfer from the foliage. Among the species analysed, Douglas fir showed potential for Hg dendrochemistry, with high tree-ring Hg concentrations, minimal radial translocation and low variability between trees. Mercury concentrations in the tree rings of cherry and Norway maple correlated with atmospheric Hg emissions; however, high inter-tree variability in cherry and low Hg concentrations in maple limited their reliability. Black locust was found unsuitable, as its tree-ring Hg trends were inconsistent with atmospheric Hg emissions. In urban areas where larch or fir trees are unavailable, it may still be preferable to sample these species from nearby locations rather than rely on the broadleaved species closer to the pollution source. • Different tree species were tested for Hg accumulation in tree rings and foliage • Cherry tree rings showed high inter-tree variability in Hg concentrations • Norway maple had low Hg concentrations and black locust showed radial translocation • Douglas fir and European larch both appeared to be suitable for Hg dendrochemistry