Browsing by Subject "succession"
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Item Fire Regimes of the Southern Appalachian Mountains: Temporal and Spatial Variability and Implications for Vegetation Dynamics(2012-08-31) Flatley, William 1977-Ecologists continue to debate the role of fire in forests of the southern Appalachian Mountains. How does climate influence fire in these humid, temperate forests? Did fire regimes change during the transition from Native American settlement to Euro-American settlement? Are fire regime changes resulting in broad vegetation changes in the forests of eastern North America? I used several approaches to address these questions. First, I used digitized fire perimeter maps from Great Smoky Mountains National Park and Shenandoah National Park for 1930-2009 to characterize spatial and temporal patterns of wildfire by aspect, elevation, and landform. Results demonstrate that fuel moisture is a primary control, with fire occurring most frequently during dry years, in dry regions, and at dry topographic positions. Climate also modifies topographic control, with weaker topographic patterns under drier conditions. Second, I used dendroecological methods to reconstruct historical fire frequency in yellow pine (Pinus, subgenus Diploxylon Koehne) stands at three field sites in the southern Appalachian Mountains. The fire history reconstructions extend from 1700 to 2009, with composite fire return intervals ranging from 2-4 years prior to the fire protection period. The two longest reconstructions record frequent fire during periods of Native American land use. Except for the recent fire protection period, temporal changes in land use did not have a significant impact on fire frequency and there was little discernible influence of climate on past fire occurrence. Third, I sampled vegetation composition in four different stand types along a topographic moisture gradient, including mesic cove, sub-mesic white pine (Pinus strobus L.) hardwood, sub-xeric oak (Quercus L.), and xeric pine forests in an unlogged watershed with a reconstructed fire history. Stand age structures demonstrate changes in establishment following fire exclusion in xeric pine stands, sub-xeric oak stands, and sub-mesic white pine-hardwood stands. Fire-tolerant yellow pines and oaks are being replaced by shade-tolerant, fire sensitive species such as red maple (Acer rubrum L.) and hemlock (Tsuga canadensis L. Carr.). Classification analysis and ordination of species composition in different age classes suggest a trend of successional convergence in the absence of fire with a shift from four to two forest communities.Item Leadership succession: a discourse analysis of governance dialectics in two nonprofit organizations(Texas A&M University, 2008-10-10) Duta, Andrei ConstantinNonprofit organizations across the country are faced with a leadership crisis due to unprecedented demographic changes and challenges. The en masse retirement of the baby boomers leaves leadership positions in nonprofit organizations vacant and vulnerable. This study uses discourse analysis and dialectical theory to compare and contrast the leadership succession processes in two Texas-based nonprofit organizations. Both organizations have comparable missions, religious backgrounds, and annual budgets. In addition, the two organizations share similar structures in terms of board of directors, board chair, and executive director (ED) dynamics. This research is a descriptive comparative case study. The specific purpose of this study is to examine how various board members, including the board chair and the incumbent ED, construct the leadership succession process through their discursive interactions and strategies. This study demonstrates that leadership succession in nonprofits is a convoluted process enacted discursively by conflicting or collaborating key organizational actors huddled around various leadership nuclei. These nuclei include, in various combinations, the EDs, board chairs, vice-chairs, and even spouses of some of these actors. The leadership nuclei morph over time, based on the management of dialectical tensions experienced across four stages in the succession process: presuccession, during succession A and B, and post-succession. This research contributes to the larger body of leadership succession knowledge in multiple ways. First, this study reveals that leadership is an amorphous and dynamic concept contested among organizational actors across time. Second, the study highlights the architectonic role of discourse in the leadership succession process. The succession process is enacted through the key organizational actors' discursive exchanges. Third, this research points to three types of dialectical tensions underlying the actors' discursive interactions: individual-centered dialectics ("staying/leaving"), relation-centered dialectics ("blaming/absolving," "freedom/control," and "cooperation/competition"), and organization-centered dialectics ("change/stability"). Next, this study demonstrates that organizational or individual crisis prompts the succession process, and that change, conflict, goals, and deadlines grow out of the succession process and not the other way around. Finally, this study contributes specifically to the research area of small nonprofit organizations. There is a paucity of research examining the succession process in small nonprofits, and this study addresses this need.Item Post-fire recovery and successional dynamics of an old growth red spruce forest in the southern Appalachian Mountains(2009-05-15) Krustchinsky, Adam R.Red spruce is a shade-tolerant conifer whose distribution and abundance reflect Quaternary climate history as well as natural and anthropogenic disturbances. This species once extended further south than its present localities, because of natural and anthropogenic disturbances such as logging, windthrow, and fire. Little is known about the disturbance regime of this species, because long term stand dynamics are difficult to obtain. This-long lived species is hypothesized to be suffering a decline in radial growth, density and abundance at the present time. Recent research suggests pollution, biotic stresses, climate change and natural stand dynamics are the driving forces behind these decreases. The purpose of this study is to investigate the influence of fire in a mesic ecosystem, specifically a high-elevation red spruce (Picea rubens Sarg.) forest on Whitetop Mountain in the southern Appalachian Mountains. Six plots were established in a high elevation red spruce stand to characterize the stand composition. Tree ring data were collected to investigate radial growth relations to inter-annual climatic variability and cross-sections were used to investigate fire history. Red spruce continued to establish throughout the 19th century until a severe fire occurred in 1919 and caused a new cohort of yellow birch (Betula alleghaniensis Britt.) to establish within the stand. Logging and fire caused high mortality in the stand, yet many spruce remain that outdate the past disturbances. Red spruce saplings continue to persist in the stand, showing regeneration despite the abundant hardwoods. Moisture was the main contributing factor to red spruce growth in the dendroclimatic analysis. Red spruce radial growth was significantly correlated to high precipitation and low temperatures of the previous growing season, which is similar to recent research results. This study collaborates the current literature on red spruce growth along with the results found here in creating a model to represent the growth characteristics of red spruce when inter-mixed with hardwoods after a severe disturbance.Item Spatial and Temporal Dynamics of Salt Marsh Vegetation across Scales(2010-10-12) Kim, DaehyunBiogeographic patterns across a landscape are developed by the interplay of environmental processes operating at different spatial and temporal scales. This research investigated dynamics of salt marsh vegetation on the Skallingen salt marsh in Denmark responding to environmental variations at large, medium, and fine scales along both spatial and temporal spectrums. At the broad scale, this research addressed the importance of wind-induced rise of the sea surface in such biogeographic changes. A new hypothetical chain was suggested: recent trends in the North Atlantic Oscillation index toward its positive phase have led to increased storminess and wind tides on the ocean surface, resulting in increased frequency, duration, and magnitude of submergence and, hence, waterlogging of marsh soils and plants, which has retarded ecological succession. At the mid-scale, spatial patterns of vegetation and environmental factors were examined across tidal creeks. Sites closer to tidal creeks, compared to marsh interiors, were characterized by the dominance of later-successional species, higher bulk density, and lower nutrient contents and electrical conductivity. This finding implies that locations near creeks have experienced a better drainage condition than the inner marshes, which eventually facilitated the establishment of later-successional plants that are intolerant to physical stress. At the micro-scale, this research examined how the extent and mode of facilitation and competition vary for different combinations of plant species along physical gradients. Both positive and negative relationships were spatially manifested to a greater degree on the low marsh than on the mid marsh. This insight extends our current knowledge of scale-dependent interactions beyond pioneer zones to higher zones. On the low marsh, different types of bivariate point pattern (i.e., clustered, random, and regular) were observed for different combinations of species even at similar spatial scales. This finding implies that it is difficult to generalize at which scales competition and facilitation occur. To conclude, this research stresses the need for a holistic approach in future investigations of salt marsh biogeography. For example, based on results of this current research, it would be meaningful to develop a comprehensive simulation model that incorporates salt marsh ecology, geomorphology, and hydrology observed across scales.