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▼a Human activity is a dominant force shaping the structure and function of Earth's ecosystems. Some of these impacts are direct and local: phosphorus-rich runoff from urban landscapes causes excessive siltation, eutrophication, and biodiversity reduction in waterways (Carpenter et al. 1998, Paul and Meyer 2001). Other impacts are indirect and regional: ozone precursors released from vehicles in cities can travel downwind and form ozone harmful to plants, animals, and people (Keiser et al. 2018). Still other anthropogenic impacts have profound consequences worldwide, namely fossil fuel combustion and the release of carbon dioxide, a greenhouse gas, into a globally-mixed troposphere (Hayhoe et al. 2018).Due to these teleconnections between human action and environmental impact, ecosystems that appear remote from direct human modification are still molded by our societal behavior. This is particularly true of northern high-latitude ecosystems, including arctic tundra and boreal forest. Cold temperatures and nitrogen limitation of terrestrial primary productivity make these biomes highly sensitive to warming and atmospheric nitrogen deposition, two widespread impacts of fossil fuel combustion (Elser et al. 2007, LeBauer and Treseder et al. 2008, Elmendorf et al. 2012a, Shaver et al. 2014).Background on anthropogenic changesThis dissertation will examine high-latitude ecosystem responses to warming and atmospheric nitrogen deposition. The following two subsections provides background on these topics.High latitude warmingThe arctic tundra biome is warming twice as fast as the global average (Cohen et al. 2012). Paleoclimate proxies indicate that such polar amplification has been a reliable response to rising temperatures for at least the last three million years of Earth's climate history (Miller et al. 2010). This phenomenon is largely caused by the ice-albedo feedback, but other contributing factors include changes in oceanic and atmospheric heat transfer, increased near-surface cloud cover and atmospheric water vapor, and soot deposition (Serreze and Barry, 2011).A rapidly warming arctic is a source of considerable uncertainty in climate projections due to poorly constrained biophysical carbon cycle feedbacks. Soils of high-latitude permafrost regions contain nearly 1,700 Pg of organic carbon, equivalent to roughly double the global atmospheric carbon pool (Tarnocai et al. 2009). As permafrost thaws, much of the permafrost organic carbon pool will be metabolized by microbes into atmospheric methane (in wet environments) or carbon dioxide (in dry environments). The magnitude and rate of change in land-to-atmosphere carbon fluxes from permafrost remains a topic of vigorous debate-"Methane bomb" media headlines may be too sensationalized (Petrenko et al. 2017), but a gradual and sustained carbon release (Schuur et al. 2015) may be too optimistic.In addition to permafrost carbon feedbacks, high-latitude vegetation changes could trigger both positive and negative feedbacks to climate change. For example, the expansion of woody deciduous shrubs in circumarctic tundra could accelerate climate change through positive feedback mechanisms including albedo reduction, competitive exclusion of permafrost-insulating Sphagnum spp., and the capture of deep snowpacks that increase microbial respiration through elevated winter soil temperatures (Sturm et al. 2001, Cornelissen et al. 2001, Blok et al. 2011a, Chapin et al. 2005, Lawrence and Swenson 2011). Alternatively, shrub expansion could mitigate climate change through negative feedback mechanisms including increased primary productivity, woody stem production, recalcitrant litter chemistry, and shading of carbon-rich permafrost soils in summer (Shaver 1986, Sweet et al. 2015, Cornelissen et al. 2007, Blok et al. 2011a, Nauta et al. 2015). While the balance of such positive and negative feedbacks remains an area of active research, integrating these feedbacks into earth systems models will ultimately require accurate predictions of shrub expansion rates across space. In other words, shrub expansion has not and will not proceed at a uniform pace throughout the tundra. Chapters 1 and 2 of this dissertation seek to determine which factors underlie the variable rates of shrub expansion across environmental gradients in arctic tundra.Nitrogen depositionHumans have doubled the amount of bio-reactive nitrogen entering the biosphere in the industrial era through fertilization, biomass and fossil fuel combustion, and volatilization from agricultural materials such as manure (Galloway et al. 2004, Galloway et al. 2008). Despite major improvements in agricultural production, the rapid increase in nitrogen availability has wide-ranging consequences for human health, ecosystem function, and community structure. For example, nitrate-loading in groundwater aquifers in highly fertilized agricultural regions is linked to methemoglobinemia (blue-baby syndrome) and is believed to be a risk factor for certain cancers and birth defects (Spalding and Exner 1993, Weyer et al. 2008, Brender et al. 2013). In nitrogen-limited terrestrial ecosystems, increased availability boosts primary productivity, often causing community shifts toward dominance by species with low nitrogen-use efficiencies (Stevens et al. 2004, Clark and Tilman 2008). Elevated delivery of nitrogen to nitrogen-limited (or nitrogen and phosphorus co-limited) freshwater and marine ecosystems can fuel algae blooms that harm plants and animals through light attenuation. (Abstract shortened by ProQuest). |