Theses and Dissertations

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Tropical forest root characteristics and responses to drying across environmental gradients
(Colorado State University. Libraries, 2024) Longhi Cordeiro, Amanda, author; Cusack, Daniela F., advisor; Ojima, Dennis, committee member; Cotrufo, M. Francesca, committee member; Conant, Richard, committee member
Fine roots represent the interface between plants and soils, and as such regulate all major biogeochemical cycles in terrestrial ecosystems, including tropical forests. Tropical forests play a crucial role in global carbon (C) cycling, largely due to their extensive root biomass and significant soil C stocks. However, these ecosystems have been experiencing more frequent severe droughts across some regions and are predicted to continue experiencing these extreme drought events in the future. This dissertation seeks to contribute to the understanding and synthesis of tropical root responses to drying in varying environmental conditions. In chapter 1, I gave an introduction about the importance of fine roots to ecosystem function and the impacts of drying in tropical forests. In chapter 2, I characterized root biomass, morphology, nutrient content, colonization to 1.2 meters depth as well as and arbuscular mycorrhizal fungal (AMF) to 20 cm depth in 32 plots across four distinct lowland Panamanian forests which are representative of the vast variation in soil fertility and mean annual precipitation (MAP) found across tropical forests. Root characteristics measurements, such as morphology and chemistry, at soil layers deeper than 30 cm have been rarely documented and to the best of knowledge this is the first study in tropical forests. I observed that that some root traits changed with soil depth similarly across sites while others had site-specific variation. I also observed larger variation at the soil surface and that morphological traits, in addition to root biomass can affect soil C stocks. In chapter 3, the effects of experimental and seasonal drying on fine root dynamics were explored using a partial throughfall reduction experiment across the same 32 plots as in chapter 2. I found that chronic drying impacted root biomass, productivity, morphology and arbuscular mycorrhizal fungi (AMF) colonization. Root biomass and characteristics also changed across seasons with different dynamics across depths. Chapter 4 focused on the effects of drought on tropical seedling development in a controlled chamber environment. I observed that drying decreased seedling growth, but high soil fertility and AMF inoculation mitigated these effects. I also observed changes in root morphology, leached C, new C allocation patterns, and aboveground traits in response to drought, but with usually interacting effects with fertility and AMF inoculation. Chapter 5 contributes a tropical root database (TropiRoot 1.0 database) with root data extracted from scientific papers across different countries and continents. Overall, this dissertation provides novel results and insights into the variation in root characteristics among tropical forests and their responses to climatic drying with interacting effects of fertility, symbionts and soil depth effects. It brings novel measurements that have never been published in tropical forest studies. In chapter 2, I found novel results about how different tropical forests had similar patterns of root variation with depth. It indicated differences in resources acquisition at the soil surface (likely for nutrients) and at deeper soil layers (likely for water) that are usually less investigated. I also showed a large variation of roots at surface soil across different forests that may influence forest responses to global change factors. In chapter 3, I supported some results across the literature such as drying decreasing root growth at the soil surface. However, I added new results such as drying decreasing root productivity at deeper soil layers, and changing root morphology and associations with symbionts probably to compensate the lower root growth. All together I observed that drying promoted changes in acquisition strategies and also that fertile forests may respond differently to drying. In chapter 4, I showed some clear tradeoffs in plant traits providing evidence that they are constantly changing in response to the environment. Also, I provided some novel results on the mechanisms, such as nutrient retention, on how mycorrhizal and fertility mitigated some negative effects of drying on plant growth. This aligns with the field study showing some possible resilience in the fertile forests to drying. The findings highlight the complex interactions between root traits and environmental conditions, offering important implications for predicting tropical forest responses to changing moisture and nutrient availability. All these chapters together provided a good understanding on how different forests respond to environmental changes. These impacts on soil C storage, links with root function and possible larger vulnerability of some forests are great topics for future studies.
Open Access
A modeling-experimental (ModEx) approach to advance understanding of global controls and microbial contributions to particulate and mineral-associated organic matter storage
(Colorado State University. Libraries, 2024) Hansen, Paige M., author; Cotrufo, M. Francesca, advisor; Schipanski, Meagan, committee member; Wallenstein, Matt, committee member; Trivedi, Pankaj, committee member
As soils are the largest terrestrial pool of carbon (C) and provision many ecosystem services, including nutrient cycling and maintenance of plant productivity, soil C sequestration represents a promising technology to help meet urgent needs to draw down atmospheric carbon dioxide (CO2) and prevent acceleration of climate change, as well as to help feed a rapidly growing global population. Given this, a comprehensive understanding of the mechanisms underpinning observed patterns of soil C storage is necessary to ensure a sustainable future for all. In response to this need, recent breakthroughs in our understanding of soil organic matter (SOM) dynamics have led to the development of multiple frameworks articulating how climate, soil, plant, and microbial properties interact with one another to control the formation of the two SOM constituents, particulate (POM) and mineral-associated organic matter (MAOM). Despite this, environmental controls that act on POM and MAOM storage at the global scale, as well as microbial functionality, is noticeably absent from our empirical understanding of SOM fraction formation and persistence. More advanced knowledge of these controls would enable more robust identification of where SOM is most vulnerable to loss, as well as more informed implementation of 'multi-pool' management practices aimed at enhancing C storage in both POM and MAOM. In this vein, this dissertation explores global controls on and microbial mediation of SOM dynamics at multiple scales through a combination of synthesis, modeling, and experimental (i.e., ModEx) approaches. Specifically, I first synthesized climate, soil property, and fraction C data to understand global controls on C storage in POM and MAOM. I then applied a previously developed individual-based model (Kaiser et al., 2015) to determine how emergent microbial community properties resulting from microbial social dynamics (i.e., interactions among microbes that produce enzymes at different rates) impact POM retention under varying degrees of MAOM saturation. Lastly, I investigated the relevance of hypothesized microbial copiotrophic and oligotrophic life history strategies to changes in POM and MAOM storage. Results from these projects indicate that global POM and MAOM storage is controlled by disparate suites of environmental variables, with POM being primarily controlled by variables that modulate microbial activity, and MAOM being controlled by a combination of C inputs and soil properties related to the potential to stabilize new MAOM. Additionally, flexible enzyme production in response to the availability of easily-assimilable, soluble substrates may contribute to POM retention under varying degrees of MAOM saturation and POM carbon:nitrogen ratio (C:N). However, variation in microbial function does not always result in changes in POM and MAOM storage – differences in growth rate, our proxy for copio- and oligotrophy, was unrelated to changes in POM and MAOM. Despite this, this dissertation indicates that microbial functions and environmental properties controlling microbial activity rates (i.e., controls on C outputs from the soil) mediate POM storage, but that MAOM is more reflective of C inputs to the soil. This indicates that microbial interventions to support soil C storage may want to focus on ecosystem-specific microbial manipulations that support community efficiency and modulate exo-enzyme production. In combination with other management strategies that increase soil C, these types of microbial interventions may help ensure that new soil C is retained in the soil for longer periods of time. Additionally, given that microbial activity is generally expected to increase with climate warming, these results indicate a premium need to preserve existing POM stocks.
Embargo
Isolation, interpretation, and implications of physical soil organic matter fractions in soil systems
(Colorado State University. Libraries, 2024) Leuthold, Samuel J., author; Cotrufo, M. Francesca, advisor; Lavallee, Jocelyn M., advisor; Mueller, Nathan, committee member; Schipanski, Meagan, committee member
Soil organic matter (SOM) is crucial to sustained ecosystem function, due to its role in regulating nutrient cycling, carbon (C) storage, and soil structure relevant to both food production and climate regulation. Since the early 1990s, physical fractionation methods have been used to separate bulk SOM into discrete components. The central aim of these methodologies is to simplify the complex heterogeneity of the bulk SOM pool by isolating fractions with more homogenous chemistries, formation pathways, and mechanisms of persistence. By understanding the relative distribution of C and nitrogen (N) among these various fractions, we gain appreciable insight into the mechanisms underlying fundamental soil biogeochemical processes. Despite their historic use, however, significant questions remain regarding the means of proper isolation and interpretation. This dissertation looks to these questions directly, reviewing and then interrogating the methods by which fractions separated before applying those fractionation schemes to answer key questions relating SOM to ecosystem function. The first section reviews the history and current state of physical fractionation methodologies, before using a triangulation of experimental evidence, including chemical, isotopic, and spectral indicators, to identify the best practices for laboratory use. These chapters advance our current understanding of SOM biogeochemistry by drawing an explicit link between the conceptual definitions of SOM fractions and the various procedural definitions that have been used historically. Across a range of soils representative of agricultural land in the United States, we show that fractionation methods that separate particulate organic matter (POM) fraction by density isolate fractions more in line with the conceptual definition of POM than the more frequently used size separation. This work aims to unify understanding across the field of soil biogeochemistry and allows for more robust analyses and modeling efforts. The subsequent chapters use this approach to investigate fundamental questions around SOM stability and persistence. The mineral associated organic matter (MAOM) fraction has long been understood to be relatively stable, with slower turnover times and a more homogenous composition as compared to POM. Its accumulation has thus been discussed as a target for climate change mitigation. We leveraged a unique long-term experimental site with archived samples stretching back over 60 years to test this assumption, aiming to identify a dynamic fraction of MAOM by comparing the SOM composition of plots that had not received organic inputs over the course of the experiment against plots that had received regular inputs for six decades. Our spectral and isotopic analyses showed that a dynamic fraction of the MAOM existed and was primarily composed of plant derived compounds. As the exchangeable MAOM pool was exhausted due to a lack of fresh C inputs, we found that the composition of the MAOM pool became more strongly dominated by microbial byproducts. This work represents useful evidence towards a holistic understanding of the dynamic nature of SOM, and forces reimagining of long-held paradigmatic views. One challenge in the current SOM biogeochemistry landscape is that often questions exist downstream of methodologies, such that the fractions that can be isolated drive the research that is conducted. By first identifying robust methodologies, in the second half of this dissertation we were able to ask specific questions about the link between SOM dynamics and ecosystem function. To this end, we pursued three different lines of inquiry: a field study in which the objective was to link the fractional distribution of C and N to yield stability in agricultural systems, a field study that seeks to understand the persistence dynamics of SOM over a decadal scale in grassland systems, and a laboratory incubation that aims to discern the relative contributions of POM and MAOM in regard to plant available N. The first field study used samples from 9 working farms across the Central United States to better understand how SOM might moderate the spatiotemporal stability of crop yields at the field scale. Yield instability is a major cause of economic and environmental distress in row crop systems, and regional studies have suggested that increasing SOM may be able to mitigate variation in yield across time and space. The chapter presented here is the first study that attempts to identify a mechanistic link between SOM fractions and yield stability. In disagreement with regional and county scale studies, we found that SOM abundance was not linked to increased yield stability in cropping systems. Rather, unstable yield zones had significantly higher SOM content than stable zones, particularly in regard to the POM fraction. This work indicates that at the subfield scale, interactions between climate, topography, and management may be driving spatial patterns of both yield stability and SOM accumulation. This is a key insight, implying that some of the relationships between SOM and agronomic outcomes are scale dependent, and highlighting the need for field scale work to maintain relevance to growers. The second field study produced novel insights, tracing isotopically enriched litter and pyrogenic organic matter (PyOM) through various SOM fractions over the course of a decade, one of the longest tracer experiments that has occurred in grassland ecosystems. We found that after 10 years, the majority of the remaining litter derived C and N inputs were stored in the MAOM fraction, a result well aligned with our hypotheses. Interestingly though, the litter derived MAOM fraction formed rapidly (~ 1 year) and persisted at a relatively similar concentration for the duration of the study. This suggests the potential for divergent persistence mechanisms of POM and MAOM, implying less inter-fraction transfer than previous frameworks have proposed and prompting re-evaluation of the mechanisms of MAOM formation and persistence. In contrast, the applied PyOM remained almost completely in the POM fraction over the 10-year period, reinforcing both the heterogeneity of the bulk SOM pool, and the myriad of persistence mechanisms that stabilize various SOM fractions. Given that PyOM is ubiquitous in soil regardless of burn history and can persist for hundreds of years, this result has critical importance for our understanding of turnover time of the POM fraction, and suggests that we may be underestimating the dynamic nature of POM when PyOM is not accounted for. Finally, in a lab incubation experiment, we took advantage of recent advances in isotopic measurement to prove recent theories around MAOM N accessibility. Whereas POM is often thought of as the fraction that provides nutrients in the short term, our two-week incubation showed that under certain conditions, the majority of plant available N may be derived from the MAOM fraction. This work validates proposed frameworks and is an important step towards understanding coupled C and N management in agroecosystems that could improve N use efficiency and increase producer sustainability. Overall, the work in this dissertation aims to provide a comprehensive overview of how fractions can and should be isolated, and the information gained via this fractionation. By clarifying and advancing methodology to quantify SOM components and the understanding of their contribution to critical soil functions for the sustainability of food production and the mitigation of climate change this dissertation represents a major step forward for the study, modeling and managing of SOM in agricultural systems.
Open Access
Bridging the gap between regenerative agriculture and the biological mechanisms controlling soil organic matter dynamics
(Colorado State University. Libraries, 2024) Prairie, Aaron, author; Cotrufo, M. Francesca, advisor; Fonte, Steven, committee member; Rosenzweig, Steven, committee member; Hall, Edward, committee member
This dissertation investigates the complex impacts of regenerative agriculture on soil organic matter (SOM) dynamics and soil fauna biodiversity, addressing a broad range of objectives from uncovering global patterns and policy needs to mechanistic understanding. Through global meta-analyses, policy evaluations, field studies, and mechanistic experiments, this research provides a comprehensive understanding of how regenerative practices influence soil health, carbon sequestration, and biodiversity. Chapter 2 aimed to understand global patterns through a meta-analysis quantifying the effects no-till (NT) and cropping system intensification significantly increase SOM, via impacts on both particulate organic matter (POM) and mineral-associated organic matter (MAOM). The analysis reveals that NT and cropping intensification synergize with integrated crop-livestock (ICL) systems to greatly enhance soil organic carbon (SOC) stocks, highlighting the potential of regenerative practices to mitigate climate change and promote soil health. Chapter 3 sought to evaluate the impacts of diversified agricultural systems on SOC, soil health, and yield across the United States. The findings indicate that diversified systems consistently show higher levels of SOC, improved soil health, and improved agronomic outcomes. The policy recommendations include increasing funding for soil health practices, supporting longer participation of producers in conservation programs, and tailoring these programs regionally to maximize their effectiveness. Chapter 4 focused on field-level impacts by examining the effects of varying degrees of regenerative practice adoption on SOM dynamics and soil fauna biodiversity in 22 farms within the Cheney Watershed, of central Kansas. By developing a Regenerative Farming Index (RFI), the study clearly links regenerative practices to increased carbon and nitrogen stocks in both POM and MAOM, and indicates a positive correlation between regenerative practices and soil biodiversity. Path analysis suggests that soil fauna indirectly influence SOM through their role in enhancing regenerative practices. Chapter 5 aimed to provide a mechanistic understanding of SOM dynamics by exploring the interactions between predatory mites and bacterivorous nematodes. The study highlights how these interactions shape microbial necromass accrual and MAOM formation. The findings underscore the importance of considering the entire soil food web in ecological studies to fully understand SOM formation and stabilization mechanisms. Overall, this dissertation advances the understanding of SOC dynamics under regenerative agriculture, providing valuable insights for sustainable soil management and climate change mitigation. By integrating global and local scales, it offers a holistic view of how regenerative practices can restore soil health and contribute to more resilient and productive agricultural systems.
Embargo
Cultivating collaborative adaptability in public lands social-ecological settings: linking theory, practice, and evaluation across cases and contexts
(Colorado State University. Libraries, 2024) Beeton, Tyler Andrew, author; Galvin, Kathleen, advisor; Cheng, Antony, committee member; Schultz, Courtney, committee member; Snodgrass, Jeffrey, committee member
Collaborative and adaptive forms of governance have become increasingly common in environmental management as they are claimed to help reduce conflict over resource management issues and uncertainty, increase trust, support shared understanding and buy-in for management actions, and facilitate social learning. In the USDA Forest Service, legal, regulatory, and bureaucratic challenges, and the increased emphasis on ecosystem management has increased the demand for, and investment in, collaboration as a tool to meet forest and fire management goals. Collaborative governance and adaptability scholarship has documented the key drivers and external conditions that influence collaboration, the internal dynamics that cultivate or constrain collaboration, and the outputs and outcomes of collaboration. Still, a number of research gaps remain that, if addressed, could advance the theory and practice of collaborative governance. First, the ways in which groups adapt and remain resilient to inevitable internal and external changes remains underexplored. Second, despite over twenty years of research in this space, relatively little is known about the configuration of, and relationship between, factors that comprise collaborative governance and adaptability. Third, collaboration is dynamic. As groups evolve, they create value in different ways, and their needs and priorities change. Thus, there is a need for periodic and ongoing assessments of how collaboration is working in local contexts, current challenges, and what adaptations are needed to improve collaborative processes and progress. This dissertation starts to address these research gaps and needs. I situated this work within the Collaborative Forest Landscape Restoration Program, a Forest Service-administered program first authorized in 2009 and reauthorized in 2019. The work presented in this dissertation was co-developed with the Southwest Ecological Restoration Institutes and the Forest Service Forest, Range Management, and Vegetation Ecology Program. Specifically, the work supported synthesis of lessons learned from the first 10 years of the CFLRP and the development of a standardized and longitudinal assessment of collaborative governance and adaptability for use in the CFLRP Common Monitoring Strategy. Broadly, my dissertation contributes to our understanding of the factors that facilitate or frustrate adaptation to inevitable change in collaborative settings. This is a critical line of inquiry given the increased and sustained investment in long-term collaborative environmental management in the United States and beyond. I employed a mixed-method analysis consisting of focus groups, program-wide surveys, and a systematic review, and I drew on the Integrative Framework for Collaborative Governance, collaborative adaptability and resilience literature, and organization theory to frame my analysis. In Chapter 2, I asked, how do collaborative governance regimes adapt to disruptions and what barriers constrained collaborative resilience? I found that collaboratives demonstrated the ability to mobilize social capital, learning, resources, and flexibility to respond to disruptions. Yet authority, accountability, and capacity complicated collaborative resilience. I conclude Chapter 2 with policy and practice recommendations to cultivate collaborative resilience moving forward. In Chapter 3, I developed and deployed a program-wide collaborative governance and adaptability assessment to all currently authorized CFLRP projects. I used a modified grounded theory approach to document and describe CFLRP project respondents' recommended actions and adaptations to improve collaborative processes and progress towards desired outcomes. Key recommendations included the need for: inclusive engagement throughout the process; institutional arrangements; resources and capacity; monitoring and social learning; trust, relationships, and commitment; external communication and outreach; and local autonomy in decision making. I discussed these findings in light of collaborative governance theory and practice and included relevant resources and actions that practitioners and funders of collaboratives and policy actors may consider to support collaboratives in working towards forest and fire management objectives. In chapter 4, I again used the program-wide CFLRP collaborative governance and adaptability assessment, and I used confirmatory factor analysis to test assumptions underlying the dimensional structure, reliability, and validity of measures thought to comprise collaborative governance and adaptability. I found that the components of collaborative governance and adaptability comprised six dimensions – principled engagement, shared motivation, leadership, resources, knowledge and learning, and institutional arrangements. As expected, several dimensions were significantly related, and the pattern of inter-factor relationships aligned with theoretical and empirical assumptions. We also found that the six dimensions represent statistically reliable, valid, and distinct measures that may be used to evaluate collaborative governance and adaptability. While our focus was on the CFLRP, the assessment can be adapted to other collaborative environmental governance contexts. Chapter 5 ends with a summary of findings, limitations of the work, and future research directions to address lingering questions about collaborative environmental governance.
Embargo
Quantifying and mapping tree mortality due to mountain pine bark beetles via analyses of remote sensing data in northern Colorado
(Colorado State University. Libraries, 2024) Taleb, Hamza A. S., author; Laituri, Melinda, advisor; Fassnacht, Steven, committee member; Leisz, Stephen, committee member; Grigg, Neil, committee member
In the past two decades, Mountain Pine Bark Beetle (MPBB) infestations have become more pervasive due to increasing temperatures and drought conditions related to climate change causing regional-scale mortality. Insect effects on tree die-off, fuels, and fire behavior can vary widely. A key problem in understanding insect-fire relationships is the lack of empirical maps that show interrelated changes in the distribution of insect infestations and fire zones over space and time. This study demonstrates an approach to tracking and mapping the spread of MPBB by analyses of vegetation indices calculated from Landsat TM data in the study site in northwestern Colorado. These indices were used for calculations in the Random Forest (RF) classifier algorithm and the Support Vector Machine (SVM) classifier algorithm to determine the presence or absence of MPBB and to illustrate the changes in the distribution of infestations with time. A comparison was made between the accuracy of the two classification algorithms (RF and SVM) in tracking and mapping the spread of MPBB. R2 has proved to be a reliable measure of accuracy of regression models. If the statistical accuracy of all the models, (RF vs. SVM and binary vs. regression) are compared, both the regression and binary models based on RF are more accurate. The results of this study can provide a useful tool for forest managers to make decisions about how changing conditions affect potential problems in forest management.
Open Access
Ecovoltaics and grassland responses to solar energy co-location
(Colorado State University. Libraries, 2024) Sturchio, Matthew Anders, author; Knapp, Alan K., advisor; Ocheltree, Troy, committee member; Schipanski, Meagan, committee member; Mueller, Nathan, committee member
The mitigation of climate change requires a transition to renewable sources of energy, and of all available options solar photovoltaic (PV) energy generation has the greatest potential to reduce CO2 emissions by the year 2030. Even so, ground mounted PV is land use intensive, and ideal locations for solar development often overlap with sensitive natural ecosystems and highly productive agricultural land. A scalable approach with potential to alleviate the land use tension created by solar development is the co-location of PV arrays and grassland ecosystems. While this approach has many positive implications for land sparing, the ecological consequences of PV presence above grassland ecosystems are not well understood. In this dissertation I discuss how the unique microenvironments created by PV arrays alter patterns of productivity, physiological response, and forage quality in a semi-arid grassland in Colorado, USA. I also outline a new approach to PV development, Ecovoltaics, that is informed by several fundamental ecological concepts. An Ecovoltaic approach to solar development co-prioritizes energy generation and ecosystem services by intentional design and management through all aspects of array development. With this work, I hope to inform a more sustainable future for solar energy.
Open Access
Population assessment of burrowing owls nesting on black-tailed prairie dog colonies in Colorado
(Colorado State University. Libraries, 2024) Albright, Sarah, author; Kendall, William, advisor; Conrey, Reesa, committee member; Pejchar, Liba, committee member; Boone, Randall, committee member
In North America, grassland birds have experienced steeper and more widespread declines than any other avian guild due to habitat loss resulting from grassland conversion to cropland, increasing urban and energy development, and climate change (Knopf 1994, Askins et al. 2007). The historical area of native grasslands has decreased by 62% since the 1800s and contributed to the loss of nearly 40% of grassland bird populations since 1966 (Wilsey et al. 2019). Heterogeneity in climate, grazing, and fire across the landscape have resulted in the existence of different grassland types that vary in structure and composition. The shortgrass prairie is the driest and warmest of the Great Plains grasslands and is dominated by low-growing perennial grasses, forbs, and shrubs. The shortgrass prairie provides vital nesting and foraging habitat for many grassland birds. In Colorado, approximately 50% of the historic shortgrass prairie has been converted to other land uses (Neely et al. 2006). The partial loss of shortgrass prairie habitat has ecological consequences including loss of native vegetation and decreases in populations of grassland species, including grassland birds. Black-tailed prairie dogs (Cynomys ludovicianus) are important drivers of ecosystem function in the shortgrass prairie because their colonial social structure, burrowing and foraging behaviors alter the landscape and provide areas of shorter vegetation and burrow systems that support increased biodiversity of animals and plants (Cully et al. 2010). Black-tailed prairie dogs function as a keystone species in shortgrass prairie ecosystems and create important breeding and foraging habitat for grassland birds including western burrowing owls (Athene cunicularia hypugaea: Smith and Lomolino 2004). The western burrowing owl is a small diurnal raptor that lives in grasslands, deserts, and other open habitats. It is a partially migratory species where populations in the southern parts of its range in the southwestern United States, Mexico, and portions of Central and South America are typically year-round residents. Migratory populations occur in the grasslands of North America, arriving in early spring to start breeding as far north as Canada and departing in late August to return to their wintering grounds in the southwestern United States and Mexico (Poulin et al. 2011). Burrowing owls typically nest in burrows dug by rodents such as prairie dogs and ground squirrels. In eastern Colorado, burrowing owls almost exclusively nest on black-tailed prairie dog colonies. Benefits of nesting on prairie dog colonies include increased predator detection from alarm calls, decreased predation due to the dilution effect, and reduced vegetation height. Black-tailed prairie dog populations have experienced an estimated decline of 90-98% since 1900 due to sylvatic plague outbreaks and habitat loss and alteration by human development (Miller et al. 1994, Desmond et al. 2000). Since prairie dog colonies provide critical habitat for burrowing owls and other species, population decline contributes to decreased availability of burrowing owl nesting habitat. Conservation status of the burrowing owl varies across its range. It is a species of conservation concern in the western United States, threatened in Mexico, and endangered in Canada (Sheffield 1997). The western burrowing owl is currently listed as a state-threatened species in Colorado and is designated as a Tier 1 Species of Greatest Conservation Need in Colorado's State Wildlife Action Plan (Colorado Parks and Wildlife 2015). The last burrowing owl population assessment in Colorado was conducted in 2005 (Tipton et al. 2008, 2009) and since then, only local surveys limited in spatial and temporal extent have been conducted. This has prompted the need for an updated population assessment of burrowing owls nesting in eastern Colorado, where the majority of Colorado's burrowing owls breed on black-tailed prairie dog colonies. In this study, we provide an updated status assessment for burrowing owls on Colorado's eastern plains and seek to expand the current understanding of which black-tailed prairie dog colony attributes have the highest value for burrowing owl occupancy, density, and productivity. We specifically examined how colony size, activity status, and vegetation characteristics influence these population parameters on 175 survey plots throughout eastern Colorado. We surveyed some of the same plots using similar methodology as Tipton et al. (2008, 2009) in their 2005 study, facilitating comparisons of burrowing owl populations 17–18 years later. The first chapter describes the distribution of burrowing owls nesting on black-tailed prairie dog colonies in eastern Colorado and serves to examine which black-tailed prairie dog colony characteristics drive the use of a colony by burrowing owls and the probability of successful reproduction. The second chapter focuses on burrowing owl density, productivity, and abundance in eastern Colorado to determine how many burrowing owls are present on occupied colonies and how productive they are on colonies where they do reproduce. The value of describing these components of burrowing owl populations in separate chapters comes from estimating and identifying the drivers of burrowing owl occupancy in chapter 1, then shifting to the finer scale of density to determine if the drivers of burrowing owl distribution are also driving density, productivity, and abundance. We used a black-tailed prairie dog colony shapefile prepared by the Colorado Natural Heritage Program for Colorado Parks and Wildlife in 2020 as our sampling frame. This shapefile includes polygons that represent black-tailed prairie dog colonies with digitized boundaries, created using imagery collected in 2019 by the National Agriculture Imagery Program (NAIP). We used a spatially balanced sampling design to select potential plots and selected new samples for each survey year (2022 and 2023) to maximize sample size and spatial coverage of the large study region. From early May through early August, we conducted four surveys on 175 plots in eastern Colorado, counting all burrowing owls seen, with two visits occurring prior to juvenile emergence and two occurring after. We estimated occupancy using a static multistate occupancy estimation model with two states: 'occupied' and 'occupied with successful reproduction'. We estimated density and abundance using distance sampling methods. We estimated productivity using a zero-inflated beta generalized linear model. We used occupancy data from Tipton et al. (2008) and dynamic occupancy models to evaluate burrowing owl local colonization and local extinction between 2005 and 2022 - 2023. Our analyses indicate that burrowing owl occupancy and density are highest in southern Colorado and lowest in northern Colorado. Colonies with higher prairie dog activity level had higher probability of reproduction and higher densities of adult burrowing owls. Vegetation height was the main driver of juvenile density such that colonies with taller vegetation supported lower densities of juvenile owls. We estimated burrowing owl occupancy to be 0.84 (95% CI [0.62, 0.95]) and probability of successful reproduction on occupied plots to be 0.86 (95% CI [0.70, 0.94]). We estimated an average density of 3.47 (95% CI [2.79, 4.15]) adult owls/km2 prior to juvenile emergence, 8.20 (95% CI [6.39, 10.00]) adult owls/km2 after juvenile emergence, and 18 juveniles/km2 (95% CI [13.86-23.66]). We expanded our density estimates to our sampling frame and estimated that there were 4,913 (95% CI [3,948-5,875]) adult owls prior to juvenile emergence, 11,613 (95% CI [5,333-17,893]) adult owls after juvenile emergence, and 26,580 (95% CI [19,623-33,537]) juvenile burrowing owls on black-tailed prairie dog colonies in eastern Colorado. We found that prairie dog activity had a positive effect on burrowing owl density, successful reproduction, and productivity regardless of prairie dog colony size. This indicates that burrowing owls are effectively utilizing and nesting on small prairie dog colonies in eastern Colorado, which could make them more resilient to breeding season habit loss, fragmentation, or degradation. In addition, we found that northern Colorado had lower burrowing owl occupancy and adult density, but had a similar probability of successful reproduction and juvenile density compared to south and central Colorado. If northern Colorado can sustain stable burrowing owl densities, burrowing owl populations may have enough successful reproduction to maintain stable populations. The covariates we investigated in this study did not adequately explain this spatial pattern. However, it is likely that differences in climate, prairie dog population dynamics, land use, or some other factor could cause differences in local habitat and breeding conditions across Colorado. The previous burrowing owl population assessment in eastern Colorado estimated burrowing owl occupancy to be 0.80 (95% CI [0.66-0.89]), density to be 3.04 adult owls/km2 (95% CI [2.15, 5.13]), and adult abundance to be 3,554 (95% CI [3,928-8,445]) owls in eastern Colorado. This suggests that overall, burrowing owl populations in eastern Colorado are relatively stable and are likely to remain stable if efforts continue to preserve the prairie dog colonies that are vital for burrowing owls during the breeding season. The burrowing owl is a state-threatened species in Colorado at the time of this thesis and thus we recommend future burrowing owl surveys to track population changes through time. Future monitoring efforts can help identify the drivers of burrowing owl population change and clarify the spatial patterns we found. These future efforts should occur more frequently than the ~17 year time period between this population assessment and the last assessment in 2005. We recommend conducting burrowing owl surveys every 5 years because it exceeds the time lag between black-tailed prairie dog colony local extinction and cessation of burrowing owl nesting. In addition, a 5 year time interval coincides with the timing of the Colorado Natural Heritage Program's black-tailed prairie dog mapping efforts in eastern Colorado, from which we constructed our sampling frame. Using updated mapping efforts is vital for monitoring efforts because it may decrease the probability that a plot selected from the sampling frame contains a prairie dog colony that has gone locally extinct. We recommend future efforts should select new plots to survey for burrowing owls in addition to resurveying a subset of the plots from this study. Revisiting sites from this study would be helpful in determining burrowing owl population trends through time, while selecting new plots can increase the spatial coverage of surveys. This 2-year study provides an updated status assessment of burrowing owl populations across the black-tailed prairie dog range in Colorado that will help calibrate burrowing owl population models incorporating prairie dog colony extent, inform future monitoring plans, and help guide conservation of keystone species and their communities.
Open Access
Assessing drought sensitivity across the shortgrass steppe biome
(Colorado State University. Libraries, 2024) Hedberg, Sydney Leigh, author; Knapp, Alan K., advisor; Dao, Phuong D., advisor; Mueller, Nathan, committee member
Net primary productivity (NPP) of grassland ecosystems is dependent on many biotic and abiotic factors. However, water availability is generally considered the primary determinant of NPP, as well as being key for defining grassland community structure, and thus it is imperative to understand how grasslands respond to drought in a climate where droughts are expected to become more frequent and severe. There is a well-documented negative relationship, described by the Huxman-Smith model, between drought sensitivity and mean annual precipitation (MAP) at spatial scales that span multiple biomes. In other words, drier ecosystems are usually more sensitive to drought than more mesic ecosystems. While this cross-biome pattern has been independently confirmed with a variety of research approaches, there is limited research that has explored how patterns of drought sensitivity vary with MAP within a single biome where the dominant species do not vary. My goal was to determine if this negative relationship is evident within a regionally extensive grassland biome generally dominated by a single grass species (Bouteloua gracilis or blue gramma). I characterized the spatial pattern and relationship between drought sensitivity and MAP across the shortgrass steppe biome of the North American Great Plains using satellite-derived Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) data (from 2000-2022) as proxies for vegetation productivity. Gridded annual precipitation data were obtained at a comparable spatial scale. I found a negative relationship between drought sensitivity and MAP within the shortgrass steppe biome, indicating that the Huxman-Smith model is also supported within a single biome. Thus, my results suggest that while changes in the dominant vegetation may contribute to the patterns observed between MAP and drought sensitivity at large spatial scales that include multiple biomes, gradients in MAP within a biome can also drive this negative relationship. As a result, directional changes in annual precipitation amounts have the potential to alter drought sensitivity directly, even if the dominant plant species do not change.
Open Access
Evolution at the edge: how hibernation, heat waves, and hybridization impact a range expansion
(Colorado State University. Libraries, 2024) Clark, Eliza, author; Hufbauer, Ruth, advisor; Bitume, Ellyn, committee member; Norton, Andrew, committee member; Funk, Chris, committee member
Evolutionary processes shape the diversity of life on earth. Over millennia, species diverge from one another, radiating out into the tree of life. The same processes of evolution are also acting in much shorter periods of time, selecting for traits, mixing genes across populations, and generating new mutations each generation. These rapid evolutionary processes interact with ecological processes, which are happening on similar time scales. Range expansions, or expansions of a population's geographic distribution, were once considered strictly ecological processes of populations interacting with other populations and the environment, unaffected by evolution. However, modern theory understands range expansions to be crucibles of rapid evolution. Rapid evolution shapes the process of range expansion itself, and is also integral to determining the outcomes of range expansion. During range expansions, ecological and evolutionary processes intertwine, combining to shape the dynamics of a range expansion, like where a population can establish, and how quickly the expansion moves. The study of evolution during range expansions has only just begun to make it out of the theory to be tested in wild populations in nature, so we don't yet know how common evolution during range expansion is, or how large its effects might be. Here, I explore how evolution impacts range expansions that are current and ongoing in natural systems in the wild. I focus on the tamarisk beetle (Diorhabda spp.), deliberately introduced in the United States about two decades ago for biological control of a widespread invasive weed. Through its role as a biological control agent, the tamarisk beetle has expanded its range hundreds of kilometers along rivers, colonizing new areas of the invasive weed in environments very different from its original release habitat. The range expansion of the tamarisk beetle provides a unique opportunity to study evolution during an ongoing natural range expansion across an environmental gradient. Through the following four chapters, I document evolution of dispersal ability and life history traits (Chapter 1), evolution of seasonal dormancy and genetic variation of that trait (Chapter 2), evolution of phenotypic plasticity (Chapter 3), and the impacts of hybridization (Chapter 4). Throughout, I discuss the implications for biological control and the tamarisk beetle specifically, and more generally how these results improve our understanding of how evolution is caused by, enables, and alters natural range expansions over short time periods, even in natural range expansions.
Open Access
Plant-mediated interactions between herbivory and soil microbial communities in biocontrol programs of Russian knapweed
(Colorado State University. Libraries, 2024) Matos Franco, Giovana, author; Ode, Paul, advisor; Pearse, Ian, committee member; Smith, Melinda, committee member; Trivedi, Pankaj, committee member
Russian knapweed (Rhaponticum repens) is an invasive noxious weed present in the United States and two insect biocontrol agents have been released to assist with its management: the gall midge (Jaapiella ivannikovi) and the gall wasp (Aulacidea acroptilonica). Since their establishment, no concrete impacts of biocontrol agents onto Russian knapweed have been measured, neither their impacts on interactions between Russian knapweed and local microbiomes. To address this knowledge gap, observational and manipulative studies were conducted to investigate the effects of biocontrol agents on Russian knapweed fitness as well as its associated microbiomes. We found that Russian knapweed associates with a core microbiome that can assist with invasion in the introduced range as well as, in root samples collected from sites where gall wasp were present, lower microbiome diversity was observed, indicating potential negative effects on overall plant health. In garden conditions, water availability positively correlated with plant growth, negatively correlated with insect establishment, and shaped microbiomes in root associated tissues. Results of this dissertation highlights how introduction of biocontrol agents shifts pre-established relationships between invasive plants and microbiomes as well as how such relationships could be impacting the success of biocontrol programs.
Open Access
Dynamics of stress and mortality for grass dominated ecosystems: an interplay of water limitation, heat, and erosion
(Colorado State University. Libraries, 2024) Bradfield, Scott J., author; Ocheltree, Troy, advisor; Knapp, Alan, committee member; Augustine, David, committee member; Hoffman, Chad, committee member
Grass dominated systems account for ~40% of the earth's terrestrial surface and typically occur in semi-arid and arid regions. The plant species that grow in these systems are known for their ability to withstand disturbance, including drought, grazing, and fire. While it is understood that the plants in these systems often experience multiple forms of stress in a growing season, interactions among these stress variables are not well represented in the literature. In this research, I sought to determine how combinations of stress variables influence the shortgrass steppe, this includes: long-term grazing, drought, erosion, and temperature. Specifically, I examined (1) how the interaction of long-term grazing and drought influences the recovery of the vegetation on the SGS following single-year and multi-year droughts, (2) how the interaction of grazing and erosion influence mortality following exposure to extreme surface temperatures, and (3) performed a comparative analysis of the microclimate of grass dominated systems in the United States to determine the intensity and frequency of stressful abiotic conditions that the vegetation experiences. First, I quantified the interactive effects of single and multi-year droughts with grazing pressure, because the Bouteloua species that dominate the region have been shown to be tolerant of grazing and drought independently, but the interactive effects of the two have not been well studied. Past research has focused on heavy cattle grazing but I included a mixture of moderate cattle grazing with prairie dogs, which is more intensive grazing than heavy cattle grazing. I found that the combined stress of multiple years of drought along with high grazing pressure has the potential to increase mortality in these Bouteloua species. Next, I quantified the erosion severity by ranking the amount crown exposure of the Bouteloua species during a drought on the SGS and then determined how erosion influenced bud outgrowth (production of a tiller) during the recovery year. I combined these data with environmental data collected by the National Ecological Organization Network (NEON) to determine the environmental conditions that the meristems of the plants experienced during the drought. My results showed that the temperatures at the surface of the soil, and exposed meristems, frequently reached levels thought to be lethal to plant cells. I acknowledge that it was likely a combination of water deficit and temperature that led to mortality of Bouteloua species that experienced erosion, but the high temperatures alone had the capacity to cause mortality of the meristems. Finally, I compared several near surface micrometeorological variables of grass dominated systems across the United States. Ultimately, I wanted to determine the frequency that these systems experienced temperatures near the surface that would be damaging to plants, if conventional methods for determining heatwaves represents damaging conditions to grassland plants, and what environmental factors lead to potentially damaging surface temperatures. I found that damaging temperatures occur often at arid sites, conventional heatwaves overestimate heat stress in sites that are wet or at higher latitudes, and underestimates heat stress for arid sites.
Embargo
The genomics of habitat-linked microgeographic adaptation in an island endemic bird
(Colorado State University. Libraries, 2024) Cheek, Rebecca G., author; Ghalambor, Cameron K., advisor; Funk, W. Chris, advisor; Sillett, T. Scott, committee member; Aubry, Lise M., committee member
A fundamental goal of evolutionary biology is to understand the mechanisms that maintain adaptive diversity. This dissertation focuses on the interplay of two key evolutionary mechanisms - natural selection and gene flow. While natural selection is often portrayed as a driving force of adaptive evolution, gene flow is assumed to disrupt selection by introducing maladapted alleles into locally adapted populations. Yet this paradigm is beginning to shift as a growing appreciation for the role gene flow may play in concert with natural selection to facilitate adaptative divergence. I explore this interaction of selection and gene flow in island scrub-jays (Aphelocoma insularis), a highly mobile bird experiencing local adaptation at a microgeographic scale. First, I demonstrated that observed differences in bill morphology between pine-oak ecotones are likely genetically based despite overall limited population genetic structure. Second, I found that the genetic underpinnings of divergent bill morphologies are highly parallel at higher genetic levels, which is indicative of selection acting on shared, but highly polygenic, molecular pathways. Finally, I tested alternate dispersal mechanisms potentially impacting patterns of limited gene flow and found evidence for sex-biased natal habitat preference shaping limited dispersal. Collectively, these results show how gene flow can enhance adaptive divergence at microgeographic scales.
Embargo
A case for context in quantitative ecology: statistical techniques to increase efficiency, accuracy, and equity in biodiversity research
(Colorado State University. Libraries, 2024) McCaslin, Hanna M., author; Bombaci, Sara, advisor; Hooten, Mevin, committee member; Koons, David, committee member; Hoeting, Jennifer, committee member
The current era of ecological research is characterized by rapid technological innovation, large datasets, and numerous computational and quantitative techniques. Together, big data and advanced computing are expanding our understanding of natural systems, allowing us to capture more complexity in our models, and helping us find solutions for salient challenges facing modern ecology and conservation, including climate change and biodiversity loss. However, large datasets are often characterized by noise, complex observational processes, and other challenges that can impede our ability to apply these data to address ecological research gaps. In each chapter of this dissertation, I seek to address a data problem inherent to the 'big data' that characterizes modern ecological research. Together, they extend the strategies available for addressing a problem facing many ecologists – how to make use of the large volumes of data we are collecting given (1) current computational limitations and (2) specific sampling biases that characterize various methods for data collection. In the first chapter, I present a recursive Bayesian computing (RB) method that can be used to fit Bayesian hierarchical models in sequential MCMC stages to ease computation and streamline hierarchical inference. I also demonstrate the application of transformation-assisted RB (TARB) to a hierarchical animal movement model to create unsupervised MCMC algorithms and obtain inference about individual- and population-level migratory characteristics. This recursive procedure reduced computation time for fitting our hierarchical movement model by half compared to fitting the model with a single MCMC algorithm. Transformation-assisted RB is a relatively accessible method for reducing the computational demands of fitting complex ecological statistical models, like those for animal movement, multi-species systems, or large spatial and temporal scales. Biodiversity monitoring projects that rely on collaborative, crowdsourced data collection are characterized by huge volumes of data that represent a major facet of 'big data ecology,' and quantitative methods designed to use these data for ecological research and conservation represent a leading edge of contemporary quantitative ecology. However, because participants select where to observe biodiversity, crowdsourced data are often influenced by sampling bias, including being biased toward affluent, white neighborhoods in urban areas. Despite the growing evidence of social sampling bias, research has yet to explore how socially driven sampling bias impacts inference and prediction informed by crowdsourced data, or if existing data pre-processing or analytical methods can effectively mitigate this bias. Thus, in Chapters 2 and 3, I explored social sampling bias in data from the crowdsourced avian biodiversity platform eBird. In Chapter 2, I studied patterns of social sampling bias in the locations of eBird "hotspots" to determine whether hotspots in Fresno, California, U.S.A. are more biased by social factors than the locations of Fresno eBird observations overall. My findings support previous work showing that eBird locations are biased by demographics. Further, I found that demographic bias is most pronounced in the locations of hotspots specifically, with hotspots being more likely to occur in areas with higher proportions of non-Hispanic white residents than eBird locations overall. This relationship is reinforced because hotspots in these predominantly white areas also amass more eBird checklists overall than hotspots in areas with more demographic diversity. These findings raise concerns that the eBird hotspot system may be exacerbating spatial bias in sampling and reinforcing patterns of inequity in data availability and eBird participation, by leading to datasets and user-facing maps of birding hotspots that mostly represent predominantly white neighborhoods. Then, in Chapter 3, I investigated the impacts of not accounting for socially biased sampling when using eBird data to study patterns of urban biodiversity. The luxury effect has emerged as a prominent hypothesis in urban ecology, describing a pattern of higher biodiversity associated with greater socioeconomic status observed in many cities. Using eBird data from 2015-2019, I tested whether an avian luxury effect is observed in Raleigh-Durham, North Carolina, U.S.A. before and after accounting for social sampling bias. By jointly modeling sampling intensity and species richness, I found that sampling intensity and species richness are positively correlated and sampling bias influences the estimated relationship between species richness and income. Thus, failing to account for sampling bias can hinder our ability to accurately observe social-ecological dynamics. Additionally, I found that randomly spatially subsampling eBird data prior to analysis, as recommended by existing guidelines to mitigate sampling bias in eBird data, does not reduce biased sampling related to demographics, because there are data gaps in communities of color and low-income communities that cannot be addressed via spatial subsampling. Therefore, it is paramount that crowdsourced and contributory science projects prioritize more equitable participation in their platforms, both for more ethical, equitable practice and because current sampling inequity negatively impacts data quality and project goals. Quantitative techniques can help us understand the complex observational processes influencing ecological data, and each chapter of this dissertation highlights how tailoring statistical or computing methods to these observational contexts can advance ecological knowledge – either by extending the complexity of models we can feasibly fit, as in Chapter 1, or by acknowledging and accounting for sampling inequity, in Chapters 2 and 3. We are all participants actively shaping the ecological processes we observe, and the actions, approaches, and assumptions used in our research reflect societal systems and biases. Data are never objective, and it is dangerous and false to assume that quantitative techniques can take data out of the contexts in which they were collected. Instead, quantitative frameworks that embrace, reflect, and seek to improve the ways in which social and observational contexts inform what is observed can elevate analytical techniques to tools towards more just, inclusive, and transparent ecological research and conservation.
Open Access
Effects of disturbance on tree level resistance in ponderosa pine trees along the Colorado Front Range
(Colorado State University. Libraries, 2024) Woodard, Kelby, author; Stevens-Rumann, Camille, advisor; Negrón, José, committee member; Rocca, Monique, committee member
Forest restoration treatments are being implemented across ponderosa pine systems along the Colorado Front Range with goals of reducing risk of catastrophic wildfire, returning forest structure to historical conditions, and increasing ecosystem resilience and resistance in the face of climate change. While there are studies monitoring effects of thinning and wildfires on forest structure across the Front Range, few studies assess the effects of disturbances from wildfires and thinning treatments on tree-level resistance. Here we examined forest stand structure, growth, and defense characteristics in response to treatments and wildfires through the collection of plot level data, tree-level characteristics, and tree cores. We sampled 160 plots in areas that experienced thinning treatments between 2007-2012, were burned by low-severity wildfires (2012 Hewlett Gulch and High Park Fire, 2010 Fourmile Canyon Fire and Dome Fire, 2012 Flagstaff Fire, 2012 Waldo Canyon Fire), or that were untreated and unburned (hereafter "control"). Our findings reveal that tree growth and resin duct size significantly increased following thinning treatments. Relative resin duct area and duct density were significantly higher in trees following wildfire compared to trees that experienced thinning or to those trees within control plots. Control plots exhibited the highest mean basal area and stand density index, coupled with the lowest quadratic mean diameter, indicating high inter-tree competition, which both thinning and low-severity wildfire helped alleviate. Overall, our results highlight the beneficial impacts of both thinning and low-severity wildfire on mature ponderosa pine trees by enhancing their resistance to future disturbance, such as bark beetle outbreaks and drought.
Open Access
Wildfire effects on host-parasite interactions in freshwater streams
(Colorado State University. Libraries, 2024) Svatos, Emma C., author; Preston, Daniel, advisor; Hart, Sarah, committee member; Wells, Caitlin, committee member
Wildfires are increasing in intensity and frequency globally, accentuating the need to understand the implications of fire on community interactions. While previous research has focused on fire effects on free-living species, our knowledge of how wildfires influence parasite interactions with hosts and predators remains limited, especially in freshwater ecosystems. This thesis addresses this knowledge gap and presents results from two distinct multi-year observational field studies that explore how wildfires influence interactions among parasites, hosts, and predators in freshwater streams. In Chapter 1, I used a Before-After-Control-Impact design to compare freshwater snail (Juga plicifera) host populations and trematode parasite communities in Oregon streams before and after wildfire disturbance. In Chapter 2, I investigated host-parasite-predator interactions involving mermithid nematode parasites (Family Mermithidae), mayfly hosts (Order Ephemeroptera), and trout predators (Salvelinus fontinalis, Salmo trutta, and Oncorhynchus clarkia) in the southern Rocky Mountains after severe wildfires. Despite substantial changes to stream habitat, snail host populations and trematode infection patterns and community structure remained relatively stable following fire disturbance in Oregon streams; however, I observed subtle taxon-specific responses to fire, suggesting changes in abundance or behavior of definitive hosts. In Rocky Mountain watersheds, mermithid parasite patterns varied considerably over time in burned streams, which coincided with similar responses in mayfly host densities, suggesting that wildfire indirectly affected mermithid infection patterns through host-density changes in the stream. Host attributes also influenced parasite interactions, as intermediate-size Baetidae mayflies experienced the highest probability of infection. Furthermore, infection prevalences of mayflies consumed by trout were positively related to mayflies in the benthos; yet, infection prevalences in trout stomachs were lower on average, potentially due to parasite-induced behavioral changes in infected mayfly hosts that reduced susceptibility to predation. Wildfire did not seem to affect rates of this predator-parasite interaction, as instances of concomitant predation remained consistent in burned streams over time. Comparing two different host-parasite systems sheds light on how environmental variables and host-parasite ecologies mediate wildfire effects on parasite interactions with hosts and predators. Together these findings expand our knowledge of parasite ecology in aquatic macroinvertebrate hosts, offer insights into the role of parasites in energy flow through food webs and as bioindicators of environmental change, and help integrate parasite interactions into our understanding of disturbance ecology in freshwater streams.
Open Access
Evaluating Bouteloua gracilis cultivars' performance after drought; The role of the soil microbiome
(Colorado State University. Libraries, 2024) Donne, Carina, author; Smith, Melinda, advisor; Havrilla, Caroline, committee member; Trivedi, Pankaj, committee member; Metcalf, Jessica, committee member
Drought has affected the Great Plains throughout history, most notably during the Dust Bowl of the 1930's. While most drought events are not as severe as the Dust Bowl, they still cause significant agricultural losses every year. As research has begun to uncover the mechanisms and responses of drought, there are still unanswered questions. For instance, the mechanisms of ecosystem recovery after drought ends remain relatively unexplored. It is possible that intervention methods such as reseeding will need to be done to help restore ecosystem structure and function after drought. After the Dust Bowl, it was a common practice to reseed native grasses, such as Blue Grama (Bouteloua gracilis), in sites severely impacted by the drought. Given forecasts of droughts on par or even more severe than the Dust Bowl, reseeding may need to be employed more frequently in the future to enhance post-drought recovery. However, with reseeding efforts, it is imperative to understand the adaptability of cultivars to the environmental conditions in which they are planted. One aspect of environmental conditions that has rarely been examined the soil microbiome. Here, I used a common garden experiment that included two cultivars of B. gracilis that were planted with soil microbial inocula extracted from either previously droughted or non-droughted soils. These soils were collected from a recently ended four-year drought experiment in the shortgrass steppe of northeastern Colorado, which caused the widespread loss of B. gracilis. The goal of the greenhouse experiment I conducted was to examine whether the post-drought legacy of altered soil microbial communities affected the growth and performance of two common cultivars of B. gracilis. I assessed plant performance by measuring weekly height to estimate relative growth rate and at the end of the experiment, I measured plant above- and belowground biomass. I found no significant differences in relative growth rate or plant biomass, and minimal differences in the bacterial community composition between the two cultivars. These results suggest that the post-drought legacy of altered soil bacterial communities did not differentially affect growth and performance of the two common B. gracilis cultivars evaluated in this study, and that the growth of these cultivars did not differ in their effects on the soil bacterial communities found under ambient vs. previously droughted conditions. Overall, both cultivars may be suitable for reseeding in the shortgrass steppe grassland after extreme drought, yet further studies are needed to examine a broader range of B. gracilis cultivars and whether soil bacterial communities previously exposed to extreme drought would allow for improved growth and performance of different cultivars to future drought conditions.
Open Access
Moving beyond the aggregated models: woody plant size influences on savanna function and dynamics
(Colorado State University. Libraries, 2008) Sea, William Brian, author; Hanan, Niall P., advisor
Historically, models have played important roles in studying aspects of savannas, including tree-grass competition, fire, and plant-herbivore interactions. The models can be categorized as either (1) "aggregated" ones that neglect size structure but have the advantage of mathematical tractability or (2) complicated process-oriented ecosystem models incorporating mechanistic ecophysiology capturing greater ecological realism but constrained to simulation modeling. The aggregated class of models can be further separated into those focusing on resource utilization and tree-grass competition ("resource-based models") and those focusing on demographic impacts of disturbances by fire and herbivory ("demographic bottleneck models"). The resource and demographic models separately consider important aspects of savanna ecology, yet the two approaches have rarely been integrated, resulting in a significant gap in our understanding of savannas. For this study, I investigated the role of woody plant size in savanna ecology. Using extensive datasets along broad resource gradients of annual precipitation in southern Africa, I examined patterns of size-abundance for woody plants in relatively undisturbed savannas to see if relationships for savannas showed similar patterns to theoretical predictions for tropical forests. Contrary to assumptions and predictions made by aggregate savanna models, I found that the percentage of wood biomass subject to fire loss actually decreases in wetter savannas. Since resource limitation and "thinning" have been mentioned as potential factors in savannas, I investigated the suitability of self-thinning in savannas. I developed a simple theoretical model hypothesizing three potential impacts of tree-grass interactions on the self-thinning relationship. Results from the analyses, testing with field data, suggest that tree-grass competition is asymmetric with respect to tree size. For the formal modeling component of my dissertation, I developed a simple savanna model that integrates demographic bottleneck and resource-based approaches. The model is unique in that the woody carrying capacity has both resource and demographic constraints. Model simulations showed that modest amounts of variation in adult mortality during fires and size-asymmetric tree-grass competition lead to very different model outcomes. The work opens up an entirely new class of ecological models for savanna ecology: analytically tractable with enough size structure to capture realistic savanna vegetation-disturbance interactions.
Open Access
Recovering spatially and temporally dynamic regional scale carbon flux estimates
(Colorado State University. Libraries, 2009) Schuh, Andrew, author; Denning, Scott, advisor
This dissertation presents two review type chapters and three new research chapters that contribute to our theoretical and practical knowledge about terrestrial carbon fluxes on the regional scale. This research expands on previous carbon dioxide inversion work by providing estimates of ecosystem respiration and gross primary productivity, as opposed to only net ecosystem exchange, and provides estimates on scales in time and space not previously available. The first two chapters provide an introduction and review material. This is necessary to provide the reader with an understanding of the relatively complex geostatistical atmospheric inversion process which uses carbon dioxide concentration data to provide terrestrial carbon flux estimates. Issues of scale are discussed as well previous work which was fundamental to the research presented here. The third and fourth chapters use simulated data to present an analysis of the methodology to a case study of North America in 2004. In particular, simulated data is used to investigate the sensitivity of the inversion to theoretical components of the inversion process and it is concluded that reasonably robust estimates of ecosystem respiration and gross primary productivity can be achieved by using a limited network of eight carbon dioxide observing towers. Chapter 4 specifically looks at the issue of small scale variability in carbon fluxes and the impact it has on obtaining larger scale regional estimates. Chapter five contains an analysis of real collected CO2 observation data from 2004 at the aforementioned eight observing sites. Results show significant seasonal and annual corrections to the a priori carbon flux estimates, in particular to the individual components of net ecosystem exchange, ecosystem respiration and gross primary productivity. Furthermore, the annual net ecosystem exchange, when presented spatially, provides clues to annual sources and sinks in 2004. Sensitivity is investigated with respect to numerous components of the inversion. Although large confidence bounds on estimates indicate statistical uncertainty in the mean estimate of net ecosystem exchange, estimates match reasonably well with previously conducted research as well as observational data. The research provides the estimates within a spatial context (and resolution) that was not previously available, allowing for the construction, and support, of much more descriptive hypotheses about carbon fluxes than was previously possible. Chapter six contains a summary of the results of the dissertation.
Open Access
Western Serengeti people shall not die: the relationship between Serengeti National Park and rural household economies in Tanzania
(Colorado State University. Libraries, 2009) Knapp, Eli J., author; Galvin, Kathleen, advisor
This research examined the relationship between Serengeti National Park and rural household economies living near (within 18 kilometers) its western borders in Tanzania. The study was based upon semi-structured household interviews with a general sample (N = 722), acknowledged poachers (N = 104), households with park-related employment (N = 50) and key informants (N = 15) in three two administrative regions and three districts. Interviews generated information about four primary social-ecological interactions which included crop destruction by wildlife, illegal hunting, park-related employment, and wildlife depredation on livestock. A cost-benefit analysis revealed that the average household generates a net profit of USD $13 from these interactions. Despite this, 84 percent of households were found to be food insecure for maize, the region's primary food crop. Moreover, 78 percent of households were found to be significantly over-budget over the preceding 12 months. These findings suggests that most households next to Serengeti National Park are generally impoverished and are lacking adaptive capacity to deal with severe environmental or socio-ecological changes. The first component of the research provided the context for western Serengeti. Significant findings included the importance of secondary education for increasing income to household economy and showed the level of dependence that households have on local natural resources. Households draw more heavily (often illegally) from the National Park with the advent of severe crop failures which were found to occur with a ten year periodicity. The second component revealed that neither crop damage nor wildlife depredation on livestock is distributed evenly. Rather, they are heavily localized with few effects on some households and severe effects on others. Although the effects of wildlife on crops and livestock generally decreased with distance from the Park, losses were particularly large for households within three kilometers of a boundary. The third component examined illegal bushmeat hunting and sales. Findings from respondents and extensive court documents suggested that fines and imprisonment had little effect on curbing illegal hunting behavior. The fourth component consisted of a synthesis of the cost-benefit analysis with a focus on food security and its effects on adaptive capacity. Implications of these findings are made for the resilience of the coupled socio-ecological system in western Serengeti.