Mountain Scholar :: Browsing by Author "Knapp, Alan K., advisor"

Browsing by Author "Knapp, Alan K., advisor"

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Open Access
Assessing drought sensitivity across the shortgrass steppe biome
(Colorado State University. Libraries, 2024) Hedberg, Sydney Leigh, author; Knapp, Alan K., advisor; Dao, Phuong D., advisor; Mueller, Nathan, committee member
Net primary productivity (NPP) of grassland ecosystems is dependent on many biotic and abiotic factors. However, water availability is generally considered the primary determinant of NPP, as well as being key for defining grassland community structure, and thus it is imperative to understand how grasslands respond to drought in a climate where droughts are expected to become more frequent and severe. There is a well-documented negative relationship, described by the Huxman-Smith model, between drought sensitivity and mean annual precipitation (MAP) at spatial scales that span multiple biomes. In other words, drier ecosystems are usually more sensitive to drought than more mesic ecosystems. While this cross-biome pattern has been independently confirmed with a variety of research approaches, there is limited research that has explored how patterns of drought sensitivity vary with MAP within a single biome where the dominant species do not vary. My goal was to determine if this negative relationship is evident within a regionally extensive grassland biome generally dominated by a single grass species (Bouteloua gracilis or blue gramma). I characterized the spatial pattern and relationship between drought sensitivity and MAP across the shortgrass steppe biome of the North American Great Plains using satellite-derived Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) data (from 2000-2022) as proxies for vegetation productivity. Gridded annual precipitation data were obtained at a comparable spatial scale. I found a negative relationship between drought sensitivity and MAP within the shortgrass steppe biome, indicating that the Huxman-Smith model is also supported within a single biome. Thus, my results suggest that while changes in the dominant vegetation may contribute to the patterns observed between MAP and drought sensitivity at large spatial scales that include multiple biomes, gradients in MAP within a biome can also drive this negative relationship. As a result, directional changes in annual precipitation amounts have the potential to alter drought sensitivity directly, even if the dominant plant species do not change.
Open Access
Climate change consequences of geographic variation in growth and penology of two dominant central US grasses
(Colorado State University. Libraries, 2012) Giuliani, Amanda Lease, author; Knapp, Alan K., advisor; Kelly, Eugene F., committee member; Wall, Diana H., committee member
Species can exist in a given range of climatic conditions, and these ranges have shifted in response to geologic climate change. Plant species with slower migration rates, however, may not be able to keep up with the current predicted rate of climate change. Thus, populations located peripherally to a biometry may play a more significant role in sourcing future generations than previously thought. As a result of many studies, we know a lot about populations that exist central to their range, or dominant populations (DPs), of two key C4 grass species, Bounteous gracilis and Andropogon gerardii, that account for much of the biomass of the shortgrass steppe and tallgrass prairie, respectively. However, we know little about their corresponding peripheral populations (PPs). This study examines ecophysiological, morphological, and whole plant differences between DPs and PPs of B. gracilis and A. gerardii under well-watered and water-stressed conditions in a reciprocal common garden experiment. Traits that were measured included predawn and midday leaf water potential, total biomass, reproductive biomass percentage, and individual reproductive and vegetative tiller mass (A. gerardii only), specific leaf area, fluorometer, height, and reproductive tiller density. We found that key traits differed between DPs and PPs in both species, but these key traits were unique to each species. For B. gracilis phenological traits of DPs and PPs were primarily different, whereas productivity traits were significantly different between populations of A. gerardii. If, in fact, PPs of these two C4 grasses are the propagules of future generations, the differences observed in this study may have implications as we look ahead to predicted climate patterns. In B. gracilis, an understanding of the differences in phenological traits may be important when we account for future adaptation, whereas knowledge about productivity differences in A. gerardii may help us better predict effects on ecosystem function. In both cases, more research is necessary to further develop our understanding of PPs and the potentially significant role they will play in the future.
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 member
Climate 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.
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
Grassland responses to seasonal shifts in water availability
(Colorado State University. Libraries, 2023) Hajek, Olivia Louise, author; Knapp, Alan K., advisor; von Fischer, Joseph, committee member; Cusack, Daniela, committee member; Schumacher, Russ, committee member
Climate change is altering seasonal dynamics across a wide range of ecosystems with consequences that include shifts in phenology, timing of nutrient availability, and changes in plant community composition. Current research has primarily focused on temperature as the key driver for these shifts because of the strong directional trend with climate warming, however, alterations in the availability of water across seasons is an unappreciated aspect of climate change that can significantly influence ecosystem functioning. While changes in the seasonal availability of water are expected to be globally pervasive, grasslands may be particularly vulnerable because these ecosystems are often water-limited and have species with distinct seasons of growth. Therefore, my dissertation examined how seasonal patterns of water availability may shift with climate change in the grasslands of the US Great Plains and the ecological consequences of these shifts. I first explored several mechanisms by which climate change is altering the seasonal water balance, using the Great Plains as a case study. Building on that, I then designed two field experiments in semi-arid grasslands that altered seasonal patterns of water availability to understand how these shifts affected ecosystem function and structure (primarily C3 vs C4 grasses). Overall, the results from both field experiments suggest that shifts in the seasonality of water availability with climate change will alter carbon cycling dynamics, shift seasonal patterns of canopy albedo, and differentially impact C3 vs. C4 species in the semi-arid grasslands of the US Great Plains. Thus, my research confirms the importance of this aspect of climate change and provides evidence that seasonal shifts in water availability can alter ecosystem processes and drive compositional changes. Since grasslands provide many economically and ecologically valuable services, understanding how climate change will impact these systems is critical for land managers and policymakers to make informed decisions.
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 member
Climate 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.
Open Access
Sensitivity of a semi-arid grassland to extreme precipitation events
(Colorado State University. Libraries, 2021) Post, Alison Kathryn, author; Knapp, Alan K., advisor; Hoover, David L., committee member; Klein, Julia A., committee member; Ocheltree, Troy W., committee member
Rising global temperatures due to climate change are intensifying the hydrological cycle, resulting in larger and more frequent extreme rain events, or deluges. Dryland ecosystems are predicted to be especially sensitive to this change in precipitation pattern because, in these water-limited systems, ecological processes are largely controlled by infrequent water pulses and deluges represent an extreme precipitation pulse. Therefore, my dissertation examined how the semi-arid shortgrass steppe of eastern Colorado responds to deluge timing and size. Using field experiments, I applied deluge events to grassland plots that varied in seasonal timing (early, middle, or late growing season) and magnitude (moderate – extreme event sizes), and quantified ecosystem response. I also conducted an observational study to determine if these plot-level results could be scaled to the larger shortgrass steppe landscape. I identified natural deluges in the historical precipitation record and related spatial variation in precipitation to post-deluge canopy greenness via satellite imagery. My field experiments showed that the shortgrass steppe is extremely responsive to large rain events, with most measured variables exhibiting a substantial increase following an applied deluge event. Measured variables included soil moisture, soil respiration, and above and belowground net primary production (ANPP & BNPP), as well as growth and flowering of the dominant grass species, Bouteloua gracilis (blue grama). However, response magnitude depended on both deluge timing and size. The shortgrass steppe was most responsive to a mid-growing season (July) deluge, and ecosystem processes generally increased linearly with increasing deluge size, with limited evidence for response saturation. My observational study exhibited similar patterns at the landscape-scale, suggesting that these experimental plot-level results can be scaled to the larger shortgrass steppe landscape, despite greater variation in soil texture, grazing regime, and plant community. Overall, my dissertation research suggests that semi-arid ecosystems could be well-adapted to the increase in rainstorm size and frequency predicted with climate change; however, the magnitude of the ecosystem response depends on intra-seasonal precipitation patterns, including deluge timing and size. These findings have important implications for predicting both local ecosystem services (e.g., forage production) and global carbon cycling under altered precipitation regimes.
Open Access
Sensitivity of grassland ecosystems across the Great Plains to present and future variability in precipitation
(Colorado State University. Libraries, 2008) Heisler, Jana Lynn, author; Knapp, Alan K., advisor
Patterns and controls of aboveground net primary productivity (ANPP) have been of long-standing interest to ecologists because ANPP integrates key aspects of ecosystem structure and function through time. In many terrestrial biomes, water availability is a primary constraint to ANPP, and it is an ecosystem driver that will be affected by future climate change. To understand the sensitivity of temperate grasslands to inter- and intra-annual variability in precipitation, I analyzed long-term ANPP data, conducted a multi-site experimental manipulation in which the number of growing season rainfall events was varied, and simulated the effects of altered rainfall regimes using a terrestrial ecosystem model (DAYCENT). I conducted this research within the Great Plains of North America-a region characterized by a strong west-east precipitation-productivity gradient and three distinct grassland types-the semi-arid shortgrass, the mixed-grass prairie, and the mesic tallgrass prairie. My results demonstrate that temperate grasslands are indeed sensitive to both inter- and intra-variability in precipitation, but the ANPP response is contingent upon ecosystem structure and typical soil water levels. Additionally, both management strategies and topographic location may interact with precipitation to enhance or diminish coherence in the ANPP response. At the dry end of the gradient (semi-arid steppe), fewer, but larger rain events led to increased periods of above-average soil water content, reduced plant water stress and increased ANPP. The opposite response was observed at the mesic end of the gradient (tallgrass prairie), where longer dry intervals between large events led to extended periods of below-average soil water content, increased plant water stress, and reduced ANPP. Mixed grass prairie was intermediate along the gradient, characterized by the greatest plant species richness, and the most sensitive to within-season variability in rainfall. Comparison of these experimental data to model simulations revealed key differences in soil water dynamics and ANPP patterns, suggesting that more experimental data is needed to parameterize biological and physical processes that drive model simulations. In conclusion, these results highlight the difficulties in extending inference from single site experiments to whole ecosystems or biomes and demonstrate the complexity inherent in predicting how terrestrial ecosystems will respond to novel climate conditions.
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 member
As 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.
Open Access
Tree and grass interactions governing the production and distribution of savannas: linking meta-scale patterns to underlying mechanisms
(Colorado State University. Libraries, 2015) Dohn, Justin, author; Knapp, Alan K., advisor; Hanan, Niall P., advisor; Augustine, David J., committee member; Davis, Jessica G., committee member
Savannas, characterized by the co-dominance of herbaceous and woody vegetation, support an estimated 20% of the global human population and account for ~30% of terrestrial net primary productivity. Interactions among savanna trees and grasses determine important ecosystem functions such as hydrological and biogeochemical cycles and production and transpiration rates, and impact the availability of resources (fuel-wood, grass for livestock) fundamental for human wellbeing. Additionally, interactions among trees may be an important driver of savanna vegetation structure, though few existing studies empirically estimate the intensity and importance of savanna tree-tree interactions. A clear understanding of the mechanisms that govern the coexistence of trees and grasses and their interactions in savanna landscapes is crucial to our ability to predict their responses to changing climatic and anthropogenic disturbance regimes. I present research aimed at advancing our understanding of emergent trends in savanna plant interactions and the underlying mechanisms responsible for observed patterns. First, I present the results of a meta-analysis of empirical studies that quantify the net effect of savanna trees on grass production under tree canopies relative to production away from trees. We found that the effect of trees on subcanopy herbaceous production varies predictably with climate, such that trees in arid savannas generally promote grass growth and trees in mesic regions suppress growth. This finding is consistent with a general theoretical model predicting the relative importance of facilitative processes for species coexistence. Termed the stress gradient hypothesis (SGH), the theory predicts an increasing importance of facilitation with increasing environmental stress, such as high water-stress typical of arid savannas. I then present results from two empirical studies designed to experimentally test the predictions of the SGH and infer mechanistic drivers by relating abiotic covariates to plant growth in the presence and absence of neighbors. In the shortgrass steppe (SGS) in northeastern Colorado, we found a net-neutral effect of shrubs and grasses on the other life form, contrary to expected facilitation. We suggest shrub morphology and interactive effects of topography and soil texture are primarily responsible for observed patterns of growth. At five savanna field sites situated along a rainfall gradient (i.e. water-stress gradient) in Mali, West Africa, we found the net effect of trees on grass growth to be consistent with SGH predictions. Light availability and distance to tree boles best explained shifts in herbaceous production along the rainfall gradient. Lastly, I present results from a longitudinal study in an East African savanna estimating tree growth as a function of the size and distance of neighboring woody competitors. In so doing, we quantified the magnitude of inter-tree competition and inferred its impact on stand spatial structure through spatial point pattern analysis. Overall, this research increases our understanding of biotic interactions among savanna plants. The effects of savanna trees on subcanopy grass production generally conform to the predictions of the SGH, and appear to be mediated by microclimate modification by tree canopies related to light availability and water balance. The effects of grasses on trees along environmental gradients are less clear, though we found net neutral effects on woody growth over one growing season in tropical and temperate shrub-grass systems, suggesting that active competitive and facilitative mechanisms largely offset, or that the effects of grasses on plant-available resources for woody species are negligible. Finally, we found that shrubs aggregate at local scales, despite significant neighbor competition. We suggest competition among woody plants influences production and relative species abundance, but dispersal and establishment bottlenecks are likely more important for landscape-scale spatial structure. These results have important implications for our theoretical understanding of coexistence between woody and herbaceous vegetation. Furthermore, we provide empirical data that can be used to refine and parameterize vegetation models predicting savanna ecosystem processes and the global distribution of mixed tree-grass systems.

Browsing by Author "Knapp, Alan K., advisor"

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