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Browsing by Author "Norton, Andrew, advisor"

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    Environmental predictors of annual incremental Populus deltoides growth, and riparian forest structure of the South Platte River in northeastern Colorado
    (Colorado State University. Libraries, 2020) Christensen, Cetan, author; Norton, Andrew, advisor; Katz, Gabrielle, advisor; Friedman, Jonathan, committee member; Redmond, Miranda, committee member
    Riparian forests are biologically diverse systems that provide essential ecological services, such as flood attenuation, bank stabilization, habitat, nutrient cycling, temperature regulation, etc., for the landscapes they occupy. The present-day South Platte River riparian forest is dominated by native phreatophytes (Populus and Salix species) which require hydrologic disturbance to reproduce. However, with changing water-use patterns and hydrology in the South Platte basin of Colorado, the future riparian forest status is unknown. This study describes the contemporary forest composition and age structure. Data was collected along transects from seven randomly selected sites within three randomly positioned 30-km river sections between Kersey and Julesburg, Colorado on the South Platte River. A ring width chronology was developed using cores from 237 Populus deltoides (plains cottonwood) trees and was used in linear mixed modeling to describe relationships between climate, hydrology, and site attributes that affect annual biomass production (Basal Area Increment). Populus deltoides dominate the riparian forest overstory, while later successional species (Ulmus pumila and Fraxinus pennsylvanica) are present at low densities. Though the timing of recruitment has varied among sites, overall recruitment of P. deltoides is abundant, reflecting the ongoing flow-related channel change occurring in this system. Summer mean flows, as well as November and March mean flows, and climate factors (Palmer Drought Severity Index) during the growing season were responsible for variations in P. deltoides annual tree growth, as were tree age, site, and attributes of the individual trees. Our findings contrast with previously hypothesized successional trajectories for this system, which predicted the replacement of the Populus-Salix overstory by later successional species (Ulmus pumila and Fraxinus pennsylvanica). The linear mixed model results highlight the importance of not only summer season flows and climate, but also the potential effects of off-season flow variables in a changing system.
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    Impacts and management of the invasive Russian olive (Elaeagnus angustifolia L.) in a hereogenous riparian ecosystem
    (Colorado State University. Libraries, 2017) Tuttle, Graham, author; Norton, Andrew, advisor; Katz, Gabrielle, advisor; Meiman, Paul, committee member; Paschke, Mark, committee member
    Russian olive is an exotic actinorhizal tree intentionally introduced to the U.S. in the early 1900's. It has become a dominant component of riparian ecosystems throughout the western U.S. Unlike most other riparian trees in the semi-arid west, Russian olive germinates and grows both in the open and in the understory of mature cottonwood stands. As an actinorhizal species, it forms an endosymbiosis with soil actinobacteria in the genus Frankia that allows for atmospheric N2-fixation. This leads to higher soil N concentrations and mineralization rates underneath the tree's canopy than outside. Russian olive's high abundance and impact on soil N suggest it may alter plant communities, but these impacts have not been previously demonstrated. I investigated the impacts of Russian olive on shading, soil N availability, and plant communities and documented how those impacts varied across a semi-arid riparian ecosystem along the South Fork of the Republican River in eastern Colorado. Of the suite of environmental variables I measured, presence or absence of cottonwood canopy had the largest effect on Russian olive impacts. Russian olive increased shading, soil N availability, and proportion exotic plant and forb cover more in the open than underneath a cottonwood overstory. Actinorhizal endosymbioses provides an important N source in terrestrial ecosystems, but N2-fixation rates decrease due to high exogenous N and low photosynthetically active radiation (PAR). The amount that these environmental variables reduce N2-fixation in host-Frankia symbiosis types dictates the strength and duration of those symbioses' impacts on ecosystems. To understand how the two main types of endosymbioses (Alnus- and Elaeagnus-Frankia) differ in their response to environmental variability, I conducted a greenhouse experiment comparing growth and nodulation between two genera of actinorhizal species, Elaeagnus and Alnus, across exogenous N and PAR levels. Overall, Elaeagnus species had higher nodulation rates and tissue % N than Alnus species. Nodulation rate and growth response to nodulation were both lower at low PAR than high PAR for both genera. The reduction in the growth response to nodulation at high exogenous N was lower in Elaeagnus-Frankia symbiosis than Alnus-Frankia symbiosis. These results suggest that Elaeagnus species are more likely to cause a greater and longer-lasting increase in soil N than Alnus species. A main objective of exotic species management is to increase native plant cover. However, few studies monitor plant community response to exotic species management, and the few that have suggest secondary invasion is likely, particularly when effects of the target invasive persists and management efforts cause disturbance. To measure the role of these two factors in plant community response to Russian olive removal, I monitored soil N availability and plant communities along the South Fork of the Republican River two years before and three years after the tree's removal. Russian olive's impact on soil N availability persisted, with levels staying high around removed Russian olive stems three years after removal. The plant community around removed Russian olive also had no increase in native plant cover but a dramatic increase in kochia (Bassia scoparia) cover following removal. My research demonstrates that Russian olive increases exotic plant cover in areas it invades and simply removing the tree does not promote native species recovery.
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    Initiating a plant herbivory response increases impact of fungal pathogens on a clonal thistle
    (Colorado State University. Libraries, 2019) Clark, Amy LaVonne, author; Jahn, Courtney, advisor; Norton, Andrew, advisor; Davis, Seth, committee member
    Cirsium arvense, or Canada thistle, is one of the most detrimental weeds for agricultural production and rangeland health. The autecious fungus, Puccinia punctiformis, or CT‐rust, shows potential as a control agent but rarely reaches epidemic proportions in natural populations. Manipulating plant defense hormones could alter host susceptibility and allow CT-rust to have more widespread impact. To determine if applying hormones increases the infection by the fungal pathogen, Canada thistle plants were inoculated and sprayed with jasmonic acid (JA) and salicylic acid (SA). Results show that the addition of JA at the time of inoculation increases the infection rates, both the incidence and severity, and impact of CT-rust which could make it a more effective control agent for Canada thistle. In the first greenhouse experiment, I found that JA increased infection rates by nearly 20 %. Infection consistently reduced root biomass and this reduction was 45 % greater with the addition of JA compared to water. We also found that, while JA does have a slight effect on plant growth, applying herbivory defense hormones does augment CT-rust's action as a biological control.
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    Investigating the resistance status to permethrin and temephos in Aedes aegypti (the yellow fever mosquito)
    (Colorado State University. Libraries, 2024) Arthur, Nicholas Wynne, author; Norton, Andrew, advisor; Camper, Matt, committee member; Kading, Rebekah, committee member; Saavedra-Rodriguez, Karla, committee member
    Aedes aegypti (Ae. aegypti) is the principle urban vector of several viruses of high medical significance which carry a disease burden on a global scale. Ae. aegypti is anthropophilic and lives in close association with humans. This places nearly half of the global population at risk of becoming infected with an arboviral pathogen every year. Therefore, emphasis must be placed on investigating methods for controlling this vector to combat and reduce the spread of human disease. This is especially true in areas where socioeconomic factors promote sustained transmission cycles. While vector control programs use a variety of strategies, the primary method of reducing vector populations is through insecticide use. Widespread use of insecticides has placed intense selection pressures on Ae. aegypti populations and resistance mechanisms have developed. Target site modifications and the expression of detoxifying enzymes are the most significant resistance mechanisms to date. Several single nucleotide polymorphisms resulting in amino acid changes within the voltage-gated sodium channel (VGSC) have been shown to reduce binding site sensitivity and confer resistance to pyrethroids. Specifically, mutations at the knockdown-resistant (kdr) 410, 1,016, and 1,534 sites have been associated with a reduction in pyrethroid sensitivity. I investigated the resistance status to permethrin and temephos at five locations in Hidalgo County, Texas. I determined the presence of permethrin resistance using a well-characterized susceptible colony as a reference for insecticide sensitivity. The resistant allele C1,534 reached fixation at all sites and L410 and I1,016 were found at high frequencies. The permethrin resistance was over 40-fold when compared to the reference colony. The sites were less resistant to temephos at approximately 6-fold to 12-fold, which I attributed to cessation of this insecticide in the continental United States since 2016. In the absence of selection pressures mosquito populations trend towards susceptibility, which suggests that there are potential fitness costs associated with insecticide resistance. Studying these associations is important to public health as they may support different strategies to reduce vector populations. I used two collections from Tapachula, Mexico, that were free of pyrethroid exposure since 2013, to determine the presence of two previously described fitness cost metrics: wing length and egg production. I found that the average wing length of V410L and V1,016I homozygous resistant individuals were significantly smaller compared to homozygous susceptible individuals. The interaction between wing length and genotype had no effect on egg production. Wing length had no significant effect on egg production. Most notably, L410 and I1,016 resistant alleles had no effect on egg production.
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    Plant secondary metabolites enhance survival and pathogen tolerance in the European honey bee: a structure-function study
    (Colorado State University. Libraries, 2019) Hogeboom, Alison, author; Norton, Andrew, advisor; Bjostad, Louis, committee member; Uchanski, Mark, committee member
    Adequate nutrition is essential for European honey bee (Apis mellifera) colony growth, and productivity, yet foraging limitations resulting from factors such as habitat loss often lead to dietary deficiencies. Plant secondary metabolites are key constituents of floral nectar that support physiological processes in honey bees, however, these compounds are only available to bees with access to a diversity of floral resources. Furthermore, the relationship between different classes of plant secondary metabolites and their function within honey bee diets requires further investigation. Using a structure-function framework, we evaluated whether four structurally similar plant secondary metabolites found in the nectar of common agricultural crops elicit comparable effects on honey bee survival and pathogen tolerance. The addition of plant secondary metabolites to artificial nectar solution enhanced median survival, in some cases more than doubling the lifespan of worker honey bees. Moreover, plant secondary metabolites demonstrated nutraceutical effects, and sometimes elicited medicinal effects on honey bees infected with Nosema ceranae. Our findings provide a platform to identify plant secondary metabolites which can augment current management techniques to support the long-term sustainability of the apiculture industry.
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    The ecology and evolution of plant-insect interactions among hybrid populations of the invasive plant, tamarisk (Tamarix sp.), in the western United States
    (Colorado State University. Libraries, 2012) Williams, Wyatt I., author; Norton, Andrew, advisor; Friedman, Jonathan, committee member; Hufbauer, Ruth, committee member; Nissen, Scott, committee member; Bauerle, William, committee member
    Tamarisk is one of the most abundant invasive tree species in the western United States. Several species belonging to the genus Tamarix were imported intentionally to the U.S. in the mid-nineteenth century. Currently, most U.S. populations are comprised of a hybrid swarm between T. ramosissima and T. chinensis and other species. Negative consequences of hybrid tamarisk invasion include alteration of ecosystem functioning and decreases in native biodiversity. Very few natural enemies attack this invasive plant, contributing to its success on the landscape. In an attempt to provide top-down population control, a specialized herbivore that coevolved with tamarisk in its native range was intentionally released in the introduced range (i.e. biological control). I investigated interactions between tamarisk hybrids and herbivores in order to better understand the dynamics that contribute to the control of this exotic weed. In Chapter 1, which was published in Volume 57 of The Southwestern Naturalist, I describe how a native stem-boring beetle was found attacking tamarisk populations in eastern Colorado, western Kansas, and southwest Nebraska. This is an important discovery because very few native insects have been reported to consume this plant and never at the levels of the stem-borer. The beetle may reduce tamarisk growth and fecundity on the Great Plains, providing evidence for the biotic resistance hypothesis. In Chapter 2, I investigate the interaction between drought and herbivory by the biological control agent, Diorhabda carinulata. Under which environmental conditions or geographical locations can biological control be maximized? Finally, in Chapter 3, I speculate whether hybrid tamarisk individuals or populations differ in plant performance and herbivore defense traits. Since the biological control agent coevolved with one parent species, T. ramosissima, I hypothesized that some hybrids may be more or less susceptible to attack by this herbivore. Success of biological control may hinge upon the level of species introgression, and if hybridization occurs predictably across the landscape, managers can exploit this information for tamarisk control. My research not only attempts to improve control strategies, but also addresses fundamental questions in plant-insect ecology and evolution.