Graduation date: 1991

The structure and function of coastal marshes results from a complex interaction of biotic and abiotic processes that continually influence the characteristics of marsh vegetation. A great deal of research has focused on how tidal processes influence vegetation dynamics along the Atlantic coast, but few studies have investigated the influence of similar processes in the marshes along the Gulf of Mexico. This study aims to identify the characteristic vegetation assemblages of the coastal marshes bordering the East Bay of Galveston Bay, Texas, and identify if elevation, inundation frequency and burning frequency are important to their structure. To identify characteristic vegetation assemblages, hierarchical cluster analysis was used. The cluster analysis resulted in seven statistically different vegetation assemblages that were used in diversity analysis and classification and regression analysis (CART) as dependent variables. Diversity measures were calculated at both the plot and assemblage scale using Shannon's diversity index and species richness. The resulting diversity measures were used as predictor variables in the CART analysis as well as regression analysis. Hydrologic modeling was accomplished using Mike 21, a flow and wave simulation model, along with a geographic information system (GIS), to model hourly inundation frequency at each of the sampled plots. The inundation frequency was then used as a predictor variable in the CART analysis and regression analysis. This study found that the main factor contributing to species richness was elevation. Vegetation assemblages at high elevations generally had high diversity, and assemblages at low elevations had lower diversity. Elevation and inundation frequency are inversely related, and the strong correlation between species richness and elevation also assumes that inundation frequency is important in structuring the marsh. Burn frequencies had no influence on diversity in general, but more frequent burning did result in monospecific stands of Spartina patens at Anahuac NWR.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

The Okavango Delta, an internationally recognized wetland, is undergoing natural and anthropogenic change at a variety of spatio-temporal scales. The objective of this research was to utilize remotely sensed imagery to assess the spatio-temporal distribution of flooding and fire and their subsequent influences on vegetation as represented by vegetation index trajectories in the Okavango Delta. The characterization of the spatiotemporal dynamics of vegetation spectral response via a time-series of remotely sensed data not only informs ecosystem and disturbance theory but also presents new methodological applications for multi-temporal change analysis. Disentangling these components from a signal is critical for better assessing the interrelationships among climatic oscillations, disturbance regimes, and human management on ecosystem response. This research tested six hypotheses regarding flooding and fire, and found that the largest number of fires occurred either within 5 km of the border to the Wildlife Management Areas or within the active (flooded a minimum of every two years) floodplains. These hypotheses indicate that burning is highest where people have accessinto the management areas and where the natural resources are plentiful. Periodicities from vegetation signal time-series did not confirm published climate-driven periodicities of 3, 8, and 18-years but did reveal seasonal (6 month) and quasi-decadal periodicities. Vegetation trajectories were more predictable with increasing flood frequency and duration, but were less predictable with increased fire frequency. The fact that increased burning resulted in less predictable behavior indicates the potential of quantifying the anthropogenic influence on the landscape using remotely sensed imagery. Flooding and fire were not statistically correlated to the residual dynamics, refuting the conceptualization of flooding and fire as disturbance and supporting the interpretation of flooding and fire as disturbance regimes. This research thus contributes methodologically and theoretically to the ecology literature by operationalizing tests for disturbance versus disturbance regimes via spatio-temporal characterization. Further, this work extends change detection techniques typically implemented with coarser spatial resolution but more frequently acquired imagery by using harmonic regression and wavelet analysis with Landsat data. Lastly, this work provides a temporally rich assessment of recent vegetation, flooding, and fire trends for improving management efforts of the Okavango Delta.

Higher stocking density under seasonal-rotation grazing is expected to increase plant abundance because expanded animal distribution and reduced selective grazing on forage species will reduce the spatial variation and competitive advantage of non-forage species compared to yearlong grazing. Rangeland scientists struggle with how long rangeland experiments must continue in order to detect grazing treatment effects, particularly in semi-arid ecosystems with slow responses and high spatio-temporal variability. My first study investigated grazing system effects on plant abundance (cover and density) over the short-term (12yrs) or long-term (22 or 34yrs) after accounting for covariates (mesquite and precipitation gradients). My second study assessed how grazing systems affected spatial variation in grass abundance over 22 or 34 years after accounting for precipitation gradient. The first study was a course resolution approach, looking at grazing impacts on plant abundance. The second study was a finer resolution assessment of the underlying assumption that rotational grazing systems reduce selective grazing. Using split-plot analysis of variance, with year as the split, changes in mean plant abundance and variance in grass abundance were compared between two grazing systems (yearlong vs. seasonal rotation), after accounting for covariate(s). Variance of grass abundance among sample locations within an experimental pasture was the measure of spatial variability and was expected to increase with selective grazing. Grazing systems did not influence plant abundance or spatial variation of grasses. The absence of grazing effect may be due to overriding influences of grazing intensity, large pasture sizes, temporal variation in precipitation, and few replicates. Specific to spatial variation, the absence of grazing system effect may be due to discrepancies in transect representation across ecological sites and spatial variation of grasses occurring at scales different than the 30-m transect size. Like earlier research, my studies failed to substantiate the fundamental premise for implementing rotational grazing systems. This exposes challenges that confront rangeland scientists when implementing grazing studies in spatio-temporally heterogeneous ecosystems. I recommend that research shift from comparing rigid schedule-driven grazing systems to more adaptive management approach where there are comparisons between different levels or different designs of flexible systems.

Herbivores are important drivers and have a longstanding history in shaping our terrestrial environments. However, during the past decades, changes in woody vegetation in savanna and forest systems have been observed in southern Africa. Subsequently, concerns have been raised about the loss of (tall) trees in areas with elephant. The relative effects of browsing herbivores on vegetation and the potential browsing interaction with other herbivore species remain unclear and were examined using vegetation transects and exclosure experiments in savanna woodland and Sand Forest. Rainfall, fire and elephant were important savanna determinants. Especially rainfall positively affected woody densities, which were negatively affected by a longer exposure time to elephant, but not to elephant densities itself. In general, within South Africa’s savannas, tree height classes were absent from the population demography. Different height classes were likely to be impacted by different drivers. For example, seedling and sapling densities were greater with longer fire return periods and increased rainfall. The Sand Forest exclosure experiments showed that forest regeneration was impacted by nyala and both elephant and nyala, as the absence of both species increased tree densities. Both species combined, and individually, also affected tree species assemblages. In contrast, short term elephant access to a savanna area did not affect tree densities or species assemblages. In both savanna and Sand Forest elephant displaced mesoherbivores, and in Sand Forest both elephant and mesoherbivores displaced their smaller counterparts. The presence of competitive displacement also affected recruitment (i.e. seedlings and/or saplings) of woody vegetation both in Sand Forest and savanna. Thus, elephant and mesoherbivores exert direct and indirect (i.e. competitive displacement providing a window for recruitment) impact on vegetation. Active management of the herbivore species assemblage affects both vegetation and other herbivores, which effects potentially cascade into lower trophic levels, jeopardising biodiversity and ecosystem processes. Therefore, the full herbivore assemblage present and their combined and individual browsing effects need to be considered when setting management goals to conserve habitats and biodiversity across all trophic levels. In addition some contrasting results between Sand Forest and savanna emphasise the need for caution when extrapolating results from different areas and ecosystems. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.

The vegetation along the north coast of KwaZulu-Natal has long been considered to have originally consisted of forest, scrub forest and savanna. The classical view is that in the last 600 years the early Africans and European farmers were responsible for the removal of forest and scrub forest along the coast. This view was not based on direct evidence but on the theory that the eastern part of the country has a climate "suitable" for forest and scrub forest. The present 'false' grasslands were thus thought to have developed through anthropogenic influences. All of this has its basis in the paradigm of ecological succession and the presence of a "climatic climax". This traditional view has been contested recently, based on archaeological, historical, biogeographical and ecological evidence that has become available since the 1950's. It is suggested that South Africa's grass lands have been in existence for the last two thousand years but probably for more than ten thousand years. This study aims to investigate this controversy in greater detail, using evidence from archaeological records, travellers records, transcripts, historical reviews, and diarised records. The locations of archaeological sites within the study area were determined and mapped out, followed by an analysis and interpretation of the data with reference to vegetation change. Archaeological evidence included shell middens, evidence of iron working and pottery remains. The activities of the early humans included iron smelting, agriculture and stock farming. Their activities required the selective use of vegetation for specific purposes, and vegetation was cleared for homesteads and villages. However, the density of people within the study area was low, and there was limited technological development, such that extensive clearing by relatively few people is unlikely. Furthermore, sites are concentrated along the coastline, with fewer sites away from the coast, suggesting that impacts would have been greatest along the coastline. However, this is where forests presently occur. Overall, the evidence suggests that the natural vegetation on the north coast was not modified drastically by precolonial settlers. Historical accounts of early travellers and settlers indicate a strip of forest along the coastline and a grassland/woodland mosaic away from the coast. Records of mammals suggest a fauna typical of savannas and not forest. With settlement over time, the major activity that impacted on the north coast vegetation, was agriculture. Sugar cane plantations contributed considerably to the clearing of vegetation that seems to have consisted primarily of open grasslands with patches of trees. Colonial settlement of this area resulted in various activities that required the large-scale removal of natural vegetation. It is important to know the human disturbance history of an area as this helps to assess the extent of change and to design appropriate management strategies for conservation of plant resources. The belief that the early vegetation of the north coast was forest has placed great emphasis on the conservation of forests along this coastal area. Based on this study, it seems that this vegetation type should not be the focus of conservation efforts, but that coastal grasslands with scattered bush clumps should be given much greater emphasis. Grasslands were more widespread in the region prior to European settlement, and based on this, conservationists should place greater emphasis on preserving this habitat. / Thesis (M.Sc.)-University of Natal, Durban, 2000.

This thesis addresses the effects of seasonality of grazing on vegetation dynamics. Background to the thesis is provided by the Hill Sheep and Native Woodland (HSNW) project, a system-scale experiment with the long-term aim of integrating upland sheep husbandry within native woodlands. Sheep husbandry in the HSNW project has involved a change from year-round grazing to off-wintering (grazing on upland areas from April to October only). The potential impact of this change on individual plants, plant communities and the landscape is evaluated. A cutting experiment was run to simulate herbivory in different seasons at the level of the individual plant. In grassland communities, where most species are wintergreen graminoids, interaction between species and seasons in response to cutting was minimal. In mire communities, where species with a range of life-forms and phenologies are present, there was considerable interaction between species and lifeform in response to cutting in different seasons. Non-wintergreen graminoid species recovered more rapidly than wintergreen graminoid species following cutting in spring, but the reverse occurred following cutting in autumn. Dwarf shrubs and forbs were slow to recover from cutting in all seasons in both mires and grasslands. The impacts of three grazing treatments in the HSNW project (unchanged year-round grazing, change to off-wintering, and change to zero grazing) on sward structure and species abundance were monitored. Sward height increased in the ungrazed treatment, but changes in species relative abundance were minimal in the short time-scale of the study. Literature suggests that change in plant species abundance in grazed systems is driven by herbivore selection preference for (or avoidance of) plant species and plant responses to grazing. Experts were interviewed to generate a set of seasonal data on herbivore selection preferences and plant responses, and levels of agreement between experts were assessed. Information was also collated on plant response to herbivory from the cutting experiment (above) and plant trait information from the literature. A qualitative model was developed to predict change in species abundance. It was used to: i) explore patterns of interaction between herbivore preferences and plant responses to grazing; ii) compare short-term predictions of change in abundance made using each of the information sources available; iii) make longer-term predictions using plant trait information. The short-term predictions were tested against the small changes in species abundance that had occurred in the HSNW project. Predictions made with the plant trait information were more accurate than those made with other information. A study of levels of browsing on regeneration was carried out in mature sheep-grazed birch woodlands. It was rare to find regeneration at sites grazed only by sheep, but tree regeneration did occur in the presence of sheep. Seasonal variation in browsing was found to be more related to the presence of cattle than to the presence of sheep. Finally, the findings are collated to make management recommendations for the HSNW project and other systems where seasonal grazing regimes could be beneficial.

Los Andes ecuatorianos, ubicados en el noroeste de América del Sur son considerados un “hot-spot” con una alta diversidad mundial de plantas vasculares, debido a su compleja topografía (elevación de la cordillera), variaciones de las condiciones climáticas y los distintos tipos de vegetación. A pesar de su elevado nivel de biodiversidad, los Andes ecuatorianos presentan uno de los paisajes más amenazados y poco estudiados. Especialmente los ecosistemas de páramo y montaña están sujetos a sobrepastoreo, quemas, cultivos y la deforestación causada por la expansión de la actividad humana en las últimas décadas. El conocimiento sobre paleoecología y la dinámica del paisaje es clave para entender los procesos del pasado que tuvieron un papel importante en el desarrollo de los ecosistemas y los paisajes ecuatorianos actuales. Sin embargo sólo un número limitado de estudios paleoecológicos de los Andes ecuatorianos están disponibles. En esta tesis se presentan análisis palinológicos que se llevaron a cabo en tres sitios diferentes en la región centro y sur de los Andes de Ecuador, con el fin de comprender mejor los últimos cambios en la vegetación, el clima y la dinámica del fuego; así como el impacto humano desde finales del Pleistoceno. El análisis del núcleo de sedimento El Cristal, ubicado en la ladera oriental de la cordillera Oriental en el Bosque Protector Corazón de Oro en el sureste del Ecuador, revela cambios en la distribución de la vegetación, el clima, el régimen de incendios y el impacto humano desde finales del Pleistoceno. Durante el Pleistoceno tardío el bosque montano fue el principal tipo de vegetación. Especialmente, hay evidencia de bosque de Polylepis lo que no ocurre en la actualidad. Sin embargo pruebas de proporciones relativamente altas de páramo sugiere que un bosque montano abierto ocupó la región. Por otro lado la presencia de taxones de páramo durante el Pleistoceno tardío, propone que la línea superior del bosque se encontraba a una altura inferior. Así mismo, la transición del Pleistoceno tardío al Holoceno temprano y medio se caracteriza por la presencia de bosque montano y una proporción estable de la vegetación de páramo. Sin embargo, después de aproximadamente 4000 cal yr BP, el bosque de Polylepis disminuyó, probablemente debido a un aumento en la frecuencia de incendios. Durante el Holoceno medio y tardío la composición de la vegetación cambió, el bosque montano fue menos frecuente y la vegetación de páramo se expandió. Altas proporciones de Asteraceae y Muehlenbeckia/Rumex (desde ca. 1380 cal yr BP) reflejan alteraciones del paisaje, probablemente por el impacto humano. Además, se registraron incendios durante todo el Pleistoceno tardío, pero fueron más frecuentes durante el Holoceno tardío, esto sugiere que eran de origen antropogénico. Por otro lado, El registro de polen Cajanuma valle, en la ladera occidental de la cordillera Oriental del Parque Nacional Podocarpus, sur de Ecuador, revela los cambios ambientales desde el último Glacial. Durante el último Glacial, páramo herbáceo principalmente dominado por Poaceae, Cyperaceae y Gentianaceae cubrió la zona. La línea superior del bosque se localizó a una altura más baja que la actual. El Holoceno temprano y medio se caracterizó por una sustitución parcial de páramo por bosque montano (Symplocos), el cual cambió su posición a elevaciones más altas donde está actualmente. Durante el Holoceno medio y tardío hay evidencia de un cambio de la vegetación, el páramo se re-expande con el predominio de Poaceae y alta presencia de Huperzia y Cyatheaceae. Durante el Holoceno tardío el páramo fue el principal tipo de vegetación que cubrió la zona. Los incendios se hicieron frecuentes desde el Holoceno tardío. Finalmente, el récord de polen Anteojos valle, que se encuentra en la ladera occidental del Parque Nacional Llanganates, en los Andes ecuatorianos centrales, presenta una reconstrucción ambiental detallada de aproximadamente los últimos 4100 años. La vegetación de páramo tuvo una ocurrencia dominante y estable en el área de estudio (Poaceae, Cyperaceae y Asteraceae); especialmente entre ca. 4100 - 3100 cal yr BP. Entre ca. 3100 - 2100 cal yr BP hubo una disminución de la vegetación de páramo seguido de una ligera expansión del bosque montano (Moraceae/Urticaceae, Trema, Celtis y Macrocarpaea). Desde ca. 2100 cal yr BP hasta la actualidad, la vegetación de páramo una vez más se hizo frecuente con una incidencia estable de los taxones del bosque montano. Se evidenció una baja frecuencia de incendios a lo largo del núcleo de sedimento; sin embargo, es evidente un ligero aumento entre ca. 4100 - 3100 años cal BP.

Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, Discipline of Environmental Biology, 2005? / "September 2004" Bibliography: p. 145-159.