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Tropical savannas and woodlands are the dominant vegetation cover in Southern Africa covering 4 million km2. Their large spatial extent means they are potentially a globally important store of biomass carbon with implications for global climate, and an area of high biodiversity value. They provide natural resources such as food, fuel and timber that help sustain the livelihoods of over 100 million people. The ability of these savanna woodlands to maintain these important ecological functions is under question due to increases in land use and land cover change. This thesis addresses a set of science questions aimed at (i) improving our knowledge of the amount of carbon and biodiversity stored in these ecosystems and how they co-vary, (ii) how these variables are spatially distributed at landscape scales and the factors which underlie these patterns, and (iii) how they respond over time to human disturbance. In Chapter 2 I examine how patterns in aboveground woody carbon storage (AGC) are linked to differences in forest structure, tree species diversity and floristic composition across a recently established network of 25 permanent sample plots in south-east Tanzania. Large stems were a significant contributor to plot-level AGC stocks with the top 3% of individuals (>40cm) in terms of size containing 35% of the total measured C. This data can potentially be used to simplify future measurements of biomass in these systems. Tree species diversity was positively related to AGC indicating the potential to align forest conservation efforts. The linear relationship suggests a functional relationship between the variables and is consistent with ecological theory on niche complementarity and selection effects, however based on the available data the mechanisms underlying this relationship can only be theorised. Changes in tree species composition were also noted across plots with differences in vegetation structure between plots explaining 16% of the variation in composition, with environmental differences related to climate and soils explaining only 3%. In Chapter 3, the focus shifts to understanding larger-scale spatial patterns in AGC. Field plots are spatially limited in this regard, therefore radar remote sensing data was used to generate a map of AGC in order to improve our knowledge on what principally controls its spatial variability at landscape scales. Results showed that factors related topography, climate and soils explained very little of the variation in C stocks across the landscape (r2 = 15 – 20%). Differences in slope angle and topographic position were important in discriminating between low biomass savannas and moderate biomass woodlands, while differences in annual precipitation were more important in separating woodlands and denser forests. A large proportion of the variation in C stocks (~80%) was unexplained highlighting the role of unmeasured variables. It is suggested that fire may play a key role in shaping patterns in tree species composition and C stocks across these landscapes. This data has important implications for a local REDD+ project which is aiming to generate carbon credits through improved fire management. In the second part of the thesis the attention shifts to understanding the long-term ecological impacts of shifting cultivation and the sensitivity and resilience of these woodlands to anthropogenic change. In Chapter 4 I examined how carbon stored in trees and soils recover across a 40-year chronosequence of abandoned agricultural land, and how this patchy disturbance impacts spatial pattern in tree species composition and diversity. I show that re-growing woodlands can act as carbon sinks through the accumulation of woody biomass (0.83 tC ha-1 yr-1), with soil texture having no clear impact on accumulation rates. Re-growing woodlands were also found to contain considerable biodiversity value by promoting novel species assemblages. Bulk soil carbon stocks appeared to be largely unaffected by the full cycle of shifting cultivation. However in Chapter 5 I show evidence of a previously unquantified legacy effect of land clearance on soil CO2 production with more recently abandoned fields (c. 6 years) exhibiting significantly higher efflux rates than the older abandonments (15 -25 years) and mature woodlands. Total soil nitrogen was the most important predictor of soil respiration across the plots (r2 = 0.3) followed by fine root density (r2 = 0.12). Soils in the younger sites were found to be more nitrogen rich which was used to explain the greater CO2 fluxes in these areas, however, it is still unclear why this pattern exists. The thesis concludes by discussing the wider implications of the results, as well as outlining further work needed to solidify some of the conclusions drawn in this thesis.

Amphibians are declining at alarming rates globally. Multiple factors contribute to these declines, including chemical contaminants and emergent diseases. In recent years, agrochemical use, especially fungicide applications, has increased considerably. Previous studies have demonstrated that these agrochemicals leave application sites and enter wetlands via runoff and have detrimental effects on non-target organisms. For example, exposure to contaminants can have multifarious effects on amphibians, such as reducing their ability to deal with a secondary stressor, such as disease. A pathogen that is found concomitant with chemical contaminants in aquatic systems is Batrachochytrium dendrobatidis (Bd). Bd has decimated amphibian populations worldwide. Susceptibility to this pathogen varies across amphibian life stages, and is greater in adults than larvae. Consequently, it is important to examine the effects of simultaneous and serial Bd and agrochemical exposure throughout amphibian development. I assessed the combined effects of 3 different fungicides and Bd on two amphibian species: Cuban tree frogs (Osteopilus septentrionalis) and grey tree frogs (Hyla versicolor), both simultaneously and across life stages. To elucidate the complexities of these interactions, I conducted two experiments, one in the laboratory and another in outdoor mesocosms. Frogs were exposed to most of the possible combinations of fungicides and Bd as tadpoles and metamorphs. The presence of fungicides during the tadpole stage caused no difference in timing to metamorphosis and therefore no extension of time animals were exposed to the pathogen. Fungicides did not reduce fungal growth; in fact, tadpoles exposed simultaneously to a fungicide and Bd, regardless of the specific fungicide, had increased fungal loads compared to acetone controls. Additionally, animals exposed to both stressors simultaneously had higher mortality compared to controls or any of the stressors singly. Lastly, the fungicide had persistent effects on amphibian health by affecting susceptibility to Bd later in ontogeny. Frogs exposed to any of the three fungicides as a tadpole had higher Bd prevalence, Bd abundance, and Bd-induced mortality when challenged with Bd after metamorphosis, an average of 71 days after their last fungicide exposure. In conclusion, I found no benefits of fungicides for amphibians. In fact, results indicate both immediate and delayed negative effects of exposure to fungicides and Bd. These findings highlight the importance of studying multiple potential contributors to amphibian declines, simultaneously and sequentially, to understand net effects of stressors on amphibian performance.

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A hierarchical and standardized environmental disturbance index, specifically designed for karst landscapes, was created by van Beynen and Townsend (2005). To assess the applicability of the index and provide recommendations for its refinement, the index was applied in four west-central Florida counties and interviews were conducted with local and state officials, community planners, and land resource managers. The karst disturbance index consists of 30 indicators contained within five broad categories: geomorphology, hydrology, atmosphere, biota, and culture. Data was readily available for most environmental indicators used to construct the index. Overall, levels of disturbance vary between the counties due to the level of urbanization, with the highly populated Hillsborough and Pinellas Counties having higher degrees of disturbance than less developed Pasco and Hernando Counties. While this result may seem obvious, the measure of disturbance using many indicators provides benchmarks of levels of disturbance that can be reassessed with time and highlights those aspects of the environment most in need of attention. Several minor issues arose during the testing: the need for broader indicator descriptions that encompass a variety of scenarios, a new water quality indicator, obsolete sinkhole data, and a lack of data for biota indicators. The lack of data for certain indicators suggests where future research efforts can be directed.

Dans cette étude, la composition taxonomique, la structure spatiale et les ressources végétales naturelles prélevées dans les forêts de Kigwena et de Rumonge ont été analysées. Les deux forêts sont situées au Sud-Ouest du Burundi, dans une région de forte densité de population. L’objectif global de l’étude était d’élucider des éléments de l’écologie de la forêt dense de Kigwena et de la forêt claire de Rumonge en vue de leur conservation optimale. Les résultats montrent que les deux forêts sont floristiquement différentes malgré leur proximité géographique. Les types biologiques sont dominés par les phanérophytes, les chaméphytes et les thérophytes ;ce qui traduit des adaptations des espèces aux conditions du milieu de vie forestier caractérisé par un stress hydrique et par d’importantes perturbations d’origine anthropique. Le spectre phytogéographique est en concordance avec la localisation des forêts étudiées dans le territoire phytogéographique du District du Mosso/Malagarazi, caractérisé par la présence des espèces à distribution soudano-zambézienne et des espèces de liaison soudano-zambézienne et guinéo-congolaise. La forêt dense de Kigwena et la forêt claire de Rumonge sont soumises aux mêmes influences phytogéographiques et n’ont donc pas connu d’isolement géographique. Dans la forêt dense de Kigwena, l’hétérogénéité spatiale de la végétation, reflétée par l’existence de quatre groupements végétaux distincts, obéit aux gradients d’humidité et d’anthropisation. Par contre, dans la forêt claire de Rumonge, l’hétérogénéité spatiale de la végétation se traduit par des variations stationnelles dans un même fond floristique. L’analyse de la distribution spatiale a montré que les arbres sont répartis selon le modèle spatial agrégé. Les ressources végétales prélevées dans les deux forêts par les populations riveraines servent en grande partie à des fins médicinales, matérielles, alimentaires, énergétiques et/ou socio-culturelles. Les prélèvements de ces ressources, l’agriculture intensive, l’exploitation des carrières, l’aménagement des infrastructures, les feux de forêt et les chablis ont été identifiés comme étant des facteurs de perturbation qui amplifiaient la dégradation et le risque de disparition des forêts de Kigwena et de Rumonge. En définitive, cette étude a montré que les forêts étudiées ont une richesse taxonomique et une valeur conservatoire considérables. Mais ces forêts sont menacées sous l’action d’une anthropisation croissante. Pour remédier à cette situation, des mesures et des actions de conservation ont été proposées pour un court terme. /The taxonomic composition, the spatial pattern and the botanical resources used by Man around the Kigwena and Rumonge forests were studied. Both forests are located in the south-western part of Burundi, in a zone characterized by a high population density. The overall objective of this study was to understand the ecology of the Kigwena and Rumonge forests in order to support their conservation. Results show that the flora of the two forests are different although their geographical proximity. The life forms are dominated by phanerophytes, chamaephytes and therophytes; this observation corresponds to an adaptation of the floras to water stress and anthropogenic disturbances. The phytogeographical spectrum is directly related to the presence of the forests in the Mosso/Malagarazi District. It is characterized by the presence of species with a soudano-zambezian distribution and by species of the soudano-zambezian/guineo-congolese connexion. No geographical isolation was evidenced which could have altered the composition of the Kigwena and Rumonge forests. In the Kigwena forest, the spatial floristic heterogeneity is shown by four different plant communities which occur according to anthropogenic disturbance and soil moisture gradients. But, in the Rumonge forest, the spatial floristic heterogeneity occurs with more common species, in the same flora. The analysis of the spatial pattern of the trees revealed a general tendency towards a clumped distribution. Many natural plant resources are removed from the Kigwena and Rumonge forests by neighbouring populations for medicinal use, material use, food use, energy use and cultural use. Removal of natural plant resources, intensive agriculture, quarry exploitation, infrastructure development, forest fires and windfall of trees have been identified as disturbance factors which amplify the degradation and the risk of disappearance of the forests of Kigwena and Rumonge. Ultimately, this study shows that the Kigwena and Rumonge forests are characterized by a high taxonomic richness as well as by their importance for conservation. Nevertheless, these forests are threatened by increasing anthropogenic activities. Consequently, short-term measures and actions for conservation are proposed. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished