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Browsing by Author "Ocheltree, Troy, advisor"

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    Analyzing root traits to characterize juniper expansion into rangelands
    (Colorado State University. Libraries, 2016) Chesus, Kelly A., author; Ocheltree, Troy, advisor; Comas, Louise, committee member; Knapp, Alan, committee member
    Juniper expansion into sagebrush communities is a widespread phenomenon occurring across large regions of the western U.S. over the past century. Fire suppression and increased grazing activity are commonly considered as the primary drivers of expansion but they do not explain all instances of expansion. In order to develop a complete explanation for the success of juniper we investigated the competitive abilities of J. osteosperma (Utah juniper) and A. tridentata (big sagebrush) based on fine root traits, including specific root length (SRL), fine root diameter, and fine root biomass, and spatial patterns of water use inferred from stem and soil stable oxygen isotopes (δ18O). Data were collected from three different age classes of J. osteosperma (seedling, sapling, and mature) to better understand the competitive abilities at different life stages. J. osteosperma age classes were originally determined by height and later aged from cross sections. The youngest seedling in our study was 14 years of age, therefore we refer to the seedlings in this study as 'advanced.' Advanced J. osteosperma seedlings demonstrated the ability to switch their reliance from shallow to deep water sources later in the season, potentially enhancing their survival particularly during drought events. A. tridentata had traits associated with faster root proliferation and resource acquisition (significantly greater SRL and smaller root diameter) suggesting competition for limiting resources is likely not a primary driver of expansion of J. osteosperma.
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    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.
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    Improving state-and-transition models for management of sagebrush steppe ecosystems
    (Colorado State University. Libraries, 2015) Tipton, Crystal Yates-White, author; Fernandez-Gimenez, Maria, advisor; Ocheltree, Troy, advisor; Aldridge, Cameron, committee member
    The sagebrush biome was once the most widely-distributed in North America, but has recently experienced range reductions of up to 45% and has been considered one of the most endangered ecosystems in the United States (West 1983, Noss et al. 1995, Miller et al. 2011). Management for multiple land-use goals in this biome is complex, requiring an intricate understanding of biotic and abiotic interactions, their responses to disturbance, and the potential for catastrophic ecosystem shifts in response to stress. State-and-transition models (STMs) illustrate the complex relationships between ecosystem components and convey both equilibrial and non-equilibrial dynamics, in a conceptual, visual framework (Westoby et al. 1989, Walker and Westoby 2011). Recognizing their potential to guide both research and management decision-making, the Natural Resource Conservation Service, U.S. Forest Service, and Bureau of Land Management recently signed an interagency agreement to develop and use STMs to guide rangeland management decision-making nation-wide (Caudle et al. 2013). The growing popularity of STMs has brought them under increased scrutiny (Knapp et al. 2011, Tidwell et al. 2013). Common criticisms of STMs include: 1) reliance on insufficient empirical datasets or knowledge-based data prone to bias; 2) failure to explicitly identify the spatial and temporal scale of the model and the limitations of its generalizability; 3) dependency on assumptions of linear, reversible succession toward a climax reference community while ignoring the roles of non-equilibrial change, multiple disturbance types and abiotic gradients in shaping system resilience; 4) focus on the practices associated with structural change, while overlooking the ecological process feedbacks that influence disturbance response; 5) failure to validate STMs by testing model predictions. Motivated by the need for improved sagebrush-steppe management tools, my thesis addresses these criticisms and challenges by exploring new approaches to build and refine STMs. The first chapter provides a review of sagebrush-steppe ecosystem dynamics, paradigms of vegetation change, and the application of STMs to natural resource management. The second chapter presents work to apply a collaborative, iterative approach proposed by Kachergis et al. (2013c) that integrates knowledge-based and empirical datasets to develop an STM for a Wyoming big sagebrush-steppe ecosystem in Moffat County, Colorado. The third chapter presents a pilot project to revise an existing STM by incorporating the role of specific ecological processes (nitrogen cycling) into a state transition. I conclude that the approaches employed here can address many of the challenges and criticisms of current STMs, but should be coupled with rigorous experimental testing of model assumptions and uncertainties and long-term monitoring of experimental outcomes. In addition, collaborative approaches should take care to carefully balance resource limitations with the desire to include a broad base of stakeholders and research interests, carefully manage stakeholder expectations, and explicitly define success in terms of both the collaborative process and the scientific outcomes.
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    Is it plastic or just fantastic? Understanding the role of plasticity and local adaptation in the drought tolerance of Bouteloua gracilis
    (Colorado State University. Libraries, 2017) Bushey, Julie A., author; Ocheltree, Troy, advisor; Smith, Melinda, committee member; Gleason, Sean, committee member
    The ability of an organism to tolerate or acclimate to drought may become a major driver of changes in community composition, carbon and water cycles and ecosystem services as we encounter increasing severity and frequency of droughts in the face of global change.  Drought tolerance traits allow us to quantify functional attributes of individual species, but the expected variability of drought tolerance traits within a species is uncertain.  Although some variability in plant traits are expected, it is unknown whether the local adaptation of populations can explain the expected variability.  The objective of this study is the quantification of plasticity of drought tolerance responses across populations of Bouteloua gracilis to different soil moisture levels.  B. gracilis is a C4 perennial grass that dominates grasslands across a range of climates and is a major contributor of ecosystem function and services within these systems. Populations from less arid sites showed greater osmotic adjustment and higher midday water potentials when grown under limited soil moisture conditions. Populations from arid sites did not adjust osmotic potential but showed more negative midday water potentials while maintaining higher growth rates. This variation in response to lowered soil moisture indicates a potential shift in water use strategy across an aridity gradient that has implications for land managers seeking to restore B. gracilis dominated ecosystems with drought tolerant material.
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    Novel water-use strategies of Colorado wetland plants: implications for wetland water loss
    (Colorado State University. Libraries, 2023) Wright, Anna, author; Ocheltree, Troy, advisor; Sueltenfuss, Jeremy, advisor; von Fischer, Joe, committee member
    In the arid west, there is a tension between wetland restoration and water rights. Wetlands, relied upon by humans and wildlife alike, also contribute significantly to evapotranspiration (ET) due to higher water tables and dense vegetation. It is therefore critical to understand how much water wetlands lose to evaporation and transpiration, and what affects wetland water loss. This paper quantifies the transpiration of five abundant wetland species and investigates physiological mechanisms that drive transpiration rates for each species. The focal species transpire significantly different amounts at the leaf-level and when scaled to ground area. Stomatal response to environmental stimuli differed from upland stomatal responses, which suggests that wetland plants prioritize carbon uptake over hydraulic safety and do not align with current paradigms for stomatal responses to vapor pressure deficit, leaf water potential, or turgor loss point. Understanding species' stomatal responses to extreme environmental conditions is key to managing this rare and critical ecosystem as the climate changes.
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    Relationships between anatomy and climate in members of the subfamily Pooideae
    (Colorado State University. Libraries, 2019) Spitzer, Daniel B., author; Ocheltree, Troy, advisor; Meiman, Paul, committee member; Hufbauer, Ruth, committee member
    As climate continues to change, it will become increasingly important to accurately predict the landscape-level response of plant functional groups to climatic shifts. Relationships between vascular anatomy and climate distribution have been revealed in woody plant species, but little research has been done on this subject in the Poaceae family. These relationships were studied in 21 grass species from 5 genera in the subfamily Pooideae by analyzing their anatomical traits and climate data from each species' range. Analysis showed multiple correlations among anatomical traits, such as a positive correlation between the ratio of vessel wall thickness to vessel diameter (t/b ratio) with vein density. These relationships suggest that anatomical traits are closely linked to each other and to vein density, which may be confer some advantage or be in response to selective pressure. Some correlations between climate traits and anatomical traits were found, but the strength of these relationships was less than expected and were often the result of the interaction of multiple climate variables with a single anatomical variable. This may be because of broad, multi-year averaged data obscuring important temporal and spatial factors, as well as phenological variation across species not being accounted for. Future research should focus on microclimatic conditions species occupy and noting phenology for each species to better identify the climate conditions species are adapted to.