Browsing by Author "Smith, Melinda D., committee member"
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Item Open Access Characterizing host plant-virus-vector interactions of the potato virus Y and aphid pathosystem(Colorado State University. Libraries, 2023) Pitt, William Jacob, author; Nachappa, Punya, advisor; Charkowski, Amy O., committee member; MacRae, Ian V., committee member; Peairs, Frank B., committee member; Smith, Melinda D., committee memberAphid-transmitted potato virus Y (PVY) is one of the most damaging pathogens of potato worldwide. Plant virus prevalence is influenced by landscape composition, host-use patterns of vectors, and the range of capable vector species. Regarding these influences, there are important knowledge gaps that remain within the PVY-aphid pathosystem. The overall goal of this research was to better understand host-virus-vector interactions within the PVY-aphid pathosystem at multiple levels of ecological organization. There is limited information on the effect of landscape-scale crop diversity on prevalence of insect-vectored viruses. In my dissertation, I investigate how landscape composition of crops (Shannon diversity of crops, percent crop cover) affects aphid vector communities and prevalence of aphid-transmitted PVY. I conducted a two-year field study in the San Luis Valley in Colorado where I sampled aphid communities with pan traps, quantified PVY incidence in potato crops with ELISA, and determined the association with landscape variables (Shannon diversity index of crops and percent crop cover) surrounding sampling sites. Crop diversity negatively influenced aphid species richness, but positively influenced PVY incidence. The negative association of crop diversity with aphid species richness could have been due to differences in management between crops and/or increased predation/parasitization of aphids. The positive association between crop diversity and PVY is likely because PVY has a wide host range and increasing crop diversity may positively influence PVY incidence due to increased inoculum in the landscape. Additionally, there was a positive association of potato (virus host) and a negative association of barley (virus non-host) with PVY incidence. In summary, I found that crop species diversity influenced both PVY prevalence and aphid communities, and that the virus host/non-host status of crops likely modulates this effect. Aphid vectors frequently probe upon various plants within a landscape, but the host use patterns of aphid vectors has not been adequately described. It would be useful to identify plants that aphid vectors are probing upon within a landscape in order to identify 1) vector movement within a landscape and 2) potential sources of aphid-transmitted virus inoculum. I used high throughput molecular gut content analysis (GCA) to characterize plant-aphid vector associations within a major potato producing region, the San Luis Valley, CO, where aphid-transmitted potato virus Y (PVY) is a major issue for potato production. Aphids were trapped weekly with suction traps during the growing seasons of 2020 and 2021. Plant-specific DNA in 200 individual aphids representing 9 vector species of PVY (Acyrthosiphon kondoi, A. pisum, Capitophorus elaeagni, Diuraphis noxia, Hayhurstia atriplicis, Myzus persicae, Phorodon cannabis, Protaphis middletonii, and Rhopalosiphum padi) were amplified by PCR, sequenced with the PacBio sequencing platform, and sequences were identified to genus using NCBI BLASTn. I found that all species of aphid vectors probed on plant genera that are outside of their reported host ranges, with Solanum, Medicago, Populus, Brassica, and Glycine as the most frequently detected plant genera. Moreover, aphids probed on many plant genera likely to be PVY host plants. These findings provide information that is essential to develop sustainable and effective management strategies to reduce PVY spread. With the increasing acreage of hemp (Cannabis sativa L.) (Rosales: Cannabaceae) in the United States, I was interested to know whether the cannabis aphid (Phorodon cannabis Passerini) (Hemiptera: Aphididae) is a potential vector of PVY. I conducted transmission assays and used the electrical penetration graph (EPG) technique to determine whether cannabis aphids can transmit PVY to hemp (host) and potato (non-host) (Solanum tuberosum L.) (Solanales: Solanaceace). I showed, for the first time, that the cannabis aphid is an efficient vector of PVY to both hemp (96% transmission rate) and potato (91%) using cohorts of cannabis aphids. In contrast, individual aphids transmitted the virus more efficiently to hemp (63%) compared to potato (19%). During the initial 15 min of EPG recordings, aphids performed fewer intracellular punctures and spent less time performing intracellular punctures on potato compared to hemp, which may in part explain low virus transmission to potato using individual aphids. During the entire 8-hour recording, viruliferous aphids spent less time ingesting phloem compared to non-viruliferous aphids on hemp. This reduced host acceptance could potentially cause viruliferous aphids to disperse thereby increasing virus transmission. Overall, my study shows that cannabis aphid is an efficient vector of PVY and that virus infection and host plant suitability affect feeding behaviors of the cannabis aphid in ways which may increase virus transmission.Item Open Access Ecological responses to climate extremes in a mesic grassland(Colorado State University. Libraries, 2014) Hoover, David Lewis, author; Knapp, Alan K., advisor; Smith, Melinda D., committee member; Bauerle, William L., committee member; von Fischer, Joeseph C., committee memberClimate change threatens ecosystems through altered climate means and by increasing the frequency and intensity of extreme climatic events. Such events may have greater impacts on ecosystems than shifting means alone because they can push organisms beyond critical thresholds. Thus, there is an urgent need to assess the response of ecosystems to climate extremes as well as elucidate the mechanisms underlying the observed responses. My dissertation examined the ecological impacts of two years of experimentally imposed climate extremes (heat waves and drought) followed by a recovery year, on a mesic tallgrass prairie grassland ecosystem. The broad objectives of this research were (1) to assess the resistance and resilience of this ecosystem to the individual and combined effects of heat waves and drought, and (2) to identify the ecological mechanisms driving the responses and (3) to evaluate the sensitivities of key carbon cycling process to heat waves and drought. I measured a range of biotic responses to these treatments including: ecophysiology, community dynamics, primary production, and soil respiration in order to gain a comprehensive understanding how this ecosystem responds to such extremes. During the first year of the experiment, I examined the ecophysiological and productivity responses of the dominant C4 grasses to a growing season-long drought and a midsummer, two-week heat wave. Although differential sensitivities were apparent, the independent effects of drought dominated the ecological responses for both species, with only minor direct effects of heat were observed. However, the heat wave treatments had indirect effects via enhanced soil drying, making it difficult to separate the effects of the heat wave and precipitation treatments on biotic responses. Therefore in the second year of the experiment, I controlled for heat-induced water losses during the heat wave and examined the independent effects of heat on net photosynthesis in both grass species under contrasting soil moisture regimes. Under low soil moisture, heat had no effect on net photosynthesis, while increasing temperatures moderately reduced photosynthesis under high soil moisture. Next I examined the resistance and resilience in ecosystem function (aboveground primary production) of this tallgrass prairie to the two years of extreme treatments and for one subsequent recovery year. I observed high resistance to heat but not drought, as aboveground production dropped below historic levels during the second year of the drought. Despite this extreme ecological response, productivity fully recovered in just one year post-drought due to rapid demographic compensation by the dominant grass offsetting the loss of the dominant forb. Finally, I examined the response of soil respiration to heat and drought across the three years of the experiment. As with aboveground net primary production, soil respiration was more sensitive to drought than heat, but it was less sensitive overall to drought than production. There are three main conclusions from my dissertation research. First, this tallgrass prairie ecosystem has low resistance but high resilience to extreme short-term drought, which may be an important characteristic for long-term stability in ecosystems with histories of drought. Secondly, the two most abundant species governed both community and ecosystem-level dynamics across this three-year experiment, providing evidence for the central role of dominant species during these short-term events. Finally, my results suggest that three key carbon cycling processes in this mesic grassland - photosynthesis, plant productivity and soil respiration - are all significantly more sensitive to the independent effects of an extreme drought than heat waves and there were little to no combined effects of heat waves and drought. Overall, these results suggest that in a future with more frequent and extreme heat waves and drought, this mesic grassland will be most vulnerable to water stress, either directly through precipitation deficits or indirectly through warming-induced drying, while the direct ecological effects of midsummer heat waves will be minor.Item Open Access Grassland sensitivity to extreme drought: assessing the role of community functional composition(Colorado State University. Libraries, 2019) Griffin-Nolan, Robert James, author; Knapp, Alan K., advisor; Ocheltree, Troy W., committee member; Smith, Melinda D., committee member; Tissue, David T., committee memberClimate change is expected to cause droughts that are reminiscent of the dust bowl. While all ecosystems are negatively affected by drought to some degree, grasslands are among those most sensitive. Accurate forecasting of which grasslands are most sensitive to drought is imperative to conserving the many economically and aesthetically valuable services these ecosystems provide. This dissertation utilizes both observational and experimental data, coupled with a systematic literature review, to assess the mechanisms of differential grassland sensitivity to drought. Long-term records of precipitation and aboveground net primary production (ANPP), a key metric of ecosystem function, suggest that xeric grasslands are more sensitive to drought than mesic grasslands. I provide further support for this trend using recent observations of the response and recovery of ANPP following a short-term natural drought in six grassland sites. Predicting the ecological consequences of long-term extreme drought, however, requires a mechanistic understanding of drought sensitivity beyond its climatic determinants, especially considering two sites with similar climatic means can differ dramatically in their sensitivity to climate extremes. Plant traits, which act as proxies for more complex physiological functions, can be scaled through the community (i.e. weighted by species relative abundance) to explain and forecast ecosystem responses to environmental change. Few studies, however, measure community-weighted traits in the context of altered water availability. Following a systematic review of >500 manuscripts, I identify clear knowledge gaps in the field of plant traits research and provide guidelines for using plant traits to understand ecosystem sensitivity to PPT. Specifically, plant trait surveys could be improved by a selection of traits that reflect physiological functions directly related to plant water use with traits weighted by species relative abundance. Informed by these guidelines, I test and validate a high throughput method for assessing leaf turgor loss point, a key metric of drought tolerance, using an osmometer. The osmometer method paves the way for rapid community-scale surveys of drought tolerance across functional types. Finally, I employ a coordinated, long-term rainfall exclusion experiment to assess the drought sensitivity of ANPP and community functional composition (i.e. community-weighted trait means and trait diversity) across six grassland sites. Four years of experimental drought (i.e. 66% removal of growing season rainfall) led to reduced ANPP across all six grasslands, with the sensitivity of ANPP being highly correlated with community functional composition. Specifically, functionally diverse plant communities, as well as those with a high abundance of species with conservative resource use strategies, experienced smaller relative reductions in ANPP following drought. Additionally, drought treatments led to increased functional diversity and decreased community scale drought tolerance, largely due to species re-ordering following dominant species mortality. Increased functional diversity may stabilize ecosystem functioning in response to future drought. However, the shifts in community-scale drought strategies may increase ecosystem drought sensitivity, depending on the nature and timing of recurrent drought. The role these two mechanisms will play in determining ecosystem recovery from and response to future drought will be fascinating to assess. Overall, my research demonstrates the importance of plant traits in understanding differential ecosystem sensitivity to extreme drought, especially when the appropriate traits are measured and weighted by species relative abundance.Item Open Access Juvenile tree dynamics in changing landscapes: effects of overstory-mediated microclimates on dryland tree recruitment vary across climatic gradients(Colorado State University. Libraries, 2024) Hill, Edward M., author; Redmond, Miranda D., advisor; Ocheltree, Troy W., advisor; Bradford, John B., committee member; Smith, Melinda D., committee memberClimate change impacts the future viability of plant species and communities directly through effects on demographic processes and indirectly through structural dynamics. Regeneration, establishment, growth, and survival of juvenile trees can be especially vulnerable processes in forest and woodland community development because juvenile trees are typically not able to tolerate abiotic stress as effectively as more mature trees. Because of this elevated sensitivity to climate-related stressors, juvenile establishment patterns are fundamental to understanding long-term species persistence. Overstory tree structure is an important mediating influence of the impacts of climate change in forest and woodland communities, particularly through influences on resource availability. Fine-scale variation in overstory tree size, density, and species influence primary plant resource requirements, including light availability, atmospheric heat and moisture, precipitation throughfall and soil water availability, and soil nutrient availability. Juvenile trees of different species can benefit from buffering of microclimate conditions by overstory trees, like direct radiation and extreme temperature variation, and experience competitive interactions for light and soil resources, especially in resource-limited communities. Yet, juvenile trees can span a range of sizes and physical maturity and vary in their capacity to acquire resources or tolerate resource limitations, and therefore can differ in facilitative versus competitive relationships with overstory conditions. Amplifying the complexity of these relationships, interannual variation in weather conditions, such as drier or wetter years than normal, influences the degree to which juvenile trees experience facilitative or competitive relationships with overstory trees. Indeed, the extent of microclimate effects on regeneration processes depend in part on the complex covariance of air temperature and humidity (thus, vapor pressure deficit), moisture availability (precipitation and soil moisture), and photosynthetically active radiation (i.e., light). In the absence of temperature and moisture limitations, trees may benefit from additional light availability for photosynthesis; alternatively, if temperature or moisture conditions are limiting, juveniles may benefit more from buffering influences of overstory, at the expense of decreased light availability. For dry forests and woodlands of the western U.S. which are at the forefront of climate change-driven tree recruitment vulnerabilities, greater resolution into juvenile relationships with overstory structure, and microclimate buffering, will substantially enhance the ability to evaluate and predict the effects of increasingly marginal climate space on their persistence. In this dissertation, I evaluated juvenile tree regeneration, growth, and survival in dryland forest and woodland systems relative to the mediating influences of overstory trees, across ranges of juvenile sizes, interannual weather variation, and broad climatic and elevational environmental gradients. In Chapter 1, I investigated survival and growth of ponderosa pine and Douglas-fir newly germinated seedlings, and older, larger seedlings to variation in overstory structure and associated microclimate conditions at fine-spatial scales. This study showed that while newly germinated seedlings were more sensitive to interannual variation in microclimates, overall survival and growth of younger and older seedlings were highest in microclimates with above-average warm and dry air during early-growing season months, and above-average light conditions. Importantly, the structural and microclimate influences on survival and growth over three years of study were primarily associated with the first year of study during which spring weather was abnormally cool and more humid. These results illustrated the environmental context for the initiation of survival and growth trajectories observed in this study, and demonstrate both spatially and temporally narrow conditions in which survival and growth was collectively greatest for both species. In Chapters 2 and 3, I investigated physiological and growth differences of juvenile piñon pine in dead and live overstory microenvironments over two years following experimentally-induced mortality of overstory trees. In Chapter 2, I measured photosynthetic and stomatal conductance rates of juveniles from among the smallest to largest individuals present in a middle-elevation piñon-juniper site in the core of the geographic distribution of two-needle piñon pine. Larger juveniles in dead overstory environments showed the highest photosynthetic and stomatal conductance rates. However, juveniles of all sizes were overall similarly limited by lower soil moisture and hot and dry microclimate conditions in both live and dead overstory environments. Given these limitations, the results of this study demonstrate the susceptibility of all juvenile piñon trees to hot and dry microclimates, which can be exacerbated both by mortality of overstory trees and by projections of future hotter and drier climate in these woodlands. In Chapter 3, I measured branch growth of juvenile piñon trees at six different sites spanning a gradient of latitudinal climate differences from hotter and drier southern locations to cooler and dry northern locations, and local elevation gradients from low to mid to high elevation piñon-juniper woodlands. Growth in post-overstory mortality years relative to mean growth prior to overstory mortality ("growth ratios") of juveniles across sizes was consistently higher in dead compared to live overstory environments only for middle- and high-elevation sites in our mid-latitude study region of southwestern Colorado, which had cooler and wetter post-overstory mortality weather conditions compared to other regions. Moreover, differences among sites were likely also related to typical climate differences associated both with latitude, where drier sites at southern and northern latitudes showed little growth responses to overstory mortality, and associated with elevation, where growth ratios were highest at the highest elevation site which has more moderate temperature and precipitation conditions on average. The results of this dissertation provide evidence for microclimate and juvenile tree survival outcomes in a dry forest restoration treatment and show the impact of acute structural change following overstory tree die-off on physiological and growth activity of juvenile piñon pine. The findings presented here provide ecologists and land managers with new information on the nuances of spatially and temporally narrow regeneration niches of species in dry mixed-conifer forests, and potential patterns and mechanisms of juvenile piñon pine resilience – but also potential future sensitivity – following overstory mortality. Importantly, results of these studies emphasize the role of interannual variation in weather conditions in driving specific forest and woodland development trajectories.Item Open Access The timing of growing season drought and its effects on above- and belowground production in a mesic grassland(Colorado State University. Libraries, 2014) Denton, Elsie Mariah, author; Knapp, Alan K., advisor; Hoeting, Jennifer A., committee member; Smith, Melinda D., committee memberAs a consequence of climate change, both the timing and amount of precipitation ecosystems receive are expected to be altered. In general, regions that are relatively dry are expected to get drier and the timing of seasonal drought - defined as a prolonged absence or marked deficiency of precipitation - is expected to change. Although drought in general has been extensively studied, particularly in grasslands, we know little about how natural ecosystems will respond to shifts in the timing of growing season drought. In this study I investigated the response of both above- and belowground net primary production (ANPP & BNPP) to reductions in precipitation in a mesic, tallgrass prairie in NE Kansas. Experimental plots were subjected to one of three drought treatments (25% reductions in the average growing season precipitation [GSP]) imposed either in late spring, early summer or mid-summer. A control treatment that received the mean GSP and a wet treatment that received 130% of the mean GSP were included to assess drought responses. In all treatments, I measured soil moisture, soil N and P content, canopy light interception and plant community composition in addition to ANPP and BNPP. I expected that ANPP would be more sensitive to drought than BNPP based on evidence from past studies that have almost always found a positive correlation between precipitation and ANPP, while trends with BNPP are less clear. I also hypothesized that early summer drought would cause the highest reduction in net primary production (ANPP + BNPP), because soil moisture would likely still be high in the late spring from late winter and early spring snow/rain, lessening the effect of reduced precipitation inputs. Moreover, because annual ANPP approaches its maximum by summer, I expected the mid-summer drought to affect NPP the least. I found that without considering timing, a 25% growing season drought reduced ANPP relative to the control by 18-26%, while ANPP in the control and wet treatment was not significantly different. Early summer and mid-summer drought resulted in significant reductions in ANPP (~25%) relative to control plots, but late spring drought did not reduce ANPP significantly despite similar reductions in soil moisture across all treatments. In contrast, neither drought nor wet treatments altered BNPP significantly. Because soil nutrients may increase during drought and plant functional type diversity may buffer productivity responses to drought, I investigated the role these played in determining responses to the treatments imposed. I found that soil nutrients were positively related to ANPP only in the wet treatment; conversely, diversity was negatively related to ANPP in the ambient and drought treatments, but not the wet treatment. I conclude that timing does play an important role in determining ecosystem response to drought with periods of no rain that occur earlier in the year having less of an impact than those that occur later. Furthermore, differences in responses between ANPP and BNPP were striking and need to be accounted for when projecting productivity responses of grasslands to climate change.Item Open Access Unraveling key drivers of microbial community assembly and impacts on microbial function(Colorado State University. Libraries, 2015) Rocca, Jennifer Doyle, author; Wallenstein, Matthew D., advisor; Cotrufo, M. Francesca, committee member; Knapp, Alan K., committee member; Smith, Melinda D., committee memberTo view the abstract, please see the full text of the document.