Master of Science
Environment and Sustainability
Branfireun, Brian A.
Mercury (Hg) is a naturally occurring element with a complex biogeochemical cycle. Forests act as net sinks for both carbon (C) and Hg as foliage accumulates Hg with time. Litterfall represents a main input of C and Hg into forest soils. My aim was to investigate how foliage type (coniferous, deciduous) governs the input of Hg into forests with a field-based study that measured Hg accumulation over a growing season, and then investigate the storage and release of Hg from foliar tissues into soil with a laboratory-based incubation experiment. Results from the field-based study demonstrate deciduous leaves have more linear Hg uptake rates than conifer needles after the first growing season. Results from the incubation study suggest that Hg release is a function of decomposition influenced by litter type. Understanding how vegetation influences Hg cycles in forests is important for understanding how climate change will impact forest Hg cycles.
Summary for Lay Audience
Mercury (Hg) is a metal that can be found everywhere in the natural environment. The atmosphere transports gaseous Hg globally, which can then be incorporated into foliage. As such, Hg accumulates in vegetation (e.g. leaves and needles) over time, and enters the soil system when this plant tissues die or senesce, such as during autumn litterfall events. The overall goal of my thesis was to investigate the controls on Hg input, storage, and release in tree vegetation in the boreal forest. My thesis first investigated the rates of Hg accumulation in two common boreal forest tree types representing conifer needles (black spruce) and deciduous leaves (white birch). The deciduous leaves accumulated Hg relatively linearly over the growing season, however the conifer needles did not, probably because deciduous leave display a more continuous growth pattern. My second objective was to investigate how Hg is released from leaves and needles into soil during decomposition and the influence of temperature on this release. I conducted a laboratory-based experiment to monitor Hg concentrations that were released from the leaves and needles for three months. My results suggest that litter type controls decomposition and subsequent Hg release rates from leaves and needles. Overall, because of a higher rate of Hg accumulation, larger annual litter inputs, and more rapid release from the tissues, deciduous leaves cycle Hg through forest systems at different rates than coniferous needles. Understanding Hg flux (uptake and release) in forest systems based on dominant vegetation type is important for forecasting how Hg cycles will change and lead to better predictions for the recovery times of contaminated watersheds. My results are significant because the dominate tree type in forests is changing as deciduous tree ranges shift northward under climate warming. Coupled with an increase in temperature, the implication is a greater Hg flux (uptake and release) in the boreal forest in the near future.
Anderson, Madelaine J R, "Foliage Type Controls Mercury Input, Storage, and Release in the Boreal Forest" (2020). Electronic Thesis and Dissertation Repository. 7268.
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