Thesis (M.S.)--University of Wisconsin--Madison, 1966. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph.D.)--RAND Graduate School, 2007. / Includes bibliographical references.

No description available.

Northern Africa has experienced major shifts towards aridity and extensive C4 vegetation over the late Cenozoic, but due to a scarcity of spatially and temporally extensive paleoenvironmental records, the timing, patterns, and causes of these shifts are still under debate. Both long-term aridification and large amplitude orbital-scale climate variability have been recognized, with little understanding of how these two patterns relate to each other over time. African’s climate and environmental history of the last 7 Myr is of particular interest because hydrological and vegetation variability is considered the driving selection mechanism for human evolution. In addition, the age of the initiation of desert conditions in the modern Sahara desert, Earth’s largest warm desert and the largest source of dust to the modern atmosphere, is unknown. The stable isotope ratios of carbon and hydrogen in sedimentary plant leaf wax biomarker compounds have recently been shown to quantitatively track source vegetation photosynthetic pathways and the hydrogen isotope composition of plant source water, which is dominantly controlled by the amount of precipitation in Africa. These proxies have been applied to reconstruct long-term vegetation changes in East Africa and SW Africa over the last 14 Ma, as well as orbital-scale variability from various locations around the African continent, but they have not been extended further back in time or combined in tandem to robustly assess both long-term and orbital-scale climate and vegetation variability and how they relate to each other. In this thesis, I have utilized quantitative plant leaf wax stable isotope proxies to examine both orbital-scale and long-term changes in North African aridity and vegetation from a variety of regions over the last 25 Ma, with particular emphasis on the last 4.5 Ma. In Chapter 2, I investigated the evolution of hydrological and vegetation gradients from the equator to the sub-Sahara in NW Africa over the last 25 Myr using leaf wax stable isotopes at two marine sediment core locations, producing the longest existing leaf wax stable isotope record in Africa to my knowledge, and one of the longest such records globally. In this study I found that NW African environments were remarkably similar at both latitudes from 25 – 10 Ma, but at 10 Ma C4 vegetation abruptly expanded in the north, indicating sudden aridification in the Sahara region at that time. The hydrogen isotope record was stable long-term, with variability similar to that of known orbital-scale cyclicity in the Pliocene and Pleistocene, possibly suggesting that orbital-scale cyclicity or other factors obscured or were larger than any long-term changes in the hydrogen isotope ratio of precipitation. Saharan aridification at 10 Ma is consistent with climate model predictions of aridity due to the closure of the Tethys Seaway connection between the Indian Ocean and Mediterranean Sea near that time. The 10 Ma expansion in C4 vegetation is earlier than most other regions globally. To examine long-term changes in orbital-scale variability in the Eastern Sahara and Mediterranean Sea, I constructed a record of eastern Mediterranean sedimentary leaf wax carbon and hydrogen isotopes, leaf wax abundance, lignin biomarkers, and oxygen isotope ratios of planktonic foraminifera G. ruber during two 100-kyr periods of equal eccentricity near 3.0 and 1.7 Ma (Chapter 3). I found that precession-scale variability dominates the record during both periods, and Eastern Saharan precipitation and the vegetation assemblage, which was C4-dominated, do not change on average between the two periods. Chapter 4 extended the eastern Mediterranean record of Chapter 3 by sampling leaf wax stable isotopes in sapropel sediments (deposited during North African humid periods) at ~0.25 Myr resolution back to 4.5 Ma, placing the orbital-scale Chapter 3 results in long term context. I found that Eastern Saharan environments were persistently C4-dominated (>68%) throughout the entire interval, and that long-term hydrogen and carbon variability were similar in magnitude to orbital-scale cycles back to 4.5 Ma, strongly indicating that orbital-scale variability has been the dominant environmental control in NE Africa since the early Pliocene. This record contrasts sharply with observations of a transition from C3-C4 mixed vegetation to abundant C4 grasslands in East Africa over the same period of time. The results may suggest that long-term precipitation shifts did not occur in NE Africa since the Pliocene, or that the resolution of this approach is not sufficient to detect long-term shifts. It is likely that NW Africa also experienced similarly large hydrological variability over the same period of time, which may explain the unclear long-term hydrological signal in Chapter 2. The results emphasize that East Africa has not been representative of northern Africa as a whole since the Pliocene.

Thesis (doctoral)--Swedish University of Agricultural Sciences, 2005. / Thesis documentation sheet inserted. Appendix reprints five journal articles and manuscripts co-authored with others. Includes bibliographical references. Also issued electronically via World Wide Web in PDF format; online version lacks appendix.

The world we live in is changing rapidly. Ecological, economic and social aspects and understandings are all undergoing paradigm shifts. Communities, farmers and individuals in arid zones are experiencing climate changes, more so than city dwellers. A better understanding of the current thinking in range ecology and management, especially of arid environments, is critical to the management of these delicate, complex systems. Ecosystems in equilibrium or in disequilibrium react differently to management and reclamation efforts. An understanding of the basic principles and how they evolved is important in order to apply these principles correctly in the management of arid zones. The use of keystone species and simple technologies, such as water harvesting and mulching, can all be used to reclaim and manage the arid zones. Understanding the differences between systems in equilibrium and disequilibrium can be used as a guide for planning appropriate future research in the arid zones. Searching for new indigenous species to help in the reclamation of arid zones is of the utmost importance. An ecological criterion was used to identify potential plant species for reclamation of degraded arid rangelands of southern Africa. Tripteris sinuatum and Sutherlandia microphylla were identified as possible candidates. Germination studies, with seeds harvested from naturally occurring plants, were conducted for both species. Treatments were based on the natural seed dispersal mechanisms for both species. S. microphylla has the potential to become an important plant species for reclamation purposes in arid zones of southern Africa. Not only potential new species should be sought, but also the management of species, already in use, are of critical importance. The relative palatability and survival of 16 different Atriplex species and accessions were determined at two different localities in the arid Northern Cape Province of South Africa. Significant differences were found between species at both localities. It is believed that because of the variety of species in the Atriplex genus, relative palatability and survival should be used to determine which of the different species could be useful under specific climatic and soil conditions. The establishment and reaction of Atriplex nummularia and Cassia sturti were tested for season of planting and the use of a stone mulch at two locations in the Northern Cape Province of South Africa. A. nummularia reacted the best to a moderate pruning treatments, while Cassia sturtii reacted best to severe pruning. These results should aid in the management of planted fodder plantations. Numerous attempts at improving natural veld have failed in the past. An examination of landscape function and the potential to harvest water in localized areas should drive veld improvement in arid zones. Seeds from two different plants species (Tetragonia calycina and Tripteris sinuatum) were used to inter-seed a bare patch in the Northern Cape Province of South Africa. The two species, with two treatments, (brush packing or not) in two different locally occurring eco-topes, were used to determine plant establishment. The water run-on eco-tope showed a significantly higher plant establishment percentage than the water run-off eco-tope. The establishment of perennial grass species was also found on the water run on eco-tope, three years after establishing the reclamation site. Such sites could form an important link in biodiversity conservation. / Dissertation (MSc Agric (Pasture Science))--University of Pretoria, 2007. / Plant Production and Soil Science / unrestricted

No description available.

A simple model of surface and sub -surface soil temperature was developed at the watershed scale ( -100 km2) in a semi -arid rangeland environment. The model consisted of a linear combination of air temperature and net radiation and assumed: 1) topography controls the spatial distribution of net radiation, 2) near- surface air temperature and incoming solar radiation are relatively homogeneous at the watershed scale and are available from ground stations and 3) soil moisture dominates transient soil thermal property variability. Multiplicative constants were defined to account for clear sky diffuse radiation, soil thermal inertia, an initially fixed ratio between soil heat flux and net radiation and exponential attenuation of solar radiation through a partial canopy. The surface temperature can optionally be adjusted for temperature and emissivity differences between mixed hare soil and vegetation canopies. Model development stressed physical simplicity and commonly available spatial and temporal data sets. Slowly varying surface characteristics, such as albedo, vegetation density and topography were derived from a series of Landsat TM images and a 7.5" USGS digital elevation model at a spatial resolution of 30 m. Diurnally variable atmospheric parameters were derived from a pair of ground meteorological stations using 30 -60 min averages. One site was used to drive the model, the other served as a control to estimate model error. Data collected as part of the Monsoon '90 and WG '92 field experiments over the ARS Walnut Gulch Experimental. Watershed in SE Arizona were used to validate and test the model. Point, transect and spatially distributed values of modeled surface temperature were compared with synchronous ground, aircraft and satellite thermal measurements. There was little difference between ground and aircraft measurements of surface reflectance and temperature which makes aircraft transects the preferred method to "ground truth" satellite observations. Mid- morning modeled surface temperatures were within 2° C of observed values at all but satellite scales, where atmospheric water vapor corrections complicate the determination of accurate temperatures. The utility of satellite thermal measurements and models to study various ground phenomena (eg. soil thermal inertia and surface energy balance) were investigated. Soil moisture anomalies were detectable, but were more likely associated with average near -surface soil moisture levels than individual storm footprints.

A simple model of surface and sub-surface soil temperature was developed at the watershed scale (-100 km²) in a semi-arid rangeland environment. The model consisted of a linear combination of air temperature and net radiation and assumed: (1) topography controls the spatial distribution of net radiation, (2) near-surface air temperature and incoming solar radiation are relatively homogeneous at the watershed scale and are available from ground stations and (3) soil moisture dominates transient soil thermal property variability. Multiplicative constants were defined to account for clear sky diffuse radiation, soil thermal inertia, an initially fixed ratio between soil heat flux and net radiation and exponential attenuation of solar radiation through a partial canopy. The surface temperature can optionally be adjusted for temperature and emissivity differences between mixed bare soil and vegetation canopies. Model development stressed physical simplicity and commonly available spatial and temporal data sets. Slowly varying surface characteristics, such as albedo, vegetation density and topography were derived from a series of Landsat TM images and a 7.5" USGS digital elevation model at a spatial resolution of 30 m. Diurnally variable atmospheric parameters were derived from a pair of ground meteorological stations using 30-60 min averages. One site was used to drive the model, the other served as a control to estimate model error. Data collected as part of the Monsoon '90 and WG '92 field experiments over the ARS Walnut Gulch Experimental Watershed in SE Arizona were used to validate and test the model. Point, transect and spatially distributed values of modeled surface temperature were compared with synchronous ground, aircraft and satellite thermal measurements. There was little difference between ground and aircraft measurements of surface reflectance and temperature which makes aircraft transects the preferred method to "ground truth" satellite observations. Mid-morning modeled surface temperatures were within 2° C of observed values at all but satellite scales, where atmospheric water vapor corrections complicate the determination of accurate temperatures. The utility of satellite thermal measurements and models to study various ground phenomena (e.g. soil thermal inertia and surface energy balance) were investigated. Soil moisture anomalies were detectable, but were more likely associated with average near-surface soil moisture levels than individual storm footprints.

Research was conducted over a 2-year period at the University of Arizona Campus and Oracle Agricultural Centers to evaluate the effectiveness of surface imprintation in revegetating arid lands. Introduction of forage leguminous species into arid rangelands through land imprintat ion was another objective of this study. The soil at the Campus Center is a Brasito, mixed thermic, typic torripsamment with a sandy-loam texture. This was compared with a White House, fine mixed thermic, Ustollic haplargid with a sandyloam texture at the Oracle Center. Natural rains were the only source of irrigation at Oracle. At the Campus Center, however, a sprinkler irrigation system was installed to match rains with that at the Oracle Center. Three cover treatments together with four surface treatments were used at both sites. The cover treatments included a pure stand of grasses, a pure stand of legumes, and a mixture of both grasses and legumes. The surface treatments were imprinted, mulched, imprinted-mulched, and an untreated surface as a check. Surface imprintation was performed by a land imprinter at Oracle and by a hand imprinter at Campus. The imprinted surface significantly increased soil moisture retention, number of plants per unit area, plant height, plant cover, and biomass compared to the untreated surface. At the Oracle Center, the imprinted surface improved legume germination by 800% over the untreated surface, and by 367% over the mulched one. Corresponding percentages at Campus were 48 and 4% over the untreated and the mulched surfaces, respectively. Increases in biomass production achieved through surface imprintation were 102% over the untreated surface and 35% over the mulched surface at the Oracle Center. Corresponding increases at Campus were 63 and 33% over the untreated and the mulched surfaces, respectively. Plants grown on imprinted surfaces exhibited higher transpiration rates, lower diffusive resistance, and cooler leaf temperature compared to those grown on the untreated surfaces. Addition of mulch to the imprinted surface made no significant differences with respect to the parameters measured when compared to the imprinted surface without mulch. When mulch was used as a separate treatment, however, it significantly increased the parameters measured over the untreated surface. The effect of cover treatments on growth parameters and biomass production was masked by seasonality. Grasses tended to be superior over legumes in samples taken during the fall and the opposite was true during the summer. Mixing legumes with grasses, however, resulted in significantly taller grasses compared to grasses grown as a pure stand.