Further investigations into the use of tree rings as archives of atmospheric mercury concentrations

Because there are limited locations where atmospheric mercury (Hg) measurements are made, a method is needed for characterizing air concentrations locally, regionally and globally. Research has shown growth rings of some tree species may be used as archives that record concentrations of gaseous elemental Hg. However, there are still questions regarding the most appropriate sampling method(s), which tree species to use, and the physiological factors that influence atmospheric Hg uptake and translocation to the rings. To address these questions, tree cores were collected from different tree species at two locations on the eastern side of the Sierra Nevada, California, USA. An assessment was made as to whether there was correlation of ring concentrations between co-located deciduous and coniferous trees, as well as between concentrations measured on the east versus west sides of individual trees, and whether tree ring Hg concentrations correlated with local climate, tree ring width, and total tree ring mass. Broadleaved and coniferous tree ring Hg concentrations were not correlated. Based on results obtained in this and other studies, Pinus species of relatively the same age are recommended for tree ring monitoring in the Western United States; however, the potential for radial translocation still needs to be assessed. Data collected from the two sites focused on were aligned with the overall regional trends in Hg concentrations established from previous studies. Climatic factors, including dew point, temperature, and relative humidity, were negatively correlated with ring Hg concentrations. Tree tissues are active samplers of atmospheric Hg, and are affected by climate and local growing conditions.

Automated summary

Limited locations for atmospheric mercury measurements necessitate the development of a method to characterize air concentrations at different scales. Tree growth rings have been found to serve as archives of gaseous elemental mercury concentrations, but uncertainties remain regarding appropriate sampling methods, tree species selection, and physiological factors affecting mercury uptake and translocation. This study collected tree cores from different species in California to address these questions and found correlations between ring concentrations of deciduous and coniferous trees and local climate factors.