Mountain Scholar :: Browsing by Author "Ocheltree, Troy, committee member"

Browsing by Author "Ocheltree, Troy, committee member"

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Open Access
Analysis of root growth in two turfgrass species with minirhizotron and soil coring methods
(Colorado State University. Libraries, 2015) Young, Jason Scott, author; Qian, Yaling, advisor; Comas, Louise, advisor; Ocheltree, Troy, committee member
In this study root growth of a turf-type variety of inland saltgrass (Distichlis spicata L. Greene) (a native grass with varieties in development by Colorado State University) and Kentucky bluegrass (Poa pratensis L.) (a common turfgrass planted in the arid and semi-arid west) was examined under saline conditions in a pot experiment and non-saline conditions in the field. Since turfgrass is a high user of water, the turf industry is interested in using native species that use less water and also salt-tolerant species, which may allow the industry to use marginal water (grey water) for irrigation. However, plants with different root distributions will need to have irrigation managed differently. These experiments examined root growth differences in saltgrass and Kentucky bluegrass to begin exploring how these species might need to be managed differently in saline and non-saline conditions. Two separate experiments were conducted to answer the two objectives of this research: (I) to evaluate root growth of inland saltgrass under saline conditions in a growth chamber and (II) observe unrestricted root growth in the field both over time with a minirhizotron camera system, and in stands of differing age with a soil coring method. In the first experiment, root growth in container grown saltgrass under salt stress showed increased flushes of fine root growth in response to moderate levels of salinity (8 dS/m) compared to the control. Root growth increased about 3 weeks after salt treatments began, suggesting that this time frame was long enough for ionic stress to occur in the shoots root responses were seen. In-growth root tubes placed in the soil of the salt stressed saltgrass showed trends of increasing root and rhizome growth with increasing salt stress, this was opposite the trends seen in Kentucky bluegrass. In experiment II, field-grown saltgrass plots of varying stand age (1, 4, 5, and 8 years) had less root biomass in soil layers less than 30 cm compared to bluegrass. Kentucky bluegrass root biomass was nearly zero below 30 cm, whereas saltgrass had roots down to 275 cm in stands that had been growing longer than 4 years. In soil layers up to 1.8 m, saltgrass root mass was greater with increasing stand age. Minirhizotron observations showed that 15°C was the soil temperature at which root growth began in saltgrass and dramatically slowed in Kentucky bluegrass which had a growth range of 0 to 15°C. When soil temperatures were above 15°C saltgrass roots continued to grow at a slow but steady rate during the summer months. Findings that saltgrass produced roots deeper in the soil profile and was responsive to saline soil may impact where and when it is used. If stored moisture is present deep within the soil, saltgrass has a unique ability to mine this water that would be out of reach of shallower rooted turfgrasses. Deep rooting can also have implications for slope stabilization which can be important in the arid west where bare slopes can be stripped of soil during heavy and infrequent rainstorms. The responsiveness of rooting in saline soils may be the underlying mechanism explaining the enhanced growth of saltgrass under mild saline conditions. Increased surface area from new fine root production can enhance root water uptake providing more water to growing shoots. More studies are needed to explore root responsiveness in many types of plants, including saltgrass, to discover the true benefit of fluctuations in root system architecture.
Open Access
Causes, consequences, and management of tree spatial patterns in fire-frequent forests
(Colorado State University. Libraries, 2022) Ziegler, Justin Paul, author; Hoffman, Chad, advisor; Ocheltree, Troy, committee member; Redmond, Miranda, committee member; Rocca, Monique, committee member
Increasingly, restoration treatments are being implemented to dually meet wildland fire hazard reduction alongside ecological objectives. Restoration treatments however deviate from conventional fuels treatments by emphasizing the re-creation of forest structure present prior to EuroAmerican settlement, notably the retention of single and grouped trees interspersed between canopy openings. As these historical forests persisted over cycles of fire returns, it is assumed that restoring these historical complex tree spatial patterns will, in turn, restore historical ecological processes. This includes more benign fire behavior that results in only partial tree mortality, allowing persistent and partial retention of forest cover over cycles of fire return. The qualitative description of historical forest structure, lacks, however, a clear process-based explanation detailing the interactions of heterogeneous forest structures and fire. While fires were historically frequent, it is unclear what role fire played in the genesis and maintenance of tree spatial patterns. If models of tree spatial dynamics can be improved and the interactions between tree spatial patterns and fire can be elucidated, forest managers will have an improved understanding of the implications of restoration-based fuels hazard reduction treatments both during fire-free periods and during fire events. The aims of this dissertation were to: 1) explore the causes of tree spatial patterns in dry fire-frequent forests; 2) investigate the consequences of tree spatial patterns on potential fire behavior and effects; 3) determine how alternate silvicultural strategies targeted at manipulation of tree spatial patterns can influence fire behavior and effects. In Chapter 2, I explored spatial patterns of tree regeneration over 44 years in absence of fire. In cooler periods, regeneration preferred clustering in openings, including openings following overstory mortality and away from overstory trees. Mortality risk of regeneration was heightened nearer overstory trees. In warmer periods, these trends reversed, likely because of a 'nurse effect' from the overstory. In anticipation of climate change, these results suggest silviculturists may benefit by capturing regeneration mortality in within openings while keeping regeneration near the overstory. In Chapter 3, I found that regenerating trees also form heterogeneous patterns following stand-replacing fires. In these sparse, early seral forests, all species were spatially aggregated, partly attributable to the influence of topography and beneficial interspecific attractions between ponderosa pine and other species. Results from this study suggest that scale-dependent, and often facilitatory, rather than competitive, processes act on regenerating trees. In Chapter 4, I studied the interaction between fire and tree spatial patterns, both historically and in modern forests. Tree mortality in the historical period was clustered and density-dependent because tree mortality was greater among small trees, which tended to be assembled in tightly spaced clusters. Tree mortality in the contemporary period was widespread, except for dispersed large trees, because most trees were a part of large, interconnected tree groups. Postfire tree patterns in the historical period, unlike the contemporary period, were within the historical range of variability found for the western United States. This divergence suggests that decades of forest dynamics without significant disturbances have altered the historical means of pyric pattern maintenance. In Chapter 5, I examined how fuels treatment designs with different manipulations of tree spatial patterns may influence treatment effectiveness. I simulated fires on hypothetical cuttings which manipulated the arrangement of crown fuels horizontally and vertically, either increasing the distance between tree crowns or not, and either removing small trees or not. All cutting methods reduced fire behavior and severity, but the results confirm possible tradeoffs between ecological restoration and hazard reduction; treatments that separated tree crowns reduced severity the most because these treatments reduced crown fire spread. But these can easily be overcome where restoration treatments incorporate small tree removal, because this action limits crown fire initiation. Managers could also incorporate managed fires to reduce surface fuel loads and use more aggressive cuttings to further gains in hazard reduction, regardless of cutting method used.
Open Access
Climate forecasting, climate-resistant tree species selection, and urban canopy planning for a small, semi-arid city
(Colorado State University. Libraries, 2023) Buchholz, Natalie S., author; Klein, Julia, advisor; Boone, Randall, committee member; Ocheltree, Troy, committee member
Globally, there is an increasing shift from rural to urban living, with major implications for the ecosystem services provided by these ecological systems and profound effects on the human communities that continue to migrate to them. Urban canopies and the development of green spaces in the urban environment can mitigate some of the negative effects of urban living such as high Urban Heat Indices (UHI), poor air quality, loss of emotional support from nature, and loss of sense of place. However, the poleward shift of climate envelopes due to climate change will eventually lead to urban canopy collapses as street trees become less suited to their environment. In chapter one of this thesis, I explore this shift and climate-change resistant tree species selections by using WorldClim's current (1971-2000), SSP126 (low emissions) and SSP585 (high emissions) scenarios to generate climate envelope projections for the City of Fort Collins, Colorado in the years 2061-2080. I collected recommended street tree lists from cities that currently exhibit future climate conditions in Fort Collins (hereafter, reference cities) and compared them to the current city of Fort Collins' recommended street tree list. I identified 15 shade trees that have already undergone testing by CSU arborists and are appropriate for immediate planting in Fort Collins, Colorado – these 15 shade tree species comprise the Future Fort Collins Recommended Street Tree list. I identified an additional 21 shade tree species that still require CSU arborist testing for planting suitability – these 21 shade tree species comprise the Future Fort Collins Recommended Street Tree-Test list. The Future Fort Collins Recommended Street Tree and Future Fort Collins Recommended Street Tree-Test lists will inform street tree diversity and planting strategies for the City of Fort Collins, CO, and provide a brief list of species that need further suitability testing by foresters and arborists along the Front Range. Chapter two of this thesis aims to address current canopy inequities in Fort Collins, Colorado, by examining patterns in the city's overall land cover distribution, as well as the built environments and policies that limit canopy expansion. Here I explore canopy and land cover in four census blocks that are at least 25% Hispanic and have a median household income of less than $40k per year. I further dissect these four census blocks into smaller study areas based on zoning type as designated by the City of Fort Collins and Larimer County Assessor's Office. Using a combination of satellite observation and I-Tree canopy analyses I hope to, 1) identify patterns in land and canopy cover across zoning types, and 2) identify factors in the built environment and written policy that limit urban canopy growth in the City of Fort Collins, Colorado. Results will inform canopy care and planting strategies as the city moves to create its first Urban Forest Strategic Plan in the coming years.
Open Access
Constraints in the compensatory response of a tallgrass prairie plant community to the loss of a dominant species
(Colorado State University. Libraries, 2020) Chaves Rodríguez, Francis Andrea, author; Smith, Melinda, advisor; Knapp, Alan, committee member; Ocheltree, Troy, committee member; Sala, Osvaldo, committee member; Webb, Colleen, committee member
Biodiversity loss is one of the major consequences of global change driven by human activities. The loss of a dominant species is expected to have profound consequences on ecosystem processes (e.g. aboveground productivity) given their highest relative abundance and proportionally large contribution to community biomass production. However, through competitive release, the newly available resources following its lost, are expected to be utilized by the remaining species in the community to increase in abundance and compensate for the function lost. Complete functional compensation does not occur in every ecological community following the loss of dominant species or entire functional groups, and 1) limited resource availability, 2) absence of functionally redundant species, and 3) lack of functional traits that promote compensation have been proposed as possible constraints on compensation. In this dissertation, I evaluate the effect of removing these constraints on the biomass compensation response of a tallgrass prairie plant community following the loss of the dominant species, the C4 tallgrass, Andropogon gerardii Vitman. I experimentally removed the dominant species from a native intact tallgrass prairie plant community at Konza Prairie Biological Station, Kansas, where I selected two contrasting sites, one with functionally redundant species Panicum virgatum L. and Sorghastrum nutans (L.) Nash in low abundances, and a second site where those functionally redundant were codominants with A. gerardii. The first site was irrigated to alleviate water limitation during four growing seasons and fertilized with nitrogen during the final season of the experiment. The second site did not exhibit water limitation and was fertilized during the second growing season of the two-year experiment. My results show that in the short-term removing resource limitation promoted aboveground primary productivity but not enough to produce full biomass compensation. The presence of functionally redundant species, also C4 tall grasses with similar functional effect traits as A. gerardii, did increase aboveground biomass production, but did not promote full biomass compensation, not even when they were present in high abundance. I hypothesize that additional to the constraints proposed, compensation is limited by response traits in the remaining species that limit their demographic response to the increased available space, light, water and soil resources following the loss of the dominant species. Overall, my results show the compensation approach is important to evaluate not only the effect of species loss on ecosystem processes, but also the response of the remaining species and their ability to compensate for the function lost. They also suggest the existence of additional mechanisms in play that need to be identified and tested in order to improve the understanding of how communities recover in the face of biodiversity loss.
Open Access
Defining, describing, and assessing growth determinacy as a mechanism of plant species codominance
(Colorado State University. Libraries, 2022) Gray, Jesse Edward, author; Smith, Melinda, advisor; Knapp, Alan, committee member; Ocheltree, Troy, committee member; Blumenthal, Dana, committee member
To view the abstract, please see the full text of the document.
Open Access
Eco-physiological studies on horned poppy, (Glaucium spp.)
(Colorado State University. Libraries, 2013) Getlawi, Ahmed O. M., author; Hughes, Harrison, advisor; Shahba, Mohamed, advisor; Wallner, Stephen, committee member; Ocheltree, Troy, committee member
To view the abstract, please see the full text of the document.
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 member
Predictions 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.
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
Effects of drought stress on early white pine blister rust development in limber pine
(Colorado State University. Libraries, 2022) Bertram, Jonathan H., author; Stewart, Jane E., advisor; Scoettle, Anna W., advisor; Borlee, Brad, committee member; Ocheltree, Troy, committee member
Climate change and forest pathogens are expected to interact as incidences of drought increase and affect the disease triangle between hosts, pathogens, and the environment. Trees will become physiologically affected by drought stress and primary pathogens such as fungal biotrophs will experience drought stress as mediated through the host. White pine blister rust, caused by the non-native pathogen Cronartium ribicola, is a devastating fungal pathogen, and little is known about how it will perform (measured by fungal growth or disease severity) within pine hosts experiencing unusual drought. This study aimed to address some of the unknown aspects of this interaction by performing a greenhouse drought × pathogen experiment with Pinus flexilis seedlings, measuring host physiology, quantifying specific aspects of pathogen performance, and looking for interactive effects. Watering treatments consisting of well-watered, mild chronic drought, or severe acute drought were applied to 432 seedlings; after 3 months, a subset of 198 seedlings were inoculated with C. ribicola basidiospores under ideal inoculation conditions, after which watering treatments continued for a further 9 months. Specific rust performance measurements included mycelial growth via relative rust DNA quantification and ratings of disease severity by watering treatment.
Open Access
Environmental stress recovery of horned poppy (Glaucium spp.) using growth regulator treatments
(Colorado State University. Libraries, 2015) Elghoul, Milad M., author; Hughes, Harrison, advisor; Shahba, Mohamed, committee member; Ocheltree, Troy, committee member; Krishnan, Sarada, committee member
To view the abstract, please see the full text of the document.
Open Access
Establishing Carex scopluorum seedlings to restore the vegetation of Tuolumne Meadows, Yosemite National Park, USA
(Colorado State University. Libraries, 2018) Booher, Melissa, author; Cooper, David, advisor; Ocheltree, Troy, committee member; Rocca, Monique, committee member
Wet meadows are critically altered and at-risk ecosystems globally and in the Sierra Nevada of California. The low vegetation cover created by legacy disturbances is a restoration priority due to the importance of organic-rich soils for future plant establishment, carbon storage, and water retention. Wet meadows are characterized by seasonally saturated fine-textured mineral soils with significantly more organic matter than surrounding areas, shallow groundwater (< 1 m), and vegetation dominated by herbaceous plant species. This research focused on the establishment requirements of seedlings of the native sedge Carex scopulorum in Tuolumne Meadows, Yosemite National Park, USA. I provide critical information on biomass contribution of a key wet meadow species that could also be used in other restoration efforts in similarly degraded subalpine meadows. We tested the suitability of this species for use in future restoration work and assessed the growth of C. scopulorum seedlings in a fully factorial experiment with small mammal herbivore exclosures and planting density treatments. Seedlings were planted in June 2016 and survival was high, approximately 98%, living through the summer of 2016 and 71% surviving through the end of the 2017 summer. After two seasons of growth, planted seedlings more than doubled in area (horizontal tiller spread) and nearly doubled their longest leaf lengths. Total C. scopulorum seedling biomass increased more than six-fold from the delivered seedlings in 2016 to end of the 2017 growing season. Carex scopulorum seedlings had greater mean biomass, 703.44 g/m2 ± 246.54, than all other species in our study plots and had more than twice the belowground biomass per unit area of other herbaceous species sampled. In addition, planted C. scopulorum seedlings allocated a greater portion of their biomass below-ground (higher mean ratio) than all herbaceous species (all comparisons p = <0.05) other than Carex subnigricans (p = 0.051). Our results indicate that C. scopulorum is an appropriate species for restoration in Tuolumne Meadows where increasing biomass inputs to the system is a priority and could be a valuable tool for revegetation and restoration of other degraded meadows in the Sierra Nevada.
Open Access
Exploring interactions among biological soil crusts, plant germination, and morphological seed traits: implications for plant community assembly and dryland restoration
(Colorado State University. Libraries, 2023) Bacovcin, John, author; Havrilla, Caroline, advisor; Ocheltree, Troy, committee member; Wrighton, Kelly, committee member; Paschke, Mark, committee member
Arid and semi-arid (dryland) ecosystems make up over 40% of our plant's terrestrial surface and are incredibly vulnerable to land degradation. To combat dryland degradation, active plant and soil restoration is often needed and the role of plant-soil microbe interactions can be key to dryland restoration trajectories. Within drylands, biological soil crusts (biocrusts), collections of cyanobacteria, algae, lichen, and moss are key surface communities that influence soil processes (e.g., stability, nutrient cycling, hydrology) and can thereby strongly influence recruitment of dryland plants. These biocrusts may interact with plant functional traits (i.e., seed morphological traits), and these interactions can influence germination. However, much is still unknown about mechanisms that underlie these interactions and how plant functional traits mediate effects of biocrusts on plant germination. To investigate these knowledge gaps, I conducted two studies: (Chapter 1) a global meta-analysis of the role of morphological seed traits in determining biocrust effects on germination, and (Chapter 2) a full-factorial greenhouse study examining the effects of biocrust inoculum cover treatments and plant functional traits on plant recruitment to investigate questions about how biocrust heterogeneity and biotic components of biocrusts in the context of restoration. To explore effects of morphological seed traits on plant germination responses to biocrusts (Ch. 1), we compiled a global database of 491 studies of biocrust effects on plant germination encompassing 101 unique plant species and their associated morphological seed traits. For the greenhouse study (Ch. 2) we seeded two seed mixes on three different inoculum cover treatments (i.e., 0%, 30%, and 100%) using both biologically active (live) and autoclaved biocrust inoculum, to assess effects of cover heterogeneity, biological biocrust activity, and plant functional traits on percent germination. Results from the meta-analysis showed that morphological seed traits do mediate plant germination responses to biocrusts, and that, in general, germination of smaller seeded species with appendages was increased by biocrusts. Results from the greenhouse study showed that, in a restoration context, increasing cover of biocrust inoculum increases plant germination, and that these effects were explained by physical rather than biotic effects of inoculum on germination. As in Chapter 1, we found that biocrusts effect on germination differed across plant functional groups and that seed traits also influenced germination responses to biocrust inoculum cover treatments. Together, both studies showed that morphological seed traits mediate effects of biocrusts on plant germination. These findings increase understanding of the role of biocrusts in determining dryland plant community assembly and have implications for dryland restoration.
Open Access
Forest type and burn severity affect understory response to historic wildfires
(Colorado State University. Libraries, 2023) Weimer, Kate, author; Stevens-Rumann, Camille, advisor; Rocca, Monique, committee member; Ocheltree, Troy, committee member
The fire season of 2020 was unprecedented in the Western US. In one summer, three different fires individually broke the record of Colorado's largest wildfire. Understanding the recovery following these unprecedented events can lend insight into the compounding effects of wildfire and climate change. Reorganization of the understory community after disturbance can indicate changes in conditions not yet reflected in tree communities. Understory dynamics also affect watershed characteristics and wildlife, so knowledge about the influence of wildfire on understory plants is crucial. The purpose of this study is to determine if a trend toward thermophilization is being shown in understory vegetation following 4 different wildfires in 2020 and to compare the relative importance of burn severity, forest type, and other environmental factors on understory community composition. We found a trajectory toward thermophilization in high elevation forests that burned with high severity. We also that drivers of community composition varied by forest type. These findings help assess how wildfire is affecting plant communities in the 21st Century and highlight where future management concerns may be.
Open Access
Gambel oak productivity and demography: an observational study of fundamental ecological characteristics in western Colorado
(Colorado State University. Libraries, 2020) Brazenwood, Arian, author; Ex, Seth, advisor; Ocheltree, Troy, committee member; Sibold, Jason, committee member
Gambel oak exhibits demographic and morphological variability across its range in the intermountain West. While broadly distributed, little is known about the basic ecological behavior of this species in Colorado ecosystems. We used an observational study to systematically describe variation in Gambel oak stand biometrics across a climatic moisture deficit (CMD) gradient on the Uncompahgre Plateau in western Colorado. Our analysis focused on parsing out differences in demographic characteristics including stand structure, age distribution, and productivity among varying levels of CMD with a 90-plot inventory capturing 2,312 tree measurements across a 200 mm range of CMD. To explore causal factors for variation in productivity, we used a mixed effects model derived from a systematic model selection process that identified predictors from a suite of biotic stand structure variables, and abiotic climatic and topographic variables. Metrics of tree size, stand density, and productivity varied systematically across the CMD gradient, where the driest sites had smaller trees, higher stand densities, and slower growth than wetter sites. In the driest sites, trees averaged 3.9cm of diameter at root collar (DRC) with mean densities of 9,000 trees per hectare contrary to the wettest sites where stems sizes averaged 7.5cm DRC with mean densities of 5,700 trees per hectare. However, regardless of variation in CMD, most stands were uneven-aged, as evidenced by uniform age proportions and ample regenerating cohorts of young stems across nearly all plots. Within a 60km2 study area across 500m of elevation difference, variables capturing climate and stand structure effects were the most predictive of productivity. A significant negative interaction between tree size and CMD indicates moisture limitation likely prevents trees on the driest sites sampled from attaining the larger sizes observed on wetter sites. Additionally, stands with high CMD were consistently younger than stands with low CMD, implying either more frequent stand-replacing disturbance in these locations or recent colonization of new areas. The demographic characteristics across all CMD levels indicate stands naturally achieve a more heterogeneous structure following about a century of growth. This suggests that management promoting multi-cohort stand structures could accelerate transition from homogeneous thickets to heterogeneous old forests. Given the variable growth rates across the CMD range, this would likely be most successful on wetter sites.
Open Access
Indirect interactions between galling insects as mediated by a non-native plant
(Colorado State University. Libraries, 2021) Lucci-Rimer, Hayley, author; Ode, Paul, advisor; Balgopal, Meena, advisor; Bean, Dan, committee member; Ocheltree, Troy, committee member
Russian knapweed (Rhaponticum (=Acroptilon) repens) is a non-native plant species that has spread throughout the western United States, out-competing native and crop species. Two host-specific gall forming insects - the stem-gall wasp Aulacidea acroptilonica (Hymenoptera: Cynipidae) and the tip-gall midge Jaapiella ivannikovi (Diptera: Cecidomyiidae) - have been employed as biological control agents to manage its spread. However, the nature of the intra- and interspecific interactions between these two insect species is not fully understood even though this information is vital to successfully controlling Russian knapweed. Examining these interactions also contributes to a growing body of knowledge on plant-mediated indirect interactions, which are prevalent among herbivorous insects. To determine the nature of these interactions (e.g., positive, negative, or neutral) and the effects that the insects, together and separately, have on Russian knapweed, I performed studies in the greenhouse and at field sites throughout Colorado. I found evidence for a negative or neutral interspecific relationship, a negative or neutral intraspecific relationship among midges, and a positive or neutral relationship among wasps. The nature of these interactions is likely density and resource dependent. The associations between each insect species and measures of plant growth and reproductive capacity were mixed across the two studies, at times showing positive, negative, or neutral relationships. More studies that consider insect density, water availability, and resource allocation within Russian knapweed should be performed to better determine how the insects interact with one another and what their effects on Russian knapweed may be under varying conditions.
Open Access
Management and epidemiology of Cytospora perennial canker, Cytospora plurivora, in western Colorado
(Colorado State University. Libraries, 2021) Miller, Stephan T., author; Stewart, Jane, advisor; Ocheltree, Troy, committee member; Norton, Andrew, committee member; Minas, Ioannis, committee member; Uchanski, Mark, committee member
Cytospora canker is a ubiquitous disease in deciduous fruit tree systems in western Colorado. The research conducted herein, explores the host-pathogen-environmental framework which has enabled Cytospora plurivora to thrive and become a threat to peach production in the region. My research also focuses on management strategies, both cultural and chemical, which can help mitigate pathogen infections. Chapter II, published in the Journal of Crop Protection, presents preventative control of C. plurivora through wound protection of pruned shoots. Several fungicides and sealants were evaluated either in vitro or in field trials, to explore antifungal activity. Fungicides which have been shown effective, were also evaluated for half maximum effective concentration rates (EC50) to better understand pathogen dose sensitivity. Chapter III explores the susceptibility of thirteen peach cultivars to C. plurivora infection under different abiotic conditions. Abiotic stressors such as water deficit and high-pH can be major limiting factors to tree fruit production and can increase tree susceptibility and pathogen severity. My research shows increased severity of C. plurivora infection and decreased plant water potentials when trees experienced increased soil pH and irrigation deficits. Chapter IV provides a detailed analysis of the epidemiology of C. plurivora in the field. This study estimates spore production rates and lesion infection rates over a 12-month period. Further, it evaluates possible dissemination mechanisms of C. plurivora, reporting detection of C. plurivora spores in aerial and on insect samples, although at low concentrations. The results presented herein help inform management strategies by elucidating field patterns of C. plurivora and identifying effective cultural and chemical control measures.
Open Access
Modular modeling and its applications in studies of grazing effects
(Colorado State University. Libraries, 2016) Miao, Zhongqi, author; Boone, Randall, advisor; Conant, Richard, committee member; Ocheltree, Troy, committee member
Grazing is an important ecosystem process that can affect the grazing system at different levels. Overall grazing effect can be a combination of various direct and indirect effects. It is difficult to study grazing with all of the effects considered. To have a better knowledge of grazing effects and animal-plant interactions, modeling is one important pathway to achieve this goal. People usually use a diversity of approaches when modeling grazing based on different objectives, which makes model evaluations and comparisons difficult. With modular modeling, where different model components are regarded as separate and standardized modules, this situation can be changed. An example model is developed using a modular approach. It included most of the grazing effects and switches that can turn these effects on and off. This model was designed to be capable for applications with different hypothesis and objectives. It is expected to be clearer for people who are not familiar to models to make comparisons and evaluations of grazing effects. To test the feasibility of the model, a theoretical experiment on compensatory behavior in grassland production and a realistic simulation on plant-animal interactions in Qinghai-Tibetan plateau, China, are conducted. The results of these two applications demonstrate the benefits of using modular modeling in studies of grazing effects.
Open Access
Riparian area invasive plant management along the Niobrara River, targeting yellow flag iris (Iris pseudacorus L.)
(Colorado State University. Libraries, 2016) Spaak, Jordan L., author; Meiman, Paul J., advisor; Beck, K. George, committee member; Ocheltree, Troy, committee member
Yellow Flag Iris (YFI) (Iris pseudacorus L.) is an invasive exotic species that is causing substantial changes to the ecology of the Niobrara River and the adjacent riparian area habitat. Options for invasive plant management along the Niobrara River, like most riparian wetlands, are quite limited. Currently, herbicides offer the best opportunities for successful YFI management in riparian habitats such as those along the Niobrara River but irrigation diversion and livestock grazing are prevalent which impose further limitations on management options. In this study, we analyzed the efficacy of multiple potential YFI management methods, including; chemical (glyphosate), mechanical (cutting), plant competition, and trampling. A combination of field and greenhouse studies were used. Field studies were conducted at Agate Fossils Beds National Monument, Harrison Nebraska, U.S.A. and greenhouse experiments were completed at Colorado State University, Fort Collins, Colorado, U.S.A. The greenhouse studies were conducted to determine how temperature, light, seed scarification, and trampling, affected germination, seedling growth and survival of YFI. The field studies focused on the effectiveness of glyphosate, cutting, planting native plants, and trampling on YFI. Results from field studies indicate that cutting established plants stimulates YFI growth, spring application of glyphosate resulted in a short-term reduction of YFI abundance, and planting native plants did not reduce YFI abundance. Findings indicate that YFI prefer shaded areas and cooler temperatures for emergence and warmer shaded areas for growth. Seed scarification did not affect emergence. Trampling in the field reduced YFI density by 75% and plant height by 58%. Simulated trampling that targeted the plant crown and 1-2-cm above the crown reduced survival, though trampling that targeted the crown was five times more likely to kill the plant than trampling 1-2-cm above the crown.
Open Access
Semi-arid grassland ecosystem functional collapse after effects of five years of extreme drought
(Colorado State University. Libraries, 2023) Lenners, Alicia, author; Smith, Melinda D., advisor; Havrilla, Caroline, committee member; Ocheltree, Troy, committee member
A key outcome of climate change is an increase in the frequency and intensity of drought events in many regions of the globe. The largest impacts on ecosystem structure and function are likely to occur in water-limited ecosystems, such as semi-arid grasslands, potentially leading to a collapse of ecosystem function. While short-term studies have been conducted on various grassland ecosystems, the goal of this study is to fill in the gap of the effect multi-year extreme droughts have on the semi-arid shortgrass steppe of the Central US by characterizing the change in structure and function of these ecosystems. The drought was conducted between 2018-2022, and I had conducted various measurements over the summer of 2022 within the USDA-Central Plains Experimental Range (CPER) of Northeastern Colorado. The experimental drought was imposed using four rainfall exclusion shelters, two of which blocked 66% of precipitation from entering, and the other two remaining uncovered (control plots). Ten plots in each of the four shelters were measured weekly for soil moisture (%), soil temperature (°C) and soil respiration (CO2 efflux); twice per season for soil nutrient availability; and at the end of the growing season for aboveground (ANPP; stems and leaves) and belowground net primary production (BNPP; roots). The extreme drought resulted in an ~40% reduction in growing season soil moisture and an average 2°C increase in soil surface temperatures. Within the 13 weeks of study, drought led to an ~50% reduction in soil respiration (CO2 efflux). ANPP was drastically reduced (~99%) with extreme drought, while cactus surface area increased 3-fold. The extreme drought treatment also resulted in large reductions in BNPP measured from 0-30 cm (79%); however, root growth was reduced most in the shallowest soil depth (0-10cm) when compared to control plots. Lastly, there was an increase in nitrogen availability (both NH4+ and NO3-) with extreme drought by the end of the growing season. These results suggest that extreme, multi-year drought can cause an almost complete collapse in ANPP and significantly reduce BNPP particularly in the top 10 cm of the soil profile, which could have important implications for carbon sequestration. It remains unknown what impact the dramatic reduction in ecosystem productivity but accumulation of available nitrogen in the soil will have for recovery of the shortgrass steppe ecosystem post-drought, but it is likely that recovery will be prolonged despite the increase in soil resources.
Open Access
Testing hydrologic performance standards to evaluate wetland restoration
(Colorado State University. Libraries, 2018) Sueltenfuss, Jeremy Paul, author; Cooper, David J., advisor; Ocheltree, Troy, committee member; Paschke, Mark, committee member; Rocca, Monique, committee member; Sanderson, John, committee member
Defining success in wetland restoration can be difficult and subjective, as each restoration project has distinct goals. When wetland restoration projects fail to achieve their identified goals, it is often due to inadequately restored water levels. Incorrect water levels can lead to invasion by exotic species, or can alter the type of wetland that was supposed to be restored. To provide guidance to local wetland restoration efforts I investigated the hydrologic niche of Carex pellita, a wet meadow species commonly planted in wetland restoration, along with Typha latifolia, a species of cattail which commonly invades restored sites. Restored wetlands often have higher water levels than naturally occurring wetlands, and the prevailing assumption has been that hydrologic conditions in restored wetlands are not suitable for this species of Carex. Using experimental transplants across a hydrologic gradient, I found that Carex has a much wider hydrologic niche than previously thought. It produced either the same or more biomass in the high water level transplant treatment than in the low water level control plot. Typha responded negatively to being transplanted into areas with lower water levels. My results indicate Typha have filled their entire hydrologic niche in these wetlands and have competitively excluded Carex pellita to a smaller portion of its potential distribution. I also evaluated existing hydrologic and vegetation datasets from regulatory wetland restoration projects across the United States to help inform the development of wetland mitigation policy. The objective of regulatory wetland mitigation is to restore or create wetlands to offset the losses of wetland acreage and function incurred from impacts to existing wetlands. Unfortunately, wetland acreage and function are not always successfully replaced, and performance standards are now used in hopes to improve wetland mitigation outcomes. Because of the agreement in the scientific literature about the role of hydrology in creating and maintaining wetland structure and function, hydrologic performance standards may be an ecologically meaningful way to evaluate restoration outcomes. However, a framework for hydrologic performance standards has not been created or tested to date. I analyzed existing datasets from past and ongoing wetland mitigation projects to identify the number of years it took water levels in restored wetlands to match reference sites, and to test whether similar water levels between restored and reference sites leads to higher cover of native species. Wetland types differed in the number of years it took for water levels to match reference sites. Vernal pools in California took nine years to match reference sites, fens and wet meadows in Colorado took four years, and forested wetlands in the southeastern US were hydrologically similar to reference sites the first year following restoration. Plant species cover in all three restored wetland types was related to the water level similarity to reference sites. Native cover was higher when water levels were more similar to reference sites in some vernal pools, fens, and wet meadows, and was lower in areas where water levels were different. Exotic species cover showed the opposite relationship in fens and wet meadows, where hydrologic similarity led to low cover of exotic species. Forested wetlands showed no consistent relationship between tree seedlings or species richness and hydrologic similarity with reference sites. Based on the general agreement of the importance of hydrology for wetland form and function, hydrologic performance standards should be used in wetland mitigation. My research shows that hydrologic performance standards may also lead to increased vegetation success in some wetland types.