<p> I examine how two different realizations of bed morphology affect Thwaites Glacier response to ocean warming through the initiation of marine ice sheet instability and associated grounding line retreat. A state of the art numerical ice sheet model is used for this purpose. The bed configurations used are the 1-km resolution interpolated BEDMAP2 bed and a higher-resolution conditional simulation produced by John Goff at the University of Texas using the same underlying data. The model is forced using a slow ramp approach, where melt of ice on the floating side of the grounding line is increased over time, which gently nudges the glacier toward instability. Once an instability is initiated, the anomalous forcing is turned off, and further grounding line retreat is tracked. </p><p> Two model experiments are conducted. The first experiment examines the effect of different anomalous forcing magnitudes over the same bed. The second experiment compares the generation and progress of instabilities over different beds. Two fundamental conclusions emerge from these experiments. First, different bed geometries require different ocean forcings to generate a genuine instability, where ice dynamics lead to a positive feedback and grounding line retreat becomes unstable. Second, slightly different forcings produce different retreat rates, even after the anomalous forcing is shut off, because different forcing magnitudes produce different driving stresses at the time the instability is initiated. While variability in the retreat rate over time depends on bed topography, the rate itself is set by the magnitude of the forcing. This signals the importance of correct knowledge of both bed shape and ocean circulation under floating portions of Antarctic ice sheets. The experiments also imply that different ocean warming rates delivered by different global warming scenarios directly affects the rate of Antarctic contribution to sea level rise.</p>

<p>This dissertation examined the response to termination of CO2 enrichment of a forest ecosystem exposed to long-term elevated atmospheric CO2 condition, and aimed at investigating responses and their underlying mechanisms of two important factors of carbon cycle in the ecosystem, stomatal conductance and soil respiration. Because the contribution of understory vegetation to the entire ecosystem grew with time, we first investigated the effect of elevated CO2 on understory vegetation. Potential growth enhancing effect of elevated CO2 were not observed, and light seemed to be a limiting factor. Secondly, we examined the importance of aerodynamic conductance to determine canopy conductance, and found that its effect can be negligible. Responses of stomatal conductance and soil respiration were assessed using Bayesian state space model. In two years after the termination of CO2 enrichment, stomatal conductance in formerly elevated CO2 returned to ambient level, while soil respiration became smaller than ambient level and did not recovered to ambient in two years.</p> / Dissertation

<p> Seabird populations are declining worldwide. Two major causes of decline are non-native predators and seabird (indirect or direct) overexploitation; these have been suspected but not assessed in the Caribbean region, including in the southern Grenadine islands. Using cameras, questionnaire surveys, and nest monitoring, I explored the potential of invasive predator presence and seabird harvest in affecting nesting productivity of five species in five uninhabited islands during 2014&ndash;2017. Overall, nesting productivity increased (n = 567 nests) despite the confirmed presence of non-native rats. However, my survey of Grenada residents (n = 32 responses) and physical evidence found on the islands (e.g., gun shells) suggest that seabird harvest still occur and may be responsible for previous declines. Additionally, most contributors to harvest did not seem aware of any seabird-protective laws. Therefore, I recommend establishing a community-based monitoring program that educates and empowers residents to prevent seabird harvest and continue nest monitoring. </p><p>

In 2006, the Minister of Fisheries and Oceans Canada (DFO) accepted an industry proposal called the Commercial Groundfish Integration Pilot Program (CGIPP), which integrated each of the seven commercial groundfish fisheries in British Columbia. The industry proposal, developed by the Commercial Industry Caucus (CIC), was the result of guiding principles developed by DFO that focused on the conservation of certain rockfish species on the Pacific Coast. If industry had not developed a plan, DFO would have developed an alternative fishing plan (AFP). This thesis evaluates whether DFOs conservation objectives were met under the CGIPP and if there were any social and economic impacts. These same impacts are compared to what may have occurred under the AFP. The results of this thesis indicate that the CGIPP is a sustainable template for multi-species commercial fisheriesa first step in achieving an ecosystem-based approach to fisheries management.

In 2006, the Minister of Fisheries and Oceans Canada (DFO) accepted an industry proposal called the Commercial Groundfish Integration Pilot Program (CGIPP), which integrated each of the seven commercial groundfish fisheries in British Columbia. The industry proposal, developed by the Commercial Industry Caucus (CIC), was the result of guiding principles developed by DFO that focused on the conservation of certain rockfish species on the Pacific Coast. If industry had not developed a plan, DFO would have developed an alternative fishing plan (AFP). This thesis evaluates whether DFOs conservation objectives were met under the CGIPP and if there were any social and economic impacts. These same impacts are compared to what may have occurred under the AFP. The results of this thesis indicate that the CGIPP is a sustainable template for multi-species commercial fisheriesa first step in achieving an ecosystem-based approach to fisheries management.

<p>Uptake and storage of carbon by forest ecosystems continues to be a major research topic needed for the quantification of global budgets in an increasing atmospheric carbon dioxide environment. However, there are considerable challenges in quantifying carbon budgets of forest across a wide range of spatial and temporal scales. Although general trends in the components of carbon budgets emerge when analyzed over large spatial or temporal scales, these relationships tend to weaken, or even reverse, at smaller spatial (e.g. stand level) and temporal scales. On the other hand, continuous measuring and monitoring is not a feasible or sensible approach for the range of global forests. There is growing need to identify the key variables that drive variability in these localized budgets at multiple time scales. These results will assist in upscaling stand-level observations into large-scale modeling approaches. </p><p>Forest carbon dynamics are closely-coupled with the hydrologic cycle, so an approach that attempts to bridge these dynamics must incorporate water availability and use. Water is necessary for trees to transport nutrients, maintain cellular function, and regulate stomatal conductance; however, water is also related to other biological processes, including microbial decomposition of soil carbon, and physiologically-important abiotic factors, such as atmospheric vapor pressure deficit. Thus, much of the key to understanding the variability in forest carbon cycles is identifying the sensitivity of the processes of the carbon cycle to water availability. </p><p>Therefore, my research takes the following approach: I begin by using sap flux sensors to measure tree-level transpiration over a four-year period and combine these values with other estimates of stand-level evaporation to generate an accurate estimate of total evapotranspiration, partitioned by component and tree species (Chapter 2). To assess the sensitivity of the water fluxes in the forest, I next establish a complete hydrologic budget for the forest stand over four years, including one severe and one mild drought (Chapter 3). I then focus on the flux of carbon from the soil and its variability over space and time. Using automated, high-frequency measurements of soil CO₂ flux over a 10-year period and including 3 forest stands, I assess inter- and intra-stand variability as well as inter- and intra-annual variability in soil flux in relation to climatic factors and stand characteristics representing productivity (Chapter 4). In order to assess how soil CO₂ flux may change over longer periods of time within the context of global change, I analyze how enrichment of [CO₂] independent of and combined with soil nitrogen availability alter the balance of carbon in a stand (Chapter 5). Finally, building off these previous chapters, I examine the relationship between carbon uptake, allocation, and turnover in a mixed-species forest experiencing interannual variability in water availability (Chapter 6). </p><p>I conclude that (Chapter 2) sap flux sensors can successfully be used to estimate tree- to stand-level transpiration if one accounts for both nocturnal water movement through the tree stem and spatial variability of species composition and demography within a stand. (Chapter 3) Despite reductions in transpiration by some species during water-limited (i.e. drought) periods, the magnitude and duration of these reductions results in annual water use that is similar to a non-drought year. The consequence of this invariability in transpiration and evapotranspiration for the hydrologic cycle is that changes in annual precipitation translate directly to changes in water supplied to rivers and streams. (Chapter 4) Diurnal to seasonal variability in soil CO2 flux is driven by temperature, whereas interannual variability is most-strongly influenced by soil moisture. Furthermore, spatial variability of soil CO₂ flux is directly related to forest productivity, and by proxy, leaf production, across biomes and, to a lesser extent, across stands within a region. However, within-stand variability may be inversely related to leaf production as a result of differential allocation of carbon between aboveground and belowground uses based on local resource availability. (Chapter 5) Although elevated atmospheric [CO₂] enhances productivity, it may only result in a small increase in the flux of CO₂ from soils. Instead, nitrogen availability explains much of the variability within a forest stand, regardless of [CO₂], with increasing nitrogen availability resulting in lower allocation of carbon belowground and greater aboveground productivity. (Chapter 6) Interannual variability in water availability can affect gross primary productivity in mature forests but these effects may primarily affect the following growing season. The proportionate changes in gross primary productivity appears to show greater reductions with previous year's soil moisture than net primary productivity, leading to increased carbon use efficiency following drought. Variability in leaf biomass in this relatively stable, mature stand appears to drive the interannual variability in photosynthesis as well as the demand for carbon used for biomass production and metabolic activity.</p> / Dissertation

<p>Necessary for the diversity and survival of most terrestrial plants, arbuscular mycorrhizae (AMF) are fungi that form mutualistic symbiotic relationships with approximately 90 percent of terrestrial plant families. While the biodiversity and abundance of plants and animals have received much attention, these patterns for the belowground organisms on which they rely, such as AMF, remain poorly understood. While studies have found indications that AMF are fundamental to ecosystem structure and function, relatively few of these studies have been conducted in situ. In their ability to accommodate the complexity found in natural ecosystems, in situ studies may be vital in providing information relevant to the restoration and conservation of ecosystems. This thesis sought to explore in situ how AMF diversity and root colonization changed across ecosystems along an elevation gradient in Southern California. The findings indicate that certain soil parameters may be especially influential and that intra-species competition may play a role in AMF root colonization.

<p> The Luquillo Forest Dynamics Plot (LFDP) has played a critical role in the initial discovery and subsequent investigation of many processes that govern tropical island wet forest dynamics. Previous work has identified past land use as the main factor in creating forest community compositional and structural differences across the plot. The responses of different species to past land-use intensity and to hurricane disturbances have created an evolving forest mosaic ideal for studying tropical forest successional dynamics. I revisited the interaction of land-use legacies and natural disturbance in the LFDP with new data and new approaches, with the motivation to reveal new information about the relative roles of anthropogenic disturbance and environmental-niche partitioning on tropical plant communities over time. </p><p> In the context of tropical forests and their successional dynamics, I asked how succession resulting from a history of human land use and more recent hurricanes interacts with background environmental variation to effect community structure and diversity. Community dynamics, in terms of forest structure and composition, were summarized over a twenty-one year period, noticing a decreasing trend in species richness over time and structural maturation of the forest, shown by a decline in small stems (trees &lt; 10 cm diameter) as it recovered from the compound effect of two major hurricane disturbances &ndash; Hugo, 1989 and Georges, 1998. We evaluate the magnitude of past human land use effects over time and define indicator species for areas of differing land-use pressure within the 16-Ha permanent LFDP. Using redundancy analysis, plant community-environmental relationships with respect to soils and topography are quantified. Spatial variables, computed using a principle coordinates of neighborhood matrix, explained the majority of the variability in plant community composition between areas of high and low past land-use within the LFDP, meaning environmental differences (e.g. niche differentiation among tree species) were found to be secondary to land-use legacies in determining forest community composition. </p><p> Over two decades, the effect of past land-use peaked about 15-years following the first of two hurricanes, and remained relative stable over time. Despite damaging the forest, hurricanes preserved community differences in species composition and reinforced structural asymmetries due primarily to two species; <i> Dacryodes excels</i> Vahl., a dominant primary forest tree species, and <i> Casearia arborea</i> (Rich.) Urb., an abundant secondary forest species. Abiotic environmental factors (e.g. soil resources and topographic variation) were weak at explaining differences in forest community composition. Plant community-environmental relationships were stronger in more anthropogenically-disturbed areas, suggesting long-term effects of land use on tropical forest communities on current community dynamics.</p>

<p>Microorganisms, particularly the Bacteria, are differentially impacted by metal toxicities, and will respond very quickly to changes in their environment, making them ideal bioindicators of environmental health. In this study, we evaluated the sediment-associated bacterial diversity of fifty-seven samples collected from twenty-four anthropogenically and endogenously metal impacted, geographically distinct sites in the Colorado Mineral Belt, and elucidated the factors that correlated with observed differences in the bacterial community structure. Overall, the geochemistry of all sites distinguished anthropogenic from endogenous sources of metal impact. Anthropogenic samples, on average, had higher concentrations of total recoverable and dissolved sodium and magnesium, and lower concentrations of aluminum and zinc, compared to the endogenous samples. Bacterial communities from both anthropogenically and endogenously metal impacted sites were characterized using Illumina high-throughput amplicon sequencing of the V4 region of the 16S rRNA gene. Overall, bacterial communities were remarkably diverse, with endogenously metal impacted sediments having higher diversity compared to anthropogenic sediments. The <i>Actinobacteria </i> and <i>Betaproteobacteria</i> dominated anthropogenic samples, and the <i>Acidobacteria</i> and <i>Deltaproteobacteria </i> dominated endogenous samples. Clustering of bacterial communities based on membership and structure (presence/absence and relative abundance of particular taxa, respectively) also distinguished samples based on their source of metal impact. Analysis of similarity (ANOSIM) tests indicated a significant difference between bacterial community structure based on source of metal impact (weighted UniFrac R_ANOSIM = 0.746, p = 0.001). Mantel tests indicated that total recoverable magnesium concentrations accounted for &sim;54% of variance in community structure of all bacterial communities in the study. Dissolved aluminum concentrations accounted for &sim;71% of the variation in all communities with an anthropogenic source of metal, and dissolved aluminum concentrations also accounted for &sim;41% of the variation in bacterial communities with endogenous sources of metal impact. </p><p> This study provides one of the first direct comparisons between microbial community structures of sediments based on source of metal impact. This study is also one of the first comprehensive characterizations of bacterial communities from naturally occurring iron fen systems. </p>

Field observations show daytime HONO levels in urban, rural and remote environments are greater than those expected at photostationary state, that is, balance between production by \(NO+OH\) reaction and loss by UV-photolysis and OH-oxidation. Studies have interpreted measurements of \([HONO]_{Obs} > [HONO]_{PSS}\) - or equivalently, the rate of HONO loss exceeding that of production - as evidence of a missing, sunlight-driven HONO source. Formation rate inferred from assuming photostationarity indicate a significant source of HONO, which photolyzes to yield OH. Moreover, depending on the mechanism, it may represent a pathway by which deposited nitrogen oxides are repartitioned back into the atmosphere in reactive form. The accumulation of HONO beneath the nocturnal boundary layer initiates photochemistry in the early morning prior to other \(HO_x\) precursors. Previous studies have estimated nighttime HONO production rate by attributing the increase in \(HONO:NO_x\) solely to heterogeneous HONO formation, while treating \(NO_x\) as an invariant. Moreover, because ambient \(HONO:NO_x\) exceed what is observed in automobile exhaust, combustion sources are discounted. In May of 2009, we observed HONO and \(NO_2\) mixing ratios in Houston, Texas during the SHARP campaign. We demonstrate – using a chemical box model – that photostationary state during daytime is not fully established. The reaction/transport time since emission from automobiles is short relative to the lifetime of HONO. The result of assuming PSS is a drastic over-estimation of the magnitude of the so-called missing HONO source. At night, we show that \(NO_x\) oxidation and emission are significant, thus, \(NO_x\) cannot be treated as a conservative tracer to infer secondary HONO production. Nearly-continuous observations of HONO and \(NO_2\) at Harvard Forest from December 2010 to December 2011 reveal daytime HONO levels that are comparable to what is expected from just known chemistry and much lower than has been reported in similar environments by different measurement techniques. Moreover, HONO fluxes are always below detection limit, indicating daytime HONO production contributes negligibly to the \(HO_x\) and \(NO_x\) budgets of the overlying atmosphere at Harvard Forest. Nighttime HONO enhancement is observed, but high night-to-night variability in \(HONO:NO_2\) that is not reasonably explained by the trends in HONO and \(NO_2\) fluxes, suggest a non-\(NO_2\), non-ground/canopy-surface related HONO source. / Engineering and Applied Sciences