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Browsing by Author "Hanan, Niall P., advisor"

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    Africa's fuelwood footprint and the biome-level impacts of tree harvest
    (Colorado State University. Libraries, 2014) Tredennick, Andrew T., author; Galvin, Kathleen, advisor; Hanan, Niall P., advisor; Coughenour, Michael, committee member; Leisz, Stephen, committee member
    To view the abstract, please see the full text of the document.
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    Moving beyond the aggregated models: woody plant size influences on savanna function and dynamics
    (Colorado State University. Libraries, 2008) Sea, William Brian, author; Hanan, Niall P., advisor
    Historically, models have played important roles in studying aspects of savannas, including tree-grass competition, fire, and plant-herbivore interactions. The models can be categorized as either (1) "aggregated" ones that neglect size structure but have the advantage of mathematical tractability or (2) complicated process-oriented ecosystem models incorporating mechanistic ecophysiology capturing greater ecological realism but constrained to simulation modeling. The aggregated class of models can be further separated into those focusing on resource utilization and tree-grass competition ("resource-based models") and those focusing on demographic impacts of disturbances by fire and herbivory ("demographic bottleneck models"). The resource and demographic models separately consider important aspects of savanna ecology, yet the two approaches have rarely been integrated, resulting in a significant gap in our understanding of savannas. For this study, I investigated the role of woody plant size in savanna ecology. Using extensive datasets along broad resource gradients of annual precipitation in southern Africa, I examined patterns of size-abundance for woody plants in relatively undisturbed savannas to see if relationships for savannas showed similar patterns to theoretical predictions for tropical forests. Contrary to assumptions and predictions made by aggregate savanna models, I found that the percentage of wood biomass subject to fire loss actually decreases in wetter savannas. Since resource limitation and "thinning" have been mentioned as potential factors in savannas, I investigated the suitability of self-thinning in savannas. I developed a simple theoretical model hypothesizing three potential impacts of tree-grass interactions on the self-thinning relationship. Results from the analyses, testing with field data, suggest that tree-grass competition is asymmetric with respect to tree size. For the formal modeling component of my dissertation, I developed a simple savanna model that integrates demographic bottleneck and resource-based approaches. The model is unique in that the woody carrying capacity has both resource and demographic constraints. Model simulations showed that modest amounts of variation in adult mortality during fires and size-asymmetric tree-grass competition lead to very different model outcomes. The work opens up an entirely new class of ecological models for savanna ecology: analytically tractable with enough size structure to capture realistic savanna vegetation-disturbance interactions.
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    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.
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    Woody cover in African savannas: mapping strategies and ecological insights at regional and continental scales
    (Colorado State University. Libraries, 2010) Bucini, Gabriela, author; Hanan, Niall P., advisor; Boone, Randall B., 1963-, committee member; Lefsky, Michael Andrew, committee member; Dangelmayr, G. (Gerhard), 1951-, committee member
    Savanna ecosystems are characterized by the coexistence of woody and herbaceous vegetation. They are recognized as highly heterogeneous, for their diversity of growth forms and woody plant spatial arrangements. The relative fraction of woody versus herbaceous cover is particularly important in determining ecosystem functions such as water and biogeochemical cycles and energy fluxes, availability of graze and browse resources for wild and domestic herbivores, and availability of fuel-wood and other savanna products for human societies. This dissertation research focused on woody cover in tropical African savannas, with two main objectives, i) to map woody cover at regional to continental scales across Africa, and ii) to model its dependence on biotic and abiotic factors, at landscape, regional and continental scales. Among the most important outcomes are the creation of woody cover maps for Kruger National Park (South Africa) and the African continent using combined optical and radar imagery, and the development of ecological models that provided empirical evidence for resource-competition and disturbance mechanisms. The two-scale approach allowed the identification of relationships between woody cover and spectral predictors which can successfully be scaled up to predict the continental distributions of woody vegetation across the full gradient from deserts, through grasslands and savannas, to the dense tropical forests. The ecological models identified mean annual precipitation (MAP) as the main determinant of woody cover at the continental level. Regional variations of this MAP-driven woody cover arose from dynamics dependent on perturbations such as fire frequency, herbivory, and anthropogenic activities combined with soil characteristics.