What influences Hg uptake and movement in tree rings?

Tree rings are useful archives of local source mercury (Hg) emissions and regional/global trends in atmospheric elemental mercury (Hg⁰) concentrations. Archives that record trends in atmospheric Hg deposition include sediment, peat, and ice cores. Despite the fact these archives record different processes, trends in Hg concentrations compare favorably. High temperatures and relative humidity, water vapor deficit, and low soil moisture can decrease atmospheric Hg⁰ uptake by foliage, and thus, reduce Hg uptake into tree rings formed under those conditions. Soil nitrogen promotes Hg⁰ uptake in leaves and nitrogen compounds are associated with transport to other tissues. An important confounding factor for relating trends in tree-ring and atmospheric Hg⁰ is radial translocation. A case study is presented that entailed collection of Jeffrey pine (Pinus jeffreyi) tree rings after a wildfire event. The tree rings recorded an atmospheric Hg⁰ spike associated with the fire year. Radial translocation was observed. The recorded spike was accentuated due to higher concentrations in the rings adjacent to the bark. Dendrochemical analysis of tree rings at an annual resolution is essential for accurate detection of discrete disturbance events, such as wildfire. To assess whether radial translocation has influenced Hg concentrations in a tree-ring series, comparison to known emission events or other archives is needed, due to the many physiological factors that influence Hg concentrations in tree rings.