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Assessment of the Suitability of Tree Rings as Archives of Atmospheric Mercury Pollution using Tree Cores and Results of a controlled field experiment to assess the use of tree tissue concentrations as bioindicators of air Hg
AdvisorGustin, Mae S.
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Mercury (Hg), a toxic metal and known neurotoxin, is a widespread environmental contaminant. The element, and its speciated compounds, are released naturally through geothermal and volcanic activity, but global concentrations have increased due to anthropogenic sources such as coal-fired power plants and mining activity. Because of its ability to stay aloft in the atmosphere, Hg may travel vast distances from point sources, contaminating aquatic and terrestrial ecosystems. Atmospheric Hg may be assimilated via biological processes and sequestered within the tissues of trees.The work presented in Chapter 1 explored whether tree rings are suitable archives for atmospheric Hg concentrations. Methodology, and the utility of dendrochemistry for the spatial and temporal assessment of atmospheric mercury were explored. We analyzed tree cores from California and Nevada, USA, choosing locations based on proximity to, or remoteness from, known past and present sources of Hg emissions. By revisiting National Park and BLM lands that were previously sampled, stability of Hg concentration over time was investigated through a dataset comparison. For the sites where sampling was successfully duplicated, similar historical trends were observed, suggesting that Hg concentrations are temporally consistent for the upland tree species; Pinus jeffreyi, Pinus ponderosa and Pinus monticola. Other tree species that occur in arid regions with low primary productivity (Pinus monophylla) are less suitable for use as temporal Hg archives.The work presented in Chapter 2 examines the effect of local ambient air Hg concentrations on accumulation in tree tissue, and was investigated using a 2-year field experiment that entailed movement of nursery trees (Pinus nigra) from Oregon, to strategically chosen locations in Nevada and California. Needles, bark, and tree rings were sampled at regular intervals and analyzed for THg to survey concentration differences between growth occurring pre-experimental placement and growth occurring at each discrete location. Needle Hg concentrations increased significantly relative to the control. Concentrations for spring needles were low and increased during the growing season, indicating potential resorption. In the first and second years, both inner and outer bark concentrations increased significantly, as did the outermost tree rings. HgBr₂ root spiking treatments produced no significant impacts for above ground tree tissues, following. The results from this study highlight the potential for Pinus nigra needles, bark, and tree rings to serve as widespread and cost-effective proxies for atmospheric mercury concentrations, also aiding in the scientific comprehension of tree Hg uptake and behavior, post assimilation.