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- Member of: ASU Electronic Theses and Dissertations
Description
Dissolved organic matter (DOM) is an important part of aquatic foodwebs because it contains carbon, nitrogen, and other elements required by heterotrophic organisms. It has many sources that determine its molecular composition, nutrient content, and biological lability and in turn, influence whether it is retained and processed in the stream reach or exported downstream. I examined the composition of DOM from vascular wetland plants, filamentous algae, and riparian tree leaf litter in Sonoran Desert streams and its decomposition by stream microbes. I used a combination of field observations, in-situ experiments, and a manipulative laboratory incubation to test (1) how dominant primary producers influence DOM chemical composition and ecosystem metabolism at the reach scale and (2) how DOM composition and nitrogen (N) content control microbial decomposition and stream uptake of DOM. I found that differences in streamwater DOM composition between two distinct reaches of Sycamore Creek did not affect in-situ stream respiration and gross primary production rates. Stream sediment microbial respiration rates did not differ significantly when incubated in the laboratory with DOM from wetland plants, algae, and leaf litter, thus all sources were similarly labile. However, whole-stream uptake of DOM increased from leaf to algal to wetland plant leachate. Desert streams have the potential to process DOM from leaf, wetland, and algal sources, though algal and wetland DOM, due to their more labile composition, can be more readily retained and mineralized.
ContributorsKemmitt, Kathrine (Author) / Grimm, Nancy (Thesis advisor) / Hartnett, Hilairy (Committee member) / Throop, Heather (Committee member) / Arizona State University (Publisher)
Created2018
Description
Rangelands are an extensive land cover type that cover about 40% of earth’s ice-free surface, expanding into many biomes. Moreover, managing rangelands is crucial for long-term sustainability of the vital ecosystem services they provide including carbon (C) storage via soil organic carbon (SOC) and animal agriculture. Arid rangelands are particularly susceptible to dramatic shifts in vegetation cover, physical and chemical soil properties, and erosion due to grazing pressure. Many studies have documented these effects, but studies focusing on grazing impacts on soil properties, namely SOC, are less common. Furthermore, studies testing effects of different levels of grazing intensities on SOC pools and distribution yield mixed results with little alignment. The primary objective of this thesis was to have a better understanding of the role of grazing intensity on arid rangeland soil C storage. I conducted research in long established pastures in Jornada Experimental Range (JER). I established a 1500m transect in three pastures originating at water points and analyzed vegetation cover and SOC on points along these transects to see the effect of grazing on C storage on a grazing gradient. I used the line-point intercept method to measure and categorize vegetation into grass, bare, and shrub. Since soil adjacent to each of these three cover types will likely contain differing SOC content, I then used this vegetation cover data to calculate the contribution of each cover type to SOC. I found shrub cover and total vegetation cover to decrease, while grass and bare cover increased with decreasing proximity to the water source. I found areal (g/m2) and percent (go SOC to be highest in the first 200m of the transects when accounting for the contribution of the three vegetation cover types. I concluded that SOC is being redistributed toward the water source via foraging and defecation and foraging, due to a negative trend of both total vegetation cover and percent SOC (g/g). With the decreasing trends of vegetation cover and SOC further from pasture water sources, my thesis research contributes to the understanding of storage and distribution of SOC stocks in arid rangelands.
ContributorsBoydston, Aaron (Author) / Sala, Osvaldo (Thesis advisor) / Throop, Heather (Committee member) / Hall, Sharon (Committee member) / Arizona State University (Publisher)
Created2018
Description
Xylem conduits, a primary feature of most terrestrial plant taxa, deliver water to photosynthetic tissues and play a critical role in plant water relations and drought tolerance. Non-succulent woody taxa generally follow a universal rate of tip-to-base conduit widening such that hydraulic resistance remains constant throughout the plant stem. Giant cacti inhabit arid regions throughout the Americas and thrive in water-limited environments by complimenting water-storing succulent tissues with resource-efficient Crassulacean Acid Metabolism. Considering these adaptations, the objectives of this study were threefold: 1) determine whether xylem conduits in columnar cacti follow universal scaling theory as observed in woody taxa; 2) evaluate whether xylem hydraulic diameter is inversely correlated with xylem vessel density; and 3) determine whether xylem double-wall thickness-to-span ratio and other hydraulic architectural traits are convergent among phylogenetically diverse cactus species. This thesis investigates the xylem anatomy of nine cactus species native to the Sonoran Desert of Arizona and Mexico, the tropical dry forests of southern Mexico, and the Alto Plano region of Argentina. Soft xylem tissues closest to the stem apex underwent a modified polyethylene glycol treatment to stabilize for sectioning with a sledge microtome. Across all species: hydraulic diameter followed a basipetal widening rate of 0.21 (p < 0.001), closely matching the universal rate of 0.20 for woody taxa; and xylem vessel density was inversely correlated with both length from stem apex (p < 0.001) and hydraulic diameter (p < 0.001). Double-wall thickness-to-span ratio had little to no significant correlation with either length from stem apex or hydraulic diameter. There was no significant difference in hydraulic architectural trait patterns between phylogenetically diverse species with various stem morphologies, nor was there a significant correlation between conduit widening rates and volume-to-surface-area ratios.
This study demonstrates that giant cacti follow similar internal anatomical constraints as non-succulent woody taxa, yet stem succulence and water storage behavior in cacti remain separate from internal hydraulic architecture, allowing cacti to thrive in arid environments. Understanding how cacti cope with severe water limitations provides new insights on evolutionary constraints of stem succulents as they functionally diverged from other life forms.
ContributorsCaspeta, Ivanna (Author) / Hultine, Kevin (Thesis advisor) / Throop, Heather (Thesis advisor) / Hernandez, Tania (Committee member) / Arizona State University (Publisher)
Created2023
Description
Drylands cover over 40% of the Earth’s surface, account for one third of global carbon cycling, and are hotspots for climate change, with more frequent and severe droughts coupled with deluges of novel magnitude and frequency. Because of their large terrestrial extent, elucidating dryland ecosystem responses to changes in water availability is critical for a comprehensive understanding of controls on global aboveground net primary productivity (ANPP), an important ecosystem service. The focus of this dissertation is to investigate cause-effect mechanisms between altered water availability and ecosystem processes in dryland ecosystems. Across a network of experimental rainfall manipulations within a semiarid Chihuahuan Desert grassland, I examined short- and long-term dynamics of multiple ecosystem processes—from plant phenology to nitrogen cycling—in response to directional precipitation extremes. Aboveground, I found herbaceous plant phenology to be more sensitive in greenup timing compared to deep-rooted, woody shrubs, implying that precipitation extremes will disproportionately affect grass-dominated compared to woody ecosystems. Surprisingly, after 14 years of experimentally adding water and N, I observed no effect on ANPP. Belowground, bulk soil N dynamics remained stable with differing precipitation amounts. However, mineral associated organic N (MAOM-N) significantly increased under chronic N inputs, indicating potential for dryland soil N sequestration. Conversely, the difference between low- and high-N soil N content may increase a drawdown of N from all soil N pools under low-N conditions whereas plants source N from fertilizer input under high-N conditions. Finally, I considered ecosystem-level acclimation to climate change. I found that N availability decreased with annual precipitation in space across continents, but it posed initially increasing trends in response to rainfall extremes at the Jornada that decreased after 14 years. Mechanisms for the acclimation process are thus likely associated with differential lags to changes in precipitation between plants and microorganisms. Overall, my dissertation demonstrates that examining linkages between multiple ecosystem processes, from aboveground phenological cycles to belowground N cycling dynamics, can provide a more integrative understanding of dryland response to climate change. Because dryland range is potentially expanding globally, water limited systems provide a unique and critical focus area for future research that revisit and revise current ecological paradigms.
ContributorsCurrier, Courtney (Author) / Sala, Osvaldo (Thesis advisor) / Collins, Scott (Committee member) / Reed, Sasha (Committee member) / Throop, Heather (Committee member) / Arizona State University (Publisher)
Created2023
Description
Flowering phenology offers a sensitive and reliable biological indicator of climate change because plants use climatic and other environmental cues to initiate flower production. Drylands are the largest terrestrial biome, but with unpredictable precipitation patterns and infertile soils, they are particularly vulnerable to climate change. There is a need to increase our comprehension of how dryland plants might respond and adapt to environmental changes. I conducted a meta-analysis on the flowering phenology of dryland plants and showed that some species responded to climate change through accelerated flowering, while others delayed their flowering dates. Dryland plants advanced their mean flowering dates by 2.12 days decade-1, 2.83 days °C-1 and 2.91 days mm-1, respectively, responding to time series, temperature, and precipitation. Flowering phenology responses varied across taxonomic and functional groups, with the grass family Poaceae (-3.91 days decade1) and bulb forming Amaryllidaceae (-0.82 days decade1) showing the highest and lowest time series responses respectively, while Brassicaceae was not responsive. Analysis from herbarium specimens collected across Namibian drylands, spanning 26 species and six families, revealed that plants in hyper-arid to arid regions have lower phenological sensitivity to temperature (-9 days °C-1) and greater phenological responsiveness to precipitation (-0.56 days mm-1) than those in arid to semi-arid regions (-17 days °C-1, -0.35 days mm-1). The flowering phenology of serotinous plants showed greater sensitivity to both temperature and precipitation than that of non-serotinous plants. I used rainout shelters to reduce rainfall in a field experiment and showed that drought treatment advanced the vegetative and reproductive phenology of Cleome gynandra, a highly nutritional and medicinal semi-wild vegetable species. The peak leaf length date, peak number of leaves date, and peak flowering date of Cleome gynandra advanced by six, 10 and seven days, respectively. Lastly, I simulated drought and flood in a greenhouse experiment and found that flooding conditions resulted in higher germination percentage of C. gynandra than drought. My study found that the vegetative, and flowering phenology of dryland plants is responsive to climate change, with differential responses across taxonomic and functional groups, and aridity zones, which could alter the structure and function of these systems.
ContributorsKangombe, Fransiska Ndiiteela (Author) / Throop, Heather (Thesis advisor) / Sala, Osvaldo (Committee member) / Vivoni, Enrique (Committee member) / Pigg, Kathleen (Committee member) / Hultine, Kevin (Committee member) / Kwembeya, Ezekeil (Committee member) / Arizona State University (Publisher)
Created2023
Description
Physical and structural tree measurements are applied in forestry, precision agriculture and conservation for various reasons. Since measuring tree properties manually is tedious, measurements from only a small subset of trees present in a forest, agricultural land or survey site are often used. Utilizing robotics to autonomously estimate physical tree dimensions would speed up the measurement or data collection process and allow for a much larger set of trees to be used in studies. In turn, this would allow studies to make more generalizable inferences about areas with trees. To this end, this thesis focuses on developing a system that generates a semantic representation of the topology of a tree in real-time. The first part describes a simulation environment and a real-world sensor suite to develop and test the tree mapping pipeline proposed in this thesis. The second part presents details of the proposed tree mapping pipeline. Stage one of the mapping pipeline utilizes a deep learning network to detect woody and cylindrical portions of a tree like trunks and branches based on popular semantic segmentation networks. Stage two of the pipeline proposes an algorithm to separate the detected portions of a tree into individual trunk and branch segments. The third stage implements an optimization algorithm to represent each segment parametrically as a cylinder. The fourth stage formulates a multi-sensor factor graph to incrementally integrate and optimize the semantic tree map while also fusing two forms of odometry. Finally, results from all the stages of the tree mapping pipeline using simulation and real-world data are presented. With these implementations, this thesis provides an end-to-end system to estimate tree topology through semantic representations for forestry and precision agriculture applications.
ContributorsVishwanatha, Rakshith (Author) / Das, Jnaneshwar (Thesis advisor) / Martin, Roberta (Committee member) / Throop, Heather (Committee member) / Zhang, Wenlong (Committee member) / Ehsani, Reza (Committee member) / Arizona State University (Publisher)
Created2022
Description
The southwestern US will experience more frequent heat waves, prolonged droughts, and declining water supply. Riparian ecosystems are particularly at-risk under climate change predictions, but little is known about the thermal tolerance of plant species inhabiting these ecosystems. Populus fremontii, a pioneer and foundation tree species in riparian ecosystems throughout the southwest, is of concern given its importance in driving community structure and influencing ecosystem processes. This study compared leaf thermal tolerance across populations of P. fremontii to determine if local adaptation affects leaf thermal tolerance. I hypothesized that warm-adapted (low-elevation) populations would have larger leaf thermal tolerance thresholds, thermal safety margins, and thermal time constants than cool-adapted (high-elevation) populations. I expected warm-adapted populations to maintain lower maximum leaf temperatures due to local adaptation affecting leaf thermal regulation. Using a common garden at the warm edge of this species’ range, I measured leaf thermal tolerance metrics in eight populations spanning a 1,200 m elevational gradient. Data collection occurred in May, during mild air temperatures, and in August, during high air temperatures. The first two metrics were leaf thermal tolerance thresholds. The critical temperature (Tcrit) is the temperature at which the electron transport capacity of PSII is disrupted. T50 is the temperature at which the electron transport capacity decreases to 50%. The next metric was thermal safety margins (TSMs), which reflect a leaf’s vulnerability to reaching thermal tolerance thresholds. TSMs are the difference between either Tcrit or T50 and an experienced environmental variable such as leaf or air temperature. The last metric was the thermal time constant (?), which is a trait that represents how quickly leaf temperatures respond to changes in air temperatures. Tcrit, T50, and ? were not correlated with elevation regardless of season, suggesting that acclimation or phenotypic plasticity is affecting these metrics. Conversely, TSMs using maximum leaf temperature were negatively correlated with elevation in August because warm-adapted populations maintained lower maximum leaf temperatures. These findings suggest that warm-adapted populations are locally adapted to maintain cooler leaf temperatures, which is critical for their future survival since they do not maintain higher leaf thermal tolerance thresholds than cool-adapted populations.
ContributorsMoran, Madeline (Author) / Hultine, Kevin (Thesis advisor) / Throop, Heather (Thesis advisor) / Butterfield, Bradley (Committee member) / Arizona State University (Publisher)
Created2022
Description
Climate change is making the arid southwestern U.S. (“Southwest”) warmer and drier. Decreases in water availability coupled with increases in episodic heat waves can pose extraordinary challenges for native riparian tree species to persist in their current ranges. However, the morpho-physiological mechanisms that these species deploy to cope with extreme temperature events are not well understood. Specifically, how do these species maintain leaf temperatures within a safe operational threshold in the extreme conditions found across the region? Morpho-physiological mechanisms influencing intraspecific local adaptation to thermal stress were assessed in Populus fremontii using two experimental common gardens. In a common garden located near the mid-point of this species’ thermal distribution, I studied coordinated traits that reflect selection for leaf thermal regulation through the measurement of 28 traits encompassing four different trait spectra: phenology, whole-tree architecture, and the leaf and wood economic spectrum. Also, I assessed how these syndromes resulted in more acquisitive and riskier water-use strategies that explained how warm-adapted populations exhibited lower leaves temperatures than cool-adapted populations. Then, I investigated if different water-use strategies are detectable at inter-annual temporal scales by comparing tree-ring growth, carbon, and oxygen isotopic measurements of cool- versus warm-adapted populations in a common garden located at the extreme hottest edge of P. fremontii’s thermal distribution. I found that P. fremontii’s adaptation to the extreme temperatures is explained by a highly intraspecific specialized trait coordination across multiple trait scales. Furthermore, I found that warmer-adapted populations displayed 39% smaller leaves, 38% higher midday stomatal conductance, reflecting 3.8 °C cooler mean leaf temperature than cool-adapted populations, but with the tradeoff of having 14% lower minimum leaf water potentials. In addition, warm-adapted genotypes at the hot edge of P. fremontii’s distribution had 20% higher radial growth rates, although no differences were detected in either carbon or oxygen isotope ratios indicating that differences in growth may not have reflected seasonal differences in photosynthetic gas exchange. These studies describe the potential effect that extreme climate might have on P. fremontii’s survival, its intraspecific responses to those events, and which traits will be advantageous to cope with those extreme environmental conditions.
ContributorsBlasini, Davis E (Author) / Hultine, Kevin R (Thesis advisor) / Day, Thomas A (Thesis advisor) / Ogle, Kiona (Committee member) / Throop, Heather (Committee member) / Gaxiola, Roberto (Committee member) / Arizona State University (Publisher)
Created2022
Description
Climate change is increasing global surface temperatures, intensifying droughts and increasing rainfall variation, particularly in drylands. Understanding how dryland plant communities respond to climate change-induced rainfall changes is crucial for implementing effective conservation strategies. Concurrent with climate change impacts on drylands is woody encroachment: an increase in abundance of woody plant species in areas formerly dominated by grasslands or savannahs. For example, the woody plant, Prosopis velutina (velvet mesquite), has encroached into grasslands regionally over the past century. From an agricultural perspective, P. velutina is an invasive weed that hinders cattle forage. Understanding how P. velutina will respond to climate change-induced rainfall changes can be useful for management and conservation efforts. Prosopis velutina was used to answer the following question: Is there a significant interactive effect of mean soil water moisture content and pulse frequency on woody seedling survival and growth in dryland ecosystems? There were 256 P. velutina seedlings sourced from the Santa Rita Experimental Range in southern Arizona grown under four watering treatments where mean and pulse frequency were manipulated over two months. Data were collected on mortality, stem height, number of leaves, instantaneous gas exchange, chlorophyll fluorescence, biomass, and the leaf carbon to nitrogen (C:N) ratio. Mortality was low across treatments. Pulse frequency had less impact across response variables than the mean amount of water received. This may indicate that P. velutina seedlings are relatively insensitive to rainfall timing and are more responsive to rainfall amount. Prosopis velutina in the low mean soil moisture treatments lost a majority of their leaves and had greater biomass allocation to roots. Prosopis velutina’s ability to survive in low soil moisture conditions and invest in root biomass can allow it to persist as drylands are further affected by climate change. Prosopis velutina could benefit ecosystems where native plants are at risk due to rainfall variation if P. velutina occupies a similar niche space. Due to conflicting viewpoints of P. velutina as an invasive species, it’s important to examine P. velutina from both agricultural and conservation perspectives. Further analysis on the benefits to P. velutina in these ecosystems is recommended.
ContributorsDavis, Ashley R. (Author) / Throop, Heather (Thesis advisor) / Hultine, Kevin (Committee member) / Sala, Osvaldo (Committee member) / Arizona State University (Publisher)
Created2020
Description
Ecological phenomena act on various spatial and temporal scales. To understand what causes animal populations to build and decline depends heavily on abiotic and biotic conditions which vary spatiotemporally throughout the biosphere. One excel- lent example of animal populations dynamics is with locusts. Locusts are a subset of grasshoppers that undergo periodical upsurges called swarms. Locust swarms have plagued human history by posing significant threats to global food security. For example, the 2003-2005 desert locust (Schistocerca gregaria) swarm destroyed 80%-100% of crops in the impacted areas and cost over US $500 million in mitigation as estimated by the Food and Agriculture Organization of the United Nations. An integrative multi-scale approach must be taken to effectively predict and manage locust swarms. For my dissertation, I looked at the ecological causes of locust swarms on multiple scales using both the Australian plague locust (Chortoicetes terminifera) and desert locust as focal species. At the microhabitat scale, I demonstrated how shifts in the nutritional landscape can influence locust gregarization. At the field level, I show that locust populations avoid woody vegetation likely due to the interactive effect of plant nutrients, temperature, and predators. At the landscape level, I show that adaptations to available nutrient variation depends on life history strategies, such as migratory capabilities. A strong metapopulation structure may aid in the persistence of locust species at larger spatial scales. Lastly, at the continental scale I show the relationship between preceding vegetation and locust outbreaks vary considerably between regions and seasons. However, regardless of this variation, the spatiotemporal structure of geographic zone > bioregion > season holds constant in two locust species. Understanding the biologically relevant spatial and temporal scales from individual gregarization (e.g. micro-habitat) to massive swarms (e.g. landscape to continental) is important to accurately predicting where and when outbreaks will happen. Overall, my research highlights that understanding animal population dynamics requires a multi-scale and trans-disciplinary approach. Into the future, integrating locust re- search from organismal to landscape levels can aid in forecasting where and when locust outbreaks occur.
ContributorsLawton, Douglas (Author) / Cease, Arianne J (Thesis advisor) / Waters, Cathy (Thesis advisor) / Throop, Heather (Committee member) / Wu, Jianguo (Committee member) / Arizona State University (Publisher)
Created2021