Browsing by Author "Knapp, Alan K., committee member"
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Item Open Access Cascading effects of changing climate and land use on alpine ecosystems and pastoral livelihoods in central Tibet(Colorado State University. Libraries, 2015) Hopping, Kelly A., author; Klein, Julia A., advisor; Galvin, Kathleen A., committee member; Knapp, Alan K., committee member; Leisz, Stephen J., committee memberChanging climate and land use practices are re-shaping the dynamics of social-ecological systems globally, with alpine regions and subsistence-based communities likely to be among the most vulnerable to the impacts of these changes. The Tibetan Plateau exemplifies a system in which climate warming and projected increases in snowfall, coupled with natural resource management policies that reduce livestock herd sizes and mobility, will have cascading effects not only on the livelihoods of local pastoralists, but also on other globally important ecosystem services that Tibet’s alpine meadows provide. To improve our understanding of the impacts of altered climate and grazing restrictions in central Tibet, I conducted interviews with local herders about their knowledge of environmental changes and the ways in which this knowledge is produced and transmitted within the community, performed a 5-year climate change and yak grazing experiment, and carried out observational measurements in plant communities around the landscape. I found that herders are well attuned to the changes that are the most threatening to their livelihoods, and they transfer this knowledge of environmental change within their village primarily as a means for seeking adaptive solutions, rather than for learning from others. Results from the experiment and landscape observations corroborate much of the herders’ understandings of the factors driving undesirable changes in the alpine meadows. From the experiment, I found positive feedbacks between yaks, vegetation, and nitrogen cycling, indicating that these meadows are well adapted to moderate grazing under ambient climate conditions. However, they are particularly sensitive to warming-induced reductions in soil moisture. Although decreased plant production and ecosystem CO₂ fluxes with warming were partially mitigated by additional snow before the start of the growing season, results from the landscape observations suggest that in the longer term, climate warming will likely decrease the quantity and quality of forage available to livestock and wildlife, while also reducing the carbon sink strength of alpine meadows in central Tibet. Therefore, my results indicate that instead of continuing to mandate livestock removals, which will do little to reverse undesirable ecological trends, more consideration needs to be given to climate change adaptation strategies for pastoral social-ecological systems in Tibet.Item Open Access Climate change and plant species composition and community structure in the Central Grassland Region of North America(Colorado State University. Libraries, 2012) Byrne, Kerry M., author; Lauenroth, William K., advisor; Klein, Julia A., advisor; Adler, Peter B., committee member; Knapp, Alan K., committee memberPrecipitation and temperature are recognized as important drivers of plant community structure and function across ecosystems worldwide. The seasonality and quantity of precipitation combine with temperature to influence soil water balance, which is a primary determinant of terrestrial vegetation. Aspects of soil water balance have been shown to affect many properties of plant communities. The distribution of the earth's major biomes, for example, can be largely predicted from temperature and precipitation. Abundant evidence supports a strong relationship between actual evapotranspiration (AET) and aboveground net primary production (ANPP), and strong relationships exist between precipitation and species richness as well. Yet recent predictions of an increase in mean global temperature and changes in precipitation timing and quantity have the potential to alter terrestrial communities in novel ways by changing both the strength of abiotic controls on ecosystem processes as well as changing biotic interactions such as predation, competition, and trophic interactions in plant communities. As strongly water-controlled systems, grasslands may be particularly sensitive to predicted changes in climate. Using the central grasslands of North America as my study region, I examined how predicted changes in climate will affect soil water availability, net primary production, and species composition and community structure at study sites located in the shortgrass steppe and mixed grass prairie. My results demonstrate that ecosystems located within the same biome may respond differently to similar changes in precipitation and temperature, primarily due to differences in community structure, interspecific competition, and patterns of soil water availability. Simulated future soil water availability revealed greater temporal and spatial changes in available water at the mixed grass prairie than at the shortgrass steppe site. Using a soil water manipulation experiment, I found that ANPP at the shortgrass steppe was insensitive to changes in soil water, while belowground net primary production (BNPP) was sensitive to changes in soil water, although the direction of the response differed between years. I observed the opposite pattern at the more mesic mixed grass prairie site. Here, there was a rapid ANPP response to the water manipulation treatments, but BNPP was insensitive to changes in soil water availability. Likewise, the shortgrass steppe plant community was fairly insensitive to manipulated soil water, while the mixed grass prairie plant community responded rapidly to manipulated soil water. The differences in community responses between my two sites highlight the importance of multi-site studies to refine our knowledge of the mechanisms and generalities of community response to climate change at the biome level.Item Open Access Climate change impacts on population dynamics in tallgrass prairie: implications for species codominance(Colorado State University. Libraries, 2017) Gray, Jesse E., author; Smith, Melinda D., advisor; Knapp, Alan K., committee member; Ocheltree, Troy W., committee memberTwo grass species, Andropogon gerardii and Sorghastrum nutans, together account for the great majority of individuals, biomass, and possibly genetic diversity in plant communities of the tallgrass prairies of the Great Plains, US. As competitors with similar functional traits and what appears to be overlapping niches, it is not clear what mechanisms facilitate their co-dominance, but it may rely on the high variability of environmental conditions that characterize grassland ecosystems. Because these abundant grasses strongly influence plant community structure and ecosystem function, it is critical that we understand the factors influencing the population dynamics of these species, and how climate change might alter those relationships. We found an asynchrony in population dynamics in which A. gerardii begins each growing season at higher tiller densities, with attrition of tillers starting mid-season. Concurrent gains of S. nutans tillers results in A. gerardii becoming the less abundant by the end of most growing seasons. We hypothesized that this differentiation in tillering strategies causes each species to be vulnerable to unfavorable environmental conditions during different parts of the growing season, thus enabling their coexistence by preventing an inter-annually consistent competitive advantage of either species. We found that greater tiller density asynchrony was associated with higher population densities of S. nutans and of aggregate tiller densities of both species. Experimental increases in temperature and rainfall variability reduced population-level asynchrony while exacerbating population declines and overall community productivity, suggesting this mechanism of co-dominance may rely on current levels of environmental variability, and may be vulnerable to projected increases in that variability with climate change.Item Open Access Ecology of bison herbivory in North Rim Grand Canyon grasslands(Colorado State University. Libraries, 2023) Musto, Dana Theresa, author; Aldridge, Cameron L., advisor; Schoenecker, Kathryn A., advisor; Knapp, Alan K., committee member; von Fischer, Joe, committee memberThe American Plains bison (Bison bison bison) is a grassland ungulate herbivore that historically played a keystone role in the structure and function of grasslands throughout North America. The mechanisms by which bison influence grassland characteristics are both direct (i.e., via plant consumption and highly nutrient-rich waste deposition) and indirect (i.e., plant community responses), having the capacity to alter entire ecosystems. The ungulate herbivore-grassland relationship has been studied across the globe in a wide range of ecosystems from the tallgrass prairie, montane temperate, and semi-arid grasslands of North America, to the savannah plains of East Africa. My research aims to contribute to this body of knowledge by exploring the effects of bison grazing on the semi-arid, high elevation grasslands of the Southwestern United States in the southern edge of the historic range of the Plains bison in northern Arizona. With recent expansion of Plains bison into the North Rim area of Grand Canyon National Park, I sought to assess the potential effects of bison on grassland structure and function in an ecosystem where this relationship had yet to be assessed. I used a replicated herbivore exclusion experiment in grassland meadow habitats, employing both long-term grazing exclosures (0.40-ha) and temporary grazing exclusion cages (1-m²) to quantify herbaceous productivity and consumption by free-ranging bison. I established six sites in Grand Canyon National Park (GRCA) in areas with high bison density, and six additional sites in similar meadow habitat in Kaibab National Forest (KNF) with low to no bison density. Chapter 1 is largely composed of literature review exploring the importance of grassland ecosystems, bison populations, and the interactive history of the effect of bison across North America. I also provide relevant information regarding the ecological and historical aspects of my study area within the Kaibab Plateau, northern Arizona, including a summary of significant biological and cultural aspects, its history with grazing, and current research goals that include bison management plans. The goal of Chapter 1 is to provide the context for my research as well as to provide background for my research objectives and how I conducted the research, which are described in the following chapters. In Chapter 2, I conducted an experimental study among meadows of varied bison density to evaluate vegetation offtake, the effect of bison herbivory on aboveground primary productivity and its effect on vegetation ground cover. Using quadrat clipping rings inside and outside of grazing cages, I sampled plant biomass (which I used to calculate offtake and productivity) and measured percent ground cover twice each year in 2021 and 2022, where sampling events occurred in summer and fall to capture peak production of both cool (C3) and warm (C4) season plants. I compared the effects of grazing at various intensities on grassland productivity and plant percent cover by taking measurements between treatments (experimental grazed plots vs. exclosure control plots), stratum (high bison use areas in GRCA vs. low bison use areas in KNF), as well as between years (2021 vs. 2022). I calculated aboveground herbaceous production of grazed treatments (ANPPg) and exclosure treatments (ANPPug), as well as total annual offtake of grazed treatments (Ot) using the Sum of Significant Positive Increments (SSPI) method, where only significant (p<0.1 due to limited sample size) and positive increments of vegetation biomass change between sampling occasions were summed to the total annual productivity (ANPP) value (measured in g/m2). I used a linear mixed model to evaluate the influence of treatment, stratum, and year on annual primary productivity. As expected, GRCA grazed treatments had higher annual offtake and grazing intensity when compared to KNF grazed treatments. Annual aboveground herbaceous production of grazed plots (ANPPg) was significantly higher in GRCA than in KNF and a positive relationship was observed between herbivory utilization and ANPP in GRCA. These results are partially explained by the "Grazing Optimization Hypothesis," which predicts an increase in aboveground production and nitrogen yield of grazed plants compared to ungrazed plants under intermediate grazing; however, my results indicated a continuous increase in aboveground production past intermediate levels of grazing intensity. My results may be better explained by the "Compensatory Continuum Hypothesis," which theorizes that the ability for a plant to tolerate or compensate for losses from herbivory are likely driven by a complex of interactions among the affected plant and its environment (Maschinski and Whitman 1989). Annual herbaceous production inside exclosure plots (ANPPug) was nearly identical between the two ungulate stratum (high density and low density bison herbivory) and no difference could be detected. When evaluating the relative proportion of ground cover classes, I found no differences between treatments, but differences between strata. Sites within GRCA supported significantly higher coverage of forbs and bare ground, whereas sites in KNF supported significantly higher coverage of graminoids and litter. Additionally, I deployed a temperature and precipitation gauge at ten sites to collect local climate information. Climate information obtained from weather stations was organized by temperature and precipitation seasonal windows and used in the linear mixed model as predictor variables, where spring temperature was the single most influential weather variable. Twelve motion sensor wildlife cameras (one at each site) were installed to assess herbivore type (bison vs. cattle) and frequency of visits to sites. Results from photos indicated that 1) bison were observed in high proportions throughout GRCA during the growing season, 2) cattle grazing occurred at three KNF sites at low frequencies, 3) bison were observed several times at only two KNF sites, and 4) the camera data was mostly consistent with the data from GPS collared bison that shows seasonal migratory behaviors. In Chapter 3, I present results of soil conditions between treatments and strata. I took measurements to evaluate soil structure (erodibility) and function (nutrient availability) using soil corers and in-situ soil probes in both grazed and exclosure treatment plots at each of the established twelve sites. Soil condition measurements of stable aggregates and soil nutrients were measured once during the onset of the study in spring of 2021. Additionally, soil probes were deployed after exclosure construction and prior to most bison arriving in my study area, thus, grazing treatment had yet to take effect. Consequentially, soil nutrient measurements were primarily used to assess baseline soil nutrient availability and distribution while also providing insight during the evaluation of spatiotemporal variability in production across the landscape. I used a paired and two-sample t-test to evaluate differences in aggregate stability between treatments and strata, and no difference in the proportion of stable aggregates (erosion potential) was detected at any spatial scale throughout the study area. Soil nutrient analysis using an ANOVA test revealed significantly higher phosphorus concentrations in KNF vs. GRCA and higher nitrate in exclosures vs. grazed plots. When soil nutrients nitrate, ammonium, total nitrogen, and phosphorus (measured in µg/cm2) were included in the linear mixed model, soil ammonium was the most influential nutrient predictor variable on ANPP; however, the lack of treatment effect limited my ability to assess the effect of bison herbivory on soil nutrients, and thus, aboveground productivity. Subsequently, I conducted elemental analysis on aboveground clipped herbaceous biomass. This revealed significantly higher nitrogen yield in grazed plants compared to ungrazed plants, consistent with the Compensatory Continuum Hypothesis. Prior to my research, there was only a single study that explored ecological effects of the bison herd on the Kaibab Plateau; however, that studies' focus was on riparian areas and riparian vegetation. My research provides a novel evaluation of the effect of the Kaibab Plateau bison herd on soil and plant structure and function within grasslands of the North Rim, Grand Canyon. This unique ecosystem has been protected since 1919, when Grand Canyon National Park was established from the surrounding Kaibab National Forest Service lands (Merkle 1962). Its richness in historical, cultural, and biological resources have given this Park its reputation as a natural wonder of the world. With the establishment of Plains bison in this unique ecosystem, my hope is that the results of this study will support resource managers in their efforts to manage and conserve the natural integrity of the Grand Canyon ecosystem while also promoting the welfare and conservation of the American bison, declared the United States' first National Mammal in 2016 (NPS 2016).Item Open Access Ecology of bison, elk, and vegetation in an arid ecosystem(Colorado State University. Libraries, 2012) Schoenecker, Kathryn Alyce, author; Hobbs, N. Thompson, advisor; Swift, David M., committee member; Coughenour, Michael C., committee member; Knapp, Alan K., committee memberTo view the abstract, please see the full text of the document.Item Open Access Ecosystem responses to precipitation extremes(Colorado State University. Libraries, 2018) Felton, Andrew Jennings, author; Smith, Melinda D., advisor; Knapp, Alan K., committee member; Ocheltree, Troy, committee member; Sala, Osvaldo, committee memberPredictions and recent observations of changing frequencies and intensities of climate extremes have prompted ecologists to increasingly study their ecological impacts. Rising interest in this field of research reflects growing recognition that changing climatic variability can impact ecological dynamics independent of climatic means, and that the ecological impacts of climate extremes may be of equal or greater magnitude than gradual changes in mean climate. However, recent concerns have emerged that traditional approaches used to understand and quantify relationships between climate and ecological processes may not be predictive of responses to extreme climatic conditions with no historic analog. In this dissertation, I describe tests of current knowledge about how precipitation impacts ecosystem processes by considering how changing extremity at both intra-annual and interannual timescales impacts different components of the carbon cycle. To achieve this, I employed a novel experimental design that imposed multiple levels (n = 11 levels, n = 4 replicates), and thus a gradient, of precipitation amount and extremity within a single growing season. These manipulations were imposed within two intact ecosystems of opposing climatic backgrounds; the semi-arid steppe of Colorado (low mean productivity) and the mesic tallgrass prairie of northeastern Kansas (high mean productivity). I show that despite these ecosystems harboring differing ecological characteristics, aboveground net primary productivity was consistently more sensitive to extreme wet years than severe drought, and thus carbon gains during wet years were greater than drought-induced productivity reductions. Despite asymmetrical productivity responses to precipitation extremes in both systems, there was consistent evidence for an underlying linear relationship as best describing the response of productivity to changes in growing season precipitation within these grasslands, in agreement with current models. Coupling this experimental data with long-term records within the mesic grassland revealed strong interactions between variability in rainfall patterns within and among years. Variability in intra-annual rainfall patterns, and in particular large and more variable event sizes, acted to magnify the reductions in ecosystem functioning during drought. A systemic review of the literature adds further complexity to these dynamics from an organizational perspective, suggesting that both the response and recovery of ecosystems to climate extremes are mediated by ecological responses and interactions that propagate from the individual, population, to the community-level to collectively impact ecosystem-level functioning. Overall, my research demonstrates a critical role for changes in precipitation extremity at both intra and interannual timescales and levels of ecological organization with respect to predicting the dynamics of ecosystem functioning amid climate change.Item Open Access Environmental change impacts on carbon and nitrogen dynamics in soils and vegetation: from global synthesis to local case studies(Colorado State University. Libraries, 2022) Rocci, Katherine, author; Cotrufo, M. Francesca, advisor; Baron, Jill S., committee member; Knapp, Alan K., committee member; Lavallee, Jocelyn M., committee memberHuman-induced changes in the Earth system, known collectively as global environmental changes, are modifying terrestrial ecosystems. Feedbacks between land biogeochemistry (e.g., the cycling of elements) and global change are one of the key uncertainties in global climate models, and thus understanding land (e.g., soils and plants) responses to global change will help us predict future climate. In order to advance understating of how soils and plants respond to global changes, we need to work across scales by synthesizing global findings, using experimental networks, and studying context dependent responses at individual sites. Specifically, this dissertation uses this framework to investigate: (1) the responses of carbon (C) in total soil organic matter (SOM) and its fractions to warming, elevated atmospheric carbon dioxide (CO2), altered precipitation regimes, and nitrogen (N) fertilization globally using meta-analysis; (2) SOM C and N stoichiometry and distribution in response to nutrient fertilization in globally-distributed grasslands; (3) plant and soil biogeochemical responses to increased precipitation at a mesic grassland; (4) bulk (wet and dry) N deposition in response to proximity to the road in a topographically complex, subalpine forest. Soil organic matter stores carbon (C) and N and thus helps to control climate and provide energy and nutrients for ecosystem function. Thus, understanding SOM responses to global change will help determine future climate and ecosystem processes. However, SOM is made up of a diverse pool of molecules, and separating SOM into more homogenous functional pools (e.g., particulate and mineral-associated organic matter [POM and MAOM]) can provide clearer understanding of SOM responses to perturbations. By synthesizing global-scale understanding, Chapter 2 showed that POM and MAOM C responded differently to global changes and these responses depended on experiment length, soil depth, and experiment methodology. By investigating how SOM responses to global change varied across a global distribution of grasslands, Chapter 3 found that addition of macro- and micronutrients modified POM and MAOM C:N, depending on ambient environmental conditions, and consistently reduced SOM C stability. By investigating C and N cycling under altered precipitation at a local scale, Chapter 4 showed that studying SOM fractions provided clearer understanding of the mechanisms underlying grassland biogeochemical responses to increased precipitation. Chapters 2-4 all show the value of investigating soil fractions rather than solely the total SOM pool, as studying these fractions provided unique information and greater functional understanding. Global changes are not felt equally by all ecosystems. Ecosystems near sources of N deposition may be especially vulnerable to this global change. The fifth chapter of this dissertation, like the fourth chapter, focused on understanding local responses to global change. The vast majority of roadside N deposition studies find increased N deposition adjacent to roadways, but we did not find this, potentially due to the complex topography at our site or insufficient vehicle emissions. This suggests higher roadside N deposition cannot be assumed for all ecosystems. Altogether this dissertation synthesized and advanced our understanding of global change effects on plant and soil C and N pools and cycling.Item Open Access Evolutionary and chemical ecology of Verbascum thapsus reveal potential mechanisms of invasion(Colorado State University. Libraries, 2011) Alba, Christina, author; Hufbauer, Ruth A., advisor; Detling, James K., committee member; Bowers, M. Deane, committee member; Knapp, Alan K., committee memberBiological invasions, which occur when introduced species achieve pest status due to dramatic increases in performance, cause substantial environmental and economic damage. Invasion dynamics are extremely complex, varying in space and time, and as a function of the associations that form between introduced species and the biota present in the communities they invade. For plants, herbivores play a central role in shaping the outcome of introduction events. In particular, when plants are introduced to novel ranges, they often leave behind coevolved specialist herbivores (typically insects) that act to suppress populations in the native range. This can lead to increases in plant performance, for example when introduced plants evolving in communities devoid of enemies reallocate resources from defenses to growth and reproduction. Because of the important biological associations that exist between plants and insect herbivores, as well as the dramatic shifts in these associations that characterize biological invasions, this research places a particular emphasis on the evolutionary and chemical ecology of plant-insect interactions. More broadly, this research quantifies several aspects of the invasion dynamics of the introduced weed Verbascum Thapsus L. (Scrophulariaceae, common mullein). I first present data from a biogeographic comparison in which a survey of more than 50 native (European) and introduced (United States) mullein populations confirms a marked increase in population- and plant-level performance in the introduced range. I also document several ecological differences between ranges, including shifts in the abundance, identity, and degree of damage caused by insect herbivores, as well as differences in the abundance and identity of plant competitors and precipitation availability. A greenhouse experiment revealed that the increased performance observed in the field is maintained when native and introduced plants are grown from seed in a common environment; thus, a component of the performance phenotype is genetically based, or evolved. However, this increase in performance is not associated with an evolved decrease in defense investment as predicted by the evolution of increased competitive ability (EICA) hypothesis. Indeed, despite significant population-level variation in several defenses (trichomes, leaf toughness and iridoid glycosides), there is no evidence for the evolution of range-level differences in defense investment. I further explored how mullein's investment in chemical defense varies in natural populations and in relationship to damage by chewing herbivores. Based on this exploration, I developed new predictions for how changes to defense allocation may result in increased performance. Natural mullein populations exposed to ambient levels of herbivory in the introduced range exhibit significant population- and plant-level variation in iridoid glycosides. In particular, young (highly valuable) leaves are more than 6 better defended than old leaves, and likely because of this incur minimal damage from generalist herbivores. The limited ability of generalists to feed on mullein's well-defended young leaves results in negligible losses of high-quality tissue, suggesting a mechanism for mullein's increased performance in North America. Indeed, the within-plant distribution of iridoid glycosides significantly differs between native and introduced plants exposed to the different insect communities present in each range. Importantly, introduced mullein invests significantly more in the chemical defense of valuable young leaves than does native mullein, which leads to a dramatic reduction in the attack of young leaves in the introduced range relative to the native range. This optimization of within-plant investment in defense reflects the fact that introduced mullein has been released from the evolutionary dilemma posed by simultaneous attack by specialist and generalist herbivores (with specialists often being attracted to the same chemicals used to deter generalists from feeding, resulting in stabilizing selection on defense levels). In summary, this research provides evidence for a dramatic increase in the performance of introduced common mullein that is associated with several ecological differences between ranges as well as potentially adaptive shifts in mullein's chemical defense investment under natural conditions.Item Open Access Fire disturbance belowground: untangling consequences for soil food webs and organic matter(Colorado State University. Libraries, 2019) Pressler, Yamina, author; Moore, John C., advisor; Cotrufo, M. Francesca, advisor; Knapp, Alan K., committee member; Balgopal, Meena M., committee memberSoils and the ecological communities they house provide a diverse array of ecosystem services including the provisioning of food and fiber, decomposition and nutrient cycling, water filtration, and the maintenance of terrestrial biodiversity. These complex belowground communities, and therefore the ecosystem processes they regulate, are increasingly threatened by fire due to climate, land use, and management changes. Fires can have profound effects on the physical and chemical soil environment, with consequences for soil biological communities. Fires cause mortality of soil organisms during the disturbance event, change the soil pH, and alter the quantity and quality of soil organic matter (SOM). In particular, fires transform organic matter into pyrogenic carbon (PyC), a recalcitrant material with a dense aromatic structure and long residence times in soils. In natural ecosystems, soil food webs interact with PyC produced after a fire. In agroecosystems, PyC, in the form of biochar, is also used as a tool to manage soil carbon and fertility. Given the widespread effects of fire on biological, chemical, and physical components of the soil, and the importance of soil communities for the provisioning of ecosystem services, understanding the consequences of fire disturbance for soil food webs and organic matter is an important research objective. My dissertation leverages several different scientific inquiry approaches to understand the consequences of disturbance and management for the ecology of soils. I take a multifaceted approach by considering soil organisms, food webs, and organic matter in the context of fire disturbance and agricultural management. I begin by presenting results from a meta-analysis investigating the effect of fire on soil biota biomass, abundance, richness, evenness, and diversity. Overall, I found a pervasive negative effect of fire on soil microorganisms and conclude that soil fauna are more resistant to fire than soil microorganisms. Then, I present results from a field study investigating the effect of fire frequency on soil food web structure, function, stability, and resilience in an oak-pine savanna. Here, I found that while soil biota biomass and food web function did not differ with fire frequency, food web structure, stability, and resilience did. In particular, soil food webs at intermediate fire frequencies (4-year fire return interval) were the least stable and least resilient to fire. Thereafter, I consider the consequences of fire for SOM composition through the lens of PyC. I seek to understand where and why PyC persists in soils at a continental scale by using multiple analytical techniques to quantify PyC across Europe. I found that PyC may contribute a smaller component of soil organic carbon than previously thought and that organic carbon is the best predictor of PyC at a continental scale. I then consider how agricultural management and PyC in the form of biochar, impacts soil food webs in a semi-arid corn agroecosystem. I did not find any measurable effects of biochar on soil food web structure or function. I conclude that the long-term impact of historical land management on soil food webs far outweighs any impact of short-term management practices involving biochar. I then use this field study as an opportunity to integrate scientific inquiry in middle school classrooms. I present a collection of classroom activities co-developed with secondary educators that lead students to investigate the effect of biochar on soils and plants. I conclude by discussing the themes, patterns, and ideas that emerge from the preceding chapters. I found that the responses of soil ecological communities to disturbance are highly context dependent. This context dependency leads to hidden, unexpected, and even contradictory patterns. I end by reflecting on how completing this work has informed my non-linear approach to science.Item Open Access Selected techniques in radioecology: model development and comparison for internal dosimetry of rainbow trout (Oncorhynchus mykiss) and feasibility assessment of reflectance spectroscopy use as a tool in phytoremediation(Colorado State University. Libraries, 2014) Martinez, Nicole, author; Johnson, Thomas E., advisor; Pinder, John E., committee member; Duff, Martine C., committee member; Kuhne, Wendy W., committee member; Knapp, Alan K., committee memberTo view the abstract, please see the full text of the document.Item Open Access The effect of timing of growing season drought on flowering of Andropogon gerardii(Colorado State University. Libraries, 2015) Dietrich, John David, author; Smith, Melinda D., advisor; Knapp, Alan K., committee member; Ocheltree, Troy W., committee memberTiming of precipitation is equally important as amount for determining ecosystem function, especially aboveground net primary productivity (ANPP), in a variety of ecosystems. The particular precipitation period(s) of greatest importance varies between ecosystems. In tallgrass prairie of the central US, the relative importance of different precipitation periods is dictated by the phenology of the dominant C4 grasses, in particular Andropogon gerardii which can contribute >80% to ANPP in this ecosystem. It is predicted that precipitation periods with the greatest impact on the highly variable flowering rates of A. gerardii are likely to be particularly important for determining ANPP, as flowering individuals are much larger (>2-fold) than non-flowering individuals. The potential for flowering may be affected by precipitation at different times via different mechanisms (e.g. carbon gain via rapid growth early in the growing season vs. direct effects on stalk elongation later in the growing season). In order to test the differential effects of precipitation timing, rainfall deficits (100% exclusion) at different periods of the growing season were imposed on native tallgrass prairie in Kansas, USA. Contrary to expectations, the most sensitive period in terms of flowering for A. gerardii did not coincide with the highest potential photosynthetic rates early in the growing season. Rather the most sensitive period was mid to late summer immediately preceding, and concurrent with, the initiation of flowering stalks. Growth rate, leaf water potential and carbon assimilation of A. gerardii were all most sensitive to drought late in the growing season, suggesting that growth regulation in response to plant water status, not current year’s carbon accumulation is the critical factor determining flowering responses to precipitation or lack thereof. Flowering, in addition to influencing ANPP, controls rates of sexual reproduction which in turn limit adaptation and migration, and thus understanding how flowering will be influenced by a changing climate is critical for predicting plant community and ecosystem responses in tallgrass prairie. My study suggests that increased frequency of growing season droughts forecast with climate change could result in reduced ANPP and reproductive success of the dominant grasses in the tallgrass prairie ecosystem.Item Open Access The effects of grasshoppers on soil animal communities in the Shortgrass Steppe of northern Colorado(Colorado State University. Libraries, 2015) Post, Keith Harrison, author; Wall, Diana H., advisor; Knapp, Alan K., committee member; Kondratieff, Boris C., committee member; Ocheltree, Troy W., committee memberA burgeoning area of research in ecology is on the linkages between aboveground and belowground components of terrestrial systems. Leaf-feeding insects can affect soil communities directly via frass deposition or indirectly through alterations in the quantity or composition of plant roots or the amount of labile carbon exuded belowground. These pathways can affect the three soil energy channels (i.e., root, soil bacterial, and soil fungal) by altering the absolute and/or relative amounts of their source materials and, in turn, impact soil microbial community composition and higher trophic levels, including soil nematodes and microarthropods. This aboveground-belowground interaction is important to fully understanding the functioning of terrestrial ecosystems, especially in the context of global climate change. This study investigated the effects of short-term grasshopper exclusion in the shortgrass steppe of northern Colorado on plant abundance and temporal changes in trophic groups of soil animals. Above- and belowground plant biomass, soil nematode, and soil microarthropod responses to altered grasshopper abundances were determined using grasshopper exclosures and caged controls from late summer–early fall 2014. Plant community composition during the study was drastically different than long-term data. Bouteloua gracilis, a co-dominant grass, was reduced to an average of 5.75% of total aboveground biomass, whereas the typically rare, annual grass Vulpia octoflora exploded to over 93%. Total above- and belowground plant biomass and aboveground biomass from V. octoflora and other grasses (mainly B. gracilis) were unaffected by grasshopper exclusion. Grasshopper feeding enhanced the ratio of bacterivorous to fungivorous nematodes, which remained similar through time in exclosures. Proportions of bacterial-feeding nematodes increased in caged controls but decreased in exclosures, while there was a trend for the opposite pattern for plant parasitic nematodes. Temporal changes in the densities of soil microarthropods, mites, and mite trophic groups were similar between cage types. Results indicate that grasshoppers enhanced the relative dominance of the soil bacterial energy channel, likely through greater frass deposition. Apparent exclosure effects on plant parasitic nematodes suggest a possible belowground plant response to altered grasshopper populations, which could have been weak because these effects were specific to the then-rare B. gracilis, which was about to enter senescence. Implications of this research in the context of global climate change, particularly droughts in the shortgrass steppe, are discussed.Item Open Access The influence of moisture availability on terrestrial ecosystems: effects on soil animal communities along a regional/global scale climate gradient(Colorado State University. Libraries, 2013) Sylvain, Zachary Adam, author; Wall, Diana H., advisor; Cotrufo, M. Francesca, committee member; Kelly, Eugene F., committee member; Knapp, Alan K., committee member; Seastedt, Timothy R., committee memberEarth's climate is being altered at an alarming rate, and the consequences of these changes on the planet's ecosystems are unclear. In addition to increased warming due to rising CO2 concentrations, alterations to precipitation patterns will influence soil moisture availability in terrestrial ecosystems and this will have important consequences for plant growth and the ability of soil systems to perform functions such as decomposition and nutrient cycling. The effects on soil systems are especially poorly understood, partly due to the many interactions between environmental conditions and the numerous species found within soil ecosystems, ranging from microbial organisms such as bacteria, archaea and fungi to soil animals including mites and nematodes. With chapter 2, I provide an overview of the role of soil biodiversity and the implications climate and land-use changes may have for ecosystems as a consequence of their effects on soil biodiversity. I then examine the current state of understanding for the influence of soil moisture availability on plant and soil communities of temperate ecosystems in chapter 3, and highlight challenges for future research such as the inclusion of diversity metrics and soil animal community responses in climate change experiments as well as studies that operate at scales larger than single sites in order to better capture the dynamics of ecosystem changes. My research focused on one aspect of climate change, the alteration of soil moisture availability within ecosystems due to changes in precipitation regimes, and whether it affected soil organisms, particularly soil mites and nematodes (Chapter 4 of this dissertation). These two groups were selected because of their high abundances and diversity within soil ecosystems, their dependence upon soil water availability as a consequence of life history traits and their contributions to decomposition and nutrient cycling processes. To examine the effects of changing moisture availability on communities of mites and nematodes I analyzed soil samples along a large scale regional/global climate gradient made up of four long-term ecological research (LTER) sites including Konza Prairie LTER (KNZ), Kansas, Shortgrass Steppe LTER (SGS), Colorado, Jornada Basin LTER (JRN), New Mexico and McMurdo Dry Valleys LTER (MCM), Antarctica. I established elevation transects across hill slopes to obtain landscape-scale gradients of soil moisture availability within each of these ecosystems and sampled existing experimental manipulations of moisture availability from 2009-2011. Mites and nematodes were sorted to trophic groups to determine their ecological role and how changes to their abundances may affect ecosystems. Mite and nematode abundances responded strongly to changes in moisture availability. Across the large-scale climate gradient of all four sites, a positive non-linear response was found with particularly large increases in animal abundances corresponding to incremental moisture increases at the lower limits of moisture availability. Within each of the ecosystems, however, the responses of soil animal trophic group abundances to moisture availability were very similar and were largely negative. In chapter 5 of this dissertation, I further explore the effects of soil moisture and top-down or bottom-up community dynamics on mite and nematode abundances. To do this, I constructed a structural equation model examining the direct and indirect effects of soil moisture availability and trophic interactions on soil animal trophic group abundances. Results of this model suggest that soil moisture strongly controls populations of these organisms. Additionally, predatory mite and nematode trophic groups have top-down controls on lower trophic groups, although these interactions do not appear to be due to predation and instead suggest the influence of additional, unmeasured environmental factors acting indirectly on lower-level soil animal trophic groups. With this dissertation, I demonstrate that changes to soil moisture regimes can have important effects on soil animal communities. A review of the literature (Chapter 3 of this dissertation) showed altered soil moisture availability had the clearest effects on plants, with effects on soil organisms being more idiosyncratic, likely as a result of stronger indirect than direct effects. Experimental evidence along a regional/global climate gradient of two desert and two grassland sites (Chapter 4 of this dissertation) show that increases to moisture availability have strong positive effects on mite and nematode communities, especially at low levels of moisture availability across this large, multi-site scale. At smaller scales (within individual ecosystems) this response becomes weaker and results in declines to animal groups at most sites. These results suggest that as precipitation regimes are altered as a consequence of climate change, the resultant alterations to soil moisture availability may have important feedbacks to terrestrial ecosystems. Observed changes to trophic group structuring in response to changes in moisture availability (Chapters 4 and 5 of this dissertation) show that food webs may be restructured due to future changes in moisture availability, leading to increases to root herbivory and increasing the amount of energy flowing through bacterial rather than fungal decomposition pathways. These changes to food webs can result in alterations to nutrient cycling pathways and shifts in carbon allocation within plant communities, which will further influence ecosystem dynamics.Item Open Access Unraveling key drivers of microbial community assembly and impacts on microbial function(Colorado State University. Libraries, 2015) Rocca, Jennifer Doyle, author; Wallenstein, Matthew D., advisor; Cotrufo, M. Francesca, committee member; Knapp, Alan K., committee member; Smith, Melinda D., committee memberTo view the abstract, please see the full text of the document.Item Open Access Variation in carbon cycling among four tree species in a tropical rain forest(Colorado State University. Libraries, 2014) Asao, Shinichi, author; Parton, William J., advisor; Ryan, Michael G., advisor; Bauerle, William L., committee member; Knapp, Alan K., committee memberTo view the abstract, please see the full text of the document.