The microclimate of plants growing close to the ground is strongly influenced by the orientation of a soil/vegetation surface with respect to the sun's rays (slope and aspect). In chalk grassland in the UK, slopes of contrasting aspect frequently have distinctive patterns of vegetation. A series of climatic and microclimatic measurements were made at field sites indifferent regions on the English chalk (North Dorset and the Yorkshire Wolds) during the period June 2000 to September 2002. Using digital terrain models (DTMs), process-based models of microclimatic variables at different points in the landscape were developed. The mechanisms through which topography may influence vegetation and species distribution were investigated with field experiments and measurements. Both existing vegetation data from Perring (1956) and new data collected from the field sites were analysed using detrended correspondence analysis (DCA) and generalised additive models (GAMs), to elucidate the relationships between vegetation and soil, topography and climate. A consistent gradient in chalk grassland vegetation was found across spatial scales, associated with the frequency of species with a "stress tolerant” strategy. This gradient in vegetation is apparently driven by species' responses to several separate, but often correlated, variables including soil moisture, maximum summer temperatures and soil fertility. Over the past 50 years, stress tolerant species have declined in frequency at the North Dorset field site, Ellenberg fertility indices have increased and light indices have decreased. The observed changes are consistent with fertilization from atmospheric N deposition and/or relaxation of rabbit grazing after the myxomatosis outbreak in the 1950s. Plots on sloping ground, and in particular, south-west facing slopes, were least affected by these changes, suggesting that high temperatures, phosphorus and water limitation have acted as a buffer against vegetation change, and that complex topography creates refuges for stress tolerant species in the landscape.

Gaps differing in size, form and season of origin (September 1978 and March 1979) were created artificially in adjacent north- and south-facing grasslands in North Derbyshire. Detailed measurements revealed marked differences between the two sites in patterns of vegetative regrowth and regeneration from seed. Specific effects of gap type and time of origin were detected on both slopes. Creation of gaps exerted pronounced effects upon the relative abundance of established components of the north-facing turf. Accumulations of bryophytes were considerably less after three years than in undisturbed turf and Avenula pratensis tended to be reduced in comparison with potentially faster-growing species such as Plantago lanceolata and Holcus lanatus. Effects of comparable magnitude were not induced on the southfacing slope and it is suggested that this may be related to the high level of disturbance normally experienced by the vegetation on this slope. On the north-facing slope where seedlings are comparatively scarce in undisturbed turf, gap creation promoted spring germination in species which maintain relatively persistent seed reserves at or near the soil surface. In marked contrast on the south-facing slope, large numbers of seedlings appeared in the autumn in both gaps and control areas. Such patterns are consistent with previous laboratory studies which revealed differences in germination requirements between the species concerned. On the south-facing slope seedling survival was often highest in areas where resprouting vegetative shoots were relatively dense. However, the benefit of this apparent 'shelter effect' rarely extended to seedling size which tended to decline in close proximity to vigorous vegetative shoots. In the concluding section of the thesis, the main results of the investigation are reviewed in relation to the influence of aspect upon vegetation structure, current theories on the determinants of 'species richness' and opportunities to control species composition by more informed grassland management.

Large scale drainage and pasture improvement in the past and present-day neglect of the floristically diverse wet grasslands in south west England have caused a significant decrease, approximately 92% since 1900. in the total area of this ecologically valuable semi-natural plant community. The plant community, which is locally called Culm grassland, consists of acid purple moor grassland, soft rush meadows and wet heaths. Conservation and restoration of these grasslands requires careful planning and efficient use of resources. This study was aimed at developing strategies for regional planning authorities and nature conservation agencies and developed a management tool for the selection of potential Culm grassland restoration locations and for the prediction of species composition based on the environmental characteristics of a site. Three separate sections of the research could be distinguished. The first section studied the relationships between the wet grassland location and the landscape topography, catchment hydrology and soil physical characteristics, with the use of Geographical Information Systems. Landscape topography was expressed as the ln(a/tan^) topographic index, in which 'a' is the upslope area draining through the point for which the index is calculated and 'fi' is the local slope angle. Culm grassland sites were generally found on positions with a topographic index larger than eight. A quantification of soil saturation periods was carried out by modelling the surface water dynamics with the hydrological model TOPMODEL, which was based mainly on the topographic index. Soil hydrological characteristics were described by applying the Hydrology Of Soil Types (HOST) classification to the soil map of the area. Results indicated that Culm grassland was mainly associated with poorly drained soils and topographic hollows on level or gently sloping grounds. The second section investigated the species composition In relation to environmental parameters and grazing regime. A field study involving collection of vegetation, soil and site data was carried out on existing Culm grassland sites. Multivariate statistical techniques were applied to relate vegetation gradients to the environmental parameters. Results showed that grazing pressure, soil pH and soil water were the factors most responsible for species composition within the Culm grassland communities. The third section integrated the results into a decision support system, which indicated where potentially suitable restoration sites were located and the species composition that could develop based given the environmental parameters. The procedure was tested by application of the decision rules to an independent area and comparison of the potentially suitable sites to historical data, field observations and land use information. From this study, given readily obtainable soil and topographic data a, good first selection of areas for further Culm grassland development could be made. However, relationships between vegetation and environmental parameters will need more detailed field investigation to obtain completely reliable results.

No description available.

Thesis (M. S. - Soil, Water, and Environmental Science)--University of Arizona, 2002. / Includes bibliographical references (leaves 50-52).

Thesis (M. Ag. Sci) -- University of Adelaide, Dept. of Agronomy, 1964. / [Typescript].

Thesis (Ph. D.)--University of Wisconsin--Madison, 1949. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves [i]-iii).

Thesis (Ph. D.)--University of California, Davis, 2005. / "June 2005"--Leaf 1. Degree granted in Ecology. Includes bibliographical references. Also available via the World Wide Web. (Restricted to UC campuses)

Thesis (Ph.D.)--University of Iowa, 2006. / Supervisor: Frank H. Weirich. Includes bibliographical references (leaves 152-160)

Thesis (M.S. in crop science)--Washington State University, August 2010. / Title from PDF title page (viewed on July 30, 2010). "Department of Crop and Soil Sciences." Includes bibliographical references.