In the 400-600 mm annual rainfall zone of the Sahel, soil fertility is the main determinant of yield in rainfed millet cropping systems in all but the driest years. Numerous on-farm and on-station experiments have addressed the issue of improving soil fertility. Yet the widespread use of the experimental results is restricted by the highly site specific millet response to fertility management practices due to high spatially variable soil properties as well as high intra- and inter-annual rainfall variability. Mathematical soil-crop growth simulation models could therefore suitably complement experimental research to support decision making regarding soil fertility under variable rainwater supply conditions. The objective of this thesis was therefore to develop the biophysical basis for the use of crop-soil models in decision support regarding water and soil fertility management and risk mitigation strategies in rainfed millet-based systems of Sahelian Niger. Because farmers rely on multiple cultivars with variable length of growing cycle due to sensitivity to temperature and photoperiod as part of their risk management strategies we first characterized seven Sahelian millet genotypes and parameterized the Agricultural Production Systems Simulator (APSIM-millet model). The cultivars include three improved cultivars (CIVT, ICMV-IS-89305, ZATIB) and four landraces (Ankoutes, Hainikirey, Maewa and Zongo). Our research showed that only one of the cultivars, Maewa, was very photosensitive contrary to the six others. The majority of the agronomic state variables (leaf number, leaf area, biomass and grain yield) were negatively affected by late sowing (associated with lower air temperatures). This characterization enabled to compute for the first time in the Sahel the principal eco-physiological or genetic millet parameters (thermal times of development phases, leaf area dynamics) of crop growth models (e.g. APSIM, DSSAT). To gain confidence in the use of the APSIM model for decision support in the Sahelian environment, it was successfully tested to reproduce the agronomic state variables under non-limiting water and nutrient supply conditions. Moreover the APSIM model satisfactorily reproduced the millet CIVT cultivar response to water x N interaction from the combined application of crop residue, cattle manure and mineral fertilizer during two years and for contrasted rainfall conditions. Using the model with site and cultivar specific parameterization, we implemented two applications for decision support. A 23-year, long term factorial numerical experiment showed that a moderate N application of 15 kg N/ha is more appropriate for smallholder, subsistence farmers than the usual 30 kg N/ha recommendation. Although it implies a lower long term average yield than at 30 kg N/ha, the application of 15 kg N/ha guarantees both a higher minimum yield in extreme dry years and a lower inter-annual variability, thereby increasing food security and reducing farmers vulnerability. In the second model application, we integrated GIS information (land tenure, spatially distributed weather data, fertility management) and the APSIM model in a 12-year yield simulation to show that the spatial dispersion of fields of a household throughout the village territory (farmer risk management strategy) leads to more uniform yields across households and reduces the inter-annual yield variability in the Fakara region of Niger. Our research breaks the ground for several other applications of the use of crop-soil simulation models in millet-based systems in the Sahel, e.g. climate change impact and food crisis mitigation. / Dans la zone Sahélienne avec 400 à 600 mm de précipitation annuelle, la fertilité des sols est le principal facteur déterminant des rendements du mil pluvial hormis lors des années plus sèches. De nombreuses expérimentations au champ et en station ont abordé la question de l’amélioration de la fertilité des sols. Cependant, l’extrapolation de ces résultats, et a forciori leur utilisation par les agriculteurs, est limitée par le fait que la réponse du mil à ces pratiques de fertilité dépend fortement des propriétés des sols très variables dans l’espace ainsi que de la pluviométrie annuelle et sa répartition intra-annuelle. Les modèles mathématiques et dynamiques de simulation de la croissance des plantes peuvent donc utilement compléter la recherche expérimentale pour l’aide à la décision en ce qui concerne la gestion de la fertilité des sols dans diverses conditions d’alimentation hydrique. L’objectif de cette thèse était donc de développer les bases biophysiques pour l’utilisation de modèles de croissance des cultures en vue de leur utilisation comme outils d’aide à la décision en matière de gestion de l’eau et la fertilité des sols dans les systèmes de culture à base de mil en zone sahélienne du Niger. Puisque les paysans utilisent de multiples variétés de mil avec des cycles de croissance variables en tant qu’élément dans leurs stratégies de gestion des risques, nous avons d’abord caractérisé sept génotypes de mil Sahelien en vue de la paramétrisation du model dynamique APSIM (Agricultural Production Systems Simulator). Trois variétés améliorées (CIVT, ICMV-IS-89305, ZATIB) et quatre variétés locales paysannes (Ankoutes, Hainikirey, Maewa et Zongo) ont été étudiées. Notre recherche a montré que seule une des variétés, l’écotype Maewa, est très photosensible contrairement aux six autres. La majorité des variables agronomiques (nombre de feuille, surface foliaire, biomasse et rendement en grain) ont été négativement affectées par un semis tardif (associés à des températures de l’air plus faibles). Cette caractérisation a permis de calculer pour la première fois au Sahel les principaux paramètres éco-physiologiques du mil (durée thermique des phases de développement, dynamique de la surface foliaire), indispensables aux modèles dynamiques de croissance des plantes tels qu’APSIM et DSSAT. Le modèle APSIM a permis de reproduire avec succès les variables agronomiques de 6 des 7 variétés de mil en condition nonlimitante d’apport en eau et en nutriments. De plus, le modèle APSIM a reproduit de manière satisfaisante la réponse de la variété améliorée CIVT à l’interaction de l’eau et de l’azote suite à l’apport combiné de résidus de récolte, de fumier de bétail et d’engrais minéral sur deux années ayant des pluviométries contrastées. Ceci a permis de renforcer la confiance dans l’utilisation du modèle APSIM comme outil d’aide à la décision dans l’environnement Sahélien. Sur base du modèle APSIM ainsi paramétrisé pour des conditions spécifiques de site et de variété, nous avons développé deux applications en matière d’aide à la décision. Une expérimentation factorielle numérique à long terme (23 ans) a révélé que l’application d’une dose modérée d’azote (15 kg N/ha) est plus appropriée dans le contexte d’une agriculture de subsistance que la recommandation habituelle de 30 kg N/ha. Bien qu’elle implique un rendement moyen à long terme inférieur à celui obtenu avec 30 kg N/ha, l’application de 15 kg N/ha garantit un plus grand rendement minimum pendant les années sèches extrêmes et une variabilité inter-annuelle plus faible, ce qui permet de garantir une meilleure sécurité alimentaire tout en réduisant la vulnérabilité des paysans. Dans la deuxième application du modèle, nous avons intégré des données SIG (parcellaire villageois, données climatiques et de pratiques de gestion de fertilité distribuées dans l’espace) et le modèle APSIM dans une simulation de rendement de mil sur 12 années. Ceci a permis de montrer que la dispersion spatiale des champs d’un ménage dans le terroir villageois (stratégie paysanne de gestion du risque) permet l’obtention de rendements plus uniformes entre ménages au sein du même village et de réduire la variabilité inter-annuelle des rendements de chaque ménage dans la région de Fakara au Niger. Notre recherche ouvre la voie à plusieurs autres applications de l’utilisation des modèles dynamiques de croissance des plantes dans les systèmes à base de mil au Sahel, par exemple dans l’étude de l’impact des changements climatiques et de prévention des crises alimentaires.

Organohalide-respiring Bacteria (OHRB) play key roles in the reductive dehalogenation of natural organohalides and anthropogenic chlorinated contaminants. Reductive dehalogenases (RDases) catalyze the cleavage of carbon-halogen bonds, enabling respiratory energy conservation and growth. Large numbers of RDase genes, a majority lacking experimental characterization of function, are found on the genomes of OHRB. In silico genomics tools were employed to identify shared sequence features among RDase genes and proteins, predict RDase functionality, and elucidate RDase evolutionary history. These analyses showed that the RDase superfamily could be divided into proteins exported to the membrane and cytoplasmic proteins, indicating that not all RDases function in respiration. Further, Hidden Markov models (HMMs) and multiple sequence alignments (MSAs) based upon biochemically characterized RDases identified previously uncharacterized members of an RDase superfamily, delineated protein domains and amino acid motifs serving to distinguish RDases from unrelated iron-sulfur proteins. Such conserved and discriminatory features among RDases may facilitate monitoring of organohalide-degrading microbial communities or improve accuracy of genome annotation. Phylogenetic analyses of RDase superfamily sequences provided evidence of convergent evolution and horizontal gene transfer (HGT) across distinct OHRB genera. Yet, the low frequency of RDase transfer outside the genus level and the absence of RDase transfer between phyla indicate that RDases evolve primarily by vertical evolution or HGT is restricted among related OHRB strains. Polyphyletic evolutionary lineages within the RDase superfamily comprise distantly-related RDases, some exhibiting activities towards the same substrates, suggesting a longstanding history of OHRB adaptation to natural organohalides. Similar functional and phylogenetic analyses provided evidence that nitrous oxide (N₂O, a potent greenhouse gas) reductase (nosZ) genes from versatile OHRB members of the Anaeromyxobacter and Desulfomonile genera comprised a nosZ sub-family evolutionarily distinct from nosZ found in non-OHRB denitrifiers. Hence, elucidation of RDase and NosZ sequence diversity may enhance the mitigation of anthropogenic organohalides and greenhouse gases (i.e., N₂O), respectively. The tetrachloroethene-respiring bacterium Geobacter lovleyi strain SZ exhibited genomic features distinguishing it from non-organohalide-respiring members of the Geobacter genus, including a conjugative pilus transfer gene cluster, a chromosomal genomic island harboring two RDase genes, and a diminished set of c-type cytochrome genes. The G. lovleyi strain SZ genome also harbored a 77 kbp plasmid carrying 15 out of the 24 genes involved in biosynthesis of corrinoid, likely related to this strains ability to degrade PCE to cis-DCE in the absence of supplied corrinoid (i.e., vitamin B₁₂). Although corrinoids are essential cofactors to RDases, the strictly organohalide-respiring Dehalococcoides mccartyi strains are corrinoid auxotrophs and depend upon uptake of extracellular corrinoids via Archaeal and Bacterial salvage pathways. A key corrinoid salvage gene in D. mccartyi, cbiZ, occurs at duplicated loci adjacent to RDase genes and appears to have been horizontally-acquired from Archaea. These comparative genome analyses highlight RDase dependencies upon corrinoids and also suggest mobile genomic elements (e.g., plasmids) are associated with organohalide respiration and corrinoid acquisition among OHRB. In summary, analyses of OHRB genomes promise to enable more complete modeling of metabolic and evolutionary processes associated with the turnover of organohalides in anoxic environments. These efforts also expand knowledge of biomarkers for monitoring OHRB activity in anoxic environments, and will improve our understanding of the fate of chlorinated contaminants.

The anthropogenic introduction of species into new ecosystems is a global phenomenon, and identifying the mechanisms by which some introduced species become dominant in their introduced ranges (i.e., invasive) is crucial to predicting, preventing, and mitigating the impacts of biological invasions. Introduced perennial C₄ grasses are invading semi-arid grassland and savanna ecosystems throughout the south-central U.S. We hypothesized that in these semi-arid ecosystems, where variable precipitation patterns strongly influence vegetation dynamics, the success of an invasive plant species may be due in part to ecophysiological traits that enable high performance in response to unpredictable water availability. We also hypothesized that increased primary productivity and decreased plant input quality associated with these grass invasions have the potential to alter ecosystem carbon and nitrogen cycling and storage by altering the ratio of inputs (productivity) to outputs (decomposition/respiration). We tested the first hypothesis by quantifying ecophysiological performance differences between an invasive C₄ grass, Bothriochloa ischaemum, and co-occurring C₃ and C₄ native grasses under wet and dry conditions in the field and under two levels of simulated precipitation frequencies in a greenhouse experiment. We tested the second hypothesis by examining whether increased primary productivity and decreased C₃:C₄ grass ratios in savanna grass-matrices associated with B. ischaemum invasion altered (1) plant input quality and thus nutrient cycling and/or (2) net ecosystem carbon uptake in invaded areas. B. ischaemum's success as an invader was not directly related to its ability to cope with precipitation variability and availability, but its ability to rapidly produce large amounts of biomass may allow it to directly out-compete native species. B. ischaemum invasion decreased plant input quality and soil nitrogen availability. B. ischaemum invasion shifted ecosystem C-uptake from being nearly year-round to occurring predominantly in the summer. Greater C-uptake during the summer and under drier conditions compensated for a shorter growing seasons in B. ischaemum-invaded areas and cumulative annual NEE was similar between invaded and native-dominated areas. We conclude that B. ischaemum's impacts on soil nitrogen availability and plant-canopy microhabitat may allow it to exclude native species from invaded areas, but that its impacts on ecosystem C sequestration may be small. / text

Shallow lakes are an important ecological and socio-economic resource. However, the impact of human pressures, both at the lake and catchment scale, has precipitated a decline in the ecological status of many shallow lakes, both in the UK, and throughout Europe. There is now, as direct consequence, unprecedented interest in the assessment and monitoring of ecological status and trajectory in shallow lakes, not least in response to the European Union Water Framework Directive (2000/60/EC). In this context, the spatially-resolving and panoramic data provided by remote sensing platforms may be of immense value in the construction of effective and efficient strategies for the assessment and monitoring of ecological status in shallow lakes and, moreover, in providing new, spatially-explicit, insights into the function of these ecosystems and how they respond to change. This thesis examined the use of remote sensing data for the assessment of (i) phytoplankton abundance and species composition and (ii) aquatic vegetation distribution and ecophysiological status in shallow lakes with a view to establishing the credence of such an approach and its value in limnological research and monitoring activities. High resolution in-situ and airborne remote sensing data was collected during a 2-year sampling campaign in the shallow lakes of the Norfolk Broads. It was demonstrated that semi-empirical algorithms could be formulated and used to provide accurate and robust estimations of the concentration of chlorophyll-a, even in these optically-complex waters. It was further shown that it was possible to differentiate and quantify the abundance of cyanobacteria using the biomarker pigment C-phycocyanin. The subsequent calibration of the imagery obtained from the airborne reconnaissance missions permitted the construction of diurnal and seasonal regional-scale time-series of phytoplankton dynamics in the Norfolk Broads. This approach was able to deliver unique spatial insights into the migratory behaviour of a potentially-toxic cyanobacterial bloom. It was further shown that remote sensing can be used to map the distribution of aquatic plants in shallow lakes, importantly including the extent of submerged vegetation, which is central to the assessment of ecological status. This research theme was subsequently extended in an exploration of the use of remote sensing for assessing the ecophysiological response of wetland plants to nutrient enrichment. It was shown that remote sensing metrics could be constructed for the quantification of plant vigour. The extrapolation of these techniques enabled spatial heterogeneity in the ecophysiological response of Phragmites australis to lake nutrient enrichment to be characterised and assisted the formulation of a mechanistic explanation for the variation in reedswamp performance in these shallow lakes. It is therefore argued that the spatially synoptic data provided by remote sensing has much to offer the assessment, monitoring and policing of ecological status in shallow lakes and, in particular, for facilitating the development of pan-European scale lake surveillance capabilities for the Water Framework Directive (2000/60/EC). It is also suggested that remote sensing can make a valuable contribution to furthering ecological understanding and, most significantly, in enabling ecosystem processes and functions to be examined at the lake-scale.

The two main goals of my thesis were to further our understanding of how parental effort is related to life-history trade-offs and to see how parental investment is reflected in various potential measures of nestling quality. I looked at how fitness is maximized by examining (1) the trade-off between current and future reproduction, and (2) the trade-off between quantity and quality of offspring. To see how parents responded to energetic demands and whether each sex reacted in a similar way, I experimentally manipulated brood sizes and quantified provisioning rates. Both male and female parents with enlarged broods increased their feeding rates, but provisioning on a per nestling basis declined, so that parents fledged lighter nestlings with shorter wings. Although the incidence of mortality did not differ between control and enlarged broods, nestlings from enlarged broods were lighter than those from control broods with the same brood size, suggesting that clutch size may be individually optimized. I also looked at how nestlings responded to different levels of nutritional stress in the manipulated broods by quantifying size and body condition, plumage colouration, and the physiological measures of T-cell mediated immune responses, and corticosterone levels in nestling feathers as a long-term integrated measure of stress physiology. The size of melanin ornaments on feathers and the saturation and brightness of carotenoid colouration was associated with nestling mass in such a way that suggested that plumage characteristics reflect nestling quality. The immune function of nestlings was negatively related to brood size and nestlings in better body condition could mount greater immune responses to foreign antigens suggesting that immune responses are energetically costly. Corticosterone levels in the feathers were not related to nestling body condition and were unaffected by the experimental brood manipulation. The ii mass of male nestlings, which are the larger sex, was more compromised by brood size than female mass was. I also found sex-specific relationships between plumage characteristics and measures of physiological performance. These findings help to explain optimal clutch size and the classic trade-off between quality and quantity of offspring. They also offer new insights into the reliability of putative measures of quality in nestlings and relationships between physiological and morphological traits.

The leopard tortoise (Geochelone pardalis) is the largest of the southern African tortoise species and has a wide distribution range. However, there is a lack of ecological and physiological information about the species, especially arid and semi-arid regions. The Nama-Karoo, an arid region of South Africa, is subject to large fluctuations in rainfall, food availability and ambient temperatures (Ta). This study focused on the thermal behaviour, thermoregulatory, digestive and metabolic plasticity of the leopard tortoise within the Nama- Karoo biome. Seasonal changes in activity patterns and body temperature (Tb) were investigated in free ranging leopard tortoises in the Nama-Karoo. Leopard tortoises had unimodal daily activity patterns in winter, bimodal in summer, and there were daily and seasonal differences in the extent to which certain behaviours were practiced. Daily activity behaviours were executed at lower Tb and at lower Ta in winter compared to summer. In summer, core Tb of all tortoises oscillated on a daily basis well below maximum Ta, while core Tb of all tortoises in winter oscillated well above the daily Ta range. Tortoises were therefore able to maintain their Tb independently of Ta. Differences in Tb as measured from various positions on the tortoises body was investigated in relation to Ta. There was a strong seasonal and temporal influence on the relationship between various Tb’s, with the skin and external shell temperatures being more variable in response to fluctuating Ta’s compared with cloacal and core Tb. Cloacal temperatures were significantly different to other Tb measurements suggesting that it should be treated with circumspection as an exclusive measure of Tb. Heating and cooling rates of leopard tortoises were investigated in the field and under controlled laboratory conditions to determine if the tortoises maximise operational daily activity periods, and to determine the effect of behaviour and size on the rate of heat flux. In the laboratory, cooling rates were faster than heating rates in summer and winter for all size classes and decreased with increasing body mass. Leopard tortoises had significantly faster heating and cooling rates in winter than in summer. Free-ranging leopard tortoises had faster heating rates than cooling rates and their heat flux was largely independent of Ta. Heating and cooling rates were dependant on body mass and surface area-to-volume ratio of individuals. Under experimental conditions, tortoises physiologically adjusted their rate of heat flux, while free-ranging tortoises used physiological and behavioural mechanisms to minimise the risk of overheating, to aid thermal inertia and maximise operative activity time. Seasonal climatic cycles and fluctuating daily temperatures influence the oxygen consumption (VO2) of reptiles, however the result of these effects on metabolism in chelonians is poorly understood. The effect of seasonal and daily differences in Ta on VO2 was investigated. Leopard tortoises’ VO2 was slightly higher than reported for other chelonians. There were significant differences in tortoise VO2 at different Ta’s during the day and night and in different seasons. This metabolic plasticity is possibly an adaptive mechanism to cope with unpredictable environmental conditions. Unpredictable climatic conditions lead to unpredictable food and water availability. Little is known how tortoises adjust dietary parameters in response to food type and water availability, and if this affects body mass, energy and water balance. Therefore this study also considered whether leopard tortoises adjusted food transit rate, food intake and water loss to cope with a diet fluctuating in fibre and water content, and whether body mass, energy and water balance were maintained. Leopard tortoises fed a high fibre, low water content diet had lower food intake rates, longer food transit times, but lower daily energy assimilation compared with tortoises fed a low fibre, high water content diet. Tortoises fed a high fibre, low water content diet had lower urine osmolality, but similar total water loss to those fed a high fibre, low water content diet. The results indicate that tortoises can adjust digestive parameters according to diet composition and exercise some control over energy and water balance. It is concluded that leopard tortoises show a high degree of plasticity in their thermal behaviour and physiology which allows survival in an unpredictable environment, particularly where there are fluctuations in rainfall, food availability and Ta’s. Seasonal and daily variation in thermoregulation, metabolic rate and the uptake of energy allows the leopard tortoise to maximise the duration of operative temperature, to minimise energy loss and to use variable and unpredictable seasonal resources. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.

The long-term economic viability and ecological integrity of savanna rangelands is being undermined by increasingly dense woody thickets at the expense of palatable herbaceous cover. This process is known as shrub- or bush-encroachment. Bush encroachment is a subset of a broader ecological riddle underlying the coexistence of woody and herbaceous vegetation that has been the subject of many ecological models. The ecophysiological mechanisms and interactions between trees and grasses on which most assumptions of ecological models are premised have seldom been tested empirically. This document synthesises the results of greenhouse and field-based investigations of the underlying ecological mechanisms and ecophysiological interactions between encroaching Acacia mellifera trees and grasses in a semi-arid environment. In a greenhouse study, I determined the contribution of N2 fixation to the N-budget of Acacia mellifera under conditions of both varying N availability and competition from grass. Tree seedlings had longer shoots and greater total dry mass in the absence of grass. The leaf δ15N values were lower with grass than without grasses. Thus, trees were more reliant on N2 fixation in the presence of grasses. N2 fixation may enable the tree seedlings to survive competition with grass at critical and vulnerable developmental stages of germination and establishment. In a field removal experiment, I monitored the growth rates, water relations and mortalities of shrubs around which neighbouring woody plants were removed (target) and control shrubs over three years. Results showed target trees to have benefitted from removal of neighbours, which was manifested in significantly faster growth rates, less negative predawn water potential and a relatively small degree of canopy die-back. Nonetheless, neighbouring trees appeared to prevent the whole plant mortalities resulting from severe environmental stress. Growing in close proximity with neighbours could therefore yield positive and negative ecophysiological effects. In another greenhouse experiment, I tested the effects of the separation of moisture uptake with depth between tree seedlings and grasses on two common substrate types. I also examined the influence of repeated grass clipping on the persistence of soil moisture. Results indicated a three-tier rooting pattern with a top layer exclusively exploited by grasses, an intermediate layer occupied by both grass and tree roots and deeper layers exclusively tapped by trees. Tree seedling biomass was negatively affected by grass competition although the biomass of grass was enhanced in the presence of tree seedlings on sandy substrates only. The repeated clipping of grass benefitted tree seedlings on rocky substrate more than it did on sandy substrate. The effects of heavy grazing on soil moisture availability to woody shrubs and thus bush encroachment may be contingent on substrate type, being more acute on rocky terrains. Grass competition suppresses tree seedlings but the removal of grass by grazing weakens this suppressive effect, particularly on rocky substrates. The insufficiency of space and soil resources on rocky substrates may necessitate increased investment in root biomass by plants. It is not known why grasses have lower densities on rocky substrates than on sandy substrates, but the obstruction by rocks disadvantages grasses against tree seedlings, leaving grasses vulnerable to grazing pressure. This may allow the woody plants on rocky substrates to benefit more from grass removal than on sandy substrates. Root restriction by rock barriers and, perhaps, sparse soil volume further lead to small tree sizes on the rocky substrate. Small shrubs are less likely to compete intensively for resources and cause density-dependent mortality. Intraspecific competition may maintain shrub sizes within the threshold that can be supported by available resource pools. I conclude from my results that the two-layer hypothesis of niche separation between savanna vegetation is valid although there is an overlap in the grass and tree rooting depth/moisture uptake. An additional factor that affects the success of A. mellifera is the substrate.Trees are more dense on rocky substrates but grow larger on sand. I have further shown that A. mellifera trees fix nitrogen when competing with grasses but do not do so when grasses are absent. A mechanistic model of savanna dynamics will need to integrate water use patterns, substrate and nutrients to make effective predictions about encroachment patterns. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

As restoration ecology has matured as a science there has been increased interest in the relationship between species diversity and landscape health. Degraded landscapes tend to be resource poor, which limits species diversity as only species which are capable of growing and reproducing in these resource limiting environments can inhabit the area. Additionally, the established species are strong competitors for resources and will exclude, by way of inter-specific competition, weaker competitor species attempting to invade the degraded area. Several studies have demonstrated that with increased species diversity the overall productivity and functionality of the grassland increases. Seedling development and competitive interactions between grass seedlings has a significant impact on the final community structure and species diversity. It is for this reason that aspects of the ecology of grass seedlings were investigated. The growth and competitiveness of Chloris gayana, Cynodon dactylon, Digitaria eriantha, Eragrostis curvula and E. tef seedlings were determined under three environmental stimuli, namely nitrogen availability, light availability and exposure to plant-derived smoke (in the form of smoke-infused water). The primary conclusion from the competition experiments was that the species can be split into superior and inferior competitors at the seedling stage. Chloris gayana, E. curvula and E. tef were the most competitive seedlings as they had the largest negative effect on the growth of other species (high nitrogen Relative Interactive Index (RII) = -0.449, -0.203 and -0.379 respectively) and they were least affected by competition (high nitrogen RII = -0.251, -0.168 and -0.248 respectively). The calculated RII indicates the strength of the competitive interactions, the more negative the RII the stronger the competitive interaction. Nutrient availability had limited effect on the competitive hierarchy of the tested species. Chloris gayana seedlings, however, increased in competitiveness with an increase in available nutrients. In other words, there was a decreased negative response to competition in a high nutrient environment (high nitrogen RII -0.251, no nitrogen RII -0.605). When D. eriantha was grown under varying shade, nutrient and competition levels it was evident that the primary stress factor was light deficiency (p<0.001), and nutrient availability had no affect on seedling growth (p=0.069). Smoke-infused water had no consistent affect on the germination success or the seedling’s root and shoot vigour for the five grasses. These results indicate that the introduction of a “2-phase” or “multi-phase” restoration plan may be beneficial for the development of species diverse rehabilitated grasslands. Manipulating the time and space that the different species are planted, or the distribution of nutrient concentration over the area, may increase the survivorship of all the species that are introduced to a restoration site. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

[Truncated abstract] Weeds which invade native communities can have major impacts on biodiversity and ecosystem processes. However, these impacts are rarely quantified, and the mechanisms behind these impacts are rarely investigated. Asparagus asparagoides (L.) Druce (Asparagaceae; common name: bridal creeper), a plant native to southern Africa, is a significant environmental weed in southern Australia. Bridal creeper can invade both disturbed and undisturbed native ecosystems and then dominate native communities. As is the case for many environmental weeds, there has been little work conducted on the impacts of this plant. This lack of knowledge has hampered restoration efforts of invaded areas because very little is known about the potential for invaded communities to recover prior to undertaking weed management. There is a need to improve our understanding of how to manage ecosystem recovery during and after weed control. This can be achieved by (i) determining the impacts caused by the weed; (ii) assessing the condition of invaded communities; and (iii) predicting the impacts that weed management itself will have on the native communities. These three prerequisites to environmental weed control have been determined across sites invaded by bridal creeper in southern Australia. The impacts of this invasive geophyte have been determined through multi-site comparisons, weed removal experiments and controlled glasshouse and laboratory experiments. ... Without additional restoration, we will see those species that readily germinate and those that respond positively to increased soil fertility, replacing bridal creeper after control. This will be dominated by other weeds as the invaded sites have large exotic seed banks that will readily germinate. The tuberous mats of older bridal creeper plants will also leave a legacy as they will remain many years after control and still impact on vegetation, even if control has killed the plant. These impacts will be highest at sites where bridal creeper has dominated over the longer term. Environmental weeds, such as bridal creeper, that are capable of altering ecosystem functions can lead to substantial declines in biodiversity. Therefore, it was fortunate that bridal creeper became a target for biocontrol in Australia even though the impacts of the weed were not quantified when this decision was made. There are areas in southern Australia that are still free of bridal creeper or have sparse populations, and it is highly likely that this biological control programme has lead to the protection of these areas. This protection would not have been possible if other control measures were chosen over biological control, given that biocontrol agents can self-disperse and are able to give continuous control. This means that biological control of weeds in conservation areas can be very effective and is the only economically viable option for the control of widespread environmental weeds such as bridal creeper.

Typescript. Includes vita and abstract. Bibliography: Includes bibliographical references (leaves 54-60). Description: xi, 60 leaves : ill. ; 29 cm.