Browsing by Subject "Vegetation dynamics"
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Item Fire effects on Matagorda Island vegetation(Texas Tech University, 1995) Brown, Steven Delmas; Wester, David B.; Allen, Bonnie L.Not availableItem Restoration of central Texas savanna and woodland : the effects of fire, deer, and invasive species on plant community trajectories(2014-05) Andruk, Christina Marie; Fowler, Norma L.Prescribed fire is a common tool used to restore native diversity, control invasive species, and reduce fuel loads. However, fire alone can be insufficient to restore pre-settlement vegetation; other factors such as differences in native and invasive species pools, deer herbivory, seed availability, and the spatial pattern of the fire can influence vegetation trajectories and restoration outcomes. Central Texas is a mosaic of savanna and mixed woodlands co-dominated by Quercus buckleyi (Texas red oak) and Juniperus ashei (Ashe juniper). In a savanna, I studied the joint effects of initial species composition (native-dominated or invasive-dominated) and disturbance (high-intensity fire, clipping, or control) on the ability of native species to establish, survive, and resist invasion by Bothriochloa ischaemum, an invasive C4 grass (ch. 1). Native savanna patches were resistant to invasion following high-intensity fire; fire can be used to selectively control B. ischaemum. In central Texas savanna and woodlands, under fire suppression and overabundant white-tailed deer, Quercus spp. are failing to regenerate, while J. ashei is increasing in abundance. To better understand vegetation trajectories following J. ashei removal in savanna, I studied the soil seedbank along a chronosequence of J. ashei invasion (ch. 5). In woodland, I studied the joint effects of prescribed fire and deer (ch. 2), clearing of J. ashei followed by high-intensity slash-pile burns (ch. 3), and wildfires (ch. 4) on the abundance and size of J. ashei and of hardwoods. Hardwoods resprouted vigorously after fire; J. ashei individuals of all sizes were killed by fire and slow to re-colonize. These management interventions failed to increase Q. buckleyi seedling abundance. It is likely that deer control is necessary to allow fire to have positive effects on the regeneration of oaks in this region, and wherever deer are over-abundant. However, deer can indirectly benefit hardwoods by reducing competition with palatable forbs (ch. 3). In general, these results show that fire suppression in central Texas oak-dominated woodlands is causing a shift not to more mesic-adapted species, as observed in the eastern US, but to J. ashei, which is at least as xeric-adapted as oak, a process I termed 'juniperization'.Item Seasonal Precipitation Variability and Its Impact on Vegetation Dynamics under Climate Change and Aridity Spectra of the Southwest United States Ecosystems(2015-01-16) Sohoulande Djebou, Dagbegnon ClementThis study combines hydro-climatological and biological components for addressing variability in precipitation and vegetation patterns under climate change. We explore the marginal and interactive effects of vegetation and atmospheric variables in order to better understand the plausible changes in terrestrial hydrological processes. We target the southwest United States, known for its diversified ecosystem and depleting water resources. Specifically, we employ an entropy-based disorder index to address precipitation variability and evaluate the marginal effect of watershed topography. Results show that the variability gradually increases westward. We concluded a significant watershed topography effect, which suggests that hilly reliefs have a stabilizing effect on seasonal precipitation variability in time and space. We conclude the necessity to include watershed topography information in climate model parameterizations. However, the implication of a spatial precipitation gradient raises questions regarding vegetation dynamics. In order to understand these dynamics, we analyze the inclusion of precipitation variability in conjunction with the Normalized Difference Vegetation Index (NDVI) during the growing season. We identify three climatic regions based on the United Nations Aridity Index (AI): a relatively humid region with AI?0.65, an intermediate region with 0.50?AI<0.65, and a relatively dry region with AI<0.50. We target four types of vegetation covers: deciduous forest, shrubland, pasture, and grassland. We conclude significant positive trends in the NDVI series for both relatively humid and intermediate climatic regions. In the arid region, we find distinct responses to precipitation for perennial vegetation versus annual vegetation types. The magnitude of these responses tends to increase with environmental aridity. Later we apply the entropy theory to investigate the joint inclusion of precipitation, soil moisture, and temperature in vegetation dynamics analysis. Results reveal trends toward maximum entropy; however, the variable precipitation remained particularly determinant from a marginal point of view. We use a probabilistic approach to analyze the climate change impact on future precipitation patterns. We conclude significant drifts in seasonal precipitation regimes and a meaningful spatial weight. Finally, we emphasize the plausible implications of our findings for future water management. Nevertheless, we suggest further studies on the topic particularly at a global scale.Item Vegetation manipulation strategies for playa lakes(Texas Tech University, 1991-08) Haukos, David A.The Playa Lakes Region of the Southern Great Plains is an important wintering area for waterfowl. The goal of this study was to develop and test management options for vegetation important to waterfowl wintering in playa lakes. Seed-bank composition was examined under 2 environmental conditions, drawdown and submerged. Seed banks of playa lakes are dominated by annuals. Seed-bank density and composition differed among 8 lakes. Plant community composition had a higher degree of similarity among lakes in the submerged treatment. Many species showed temporal emergence patterns throughout the growing season. Ability to predict playa vegetation from seed-bank composition decreased as the growing season progressed. Vegetation management in playa lakes (moist-soil management) involved the dewatering or irrigation of playas to create saturated, exposed soil. This promoted germination, growth, and seed production of moist-soil plants. The plant species targeted for management were Echinochloa crusgalli, Persicaria iapathjfolJa,- P. pensyivanica, Rum^x crispus^ and Eleocharis spp. Management created moist-soil conditions during early April, mid- to late June, and early August with maintenance of water in lakes over winter. Compared to unmanaged lakes, moist-soil managed lakes had greater above-ground standing crop and seed production and vertical structural cover. Moist-soil management of playa lakes favored Echinochloa crusgalli, Persicaria japathifolia. and £. pensylvanica: had no effect on Rumex crispus; and disfavored Eleocharis spp. Soil variables of conductivity, moisture, strength, nitrogen, and phosphorus were not affected by management of playa lakes. Wintering waterfowl were found more frequently on managed lakes. Chemical constituents of ash, soluble carbohydrates, crude protein, crude fat, hemicellulose, cellulose, lignin, and cutin-suberin were estimated in seeds from the 5 plant species listed above. Although chemical constituents differed among species, only seed of Eleocharis spp. Was undesirable because of high cutin-suberin levels and low protein and soluble carbohydrate values. Moist-soil management of playas is compatible with farming irrigation schedules and does not interrupt farming practices. Landowners are able to manage their lakes for lease hunting of waterfowl as an additional source of income Winter flooding of playa lakes without moist-soil management during the growing season is an inefficient use of valuable water.