Submitted by Mario BC (mario@bc.ufrpe.br) on 2016-08-05T14:42:31Z No. of bitstreams: 1 Elis Regina Guimaraes Camara.pdf: 4238472 bytes, checksum: db9cbd9a8d649eff24b6588125b239b5 (MD5) / Made available in DSpace on 2016-08-05T14:42:31Z (GMT). No. of bitstreams: 1 Elis Regina Guimaraes Camara.pdf: 4238472 bytes, checksum: db9cbd9a8d649eff24b6588125b239b5 (MD5) Previous issue date: 2016-02-24 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / In the Northeast, the semiarid region presents areas susceptible to desertification. Are areas known as Nuclei of Desertification and these compromotem 20% of the region. The Cabrobó desertification Center, belongs to the state of Pernambuco and is inserted in the geo-environmental unit Country Depression. In this environment the predominant soils and associated with the most impacted areas are Luvisols and Planosols. The study aimed to characterize morphological, physical, chemical, mineralogical and micromorphologically Luvisols and Planosols spread over a segment located in Cabrobó desertification core inserted in the Country Depression from a lithological variation, as well as understand the pedogenic processes involved in the formation of these soils. chemical analysis (pH, exchangeable cations, available phosphorus, exchangeable aluminum, potential acidity, organic carbon); physical (grain size, water dispersible clay and flocculation), selective extractions with DCB and ammonium oxalate; chemical composition analysis by fluorescence X-rays; mineralogical analysis by diffraction of X-rays (fraction sand and clay); micromorphological analyzes and petrographic analyzes. Despite quite different lithologies, soils presented mineralogical composition very similar clay minerals. The sand fraction presented quarzto and feldspar and clay fraction with smectite (beidelita, nontronite and montmorillonite), and mica, vermiculite and kaolinite. The distribution of Luvisols and Planosols in the study area is associated with the lithology, regardless of their position in the landscape, where Luvisols are formed by basic rocks and Planosols by acid rocks. The clay formation in situ (argilic) seems to be the main process responsible for the formation of the B horizon in both soils. The expressive textural difference between the A and B horizons is possibly related to the elutriation and pedimentation process on the horizon training A. The smectite main mineral found in the clay fraction of the subsurface layers Luvisols was nontronite. / No Nordeste, a região semiárida, apresenta áreas susceptíveis à desertificação. São áreas conhecidas como Núcleos de Desertificação e estas compromotem 20% da região. O Núcleo de desertificação de Cabrobó, pertence ao estado de Pernambuco e está inserido na unidade geoambiental da Depressão Sertaneja. Neste ambiente os solos predominantes e associados às áreas mais impactadas são Luvissolos e Planossolos. O trabalho teve como objetivo caracterizar morfológica, física, química, mineralógica e micromorfologicamente Luvissolos e Planossolos, distribuídos ao longo de um segmento localizado no Núcleo de desertificação de Cabrobó inserido na Depressão Sertaneja a partir de uma variação litológica, bem como compreender os processos pedogenéticos envolvidos na formação desses solos. Foram realizadas análises químicas (pH, cátions trocáveis, fósforo disponível, alumínio trocável, acidez potencial, carbono orgânico); física (granulometria, argila dispersa em água e grau de floculação), extrações seletivas com DCB e oxalato de amônio; análise da composição química por fluorescência de raios-X; análises mineralógicas por difração de raios-X (fração areia e argila); análises micromorfológicas e análises petrográficas. Apesar das litologias bastantes distintas, os solos apresentaram composição mineralógica de argilominerais bastante semelhantes. A fração areia apresentou quarzto e feldspatos e a fração argila com esmectitas (beidelita, nontronita e montmorilonita), além de mica, vermiculita e caulinita. A distribuição de Luvissolos e Planossolos na área estudada está associada à litologia, independente de sua posição na paisagem, onde Luvissolos são formados pelas rochas básicas e os Planossolos pelas rochas ácidas. A formação de argila in situ (argilização) parece ser o principal processo responsável pela formação do horizonte B em ambos os solos. A diferença textural expressiva entre os horizontes A e B possivelmente está relacionada ao processo de elutriação e pedimentação na formação do horizonte A. O principal mineral esmectítico encontrado na fração argila nos horizontes subsuperficiais dos Luvissolos foi nontronita.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / A desertificaÃÃo à um fenÃmeno que ocorre em zonas Ãridas, semi-Ãridas e subÃmidas secas, desencadeado por efeitos climÃticos e intensificado pela aÃÃo do homem, devido a fatores econÃmicos sociais e culturais e tem como conseqÃÃncias perda dos solos e desaparecimento da biodiversidade. A aÃÃo antrÃpica à responsÃvel pela intensificaÃÃo da degradaÃÃo ambiental, tanto nos paÃses desenvolvidos quanto em vias de desenvolvimento. O impacto das atividades humanas sobre o ambiente natural, objeto de estudo das ciÃncias naturais, que buscam soluÃÃes para problemas como a poluiÃÃo do ar e da Ãgua, desmatamento, erosÃo dos solos, perda da biodiversidade e a desertificaÃÃo, afeta o equilÃbrio ecolÃgico do planeta. A pesquisa âAnÃlise da desertificaÃÃo nos sertÃes dos Inhamuns â CearÃ, no contexto das polÃticas pÃblicas e o papel da mÃdiaâ tem sua Ãrea de concentraÃÃo em TauÃ, municÃpio aprovado pelo Unicef por suas aÃÃes em favor do meio ambiente. A contextualizaÃÃo histÃrico-geogrÃfica e cientÃfico-cultural das polÃticas socioambientais, o problema na histÃria, na mÃsica e na literatura e nas notÃcias veiculadas nos jornais âO Povoâ e âDiÃrio do Nordesteâ, entre 1992 e 2002, a avaliaÃÃo do nÃvel de informaÃÃo da populaÃÃo de TauÃ, o comprometimento e o conhecimento dos jornalistas sobre a temÃtica ambiental sÃo abordados neste trabalho. A desertificaÃÃo à um obstÃculo para o convÃvio no sertÃo, onde as polÃticas pÃblicas emergenciais, prometidas em campanhas e anunciadas pela mÃdia, nunca chegam de forma efetiva, representando tambÃm ameaÃa ao desenvolvimento socioeconÃmico do Estado. Faz-se necessÃria, portanto, a adoÃÃo pelo Poder PÃblico de processo de educaÃÃo geral sistemÃtica e de ecologia humana, em todos os nÃveis, para se poder reverter o quadro de ameaÃa e os nÃveis de pobreza e misÃria da populaÃÃo sertaneja. Ademais, à necessÃrio melhorar a formaÃÃo do jornalista para que a cobertura dos temas ambientais saia do factual para o acompanhamento do processo. Recomenda-se, ainda, ao Governo do Estado o incremento de polÃticas pÃblicas de convivÃncia com o semi-Ãrido, como o aproveitamento das forÃas da natureza para a produÃÃo de energia eÃlica e solar, a construÃÃo de maior nÃmero de cisternas de placas para armazenar a Ãgua no perÃodo de chuvas, a utilizaÃÃo da semente oleÃfera para limpeza e conservaÃÃo da Ãgua e a construÃÃo de microbarramentos com pedras, abordagem sistÃmica e novo paradigma para a produÃÃo agrÃcola nos trÃpicos secos, que ajuda a conservar os solos e a restaurar o meio ambiente. / The desertification is a phenomenon that occurs in arid zones, semi-arid and subhumid droughts, caused by climatic effects and intensified by the action of the human kind, due to social, cultural and economic factors, and its consequences are ground losses and disappearance of biodiversity. The anthropic action is responsible for the intensification of the environmental degradation in as much as in developed countries as it is in developing ones. The impact of human activities on the natural environment, object of study of the natural sciences, which seek solutions to problems such as air and water pollution, deforestation, soil erosion, loss of biodiversity and desertification, affects the ecological balance of the Planet. The research âAnalysis of desertification in the hinterlands of Inhamuns â CearÃ, in the context of public policies and the role of the mediaâ has its convergence area on TauÃ, council approved by UNICEF for its actions in favor of the environment. The historic-geographical and scientific-cultural contextualization of the socio-environmental policies, the approach of the problem in history, music and literature and in the news of the daily papers O Povo and DiÃrio do Nordeste, between 1992 and 2002, the evaluation of the level of information the population of TauÃ, and the commitment and knowledge of journalists about the theme are some prospects proposed for this work. The desertification is an obstacle to a normal life in the hinterlands, where the casual public policies, promised by the politicians during the political campaigns and announced by the media, never arrive or accomplish, is also a menace to the socio-economical development of the State. Hence, a process of a systematic general education and of human ecology, at all levels, is the way to change this menace and the standard of poverty and distress of the semi-arid population. Moreover, it is necessary to improve the training of journalists for the coverage of environmental issues, and to change it from factual issues to monitoring of the process. It is also recommended that the State Government increments the public policies for the coexistence with the semi-arid, such as the use of the forces of the nature for the production of wind and solar energy, construction of a greater number of tanks to restore the water during the period of rain, the use of the seed oleÃfera for cleaning and conservation of water and construction of micro dumps with stones, systemic approach and new paradigm for agricultural production in the dry tropics, which helps conserve the soil and restore the environment.

Esta pesquisa apresenta a aplicação de uma nova metodologia de medição de carreamento de solo, que foi utilizada durante o período de um ano, na área urbana de Paranavaí, no estado do Paraná. Teve como objetivo avaliar a fragilidade do solo e analisar os processos de erosão e arenização. Durante o seu desenvolvimento foram analisados principalmente os dados climáticos que envolveram desde as temperaturas médias anuais, índices de pluviosidade até direção e velocidade dos ventos de superfície. A partir da análise dos dados obtidos foram elaboradas cartas temáticas que permitiram um melhor entendimento do fenômeno da arenização, cerne deste trabalho. Uma série de fotos obtidas no campo auxiliou na análise e avaliação desses processos, gerando uma discussão de resultados que possibilitou tecer considerações sobre a recuperação de certas áreas onde ocorrem esses fenômenos. / This research aims to present the application of a new methodology of measurement of soil carrying over used during one-year term in the urban area of Paranavaí, in the state of Paraná-Brazil. Aiming to evaluate the soil fragility and analyzing the erosion processes and arenization, it was analyzed during its development, the climatic data from the average annual temperatures, pluviometric rates and even the direction and speed of the winds. Starting from the analysis point of the data collected, thematic letters were elaborated allowing a better comprehension of the arenization phenomenon, core of this work. A series of pictures were taken in field that helped us in the analysis and evaluation of these processes, arousing a discussion of the results, which let us talk about the recovery of certain areas where these phenomena occur.

La dégradation des terres dans le monde affecte directement plus d’un milliard d’hectares. Elle menace près de 35% des terres de la planète ainsi que les moyens de subsistance de deux milliards d’individus. Les estimations des pertes financières sont considérables chaque année. La dégradation des terres arides et semi-arides est une entrave au développement des pays concernés. Ce problème demeure insoluble et la communauté internationale organisée au sein de plusieurs instances n’ayant pas trouvé de solution pérenne, assiste impuissante à la progression du fléau malgré les efforts et expérimentations menées à travers le monde. L’urgence est de mise pour trouver des solutions qui soient à la fois techniques, sociales, économiques et culturelles.Ce constat appelle des réponses et la nécessité d’exploiter des approches basées sur les avancées des nouvelles techniques. La robotique mobile est une piste qui peut contribuer dans un avenir proche à freiner ce phénomène. Cette technologie recèle d’immenses possibilités dans plusieurs domaines : médical, aide à la personne, militaire, agricole et bien entendu dans la restauration des écosystèmes. La thèse s’inscrit dans le projet R-Stepps pour évaluer les capacités de résilience d’une solution basée sur les techniques robotiques. Elle expérimente une flotte de robots pour lutter contre la dégradation des terres et l’avancée de la désertification. Nous avons étudié et exploré les moyens de mettre au point une flotte de robots nomades pour remplir des missions de restauration des écosystèmes dégradés. Pour construire des systèmes mécatroniques simples et robustes nous avons mis au point des robots limités à une seule tâche.Nous avons développé une plateforme expérimentale composée de cinq robots capables de se coordonner et d’accomplir chacun une tâche spécifique (forage, plantage, arrosage, surveillance, guidage). Le démonstrateur sert à valider les modèles de navigation, de localisation, de planification ainsi que les procédures de plantation et de surveillance de l’environnement / Land degradation directly affects over a billion hectares throughout the world. It is a threat to almost 35% of the planet’s land as well as to the means of subsistence of two billion people. Estimations of heavy financial losses are increasing year after year. The degradation of arid and semi-arid lands is a serious drawback to the development of the countries concerned. This problem is today unsolved and since a number of organizations within the international community have not found an appropriate, perennial solution, we are powerlessly witnessing the progression of this plague despite the efforts and experiments carried out all over the world. It has become extremely urgent to find solutions that are not only technical, but also social, economic and cultural.This assessment calls for action and the need to exploit different approaches based on the most recent advanced technology. Mobile robotics are a lead to explore and may well contribute to slowing down this phenomenon in the near future. This technology offers endless possibilities in many fields: healthcare, personal assistance, military, agriculture and of course the restoring of ecosystems. It will therefore be necessary to follow the evolution of this sector and observe its effects on the environment and society in general in those regions affected by the phenomenon. Our findings aim to propose a technical solution, and the more general questions regarding societal consequences are not examined; our main concern is to demonstrate and evaluate the resilience capacity of a solution based on robotic techniques. This document is part of the R-Stepps project to experiment a fleet of robots to combat land degradation and reverse the progress of desertification. Its main mission is reforestation, maintenance and surveillance of such zones. The fleet of robots is composed of various platforms, each with a specific function. By limiting each robot to one task, this will ensure their solidity and simple design

O processo da desertificação é um problema de importância mundial, pois reduz a produtividade das terras como também a função ecológica dos ecossistemas onde acontece este processo. A desertificação é o resultado dos processos de degradação ambiental nas zonas áridas, semiáridas e sub-úmidas secas produto de fatores biofísicos como a variação climática e também das atividades humanas. A região do Vale de Villa de Leyva, Boyacá, Colômbia, tem paisagens em processo de transformação por desertificação porque esta localizada num local seco, pouca precipitação e com solos frágeis. Após da conquista espanhola, esta área teve a maior transformação ambiental iniciando com a sobre exploração dos recursos naturais como as florestas, os solos e os corpos de água, acelerando ainda mais este processo de degradação. Tendo em conta esse contexto, o método de análise das dinâmicas da zona para identificar padrões e processos de desertificação a partir de séries temporais de índices de vegetação, como o NDVI e EVI, foram empregadas técnicas de análise espacial, a traves de Sistemas de Informação Geográfica SIG e ferramentas de Sensoriamento Remoto. Foi feita aquisição das imagens de Índices de vegetação NDVI e EVI do produto MOD13Q1 do sensor MODIS, entre os anos 2001 e 2016. As imagens foram filtradas como o algoritmo Savitzky-Golay para diminuir os erros da informação original, como também os vazios. As series temporais foram analisadas com o intuito de identificar as áreas e os períodos em que ocorreram as mudanças ambientais mais significativas na região com relação à camada vetorial de erosão, elaborada a uma escala 1:100.000 Posteriormente, foi feito uma análise de tendência pelo algoritmo Mann-Kendall para identificar tendências negativas dos índices de vegetação, e fazer uma sobreposição com os índices do último ano, obtendo assim as áreas com maior risco de sofrer processos de degradação ambiental e que podem gerar desertificação. Para validar esses procedimentos, foi feita uma comparação visual com o NDVI do programa Landsat 8 no mesmo período, identificando padrões de distribuição da vegetação muito semelhantes, embora a resolução espacial dos dois tipos de imagens seja muito diferente. Entre os principais resultados, foi possível identificar que as áreas em risco de sofrer processos de desertificação não estão associadas nem obedecem a um processo constante desde sua origem como foi pensado inicialmente, mas as atividades atuais de agricultura não sustentável são as que causam esses processos de degradação. Na atualidade, este processo acontece de forma mais pontual e esta associado a fatores antrópicos, principalmente pelas práticas agrícolas não sustentáveis na região, principalmente pelo incremento dos cultivos de tomate sob estufa. Também existem áreas com tendência a recuperação da vegetação por fatores naturais, por processos de crescimento de gramíneas, como também por processos artificiais, semeadura de arvores e gramado, com o fim de adequar a paisagem nas áreas de expansão urbana que limitam com o “deserto”, gerando novas dinâmicas de ocupação da região que posteriormente precisam ser estudadas. / Desertification process represents a problem of global importance, due to the reduction not only of the productivity of farmlands, but also of the ecological function of ecosystems where this process takes place. Desertification is the result of processes of environmental degradation in the arid, semi-arid and dry sub-humid areas, resulting from biophysical factors such as climatic variation and from human activities, as well. The valley of Villa de Leyva, Boyacá, Colombia, has landscapes in transformation process by desertification because it is located in a dry place, with little precipitation and with fragile soils. After the Spanish conquest, this area had the greatest environmental transformation, starting with overexploitation of natural resources such as forests, soils and bodies of water; all these factors accelerated the process of degradation. Taking into account this context, it is necessary to establish a method of analysis of the dynamics of the zone, in order to identify patterns and processes of desertification through temporal series of vegetation indexes, such as NDVI and EVI, using spatial analysis techniques, through of GIS Geographic Information Systems and Remote Sensing tools. The first step was the acquisition of the vegetation indices NDVI and EVI of the product the images of NDVI and EVI vegetation indices of the MODIS product MOD13Q1 between 2001 and 2016 were acquired. The images were filtered as the Savitzky-Golay algorithm to reduce the errors of the original information as well as the voids. The time series were analyzed in order to identify the areas and periods in which the most significant environmental changes occurred in the region in relation to the vector layer of erosion, elaborated at a scale 1:100.000 Afterwards, a trend analysis was performed by the Mann-Kendall algorithm to identify negative trends of vegetation indexes, and to overlap with the indices of the last year, thus obtaining the areas with the highest risk of environmental degradation processes that can generate desertification. To validate these procedures, a visual comparison was made with the NDVI of the Landsat 8 program in the same period, identifying vegetation distribution patterns very similar, although the spatial resolution of the two types of images is very different. Among the main results, it was possible to identify that the areas at risk of suffering desertification processes are neither associated nor obeyed a constant process since its origin as initially thought, but the current activities of non sustainable agriculture are those that cause these processes of degradation. Nowadays, this process happens in a more punctual way and is associated to anthropic factors, mainly by the unsustainable agricultural practices in the region, mainly by the increase of tomato crops under greenhouse. There are also areas where the vegetation recovers by natural factors, by processes of grass growth, as well as artificial processes, planting trees and pastures, with the purpose of adjusting the landscape in areas of urban expansion that limit with the “desert”, generating new dynamics of occupation in the region that need to be studied further.

Rocky desertification (RD) is a process of land degradation that often results in extensive soil erosion, bedrock exposure and considerable decrease of land productivity. The spatio-temporal evolution of RD not only reflects regional ecological environmental changes but also directly impacts regional economic and social development. The study area, Hechi, is a typical karst peak cluster depression area in southwest China. Remote sensing, geographic information systems (GIS) and statistical techniques were employed to examine the evolution, including the identification of driving forces, of karst RD in the Northwestern Guangxi. The results indicate that RD became most apparent between 1990 and 2005 when areas of various types of RD increased. Within the karst RD landscape, slight RD was identified as the matrix of the landscape while potential RD had the largest patch sizes. Extremely strong RD, with the simplest shape, was the most influenced by human activities. Overall the landscape evolved from fragmented to agglomerate within the 15-year timeframe. Land condition changes were categorized as five types; desertified, recovered, unchanged, worsened, and alleviated land. The largest turnover within the RD landscape was between slight and moderate RD. With regards to the driving forces all RD had been increasingly influenced by human activities (i. e., the stronger the RD, the stronger the intensity of human disturbances). Dominant impact factors of the RD landscape had shifted from town influence and bare rock land in 1990 to bare rock and grassland in 2005. Moreover, the impacts of stony soil, mountainous proportion and river density on RD increased over time, while that of others decreased. The significant factors included human activities, land use, soil types, environmental geology, and topography. However, only anthropogenic factors (human activities and land use) were reported as leading factors whereas the others acted simply as constraining factors.

Student Number : 9511039F - MSc Dissertation - School of Animal, Plant and Environmental Sciences - Faculty of Science / An estimated 91 % of South Africa’s total land area is considered dryland and susceptible to desertification. In response, South Africa has prepared a National Action Programme to combat land degradation, and this requires assessment and monitoring to be conducted in a systematic, cost effective, objective, timely and geographically-accurate way. Despite a perception-based assessment of land degradation conducted in 1999, and a land-cover mapping exercise conducted for 2000/2001, there are few national scientifically rigorous degradation monitoring activities being undertaken, due largely to a lack of objective, quantitative methods for use in large-scale assessments. This study therefore tests a satellitederived index of degradation for the Limpopo Province in South Africa, which is perceived to be one of the most degraded provinces in the country. The long-term average maximum normalized difference vegetation index (NDVI), calculated from a time series (1985-2004) of NOAA AVHRR satellite images, as a proxy for vegetation productivity, was related to water balance datasets of mean annual precipitation (MAP) and growth days index (GDI), using both linear and non-linear functions. Although the linear regressions were highly significant (p<0.005), a non-linear four parameter Gompertz curve was shown to fit the data more accurately. The curve explained only a little of the variance in the data in the relationship between NDVI and GDI, and so GDI was excluded from further analysis. All pixels that fell below a range of threshold standard deviations less than the fitted curve were deemed to represent degraded areas, where productivity was less than the predicted value. The results were compared qualitatively to existing spatial datasets. A large proportion of the degraded areas that were mapped using the approach outlined above occurred on areas of untransformed savanna and dryland cultivation. However the optical properties of dark igneous derived soils with high proportions of smectitic minerals and therefore low reflectance, were shown to lower NDVI values substantially. Overall, there was an acceptable agreement between the mapped degradation and the validation datasets. While further refinement of the methodology is necessary, including a rigorous field-based resource condition assessment for validation purposes, and research into the biophysical effects on the NDVI values, the methodology shows promise for regional assessment in South Africa.

Grasslands provide fundamental ecosystem services in many arid and semi-arid regions of the world, but are experiencing rapid increases in fire activity making them highly susceptible to post-fire accelerated soil erosion by wind. A quantitative assessment that integrates fire-wind erosion feedbacks is therefore needed to account for vegetation change, soil biogeochemical cycling, air quality, and landscape evolution. We investigated the applicability of a novel tracer technique – the use of multiple rare earth elements (REE) - to quantify aeolian soil erosion and to identify sources and sinks of wind-blown sediments in a burned and unburned shrub-grass transition zone in the Chihuahuan desert, NM, USA. Results indicate that the horizontal mass flux of wind-borne sediment increased approximately three times following the fire. The REE-tracer analysis of aeolian sediments shows that an average 88% of the horizontal mass flux in the control area was derived from bare microsites, whereas at the burned site it was derived from shrub and bare microsites, 42% and 39% respectively. The vegetated microsites, which were predominantly sinks of aeolian sediments in the unburned areas, became sediment sources following the fire. The burned areas exhibited a spatial homogenization of sediment tracers, highlighting a potential negative feedback on landscape heterogeneity induced by shrub encroachment into grasslands. Though fires are known to increase aeolian sediment transport, accompanying changes in the sources and sinks of wind-borne sediments likely influence biogeochemical cycling and land degradation dynamics. Our experiment demonstrated that REEs can be used as reliable tracers for field-scale aeolian studies. / Geology / Accompanied by one compressed .zip file: MET_Tower_Data.zip

Many grasslands in arid and semi-arid regions are undergoing rapid changes in vegetation, including encroachment of woody plants and invasive grasses, which can alter the rates and patterns of fire and sediment transport in these landscapes. We investigated the spatial distribution of sediments at the scale of vegetated microsites for three years following a prescribed fire using a multiple rare earth element (REE) tracer-based approach in a shrub-grass transition zone in the northern Chihuahuan desert (New Mexico, USA). To this end, we applied REE tracers – holmium, europium, and ytterbium on shrub, grass, and bare microsites, respectively in March 2016. Soil samples were collected from both burned and control (not burned) sites before (March) and after (June) the annual windy season, from 2016 through 2018. Results indicate that although the horizontal mass flux (HMF) of wind-borne sediment increased approximately threefold in the first windy season following the fire, and the HMF of both plots were not significantly different after three windy seasons. Comparing REE concentrations in sediments from both plots over the three years and three annual windy seasons, we observed a post-fire shift in source and sink dynamics of sediments. The tracer analysis of wind-borne sediments indicated that the source of the HMF in the burned site was mostly derived from shrub microsites following the fire, whereas the bare microsites were the major contributors for aeolian sediment in control areas. The shift in sources and sinks, and the spatial homogenization of REEs indicate that the removal of shrub vegetation resulted in sediment redistribution to the bare microsites even three years after the prescribed fire. The findings of this study will improve our understanding of post-fire geomorphic processes at a microsite scale in a grassland ecosystem undergoing land degradation induced by shrub encroachment. / Geology

In order to understand the dynamics of desertification after the 1980s droughts, the trends and changes of photosynthetically active fraction of the vegetation cover of the semi-arid region of West Africa have been analyzed across three ecoclimatic entities. This study focuses on six countries (Senegal, Mauritania, Mali, Burkina-Faso, Niger and Benin) that reflect quite well the human and environmental context of semi-arid regions. The main objective of this thesis is to check in the before-mentioned biophysical and human context, if the state of the vegetation improves because of more favourable rainfall conditions, or if, conversely, the long environmental deterioration during recent decades has a healthy dose of irreversibility for several years. The process of investigation proceeds in three stages defined by geographical scales and a geoclimatic gradient. At the regional level, we compared vegetation productivity data from 1982-1999 time series of NOAA-AVHRR NDVI data to rainfall data. We analyzed data from 315 rain gauges distributed across five countries (Senegal, Mauritania, Mali, Burkina-Faso and Niger) with annual average isohyets ranging from 100 to 900 mm. For trends analysis, we used the ratio of the integrated vegetation index (iNDVI) during the crop-growth period (June to October) to the May to October sum of rainfall (RR). This ratio (iNDVI/RR), a proxy of the Rain Use Efficiency, is widely used by ecologists as an indicator for detecting desertification processes. Overall, our results show a significant increase of the net primary production as a response of post-drought rainfall increase. However, the trends of iNDVI/RR ratio suggest that most of the studied stations (54%) in sahelian and sahelo-sudanese belts were stable and 31.4% showed strong to very strong negative change in iNDVI/RR ratio, while 13.3% of the stations showed a moderate decrease and only 1.3% showed a positive trend. At the country level, similar trends were recorded throughout 128 stations in Burkina Faso located between the 500 mm and 1100 mm isohyets. In fact, more than half the studied stations showed a stability of iNDVI/RR ratio. However, 39.8% of them show a negative trend from low to high, while only 2.4% of them show a slight positive trend. In addition, a comparison with more detailed local case studies confirmed these observed trends. However, the obtained results for wetter stations in the southern part of the country should be taken with precaution, as the relationship between NDVI and rainfall tends to weaken when annual rainfall is higher than 1000 mm. Overall, the negative trends highlight a gradual decline in plant productivity. These results recorded in 44.7% of the analyzed stations may reflect ongoing desertification processes in the sahelian and sahelo-sudanian zones over the past two decades. At the local level, a structural characterization of woody units in three bioclimatic regions of the sudanian zone (900 mm 1200 mm) in the north of Benin was conducted to assess their degradation status. We recorded the structural characteristics of stands (basal area, density), species diversity as well as disturbances type and intensity. Multivariate analysis revealed a gradient of productivity between the three regions: there was a high diversity of woody stands in the south-sudanian sector, while the north-sudanian and sudano-sahelian sectors were dominated by savannas and shrub, which had low productivity. The productivity gradient is influenced by a disturbance gradient suggesting that the decline in productivity is stronger from south-sudanian to the sudano-sahelian region. In addition, the spatial component of the observable changes in vegetation cover has been mapped by remote sensing in a restricted area of the sudanian zone in northern Benin with SPOT-XS data over the period 1986-2005. Over the past two decades, 19.6% of the woody stands have completely disappeared; 12.9% of this extinction of woody stands was due to deforestation, and 13.9 % due to degradation processes. In contrast to these trends, 21.8% of the study area were stable, while less than a third (31.7%) of the area were experiencing woody recovery (reforestation). The analysis also reveals significant disparities in the rates of change of the identified land use class units. These variations are more pronounced for the woody units and agricultural land than in villages. These results suggest that land cover degradation throughout the study area is primarily due to anthropogenic factors (livestock and agricultural expansion, logging, breeding). In fact, this area is a preferred destination for agricultural migrants fleeing the unpredictable climatic conditions of the drier semi-arid areas. Overall, our results highlight the rapid decline of vegetation resources, challenging assumptions that the impact of ongoing desertification processes is mixed, outside of the arid and semi-arid regions of West Africa. The developed framework is easily reproducible and allows the implementation of a reliable diagnosis on the state of the vegetation cover from accessible and inexpensive data. Its implementation should facilitate the development of managerial techniques for better assistance to the poor, especially vulnerable to the gradual degradation of their environment.