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 changes This dissertation will examine high-latitude ecosystem responses to warming and atmospheric nitrogen deposition. The following two subsections provides background on these topics. High latitude warming The 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 deposition Humans 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 and oxygen depletion (Howarth and Marino 2006, Conley et al. 2009, Finlay et al. 2013). There is an urgent need to assess the capability of global ecosystems to buffer this nitrogen cycle intensification through nitrogen uptake by organisms and denitrification. Understanding ecosystem response to elevated nitrogen deposition is particularly crucial at high latitudes, where nitrogen limitation is widespread across the boreal forest and arctic tundra biomes (Elser et al. 2007, LeBauer and Treseder et al. 2008, Shaver et al. 2014). Primary productivity and community composition in these systems is highly sensitive to changes in nitrogen availability (Wookey et al. 1994, Chapin and Shaver 1996, Van Wijk et al. 2004, Hobbie et al. 2005). Therefore, future changes in atmospheric nitrogen deposition, which tend to be regional in scale, could restructure ecosystem function and community composition in some areas. Strategies for studying ecosystem response to anthropogenic changes The following two subsections review methods used to research ecosystem impacts of warming and elevated nitrogen availability. The four chapters of this dissertation will both incorporate and build upon some of these methods. Shrub expansion Researchers have employed a variety of experimental and observational techniques to determine environmental drivers of shrub expansion in arctic tundra. Early factorial experiments crossing nutrient addition, warming, and insolation reduction suggested that nitrogen availability was the limiting factor to primary productivity in arctic tundra (Wookey et al. 1994, Chapin and Shaver 1996). This limitation was believed to be caused by low mineralization rates, despite large pools of soil organic nitrogen (Shaver and Jonasson 1999). Nitrogen addition appeared to lend a competitive advantage to species with low nitrogen-use efficiencies, such as deciduous shrubs (Bret-Harte et al. 2001). Subsequent experiments throughout the arctic tundra have further supported this idea (DeMarco et al. 2014). Additional evidence for tundra shrub expansion has been provided through observational techniques including repeat photography and remote sensing (Tape et al. 2006, Myers-Smith et al. 2011). Dendrochronology, the study of annual growth of woody plants, is becoming an increasingly popular observational approach for studying shrub response to climate (Myers-Smith et al. 2015a). This technique can provide a centennial-scale growth record, longer than any experimental manipulation or remote sensing measure in the arctic (Weijers et al. 2010). Further, the chronology of annual ring widths or other anatomical measurements is a vast improvement in temporal resolution over previously used methods such as repeat photography (Tape et al. 2006). Dendrochronological studies have revealed historic relationships between climate and shrub growth in a wide range of shrub growth forms, from prostrate evergreen species, e.g. Cassiope tetragona, to erect deciduous species, e.g. Alnus viridis, Salix pulchra, and Betula nana (Weijers et al. 2010, Blok et al. 2011a, Tape et al. 2012). While dendrochronology appears to be a promising method, challenges remain in connecting climate and environmental variability to individual plant response. The traditional way to analyze arctic shrub dendrochronology data is to construct a site-based, annually-resolved chronology of the central tendency of growth (usually a mean or median of individually detrended ring-width indices) for the sample site (Cook and Pederson 2011). This approach yields a uniform growth signal shared among shrubs at a site, but it discards information related to the variability in climate response among individuals (Galván et al. 2014, Young et al. 2016). An overarching goal of chapters 1 and 2 of this dissertation is to compare microsites and use individual-based statistical modeling frameworks to examine variability in the climate-growth relationship among individual shrubs on the North Slope of Alaska. Nitrogen cycling Long-term measurements of atmospheric nitrogen deposition are available for some high latitude sites, but deposition estimates remain poorly constrained for many regions of the vast arctic tundra and boreal forest biomes (Vet et al. 2014). Spatial models of nitrogen deposition tend to have low, centennial-scale temporal resolution (Galloway et al. 2004). Such models are insufficient for understanding nitrogen dynamics on shorter timescales, because interannual nitrogen deposition in remote areas of the arctic can vary widely based on stochastic weather and atmospheric transport events (Choudhary et al. 2016). This uncertainty complicates the construction of nitrogen budgets for high-latitude watersheds. Chapter 3 attempts to resolve this uncertainty by creating a spatially explicit model of global nitrogen deposition with annual- to decadal-scale temporal resolution. Little is known about the drivers of variability in nitrogen retention among watersheds at a circumpolar scale. Studies on nitrogen retention rates often focus on individual water bodies, rather than whole watersheds (Kankaala et al. 2002). Those that do examine whole watershed-scale retention at large spatial scales are limited to lower latitudes (Schaefer and Alber 2007, Howarth et al. 2012). An improved accounting of environmental controls on nitrogen retention across boreal forest and arctic tundra watersheds, the aim of chapter 4, will shed light on the fate of atmospheric nitrogen inputs to high latitude systems—how much is retained in inland systems, and how much is exported, potentially fueling algal blooms in downstream systems? Outline of the remaining chapters of this dissertation In chapter 1 I use dendrochronology to examine how hillslope position (dry upland versus moist riparian site) influences shrub climate response of the deciduous shrub Salix pulchra on the North Slope of Alaska. I build upon prior research in this field in two ways. First, I allow my calculated climate-growth relationship to vary based on second-order coefficients, as is predicted by ecological theory on diminishing returns to increases in a single growth-limiting factor. Second, I use my dendrochronology data on retrospective growth in combination with established shrub allometry models to predict changes in shrub aboveground primary production under a 2-degree celcius warming scenario. I find that both upland and riparian shrubs respond positively to June temperature, but marginal growth response to temperature is diminishing at the dry upland site, possibly indicative of temperature-induced moisture limitation in particularly warm years. Further, I find that 2 degrees of warming will increase shrub biomass by about 36% at the riparian site, but only by about 19% at the upland site, emphasizing the importance of microsite variability in understanding ecosystem response to climate. In chapter 2 I construct an individual-based linear mixed effects model that incorporates Salix pulchra dendrochronology data from sites across the North Slope that vary not only in hillslope position, but also in glacial landscape age (a proxy for soil nutrient availability). I find that ring growth is remarkably coherent among individuals and sites, responding strongly to June temperature. The strength of this climate response is not systematically related to glacial landscape age. This result indicates a regionally-coherent shrub growth response to early season temperature, with local soil properties exerting only a minor influence on the climate-growth relationship. In chapter 3 I use a global Chemical Transport Model to estimate historic rates of atmospheric nitrogen deposition in the late 20th and early 21st century. From 1984 to 2016, I find that global inorganic nitrogen deposition increased by 8%, but trends varied regionally. For example, deposition declined in the European boreal and sub-arctic zones, while deposition increased in Western Canada and Eastern Siberia. Quantifying these spatially-explicit trajectories of change at high latitudes can help us predict how recent and future trends in nitrogen deposition impact ecosystem processes like primary productivity, species turnover, and nitrogen retention. In chapter 4 I use the model results from the preceding chapter to conduct a systematized review of the environmental factors influencing nitrogen retention in watersheds throughout the circumpolar north. I find that mean annual air temperature is positively related to the proportion of atmospherically deposited nitrogen retained. Other variables including watershed area, annual runoff, and watershed soil properties did not have an apparent effect on retention rates. This outcome suggests that future warming could raise the proportion of nitrogen retained in boreal and tundra watersheds, favoring plant species with low nitrogen-use efficiencies while reducing nitrogen export. However, this effect may be reversed by rapid permafrost thaw, which could mobilize and export large stores of soil nitrogen.
University of Minnesota Ph.D. dissertation. May 2019. Major: Ecology, Evolution and Behavior. Advisors: Jacques Finlay, Daniel Griffin. 1 computer file (PDF); v, 132 pages + 1 supplementary table
Anthropogenic impacts on high-latitude ecosystems: Shrubs will grow. Will nitrogen flow?.
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