Thesis (Ph.D.)--University of the Witwatersrand, Faculty of Science, 1997.

Thesis (M.A.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 89-99). Also available on the World Wide Web.

Vegetation change, particularly from the grass to shrub lifeform, is a critical issue on the world's rangelands. The plant community present on a site is the primary determinant of the land's value for watershed protection, wildlife habitat, livestock production, and recreation. Studying past vegetation composition can help separate natural from anthropogenic sources of change and guide natural resource conservation and management decisions. Stable carbon isotope (δ¹³C) values and associated radiocarbon ages from soil organic matter (SOM) were used to evaluate vegetation change across five landscape positions at a small enclosed basin in southeastern Arizona. The utility of the carbon isotope method was verified for this site based on the clear and wide separation in δ¹³C values between grasses having the C₄ photosynthetic pathway and shrubs having the C₃ pathway. The direction and timing of vegetation dynamics differed with landscape position along a gentle elevation gradient from the basin outlet to a nearby volcanic ridge top. Warm-season C₄ perennial grasses have dominated the basin outlet, center, and toe slope landscape positions since at least 5000-6000 yr BP, except for a dramatic increase in C₃ plants at the bottom of the outlet excavation around 5000 yr BP. This isotopic change is associated with rounded cobbles that may have been a stream channel, suggesting the presence of C₃ herbaceous or woody riparian vegetation. On mid-slope and ridge top landscape positions, where semidesert shrubs now dominate, warm-season perennial grass, composition decreased from approximately 60% as recently as 400 yr BP to only 1.5% now. SOM density separates were also analyzed. The youngest SOM is represented by the <2 g/cm³ density fraction that turns over in a few years to several decades and has a post-bomb radiocarbon age. For the ridge top landscape position, this fraction yielded 39% C₄ vegetation, suggesting that the conversion from grass to shrub vegetation occurred recently.

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The Campbellton Formation, ~1 km thick, has long been recognized for early plants, arthropods, and fishes. The sedimentology of two basins is described with six facies associations to provide a framework for the fossil assemblages. The western basin contains aquatic fauna and coastal flora within a basal rhyolite breccia and coastal deltaic strata, and a braided river facies association is also present. Lower eastern basin strata primarily comprise lacustrine facies, where a thick marginal association deposited by sediment-laden underflows contains a lake-side plant assemblage. Other lacustrine associations include a fine-grained association in areas with restricted circulation, and a near-shore association. Upwards, a plant-rich fluvial landscape had wide sandstone and conglomerate channels and high-diversity wetlands containing terrestrial arthropods. A proximal alluvial facies association with hyperconcentrated flows contains plants and Prototaxites, possibly from adjoining uplands. All plants groups were well-established throughout the formation, with no apparent landscape partitioning between groups.

Measurements of surface uplift rate potentially hold the key to understanding the tectonics of mountain belts and areas of uplift. Wolfe (1993) compiled a multicharacter data set (CLAMP) of the physiognomy of woody dicotyledon leaves with a view to using it to extract climatic information from fossil floras. The limited extent of this data set, along with the anomalous behaviour of some outliers cause me to question the global relationship between physiognomy and climate which has been implicitly assumed in all analyses of the data conducted to date. Additional data collected from native vegetation in New Zealand and Australia are compared to the CLAMP data set. These data include samples along altitudinal transects and from different forest types growing in the same climatic regime. In addition taphonomic samples were collected from lake bottom sediments and their physiognomic signals compared to those of the adjacent living vegetation. The possibility that the relationship between climate and physiognomy is sufficiently non-linear that only local relationships should be sought is investigated. To estimate the climate at a certain flora, resemblance functions are used to select physiognomically similar sites. Estimations of climate are formed using only these sites. The power of this approach to estimate mean annual temperature, mean annual precipitation, mean growing season precipitation, and moist enthalpy is investigated using the modern sites, and varying the number of nearest neighbours and dimensions used as well as the type of ordination. The collection of altitudinal transects has allowed the study of physiognomic change with altitude. Because these transects were collected over a very restricted area it was possible to observe this change without the superimposed effects of changing continentality and variation in latitude. In addition studies made of adjacent floras in similar climatic regimes and taphonomic studies of leaves in lacustrine sediments has allowed the beginning of a realistic assessment of possible errors in climate estimation for fossil sites. Fossil sites examined using CLAMP and related methods are re-examined using the nearest neighbour approach.

The aim of work presented in this thesis was to explore the inter-relationships of cyclic sedimentation, relative sea-level change, and palaeoclimate as inferred from plant megafossils. To this end, the investigation focused on the classic plant-bearing Middle Jurassic succession of Yorkshire. The Middle Jurassic (Aalenian-Bathonian) Ravenscar Group of the Cleveland Basin (Yorkshire) comprises a predominantly fluvio-deltaic succession intercalated between thinner, laterally persistent marine units. There is a pronounced lateral facies change across the basin, from mainly alluvial sediments in the north to more marine deposits in the south. Although variable in character, the facies composing the sequence are described by four principal environments of deposition: alluvial, estuarine, lagoonal and marine. In an attempt to achieve a more accurate stratigraphic control on the succession, sequence-stratigraphic concepts are applied to outcrop exposures and subsurface cores. The identification of 'key surfaces' in the sequence resolves a series of lithological cycles which reflect relative sea-level fluctuations. Using this approach the Aalenian-Bathonian sequence can be subdivided into two large-scale (second-order) transgressive-regressive cycles onto which six medium-scale (thirdorder) cycles of transgression and regression are superimposed. The potential for correlating these lithological cycles regionally has been assessed by comparing coeval sections from southern Scandinavia. Plant-bearing fluvio-deltaic sequences from Bornholm and Scania were chosen as a means for appraising the lateral continuity of the cycles, and assessing what factors might have controlled their development. Study of floral remains from the Ravenscar Group within the context of this stratigraphic framework yields valuable palaeoclimatic information. Growth-ring analysis of fossil wood of Late Pliensbachian to Late Bathonian age indicates a distinctly seasonal climate with low to moderate interseasonal variation in tree growth. Significant intraseasonal influences on wood production are implicit in the abundance of false rings. Consideration of these results within a stratigraphic context suggests that conditions during the Bathonian were comparatively hostile: a finding which is interpreted to be due to more frequent and extended water shortages associated with a drier climate. These palaeoclimatic inferences are substantiated by evidence obtained from the examination of the flora using Correspondence Analysis (CA). This approach verifies the presence of a temporal fluctuation in the flora found by previous investigations and, furthermore, highlights physiognomic trends in the flora with time. The results from CA also indicate adverse growing conditions during the Bathonian, emphasized by the prevalence of xeromorphic taxa.

Early Palaeocene floras from twenty seven sites within the Raton, southern Powder River and south-western Williston Basins of the western interior of North America were collected, and their leaf physiognomy, ecological character and depositional setting compared. Such a spread of samples enabled the study of spatial and temporal vegetational and climatic variations in the region, following the Cretaceous-Tertiary boundary event. Climatic changes are observed across the Cretaceous-Tertiary boundary. Precipitation increased dramatically, and remained relatively high throughout the earliest Palaeocene. Temperatures were somewhat lower, compared to those of the Late Cretaceous, and seasonality in climate increased. Climatic and vegetation zones shifted southwards as latitudinal climatic equability decreased. Palaeotemperature and palaeoprecipitation were determined using CLAMP and leaf margin analysis. Experiments carried out to assess the robustness of CLAMP to loss of foliar physiognomic data revealed that this data loss did not drastically effect palaeoclimatic determinations but that information about leaf size and margin type had the most effect on results. Vegetation was of low diversity directly after the boundary event, but recovered to stable, but still relatively low levels, within a short time. Changes in diversity are difficult to interpret due to masking by taphonomic biases, which are important within the depositional environments analysed in this study. Climatic deterioration and the prevalence of disturbed environments ultimately facilitated expansion of the angiosperms, although their aspect was changed with a general increase in deciduous forms, in relation to increased seasonality and decreased equability. These trends cannot be related merely to the impact of a bolide at the Cretaceous-Tertiary boundary, but reflect the more global and wide-ranging changes of the period, which were punctuated by this brief, deleterious event. Previous work has tended to concentrate on the North American continent but a more global perspective reveals that the Cretaceous-Tertiary boundary event was not a world-wide catastrophe within terrestrial environments.

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