Atmospheric mercury pollution recorded in conifer tree rings: Disentangling the effects of tree-ring width, water content, and climate on mercury concentrations

Abstract

Mercury (Hg) is a contaminant of major concern for public and ecosystem health, known for its toxicity and bioaccumulation in the environment. Reconstructing past Hg pollution, as well as monitoring current Hg concentrations, is therefore essential. Tree rings offer a valuable archive for reconstructing historical atmospheric Hg pollution, yet the reliability of dendrochemical Hg reconstructions depends on understanding all factors that influence Hg accumulation in the xylem. This study investigated whether factors beyond atmospheric Hg concentrations, i.e., climatic variables (temperature and precipitation), tree-ring width, and water content, influenced Hg concentrations in the tree rings of European larch and Douglas fir growing near a chlor-alkali plant in the Czech city of Ústí nad Labem. Both tree species showed similar long-term trends in Hg concentrations, broadly reflecting historical trends in atmospheric Hg pollution. However, short-term (high-frequency) variability in tree-ring Hg concentrations remaining after detrending was associated with other factors. A positive correlation between water content and Hg concentrations suggested that a fraction of the Hg was retained in xylem sap. In European larch heartwood, Hg concentrations correlated positively with tree-ring width, likely due to heartwood formation processes. In Douglas fir, a positive correlation between August-September precipitation and tree-ring Hg suggested that this tree species’ isohydric behaviour affected Hg uptake. These findings indicated that while tree-ring Hg concentrations can serve as proxies for historical atmospheric Hg pollution, other factors, such as tree-ring width, water content, and late-summer precipitation, may modulate the signal and should be considered in dendrochemical Hg reconstructions.