Browsing by Author "Smith, Melinda D., advisor"
Now showing 1 - 8 of 8
Results Per Page
Sort Options
Item Open Access Climate change impacts on population dynamics in tallgrass prairie: implications for species codominance(Colorado State University. Libraries, 2017) Gray, Jesse E., author; Smith, Melinda D., advisor; Knapp, Alan K., committee member; Ocheltree, Troy W., committee memberTwo grass species, Andropogon gerardii and Sorghastrum nutans, together account for the great majority of individuals, biomass, and possibly genetic diversity in plant communities of the tallgrass prairies of the Great Plains, US. As competitors with similar functional traits and what appears to be overlapping niches, it is not clear what mechanisms facilitate their co-dominance, but it may rely on the high variability of environmental conditions that characterize grassland ecosystems. Because these abundant grasses strongly influence plant community structure and ecosystem function, it is critical that we understand the factors influencing the population dynamics of these species, and how climate change might alter those relationships. We found an asynchrony in population dynamics in which A. gerardii begins each growing season at higher tiller densities, with attrition of tillers starting mid-season. Concurrent gains of S. nutans tillers results in A. gerardii becoming the less abundant by the end of most growing seasons. We hypothesized that this differentiation in tillering strategies causes each species to be vulnerable to unfavorable environmental conditions during different parts of the growing season, thus enabling their coexistence by preventing an inter-annually consistent competitive advantage of either species. We found that greater tiller density asynchrony was associated with higher population densities of S. nutans and of aggregate tiller densities of both species. Experimental increases in temperature and rainfall variability reduced population-level asynchrony while exacerbating population declines and overall community productivity, suggesting this mechanism of co-dominance may rely on current levels of environmental variability, and may be vulnerable to projected increases in that variability with climate change.Item Open Access Dimensions of diversity in dominant prairie grasses(Colorado State University. Libraries, 2019) Hoffman, Ava Marie, author; Smith, Melinda D., advisor; Argueso, Cristiana T., committee member; McKay, John K., committee member; Ocheltree, Troy W., committee memberAs anthropogenic influences cause climate change to worsen, extreme events such as droughts and heat waves are expected to become more frequent. The native prairies of the Central United States have historically experienced drought, yet continue to support highly productive grassland communities. Dominant species in these grasslands, such as Andropogon gerardii in the tallgrass prairie and Bouteloua gracilis in the shortgrass steppe, are drivers of productivity in these ecosystems. Thus, it is necessary to quantify diversity within these key species in order to determine how these important grasses have been historically shaped as well as how they will respond to future climate change. This dissertation seeks to answer (1) How do functionally similar coexisting dominant grasses differ at the molecular level?, (2) How does plasticity contribute to intraspecific diversity?, and (3) how does intraspecific diversity vary across the range of one of these less studied grasses? To determine transcriptional differences between codominant species A. gerardii and Sorghastrum nutans, I performed RNA-seq on watered and droughted tissues, building both species' transcriptomes using Trinity. These codominant grasses responded differently; specifically, A. gerardii had greater regulation of stress alleviation transcripts while S. nutans tended to be more sensitive within 10 key gene-groups related to stress and abscisic acid. These results support previous work on the physiological level, and demonstrate functional diversity at the gene level within dominant species in the tallgrass prairie. To explore this community further, I documented variation in plastic traits across a gradient of water availability in three A. gerardii genotypes. I found that plasticity, in particular nonlinear plasticity, in morphological and physiological traits was widespread and differed across genotypes, highlighting the influence of relatively small changes in water availability on intraspecific diversity. These genotypes also differed in reproductive strategy (flowering versus clonal tillering), but all recovered from drought similarly. These results demonstrated that variation in plasticity patterns may help explain intraspecific diversity and patterns of selection within a population. Differences in drought response strategy, particularly in terms of transcription and plasticity diversity, could provide further niche space by which the tallgrass prairie community can mitigate the effects of future drought. Lastly, I applied an understanding of dominant species diversity in A. gerardii to the relatively understudied dominant dry steppe species B. gracilis. I performed 2b-RAD genome se- quencing and a common garden trait and plasticity analysis across both regionally and locally distributed sites to broadly assess intraspecific diversity in this ecologically and economically important species. I found substantial intraspecific diversity among sites, specifically showing that New Mexico sites were distinct in terms of biomass trait distributions and plasticity. While New Mexico sites were clearly different, all sites were at least somewhat distinct genetically, indicating some limitations to gene flow. As has been shown in A. gerardii, comprehensive analysis of intraspecific diversity in this dominant grass will help clarify mechanisms of ecosystem function as well as conservation and management of the shortgrass steppe ecosystem. Overall, these three projects highlight dimensions of diversity in dominant prairie grasses, providing useful information for predicting how these species and their associated communities are likely to respond to changing climate.Item Open Access Drought impacts on the microbiome in grasslands across the Great Plains: a story of legacy effects, resistance, and resilience(Colorado State University. Libraries, 2022) Vilonen, Leena L., author; Smith, Melinda D., advisor; Trivedi, Pankaj, advisor; Cusack, Daniela, committee member; von Fischer, Joe, committee member; Zeglin, Lydia, committee memberDrought is increasing in frequency and severity across the US Great Plains as a direct result of climate change and if nothing is done to remedy climate change, drought will only continue to get worse over the next century. Thus, understanding how drought impacts natural and rangeland systems in the US will be vital to protecting these systems from negative impacts due to drought. Further, there has been a great deal of research on the aboveground response to drought, but little research on how the belowground soil community responds to drought. Lastly, some research exists on how drought impacts systems during the drought, but even less research exists on what happens after the drought. To further complicate this, the terms used to describe the period after drought are variable and inconsistent, leading to difficulty in synthesizing this literature. This dissertation aimed to re-define and make the terms used to describe the post-drought period consistent, understand how belowground communities respond after the drought has ended at one field site, and understand how microbial communities in the greenhouse respond to drought both during and after across several sites in the US Great Plains. The first chapter of this dissertation was a literature review that examined how researchers define the terms used after a drought ends and attempted to synthesize definitions for future use. The second chapter of this dissertation examined whether there were impacts leftover after a four-year drought on nutrient cycling in a mesic grassland. The third chapter examined whether there were leftover impacts from the same drought as chapter two on the microbial community. Lastly, the fourth chapter examined how microbial communities respond during and after the drought across four Great Plains sites when the microbial community was isolated from the plant community.Item 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 memberPredictions 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.Item Open Access Effects of feral horse herds on plant communities across a precipitation gradient(Colorado State University. Libraries, 2016) Baur, Lauren, author; Smith, Melinda D., advisor; Schoenecker, Kathryn A., committee member; Meiman, Paul, committee memberFeral horse herds in the western United States are managed with the goal of maintaining "a thriving natural ecological balance" with their environment. Because rangeland ecology is complex and grazers such as horses can have different effects under different environmental conditions, more data are needed to better inform Appropriate Management Levels and other management decisions. We used long-term grazing exclosures and fenceline contrasts to evaluate the impacts of feral horses on plant communities at five sites across the western United States. These sites ranged from 229 to 413 mm mean annual precipitation and represented four different ecosystems (Great Basin desert, Colorado Plateau, Rocky Mountain grassland and mixed grass prairie). We found that feral horses significantly reduced grass biomass and total biomass at alpha=0.1, but did not have a significant effect on plant community composition, species richness, diversity, evenness, or dominance. The effects of horses did not vary by site, indicating that different precipitation levels are not driving differences in grazing effects within the range encompassed by our sites. In other words, our results imply that while feral horses do reduce plant biomass, they are not causing plant community shifts, and their effects may not be as site-specific as has been assumed. Additional multi-site studies, preferably with standardized exclosures and larger sample sizes, would increase our understanding of feral horse grazing effects.Item Open Access Response and recovery of grassland plant communities exposed to multiyear drought differs across a precipitation gradient(Colorado State University. Libraries, 2022) Ross, Maggie, author; Smith, Melinda D., advisor; Knapp, Alan, committee member; Havrilla, Caroline, committee member; Wilkins, Kate, committee memberDrought events are expected to increase in grassland ecosystems in many regions of globe due to climate change. Much is known about the effects of drought on grassland plant communities, yet it is difficult to compare responses across different grassland ecosystems because studies impose drought with varying characteristics. Further, few studies have documented plant community recovery, even though the impacts of drought can persist for multiple years. We experimentally imposed four years of extreme, growing season drought at four sites representing the major Central US grassland types (shortgrass steppe, mixed grass prairie, tall grass prairie) spanning a precipitation gradient. Growing season drought was imposed in two ways: 1) by reducing each rainfall event by 66% (chronic) or 2) by completely excluding rainfall until a similar reduction in precipitation as the chronic treatment was achieved (intense). Plant community responses to the two drought treatments were monitored for each year of the four-year drought treatments and four years following the drought to assess recovery. Overall, plant communities at the drier sites responded sooner to drought and took longer to recovery than the wetter sites. Plant composition was altered at all sites, which was largely driven by shifts in the dominant C3-C4 grasses and subsequent species reordering and to a lesser extent by changes in richness in evenness. There was a significant decrease in C4 graminoid abundance in response to drought at all sites with a corresponding increase of C3 annual grasses during the drought at the mixed grass sites but not until the recovery period at the shortgrass steppe. Cheatgrass (Bromus tectorum) invaded the shortgrass steppe during the drought and proliferated during the recovery period, which likely pushed the communities into an alternate state, and inhibited recovery after four years of ambient conditions. The northern mixed grass prairie also did not fully recovery after four years, which indicates that full plant community recovery can extend longer than the drought itself at these drier sites. While there is some indication that intense drought had a greater impact on communities than chronic drought, there is limited evidence to suggest that drought type significantly influenced plant community responses or recovery. These findings indicate that while the shortgrass steppe is water limited with drought adapted species, these xeric grassland plant communities are less resistant and resilient to multiyear drought than those in mesic grasslands.Item 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 memberA 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.Item Open Access The effect of timing of growing season drought on flowering of Andropogon gerardii(Colorado State University. Libraries, 2015) Dietrich, John David, author; Smith, Melinda D., advisor; Knapp, Alan K., committee member; Ocheltree, Troy W., committee memberTiming of precipitation is equally important as amount for determining ecosystem function, especially aboveground net primary productivity (ANPP), in a variety of ecosystems. The particular precipitation period(s) of greatest importance varies between ecosystems. In tallgrass prairie of the central US, the relative importance of different precipitation periods is dictated by the phenology of the dominant C4 grasses, in particular Andropogon gerardii which can contribute >80% to ANPP in this ecosystem. It is predicted that precipitation periods with the greatest impact on the highly variable flowering rates of A. gerardii are likely to be particularly important for determining ANPP, as flowering individuals are much larger (>2-fold) than non-flowering individuals. The potential for flowering may be affected by precipitation at different times via different mechanisms (e.g. carbon gain via rapid growth early in the growing season vs. direct effects on stalk elongation later in the growing season). In order to test the differential effects of precipitation timing, rainfall deficits (100% exclusion) at different periods of the growing season were imposed on native tallgrass prairie in Kansas, USA. Contrary to expectations, the most sensitive period in terms of flowering for A. gerardii did not coincide with the highest potential photosynthetic rates early in the growing season. Rather the most sensitive period was mid to late summer immediately preceding, and concurrent with, the initiation of flowering stalks. Growth rate, leaf water potential and carbon assimilation of A. gerardii were all most sensitive to drought late in the growing season, suggesting that growth regulation in response to plant water status, not current year’s carbon accumulation is the critical factor determining flowering responses to precipitation or lack thereof. Flowering, in addition to influencing ANPP, controls rates of sexual reproduction which in turn limit adaptation and migration, and thus understanding how flowering will be influenced by a changing climate is critical for predicting plant community and ecosystem responses in tallgrass prairie. My study suggests that increased frequency of growing season droughts forecast with climate change could result in reduced ANPP and reproductive success of the dominant grasses in the tallgrass prairie ecosystem.