Impact of Air and Soil Exposures on Hg Concentrations in Foliage, Bark, and Tree Rings and Development of a Tree Ring Lesson Plan for Middle School Students
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Authors
Arnold, Jennifer D.
Issue Date
2017
Type
Thesis
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Keywords
dendrochemistry , gaseous oxidized mercury , HgBr2 , Pinus nigra , Populus tremuloides , total gaseous mercury
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Abstract
Vegetation plays a key role in the biogeochemical cycle of mercury (Hg) as it is an important sink. The United Nations Environmental Program (UNEP) Global Mercury Assessment of 2013 estimated that 3200 Mg of Hg was deposited to land and freshwater annually. Obrist (2007) estimated 1000 Mg of Hg globally is deposited to vegetation; this means 1/3rd of the global Hg budget is taken up by vegetation. Trees assimilate this pollutant into tissues-foliage, bark, roots, and bole, but mechanisms of uptake are not clear nor is what happens to accumulated Hg. The work presented in Chapter 1 focused on assessing the influence of different air total gaseous Hg exposures and soil Hg spikes on tree tissue (bark, rings and foliage), Hg concentrations of Pinus nigra (Austrian Pine) and Populus tremuloides (Quaking aspen) saplings. Furthermore, we tested the influence of different compounds of gaseous oxidized mercury (GOM) in air, and a soil spike of HgBr2 on the uptake of Hg into tree tissues. The overall objective of work done for Chapter 1 was to investigate whether tree rings can be used as reliable spatial and-or temporal proxies of atmospheric Hg concentrations. This experiment was conducted by determining dose-response relationships in three separate greenhouse bays and one outdoor exposure that contained different atmospheric concentrations and compounds of gaseous oxidized Hg. Results from this study and others indicate soil Hg concentrations had no impact on Hg concentrations in above ground tree tissues. Air Hg concentrations had a significant effect on foliage and ring Hg concentrations measured after one year of growth in the different air exposures. We found an increase in Hg concentrations among previously formed consecutive growth years of Pinus indicating movement of Hg throughout the active sapwood. Whether this is radial translocation, defined as a change in concentrations of Hg within growth rings previously formed, or lateral re-equilibrium, defined as the movement or smearing of Hg across active growth rings needs to be further examined. The use of tree rings for monitoring of temporal Hg trends should consider translocation by way of sapwood because sapwood is still active and there are vertical flows of elements across the entire sapwood band. Foliage and the most current growth year could be used for monitoring of spatial trends as total gaseous Hg concentrations were found to influence Hg concentrations in foliage and the growth ring that grew in this experiment under the different air exposures. The work presented in Chapter 2 includes a tree ring and pollution lesson plan for the Nevada Museum of Natural History located on the University of Nevada, Reno campus. This lesson plan teaches techniques of dendrochronology and dendrochemistry as well as provides data allowing students to investigate the changes in Hg pollution over time. Teachers will be provided background information on dendrochronology, dendrochemistry, plant physiology, and Hg pollution.
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