Application of tree rings [dendrochemistry] for detecting historical trends in air Hg concentrations across multiple scales

Abstract

Tree rings are a useful long term monitor of climate change indicators and certain atmospheric constituents. Since we have no long-term historical record of air mercury (Hg) concentrations, and previous research demonstrates atmospheric uptake of Hg into vegetative biomass, we hypothesized that tree rings would be a useful archive for monitoring changes in local, regional, and global air Hg concentrations. We developed and tested a method for measurement of Hg in tree rings, and then investigated our hypothesis using species within the genus Pinus. Mercury concentrations declined along a coast-to-interior (California-to-Nevada, USA) gradient with highest mean concentrations (5.7 ng/g) at the coast, and lowest (<1.2 ng/g) at inland locations. Tree ring Hg concentrations recorded the impact of local sources and indicate increasing global concentrations over time. The data also indicate that concentrations are higher in areas impacted by marine air, and this suggests that the atmospheric chemistry is influencing assimilation. Future research should address species- and age-specific variation in the sensitivity of tree-ring samples as proxy data for atmospheric Hg concentrations. Tree rings show great potential as passive recorders of atmospheric Hg for long time periods (centuries to millennia) at time steps of ≤5 years.