Search

The Andean region is susceptible to soil erosion due to steep topography, prevalent rainfall and high probability of earthquakes and landslides. By its impressive length, continuity and height, the mountain ridges significantly disrupt the atmospheric circulation resulting in a wide variety of environmental conditions. The western flank draining to the Pacific Ocean is characterized by strong environmental gradients, rough topography with deeply incised canyons, and highly variable and extreme precipitation patterns. These environmental conditions pose particular challenges for erosion and sediment flux monitoring. The thesis presents new data on the sediment yield of Andean rivers draining to the Peruvian coast. The dataset is based on time series of river suspended load, discharge and turbidity, and sediment accumulation in reservoirs. About 55% of the observed variance can be explained by hydrometeorology and topography, while anthropogenic factors such as land use marginally improve the model predictions. The effect of climate variability on sediment transport was further studied in the Cañete catchment where the 240MW El Platanal hydropower plant is located. By calibrating a coupled hydrological-sediment transport model for the Cañete River, the effect of precipitation variability on sediment transport and reservoir sedimentation was assessed. Even under the most optimistic scenario, the lifespan of the reservoir is shorter that its officially expected functionality of 50 years. The geomorphic processes driving sediment production and transport were quantified using UAV-PPK-SfM techniques. The processing chain was optimized for 4D geomorphic monitoring in rough terrain by including co-registration techniques. By analyzing the spatial patterns in sediment yield along the western Andes, the thesis enhanced the knowledge base on sediment transport to the Peruvian coastal plain. Besides, it illustrated the vulnerability of hydroelectric facilities to climate variab, (SC - Sciences) -- UCL, 2022

The importance of tropical forests in the global climate system is widely known. Through their high productivity they represent a significant global carbon reservoir. Preserving this function needs a continuous supply of nutrients, mainly nitrogen (N) and phosphorus (P). However, the availability and biogeochemical cycling of these nutrients is highly variable in space and time and the understanding of different nutrient loss pathways is still lacking. The influence of soil erosion on the availability of these nutrients in pristine forests of the tropics is not well understood. To gain a better mechanistic understanding of the aquatic loss processes of N and P, we monitored different headwater streams in three distinct tropical forests of the Congo Basin and determined the influence of soil erosion on soil N. This knowledge is important to assess the response of these ecosystems to changing climate and land use. To assess the importance of soil erosion in tropical forests and export of sediments and nutrients from drainage basins, the first part of the thesis focussed on the fundamental understanding of the dynamics and drivers of sediment export in headwater streams of a tropical lowland forest and a deciduous Miombo woodland. We used high temporal resolution turbidity data and analysed the turbidity-discharge hysteresis patterns during storm events. Although these forest systems are situated in very old and stable landscapes, the obtained sediment yields were substantial and comparable to forests in more geomorphic active sites of the tropics. Although the Miombo only experiences a five-month wet season, similar annual sediment export was found as for the lowland forest, due to the lower vegetation cover. Furthermore, results showed that storm events exported 30% and 68% of the annual sediment yield in the lowland and the Miombo, respectively. In Chapter three, it was shown that soil erosion and storm events also play a crucial role in aquatic nutrient export from, (SC - Sciences) -- UCL, 2022

Understanding interglacial climate diversity and intra-interglacial variability can help assess the sensitivity of the Earth system to different forcings during warm periods. This study examines the individual and combined effects of astronomical parameters (obliquity, precession, and eccentricity), Greenhouse gases concentrations and ice sheets as well as internal feedback mechanisms on the East Asian climate during the last nine interglacials. It has been achieved through simulations with an atmosphere-ocean coupled general circulation model HadCM3 using two different approaches. The first approach is to use a Gaussian emulator that was developed based on 61 well-designed HadCM3 experiments, which allows quantifying the sensitivity of the East Asian precipitation and temperature to orbital parameters, CO2 and ice volume during the past 800 ka. The second is to perform two groups of snapshot experiments covering the entire period of each interglacial and by a time step of 2-ka, which leads to 468 experiments in total. Results based on the emulator approach show that in the East Asian summer monsoon domain north of 25° N, the variation of the summer precipitation is dominated by precession, leading to strong 23-ka cycles. However, in the southern part, ice sheets play a more important role than insolation, generating the 100-ka cycles, by influencing the latitude of the Intertropical Convergence Zone and the Hadley cell. Obliquity and CO2 have little effect on the summer precipitation as compared to precession and ice sheets. To study the detailed mechanisms and internal feedbacks involved in the impact of astronomical forcing, GHG and ice sheets on the East Asian climate, MIS-5 is presented as an example. Results show that during MIS-5, astronomical and GHG forcings both have positive effects on the summer temperature and precipitation. Within the range of its variability during MIS-5, CO2 can cause a similar degree of warming effect but much a lower degree of humi, (SC - Sciences) -- UCL, 2022

The quantification and monitoring of soil organic carbon (SOC) in croplands is crucial in a context of effective and precise agricultural management. Conventional methods to assess SOC changes pose some limitations on time and labor consumption to efficiently assess SOC distributions and dynamics in a spatial and temporal context. Recent developments in Uncrewed Aircraft Systems (UAS), combined with miniaturized visible-near infrared sensors bring opportunities for the rapid and low-cost field-scale SOC mapping. It shows the potential of UAS-soil sensing to bridge the gap between high-resolution plot-scale sensing at centimetre-level and moderate-resolution landscape-scale sensing at meter-level from airborne or satellite platforms. For the UAS sensing system, variety of sensor types enable the multiple dimensional monitoring (i.e., temporal, spatial and spectral) of soil properties, while it brings large uncertainties in choosing a desirable sensor specification for a monitoring purpose. Accordingly, it lacks a capability evaluation for different sensor types in the estimation of soil properties. Meanwhile, there is still room for improving the pre-processing approaches against the perturbing factors such as soil roughness and anisotropy. A comprehensive understanding of how these perturbing factors affecting soil reflectance spectra under different measurement conditions will assist in the use of SOC prediction models derived from an existing regional soil spectral library (SSL), which can be applied to UAS-based hyperspectral data to reduce the need for additional field sampling. The overall aim of this thesis was therefore to improve the spatial and spectral accuracy and effectiveness of UAS-based multi- and hyper-spectral measurement for the estimation of SOC content in croplands. In this thesis, we started with assessing the potential of a non-imaging spectrometer on UAS-borne SOC estimation. We showed that local calibration with soil sampling had good capabil, (SC - Sciences) -- UCL, 2022

Colluvial soils consist of surface soils and/or parent materials locally transported from upper section of the slopes through water erosion, tillage erosion and construction of agricultural terraces. As a consequence of the recognition that colluvial soil stores large amounts of soil organic carbon (SOC), the SOC cycle in colluvial soils has received increasing attention. The aim of this thesis was to evaluate and understand the SOC dynamics and controlling factors in colluvial soils in the context of climate change. This was done for two specific colluvial environments: eroded and terraced agricultural systems. For the eroded colluvial soils, soil deposition rate is identified as the most important independent control on SOC stability and this is independent of climate zone. In addition, the rate of deposition also indirectly affects SOC stability by changing the average weathering status and mineral matrix of colluvial (sub)soil settings which affect mineral–dependent SOC physico-chemical protection mechanisms. The remaining part of this thesis focused on the colluvial SOC dynamics in terraced systems. Firstly, we demonstrate that the soil infrared spectroscopy, combined with compositional data analysis enables efficient and reliable ‘large–scale’ assessment of terracing soil properties relevant to SOC biogeochemical cycles. Furthermore, we confirm that the terraced landforms store more SOC than non-terracing landforms, mainly due to the preservation of SOC by topsoil burial in colluvial settings. Finally, we demonstrate that the terrace soil age is a key factor controlling soil heterotrophic respiration of subsoils (e.g., burial layers), likely through driving the mineral weathering and thus determining the availability of reactive mineral surfaces that stabilize SOC. In addition, the terrace soil age is also a fundamental regulator of SOC temperature sensitivity to decomposition by governing the temporal evolution of SOC fractions and soil C:N ratio (C quality)., (SC - Sciences) -- UCL, 2022

Modelling soil erosion is hampered by the complexity intrinsic to the short-term variability and dynamics between its physical properties. Soil crust development, in particular, is one of the most important factors controlling runoff and erosion, but its properties are difficult to assess. Accurate and spatially distributed data, particularly at fine scale, are increasing in demand by the scientific community to tackle this issue. Remote sensing with multispectral imaging could meet this demand, but a series of issues related to its signal complexity must be solved to provide qualitative data. This thesis focused on the detection capability of the development dynamics of soil physical crust. The general objective was to understand whether soil physical crust development could be detected and modelled by means of multispectral remote sensing, using low-altitude uncrewed aerial vehicles (UAS), with a case study on loamy soils of the Belgian loess belt. After an assessment of spectral sensor technology and analytical methods, this thesis proposed an innovative methodology to model soil physical crust development, as defined by the spectral changes caused on the soil by cumulative amounts of rainfall kinetic energy. This was achieved by developing a data processing protocol for multispectral imaging in the visible and near-infrared range. By validating the laboratory methodology in real field conditions, the results offered some evidence that it is possible to map the development of soil physical crust with a remote sensing application. The last part of this thesis tackled a critical issue with the negative effects of soil anisotropy on multispectral imaging, proposing a semi-empirical correction methodology with good preliminary results, and easing the way for wide scale UAS-based assessment of soil properties with multispectral imaging., (SC - Sciences) -- UCL, 2022

Cosmogenic radionuclides accumulate at the Earth’ surface as a function of time and depth. They can be used to constrain the age and the deposition mode of fluvial deposits. The (eastern) Campine plateau is a low-lying plateau (NE Belgium) covered by fluvial sediments that were deposited by the braided channels of the Meuse, during the Middle Pleistocene. It resisted the overall erosion that affected the Campine area, and nowadays remarkably stands out of its environment. This singular case makes the Campine plateau an ideal topographic marker to study the geomorphic processes that determined the current landscape of the European Lowlands. In this study, we applied cosmogenic radionuclides on the Zutendaal gravels (eastern Campine plateau) to constrain their depositional and post-depositional history. Main results show that the deposition of the Zutendaal gravels occurred in different phases, interrupted by long hiatuses that correspond to climatic fluctuations. The deposition of the Zutendaal gravels would have lasted at least 2 glacial cycles. The onset post-depositional time likely started somewhere between the last 500 to 600 ka, and was probably characterized by intermittent burial of the fluvial deposits by aeolian coversands. The post-depositional erosion of the Zutendaal gravels has remained limited to a few meters. The first numerical ages of the eastern Campine plateau could be used in the future to better constrain the geomorphological evolution of the Campine region and the European Lowlands in general., (SC - Sciences) -- UCL, 2022

Understanding the causal processes determining the geographical distribution of species is a fundamental question in ecology but has relevant implications in epidemiology. Infectious diseases are a public health concern for humans, livestock, and wildlife, and their relevance has fostered the interest in tools allowing the delineation of areas at risk for pathogen transmission. In the past two decades, the prompt availability of new spatio-temporal explicit datasets and coding environments has led to extensive use of modelling tools to infer the geographical distribution of the species involved in infectious disease systems. However, the validity of these models was questioned, underlining the lack of biological realism and causal-based reasoning as the main limitations. In this doctoral dissertation, I tried to include biological realism and a causal-based perspective on correlative and mechanistic modelling approaches aiming to infer the spatio-temporal distribution of vector and host species involved in vector-borne disease systems. I first applied the modelling relation framework in a species distribution modelling exercise through the Structural Equation Modelling approach, a methodology that includes and evaluates causal pathways within a linear modelling framework. I moved towards mechanistic models and built dynamAedes, a spatially-explicit model inferring the population dynamic of four Aedes mosquito species at different spatial scales. I then explored how the choice of the model parameters and spatial scales affect the outcomes of an epidemiological model estimating the number of Chikungunya’s secondary cases. Finally, since host abundance is an epidemiological parameter as substantial as vector abundance, I presented a downscaling methodology to disaggregate livestock censuses aggregated at different administrative unit levels. The results highlighted how a causal-based approach increases the biological realism and predictive accuracy of the modelling app, (SC - Sciences) -- UCL, 2022

A major concern for sustainable development in the Tropical Andes is the growing imbalance between ecosystem service supply and demand, particularly with regard to freshwater ecosystem services. Demographic growth, urbanization and evolving socio-economic conditions coupled with future projections of climate change in the Andes, call for urgent action. This thesis contributes evidence-based research on the conservation and restoration of young volcanic ash soils through an analysis of soil-water-plant interactions at the pedon, toposequence and landscape scales. The empirical data were collected over a 5-year period in the Jatunhuayco experimental site, located in the northern Ecuadorian Andes at 4200 m a.s.l. At the soil pedon scale, the vegetation type had a measurable effect on the soil water balance as well as on solute concentrations and chemical weathering rates. At the toposequence scale, the soil pore structure and hydraulic properties of the A horizon varied by vegetation type. This has implications for soil water storage in the topsoil and subsurface lateral flow. The hydrological connectivity of the geomorphic units at the landscape scale is time-dependent. As a result, the geochemical signature of the solute export varied strongly over time. The existence of strong interlinkages between plant functional types, soil and surface hydrology, and nutrient export has important implications for the sustainable management of high Andean ecosystems. The findings of this thesis can contribute to assess the potential impacts of vegetation conservation and restoration programs on water resources in the Tropical Andes., (SC - Sciences) -- UCL, 2022

Land system science, the inter-disciplinary study of the processes that shape how land and natural resources are used, is essential to solving major societal problems such as climate change, poverty, and food insecurity. This PhD thesis presents innovative remote sensing and spatial statistical techniques to understand pathways and processes of cropland and tree plantation expansion in the emerging frontier of Northern Mozambique, where large-scale land acquisitions for commercial agriculture have risen sharply over the past two decades. The empirical results of this research improve understanding of where large-scale land acquisitions are occurring in the region, the conditions in which land acquisitions consolidate smallholder cropland or encroach upon natural vegetation, and how cropland and tree plantation expansion are impacted of Mozambique’s land tenure policies and practices. The methodological findings of this research demonstrate new approaches to mapping smallholder cropland, large-scale commercial agriculture, and tree plantations using freely available software and satellite imagery. As Mozambique and many other countries ramp up efforts to mitigate climate change and promote rural economic development by investing in new tree plantations, this research shows that it is critical to monitor not only plantation expansion, but also the expansion pathways and land use trajectories to better understand the social, economic and environmental impacts of tree plantation expansion., (SC - Sciences) -- UCL, 2021

Tree-ring widths represent the most commonly used proxy to reconstruct the climate of the last millennium at high resolution, thanks to their large-scale availability. The approach often relies on a relationship between tree-ring width series and climate estimated on the basis of a linear regression. However, the underlying linearity and stationarity assumptions may be inadequate and dendroclimatic process-based models may be able to overcome some of those limitations. In this thesis, we have included for the first time a dendroclimatic process-based model, MAIDEN, into a data assimilation procedure in order to improve large-scale climate reconstructions. We have first developed a protocol to calibrate and apply MAIDEN to potentially any sites with tree-ring width data in the extratropical region. We have shown that MAIDEN can be applied at global scale with a good performance and that the clear benefits obtained make it an appealing candidate for large-scale paleoclimate reconstructions. Subsequently, the MAIDEN model has been successfully incorporated into a data assimilation procedure as a proxy system model allowing a robust comparison of the outputs of an Earth system model with tree-ring width observations. The procedure has been tested to reconstruct the climate variability in the Southern Hemisphere over the last four centuries with encouraging results. Even though additional analysis are needed to fully take advantage of the new development, important steps have been made to demonstrate that using MAIDEN as a proxy system model is a promising way to improve the large-scale climate reconstructions with data assimilation., (SC - Sciences) -- UCL, 2021

Frozen sea water, called sea ice, is an important actor of the climate system. It covers about 12% of the world's oceans. More than reflecting the incoming light, it regulates the exchanges of heat, momentum and matter between the ocean and the atmosphere in polar regions. More extended than its Arctic counterpart, the Antarctic sea ice actively participates in the redistribution of water masses in the world’s major ocean basins. Contrary to what is commonly believed, the Antarctic sea ice has been relatively unaffected by global warming. Until recently, satellite observations even showed a small positive sea ice cover trend. This trend is punctuated by large interannual variations, with a record-high cover in 2014 and a record-low cover in 2017. This makes the Antarctic climate unique and sea ice predictions challenging. At short timescales, predictions are subject to errors originating from incorrect initial conditions (ICs), model imperfections, and by “chaos”. While we can act to reduce the first two sources of errors, chaos is inherent to fully coupled climate models. Focusing on this source of error using an idealised protocol, this thesis demonstrates that such models can provide skilful sea ice edge predictions. The predictability is accounted for by the ocean with its great thermal inertia. Unfortunately, we showed that there is still a large predictability gap between idealised and operational predictions. The dearth of observations is problematic to start a prediction. Our results suggest that the errors in the ocean–sea ice ICs could even dominate the errors coming from an incorrect representation of the atmospheric conditions. The imperfect representation of the Antarctic climate in models is another major obstacle. A better observational coverage would certainly help to fix both issues., (SC - Sciences) -- UCL, 2021

The Antarctic Ice Sheet (AIS) is the biggest reservoir of fresh water on Earth. During the 20th century, the AIS has gained mass at its surface through an enhanced snow accumulation. However, our understanding of the Antarctic snow accumulation variability is still limited, which directly impacts on our confidence in the future contribution of the AIS to the sea-level rise. The main objective of this thesis is to highlight the physical mechanisms behind snow accumulation variability by investigating the links between snow accumulation and other variables characterizing the surface climate at high southern latitudes. The present work is based on various types of data – instrumental observations, paleoclimate records and climate model simulations –, focuses on different time periods – from the last millennium to the 21st century – and uses diverse methodologies, such as data assimilation. Our results show that the variability in snowfall and temperature are strongly related over Antarctica, and therefore that the classical Clausius-Clapeyron relationship explains relatively well the multi-decadal, regional to continental-scale snow accumulation changes. Our work also highlights that data assimilation is particularly appropriate for reconstructing the surface climate changes in the Antarctic over the past centuries as the observational network is sparse and includes various types of records and because of the non-stationarity of the link between the proxy records and reconstructed variables. With this technique, we have been able to provide robust past estimates of temperature and snow accumulation but also of variables that cannot be directly obtained from the records themselves such as the atmospheric circulation and even the sea-ice coverage., (SC - Sciences) -- UCL, 2021

The general framework of this work is study of hybridization between two Rhinanthus species (Orobanchaceae), Rhinanthus minor L. and Rhinanthus major using advanced genomics tools. These species are annual hemi-parasitic plants in which hybridization and introgression have been documented before. In naturally mixed populations of both species, hybridization leads predominantly to introgression from R. minor into R. major. Furthermore, hybrids with R. major as the maternal parent (F1a) have often been reported to have low germination rates in comparison with hybrids with R. minor as maternal parent (F1m). Therefore, in order to better estimate the degree of introgression between both species, a large panel of SNP markers was developed in this thesis project and used on samples collected from several mixed populations. In order to understand the mechanisms behind the low germination rate of F1a seeds, a differential gene expression study was conducted during dormancy and germination. Using ddRAD sequencing nearly 1100 species-specific SNP markers were obtained for both species and tracked in nine mixed populations from Belgium and the Netherlands. Introgression was much more frequent than could be detected with previously used ISSR and RAPD makers. Further investigation of our species-specific markers in R. major individuals with a low admixture rate with R. minor showed that only a few of the loci tended to introgress frequently, while the others had low introgression rates. Although with ddRAD sequencing we only covered a small part of the genome, we were still able to find similarity and dissimilarity of introgressed loci among the populations. Using RNA-seq in F1a and F1m seeds during cold stratification and after germination revealed that there were large transcriptome differences between seeds with different maternal plants, but we could not detect any differentially expressed genes in the plastid genome, what would be expected in case of a nuclear-cytoplasmic i, (SC - Sciences) -- UCL, 2021

In the context of growing societal demands for land-based products, pressure on forest and agricultural area is increasing. This dissertation explains some of the complexities on agricultural and forest dynamics, namely, the indirect consequences of agricultural intensification and the interconnection with agricultural expansion, and the non-linear trajectories and feedbacks of agricultural and forest areas' trajectories through the exploration of forest transitions and agricultural frontiers. The following three research questions were explored: i) What are the causal relationships between changes in cropland intensity and expansion and contraction of cropland areas? ii) What are the causal relationships and indirect effects between changes in forest, agricultural areas and major socioeconomic drivers? iii) Which factors and conditions shape an agricultural frontier’s trajectory? Combining panel data econometric techniques, causality analysis, theoretical modelling and qualitative data from experts’ interviews, this dissertation shows the importance of distinguishing between short and long run dynamics. Furthermore, the results provide evidence of a rebound effect in low- and middle-income countries over the short run for several commodities with high price elasticity of demand and, a land sparing happened over the long run for some stable cereals. In contrast, results show a rejection of the Environmental Kuznets Curve for most of the countries groups and globally, with the exception of high-income countries in the long run and post-forest transition countries in the short run. Lastly, the forest scarcity and the state of forest policy were the most relevant forest transition pathways. When zoom in the agricultural frontier regions, this dissertation shows the important role of both, structural and human agency factors- namely, the agglomeration economies and the farmer’s expectations- on the frontier trajectory and it provides a framework to compare agricultural, (SC - Sciences) -- UCL, 2021

The exchanges between the ocean, cryosphere, and atmosphere in the Southern Ocean play a pivotal role in the Earth’s Climate by driving the storage and redistribution of heat, freshwater, and CO2. However, our understanding of the processes at play is limited due to the scarcity of in-situ observations. Besides, climate models disagree on the future of Antarctica and appear to misrepresent the current trends. Is there something we are missing? Small-scale processes might be an answer. These processes, which take place at scales smaller than 100 km, are particularly hard to observe and cannot be explicitly resolved by most climate models. In this thesis, we explore their role in the interactions between the ocean, cryosphere, and atmosphere using high-resolution numerical models. The focus is given to the Adélie Land sector, one of the few birthplaces of the Antarctic Bottom Water which fills the depths of the World Ocean. The influences of ocean-ice shelf interactions, ocean tides, small-scale katabatic winds, and mesoscale eddies in ocean-cryosphere-atmosphere interactions are investigated. We show that, despite their small spatial scales, these processes have non-negligible effects on sea ice production and transport, water masses transformations, and on the exchanges of heat, freshwater, and momentum between the ocean, cryosphere, and atmosphere. Our results suggest that these processes could have important implications for the dynamics of the Antarctic climate and the way it will respond to the anthropogenic forcing. This thesis improves our understanding of rarely observed processes and stresses the need to incorporate them in climate models., (SC - Sciences) -- UCL, 2021

Agro-food commodity consumption is intricately linked to the loss of the planet’s forests. Complex, global agro-food supply chains, which are shaped by globalization processes, tie distant places together, causing consumption in one region to affect land use in another region, in particular, to drive deforestation. In this thesis, I explore how governance interventions in agro-food commodity supply chains can foster sustainable land use and help deliver deforestation-free agro-food supply chains. I conclude that it is not a lack of governance approaches, which hinders sustainability governance of agro-food commodity supply chains. Rather, a confluence of four specific factors complicates public and private governance. Firstly, the complexity of the problem, in particular the ‘wicked’ nature of agro-food commodity-driven deforestation, causes unclear feedbacks and spatially separated cause-effect mechanisms. Secondly, companies and private sector governance play a crucial, but underappreciated role in delivering governance. I show that company characteristics affect governance, while insufficient commitment among laggards and inadequate support for leaders limit the capacity of private sector governance. Thirdly, public governance plays an increasingly important role by supporting, engaging, and enabling private governance efforts through layered governance. Finally, technological advancements can enable governance by aiding implementation, reducing complexity, increasing accountability, and empowering supply chain actors, but technology is not a silver bullet that solves all governance issues., (SC - Sciences) -- UCL, 2021

Commodity trade is central to the global economy but is also associated with socio-environmental impacts, for example, deforestation, especially in producer countries. It is crucial to understand how geographic sourcing patterns of commodities and commercial relationships between places and actors influence land-use dynamics, socio-economic development, and environmental degradation. Here, we propose a concept and methodological approach to analyze the geographic stickiness of commodity supply chains, which is the maintenance of supply network configurations over time and across perturbations. We showcase policy-relevant metrics for all Brazilian soy exports between 2003 and 2017, using high-resolution supply chain data from www.trase.earth. We find that the Brazilian soy traders with the largest market share exhibit stickier geographic sourcing patterns, and that the supply network configurations between production places and traders become increasingly sticky in subsequent years. Understanding trade stickiness is crucial for supply chain accountability, because it directly affects the effectiveness of zero-deforestation commitments., (SC - Sciences) -- UCL, 2021

The year 2017 saw Cassini plunge to its demise into Saturn's atmosphere, putting an end to a twenty-year odyssey that challenged our beliefs on the possible presence of water and life elsewhere in the Universe. The Cassini-Huygens mission left a rich legacy of observations of Saturn and its ever-evolving world of rings and moons. The intense geological activity discovered throughout Enceladus's south-polar terrain is a puzzle in itself: where are the geysers' water supplies? how abundant is water? has it always remained unfrozen, and if so, how? The purpose of this thesis is to address some of these questions using Cassini's measurements of Enceladus's gravity field, surface topography, and diurnal libration. The latter, a subtle modulation of the moon's spin rate over its eccentric orbit around Saturn, induces slight offsets in images taken by Cassini's cameras. To correctly interpret the observations, we develop new, mutually consistent isostasy and libration models, correct to the second order in the flattenings associated with the moon's triaxial shape. We then show that the liquid water reservoir must be a global ocean as opposed to a regional sea, and obtain the best post-Cassini constraints on the thickness of the icy crust, the ocean, and the core. To alleviate the mathematical burden, we present a systematic method to handle equations in near-spherical geometries based on series expansions in powers of the flattenings, and prove their convergence. By sharing our code TenGSHui, we hope to entice the community into more reproducible and intuitive modelling, and reconnect global geodynamics with its most fundamental principles., (SC - Sciences) -- UCL, 2019

Methane on Mars continues to be the subject of much debate, particularly because its generation mechanism remains unidentified. Regardless of its formation, methane could be stored in subsurface reservoirs such as clathrate hydrates. Their destabilization stimulated by pressure and temperature changes could be the source of the recent observations of martian methane. In this thesis work, a subsurface thermal model is first coupled with a thermodynamic model of clathrate hydrates in order to define global stability regions in the martian crust and to investigate methane trapping. Then, a diffusive-adsorptive model of gas transport is used to constrain the intensity, variation and duration of the surface flux resulting from shallow CH4 sources located into the stability zone of clathrate hydrates on Mars. Finally, atmospheric transport simulations based on these surface flux patterns are compared to observations. Our results show that stability conditions of methane clathrate hydrate are already met in the shallow martian subsurface (a few meters to a few tens of meters deep). However, clathrates that would form at depth in the martian crust would contain a larger fraction of methane than shallow clathrates formed from a similar gas phase. While diffusive transport is consistent with low background levels of methane measured at Gale crater, clathrate dissociation and subsequent methane diffusion through the regolith cannot be responsible for CH4 spikes or short-live methane plumes., (SC - Sciences) -- UCL, 2019

Population and income growth, urbanisation and technological advances have led to intensification of livestock production systems over the past decades. This has significant health and environmental impacts (e.g. soil pollution and spread of epidemics). A trend in intensification is observable globally, but the level of intensification varies between countries. As the characteristics and the distribution of farms appear to change as a result of intensification, the impacts of livestock production systems are expected to evolve. This creates a need for a better understanding of the spatial factors influencing intensive farming. We also need high-resolution maps of livestock production systems, which are crucial to help assess the impacts of livestock intensification process. This dissertation explores the spatial features of chicken production and elaborates methods to predict chicken farm locations. First, we characterise the spatial constraints of chicken farmers in their early stages of intensification, using survey data from western Kenya. We found a diversity of semi-intensive farms in this region and show that local conditions affect production management types. Second, we build farm distribution models (FDMs) to predict chicken farms’ location and size in four countries spread along a gradient of intensification; Nigeria, Thailand, Argentina and Belgium. We also test the ability of the FDMs to simulate the spread of highly pathogenic avian influenza H5N1 in Bangladesh. Using point pattern analysis method, we developed FDMs that account for farm clustering. This allows for a more realistic spatial representation of livestock systems. Upon further validation, this method could represent an interesting tool to model the transmission of epidemic diseases in livestock systems., (SC - Sciences) -- UCL, 2019

From measurements performed by the Galileo and Cassini-Huygens missions, there is now ample evidence of the presence of a subsurface ocean under the thin ice shell of large icy satellites such as Europa, Callisto, Ganymede and Titan. However, many physical properties (such as the thickness, the density and the rigidity) of the ocean and of the ice shell are still uncertain. In comparison with the other large icy satellites, more data have been collected about Titan thanks to the numerous flybys of this satellite by the Cassini probe and the landing of Huygens on the surface of Titan. Titan is the only satellite to possess a significant atmosphere. It also harbours methane lakes and seas on its surface. These methane lakes and seas are involved in a methane cycle similar to the water cycle on Earth. Recently, a deviation of the rotation rate of Titan from its mean synchronous value has been observed from a comparison between images taken by the Cassini probe at different times between 2004 and 2009 (Meriggiola et al. 2016). The goal of this doctoral Thesis is to study and develop methods to constrain the interior of the large icy satellites of the Solar System and Titan in particular. Due to the presence of the subsurface ocean, the interior and the shell of the large icy satellites can move differently from each other. Firstly, the oscillations of the interior and of the shell (« Slichter modes ») of the satellites are studied for a large range of internal structure models. Secondly, the rotation variations of Titan are modeled. Rotation variations include (1) variations of the speed of rotation due to the gravitational torque exerted by the central planet, or « librations in longitude »; (2) variations of the speed of rotation due to angular momentum exchanges with the atmosphere, or « length-of-day variations » and (3) variations of the orientation of the rotation axis with respect to the surface of the satellites, or « polar motion ». The diurnal librations of Ti, (SC - Sciences) -- UCL, 2018

In the global effort to achieve sustainable development, new adaptive approaches of urban planning call for a reliable understanding of Urban Morphodynamics: the evolution of the spatial structure of an urban system from elementary interactions between its components. In this thesis, theoretical and methodological contributions to Urban Morphodynamics are proposed from the integration of the two main strands of literature in quantitative urban geography: Location Theory, which studies the economic determinants of the location of economic activities, and Complex Systems, which describe how cities self-organize from the dynamic interactions of many individuals. After an historical review in the first part, its second part addresses two problems of Urban Morphodynamics, gathering concepts and methods from both Location Theory and Complex Systems. First, using mathematical modelling, it exposes the scaling properties of the equilibrium monocentric city model. Second, coupling analytical developments with agent-based simulations, it explores the dynamic growth of a non-monocentric city model. In a third part, it proposes a hybrid modelling framework that combines the behavioural foundations of Discrete Choice Theory with the nonlinear dynamic models of Synergetics. This framework is then applied to a core-periphery model of New Economic Geography in order to forecast regional development trajectories. In a fourth and last part, in light of the proposed models, this thesis argues that the theoretical foundations of Location Theory and Complex Systems can be reconciled in an operational urban morphodynamic theory., (SC - Sciences) -- UCL, 2018

The movement of sediment from mountainous uplands to continental margins, i.e. the sediment cascade, modifies the terrestrial surface and eventually drives the rate of topographic evolution. Understanding the pattern and rates of geomorphic mechanisms involved in the sediment cascade is of crucial importance to constrain biogeochemical cycles, long-term landscape evolution and potential feedbacks between tectonics, erosion and climate. Sediment cascading systems are still poorly constrained, as sediment budgets and connectivity can be highly variable in space and time due to the stochastic character of geomorphic processes. In an effort to improve our quantitative understanding of the sediment cascade in landslide-prone environments, this doctoral dissertation aims at constraining spatio-temporal sediment dynamics over annual to millennial time scales. We applied state-of-the-art geomorphic techniques to constrain sediment budgets and connectivity for a mountainous catchment located in the foothills of the Central Swiss Alps. At the annual time scale, we monitored topographic changes using 3D topographic reconstructions derived from aerial pictures acquired by a drone, and at the millennial time scale, we quantified catchment-scale denudation rates using in-situ produced cosmogenic radionuclides. By integrating sediment fluxes over different temporal scales, it becomes clear that geomorphic process rates in landslide-prone terrain are highly variable in space and time. Periods of sediment remobilisation by landsliding on the hillslopes do not necessarily coincide with sediment pulses in the river network. Only when hillslopes are geomorphically connected to the river channels, the sediment mobilised by landsliding on the slopes is effectively transferred to the channel network. Therefore, phases of low or high geomorphic activity in upland catchments are not necessarily indicative for the long-term sediment fluxes of mountainous catchments. Their spatio-temporal sed, (SC - Sciences) -- UCL, 2018

Soils are a major reservoir for organic carbon storing more carbon (C) than vegetation and the atmosphere combined. Soil organic carbon (SOC) is regarded as a key soil property due to its effects on soil functions such as water retention or soil fertility. The removal of natural vegetation for agricultural purposes intensified soil redistribution processes and associated lateral SOC fluxes. There is now increasing acceptation that accelerated lateral SOC fluxes also alter the amount of carbon that is exchanged between soils and the atmosphere. Using a process-based, spatially-explicit modelling approach linking land cover change, soil redistribution, soil evolution and SOC cycling, this thesis highlights the strong temporal and spatial variability of the C budget at the catchment scale. Under soil erosion, SOC stock can reach a dynamic equilibrium at which the hillslope C export is compensated by the atmospheric replacement of the eroded C. The dynamic equilibrium follows a transient period of several decades in which the rate of C uptake from the atmosphere is steadily increasing. The SOC released from soils to the atmosphere quickly responded to anthropogenic land conversion while the erosion-induced C fluxes exhibited a very slow response time. Our model simulations suggest that, over the past millennium and for the environmental settings considered, the C sink could have almost offset the C losses related to land cover conversion. In addition, the long-term sustainability of the C sink and the establishment of a steady state regime are negatively affected by the impact of soil erosion on biomass productivity and soil resources. Using observations from a wide range of soil conditions, we showed that including biomass productivity evolution in the C budgeting improves the prediction of SOC losses and results in increased SOC losses and a reduced C sink intensity, relative to a scenario without soil erosion-productivity feedbacks. Finally, in contrast with environm, (SC - Sciences) -- UCL, 2018

The first part addresses time series which are sampled irregularly along the time axis, as is often seen in geophysical and climatological time series. We provide a general framework for the frequency and time-frequency analysis under irregular sampling, extending and unifying existing methods within the formalism of orthogonal projections. The Lomb-Scargle periodogram is extended to the time-frequency domain through the scalogram of the continuous wavelet transform, which is well-suited for irregularly sampled time series, since it does not require the data to be interpolated in time. The theory is developed for the specific case of the Morlet wavelet, widely used in geophysics. We also propose a test to estimate the significance of deterministic periodic components against an additive stationary Gaussian continuous autoregressive-moving-average process. The second part of the thesis tackles the problem of the estimation of the wavelet power spectrum (WPS) of a continuous-time or regularly sampled nonstationary process from one of its samples. To this end, we transfer the multitaper method (MTM) of Thomson to the wavelet case. The MTM estimator efficiently reduces the variance while minimizing the leakage outside a predefined area in the time-scale plane. The tapers of the MTM are the eigenfunctions of a localization operator in the Hilbert space. The analytical expression of the tapers is only known in specific cases, and an analytical derivation is probably not generalizable to any mother wavelet. This is why we provide a numerical scheme for the estimation of the tapers of any well-localized progressive wavelet. It is also proved that a numerical approach is tractable thanks to some invariance properties., (SC - Sciences) -- UCL, 2017

Polar oceans are partly covered with sea ice that results from the freezing of seawater. Sea ice is a leading player both in the climate of polar regions and for the Earth climate system as a whole. It acts as an insulating layer between the ocean and the atmosphere and it strongly influences the upper ocean physics. Because it is moved and deformed by winds and currents, the sea ice cover turns out to be an extremely heterogeneous medium. Sea ice models have been developed for decades, reaching nowadays a high level of complexity. In particular, the most advanced of them include a representation of the small-scale spatial variability in ice properties. The heterogeneous nature of sea ice-ocean exchanges has however been so far overlooked. This hampers the simulation of potentially important processes and introduces uncertainties in the predictions based on these models. The objective of this doctoral thesis was to investigate how subgrid-scale sea ice-ocean interactions can be represented in climate models, and to assess their impacts on the oceanography of polar regions. To this end, schemes of increasing complexity have been introduced in a state-of-the-art ocean and sea ice model. Our results show that the spatial heterogeneity of ice-ocean processes has a large-scale influence on the upper layer of the ocean as well as on the sea ice cover itself. The tools developed in this study provide concrete ways of improving the physical accuracy of current models., (SC - Sciences) -- UCL, 2016

The success of widespread invasive species depends of their ability to support the selection pressures of the colonized environments. The climatic determinant is a key barrier for naturalization and spread of invasive species, especially climate seasonality in temperate area. Diapause is the prime component of the winter response in insects, and a convenient model of phenotypic plasticity. The role of diapause in the adaptive capacity of an invasive species was investigated in the Asian tiger mosquito Aedes albopictus in French Mediterranean area. We show phenotypic differentiations between tropical and temperate strains (in embryogenesis kinetics, egg width and volume, and wing length) likely linked to genotypes. The diapause preparation phase per se increased the timing of embryogenesis, while photoperiod is responsible of egg volume variability, both findings of importance for molecular research. The role plays by diapause on the Ae. albopictus temperate population dynamics was demonstrated through an integrated approach. Life history traits (egg and larval survival, adult wing length and shape) of non-diapausing individuals were not differently affected by winter than diapausing individuals, likely due to the cold acclimation capacity. Diapause initiation and termination in field occurred at constant timing despite different winter conditions. The modeling of diapause shutdown demonstrated that diapause is not strictly obligatory to the species persistence, although it procured serious temporal and abundance advantages in the Southern margins of the temperate area. It highlights that the prime role of diapause is the seasonal synchronization and the prevention of egg hatching under fluctuating winter conditions. The suitability of the Mediterranean area to the establishment of diapausing or tropical invasive mosquito species is confirmed. Diapause is a plastic response with rapid evolution, which has implications for Northward spread of Ae. albopictus, and the f, Le succès des espèces envahissantes dépend de leur capacité d’adaptation pour surmonter les pressions sélectives de leur nouvel environnement. Le facteur climatique, tel que la saisonnalité, est un obstacle majeur pour la survie et l’expansion des espèces envahissantes. La diapause est le paramètre essentiel de la réponse hivernale des insectes et constitue un modèle d’étude de la plasticité phénotypique. Cette thèse s’intéresse à la valeur adaptative de la diapause du moustique tigre Aedes albopictus sous le climat méditerranéen. La phase de préparation de la diapause allonge l’embryogenèse tandis que la photopériode maternelle modifie le volume des œufs. Une approche intégrée a permis de mettre en évidence l’influence de la diapause sur la dynamique de population d’Ae. albopictus. Les traits d’histoire de vie (survie des œufs et des larves, morphométrie alaire) des individus non diapausants ne diffèrent pas de ceux des individus diapausants après un hiver, probablement grâce à une capacité d’acclimatation au froid. L’initiation et la levée de la diapause se produisent à date quasi-constante malgré des conditions hivernales différentes. La modélisation de la suppression de la diapause démontre que la diapause n’est pas strictement obligatoire sous le climat méditerranéen mais avantage fortement l’abondance et la distribution saisonnière du moustique tigre. Le rôle fondamental de la diapause est de synchroniser l’espèce avec les saisons et d’empêcher les éclosions sous des conditions hivernales fluctuantes. Les régions méditerranéennes sont propices à l’installation d’espèces envahissantes de moustiques, capables ou non de diapause. La diapause étant une réponse plastique à évolution rapide, ce processus sera déterminant pour l’expansion septentrionnale et l’extension de la période d’activité en Méditerranée du moustique tigre., (SC - Sciences) -- UCL, 2016

Viruses are considered as major constraints to the exchange of germplasm, genetic improvement and breeding of banana and plantain (Musa spp) through the world. To date, six viruses are recognized to affect Musa spp: Banana bunchy top virus (BBTV), Banana streak viruses (BSV), Cucumber mosaic virus (CMV), Banana bract mosaic virus (BBrMV), Banana mild mosaic virus (BanMMV), Banana virus x (BVX). BBTV, the most devastating virus of banana, is currently spread in 14 countries of Africa including the Democratic Republic of Congo (DRC), where the virus was observed for the first time in 1950 in Yangambi, Kisangani region. The situation of others banana viruses remains unknown however, some of these viruses were observed in DRC bordering countries. The main aim of this work was to elucidate the epidemiological situation, the genetic variability and the origin of banana viruses in Democratic Republic of Congo. Epidemiological surveys were conducted in southwestern part of DRC, with a special emphasis on Bas Congo province, in order to search major banana viruses. Up to 666 samples were collected from at least 122 locations and analysed using PCR based method. BBTV is a multi component virus, which possesses six DNA components named DNA-R, M, N, C, U3, S. To understand the BBTV epidemiology, 52 complete sequences of DNA-R and 30 full sequences of DNA-R, U3, M, N, C, S were study. Phylogenetic analysis was carried out by isolates nucleotides sequences comparison, for all viruses detected (BBTV, BSV, CMV). Result show that BBTV is the most widespread banana virus in DRC. The main factor contributed to the spread of this threat is the human exchange of infected planting material. Data reveals a low genetic variability of BBTV isolates, which are grouped in South Pacific group. Theses BBTV isolates are close between them and with those of other sub-Saharian Africa countries. The earliest report of BBTV in Africa relates to an isolate described in Egypt in 1901. Since the earlie, Les virus constituent une contrainte majeure à l’amélioration et aux échanges de germplasm du bananier et plantain (Musa spp) dans le monde. Les études récentes rapportent que six virus affectent le bananier et plantain (Musa spp) dont le Banana bunchy top virus (BBTV), le Banana streak virus (BSV), Cucumber mosaic virus (CMV), Banana bract mosaic virus (BBrMV), Banana mild mosaic virus (BanMMV), le Banana virus x (BVX). Le BBTV est considéré comme le virus le plus dévastateur présent dans au moins 14 pays africains y compris la République démocratique du Congo (RDC) où il a été détecté pour la première fois, en 1950, à Yangambi dans la région de Kisangani. La situation des autres virus du bananier demeure inconnue malgré que certains d’entre eux aient été détectés dans les pays limitrophes de la RDC. Le principal objectif de cette thèse a été d’élucider la présence des virus impactant la culture du bananier (Musa spp.) en RDC, leurs origines et leurs diversités génétiques. Les enquêtes épidémiologiques ont été menées en vue de rechercher les principaux virus du bananier. Un accent particulier a été mis sur les zones moins investiguées en RDC, particulièrement la province du Bas Congo, à cause de sa position géographique, pour la recherche des nouveaux virus. Plus de 666 échantillons de feuilles de bananier ont été collectés dans 122 sites et analysés en utilisant la PCR comme base technique. Le BBTV est un virus à génome multipartite composé d’au moins six ADN nommés ADN-R, M, N, C, U3, S. Au total 52 séquences complètes d'ADN-R et 30 séquences complètes de M, N, C, U3, S ont été étudiées en vue de comprendre son épidémiologie. Les analyses phylogénétiques et la comparaison des séquences nucléotidiques ont été réalisées pour tous les virus détectés (BBTV, BSV, CMV). Les résultats indiquent que le BBTV est le virus du bananier le plus répandu en RDC. Le facteur humain constitue le principal moteur de dispersion de ce virus, particulièrement à travers l’échange de ma, (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2016

At the Earth surface, the terrestrial life is supported by a thin soil mantle. Human activities can alter soil physical and chemical properties to the extent that humans are now identified as the sixth soil forming factor. Yet, quantitative data on natural soil development processes are hardly needed to distinguish anthropogenic from natural changes in the soil system. This dissertation aims to provide new insights on soil development on time scales of 103 to 105 years for two critical zone observatories: the Betic Cordillera (southeast Spain) and the Rio Grande Do Sul (southern Brazil). In the first part, the coupling between chemical weathering and physical erosion is analyzed based on chemical depletion and total denudation. In the semi-arid environment of the Betic Cordillera, soil weathering extents increase with soil thickness and decrease with increasing surface denudation rates, consistent with kinetically limited weathering. In the deeply weathered soils of southern Brazil, the soil development is strongly controlled by local topography, with increasing chemical depletion downslope. In the second part, the widely accepted concept of steady state soil thickness is challenged for thin soil profiles based on analytical data from in-situ produced cosmogenic nuclides and U-series disequilibria. In the third part, meteoric 10Be is used as a geochemical tracer for quantifying long-term soil denudation rates. Given the low retention of meteoric 10Be in highly weathered soil profiles, a new laboratory protocol involving sequential extraction of Be from the mineral reactive phases is developed and tested. The dissertation demonstrates the potential of employing multiple geochemical techniques to provide new insights in the rates of soil development, chemical weathering and surface denudation., (SC - Sciences) -- UCL, 2016

Tropical forests have been increasingly targeted by protection or restoration policies and programs. Yet, as a result of international trade, national territorial activities and policies may generate unintended environmental outcomes at various spatial scales, notably through land use displacement. This dissertation aims to answer three questions based on the two natural experiments of Bhutan and Costa Rica: (i) What are the interactions between the national-scale land use dynamics, the implementation of forest protection policies and the international trade of land-based products? (ii) To what extent is land use displaced abroad through international trade of land-based products? (iii) What are the multi-scale environmental outcomes of national forest protection policies and land use dynamics, taking into account the potential land use displacement? Over the past decades, the displacement of land use abroad by Bhutan and Costa Rica, along with the protection of their national forests, led overall to global environmental benefits. However, this masks mixed outcomes at the sub-national scale in Costa Rica, as export-oriented cultivation threatens forests through its demand for cropland and for packaging wood. This dissertation demonstrates the power of international markets in shaping national land use dynamics, and in generating unexpected feedbacks on different economic sectors at various spatial scales. It further highlights the crucial role to be played by institutions and governance in moderating the relations between economic development and its potential environmental impacts, as well as the need for integrated approaches to minimize inter-sectorial and multi-scale negative feedbacks., (SC - Sciences) -- UCL, 2016

The East African rainfall is characterized by a strong interannual variability, currently superimposed on a (multi-)decadal drying trend. While observations collected over the last decades show that the East African rainfall correlates positively with the western Indian Ocean sea-surface temperatures and negatively with the eastern Indian Ocean sea-surface temperatures, the physical processes involved and the stability of those teleconnections over time are not well known. Hence, this thesis aims to improve understanding of how sea-surface temperatures in the Indian Ocean are linked to the East African rainfall variability at the interannual to the multi-decadal time scale. This is achieved through the analysis of the simulations performed by several climate models. Various complementary methodologies are used in order to minimize the potential impact of climate models biases on our results. Based on model-data comparison, we suggest that the changes in East African hydroclimate and their link to the Indian Ocean sea-surface temperature are driven at the interannual time scale by natural variability rather by external forcings over the last millennium. Using a data assimilation scheme, we demonstrate that the observed covariance between the East African rainfall and the Indian Ocean sea-surface temperatures has the potential to improve the regional to large-scale reconstructions of both variables, although biases in the physics of current climate models prevent obtaining such skillful reconstructions. Sensitivity experiments with the climate model EC-EARTH show that the Indian Ocean surface is indeed an important driver for the East African rainfall variability at the interannual time scale, through its influence on the atmospheric circulation., (SC - Sciences) -- UCL, 2016

Rhynchophorus phoenicis possède des larves de dernier stade qui sont couramment consommées et vendues sur les marchés africains à un prix relativement élevé, ce qui en fait un produit de luxe. Le but principal de ce travail était la mise au point d’un élevage de R. phoenicis à partir de coproduits agricoles afin d’accroître sa disponibilité, de diminuer son coût et de développer une activité économique d’élevage. Nous avons déterminé premièrement les espèces du genre Rhynchophorus présentes dans la région de Kisangani (RDC) et leur phénologie. Cinq types de pièges ont été testés: les pièges de type debout, de type abattu et 3 pièges bouteilles. Le piège du type debout a été concluant et utilisé pour collecter les individus. Sur les trois espèces de Rhynchophorus collectées, R. phoenicis était de loin la plus abondante et totalisait plus de 95 % des captures. R. ferrugineus, mentionné pour la première fois en Afrique centrale et R. quadrangulus ont été occasionnellement récoltées en très petits nombres. La majorité des individus récoltés de R. phoenicis étaient infestés par de nombreux acariens sous les élytres. Trois espèces ont été déterminée : Centrouropoda almeroida H&W. (1992), Uroobovella phoenicola K., T. et R. (2012) et U. marginata C.L. Koch (1839). Le statut phorétique des acariens reste à confirmer. Les femelles portent plus d’acariens que les mâles et elles possèdent aussi de plus longs élytres. R. phoenicis, a été choisie pour commencer des essais d’élevage. Celui-ci comprend 5 étapes : 1) le piégeage des adultes ou la récolte de cocons ; 2) La préparation des adultes pour la ponte; 3) l’extraction des oeufs ou de larves de premier stade ; 4) l’infestation du substrat à tester, 5) l’incubation et la récolte de larves de dernier stade. Trois types d’infestation ont été testés : les oeufs, les larves de premier stade et les adultes, ainsi que 6 substrats : la canne à sucre, le jeune et vieux stipe de palmier, deux diètes artificielles et le vieux stipe de, (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2015

Soil organic carbon (OC) has been recognised as important for soil quality, soil resilience to degradation processes, but also as playing a major role in providing ecosystem services and in mitigating climate change. In this context, there is an urgent need for up-to-date data on the status of soil organic carbon. From local to global scale, the comprehensive assessment of soil OC faces several challenges related to soil OC spatial and temporal variability, to the lack of standard soil sampling and analytical protocols and to the absence of geo-referencing of most legacy data. The aim of this thesis was to produce the most up-to-date representation of the spatial distribution of topsoil OC content for 25 Member States of the European Union and to estimate the total amount of OC in the upper layer of the soils (0-20cm) in these same countries. To do so, ca. 20,000 OC measurements and land cover observations from the first harmonised topsoil database at European scale, the so-called LUCAS survey, were used. Specifically in this thesis, (i) a spatial database was built, consisting of soil data and environmental covariates relevant to the study of OC levels in European soils; (ii) the spatial distribution of topsoil OC content was predicted and represented by applying two different digital soil mapping techniques. The uncertainties associated to the predictions were also produced; (iii) a method for the quantification of topsoil OC stocks in Europe was proposed and applied. The results confirmed the difficulty to map and account for OC levels in soils, due to its high spatial variability and the detection of organic soils was highlighted as a major limitation in the European context. Finally, recommendations are given for a future European soil survey., (SC - Sciences) -- UCL, 2015

Understanding the speciation process usually involves the study of the geographic and ecological factors influencing reproductive isolation, as well as the action of evolutionary forces. Recently it was proposed that the type of communication channel, if it offers large possibilities in the direction of evolution of the signals, could help species to differentiate in a similar way as an ecosystem with empty ecological niches would. Olfactory communication could be such a modality since compared to other modes of communication, acoustic or visual, it benefits from a large variety of signals, thanks to the tremendous diversity of chemical compounds that can be used. In addition, the specificity of the olfactory receptors necessary to detect particular chemical compounds confers a strong privacy to the information transfer, regarding illicit receivers like predators. This thesis aimed at investigating the importance of olfactory sexual communication in the evolution of reproductive isolation in the butterfly genus Bicyclus. We first compared the composition of putative male sex pheromone (pMSP) across 32 species in a phylogenetic perspective. We observed that pMSP composition was very diverse, species specific, rapidly evolving, and that stronger differences of composition likely allow species to recognise each other when sharing the same ecosystem. In a second time, we investigated the diversity of the pMSP within species and discovered similar differences as between species. This supports the hypothesis that pMSP diversification can predate speciation and can therefore be a cause of this enigmatic process. Overall, our results strongly suggest that the evolution of male sex pheromone is involved in speciation in the Bicyclus genus, and support the hypothesis that olfactory communication allows a large diversity of signals, evolving without obvious constraint, and which could rapidly lead to reproductive isolation., (SC - Sciences) -- UCL, 2015

One of the most active disciplines in ecology these last years focuses on predicting and understanding the patterns and underlying processes of changes in species distributions under global environmental changes. Species distribution models have rapidly become the most important predictive tools in ecology to forecast the response of species to future global change. This discipline is now at a key stage of its conceptual development: it is challenged to better integrate a series of ecological processes and environmental factors that shape species distributions, abundance, community structure and ecosystem functioning. Such integration is essential to improve the ecological realism and the credibility of the future model predictions. The general objective of this PhD-thesis was to provide new perspectives in this direction. With European butterflies as model organisms, each of the core chapters tested an innovative approach focussing on the incorporation of environmental factors or ecological processes that have been neglected to date when producing future predictions. With a species distribution modelling approach addressing future shifts in species ranges, we tested the combined effect of land use and climate change, the effect of within-species spatial niche variation, and the importance of dispersal capacity for the genetic structure of the populations and persistence of the species in the future. With an experimental approach focussing on ecological processes involved during range shift, we investigated the behavioural role of habitat use to improve empirical knowledge behind the pattern of species range shifts. Based on our results, we conclude that land use change scenarios should be more often integrated with climate change scenarios in species distribution models to better reflect the response of species to global changes. We also stress the need for further development of land use change scenarios to better match the requirements of the ecological modelling, (SC - Sciences) -- UCL, 2015

info:eu-repo/semantics/published

Dispersal, the movement of individuals among populations, is a life history trait that affects ecological and evolutionary dynamics of species. It has gained much interest in the context of metapopulation dynamics which aim to understand how species cope with habitat loss and fragmentation. In this thesis, I used controlled microcosms of a unicellular organism, the ciliate Tetrahymena thermophila, to experimentally investigate various aspects of dispersal ecology. Firstly, I describe how digital image and video analysis can be used to rapidly and reliably quantify traits and abundances of the model organism. Secondly, I investigated the relative roles of the genotype and the environment on dispersal in a two-patch system by comparing how 44 genetically different lines dispersed in response to conspecific density. We found genetic variation for dispersal plasticity and hence potential for dispersal plasticity to evolve. Thirdly, I report how an individual-based simulation model parameterized with characteristics of video-recorded cell trajectories accounted for 45% of the variation in dispersal observed among genotypes. Finally, I describe the dispersal syndromes (co-variation of dispersal with other traits) found in this set of genotypes, which bear important information about the costs and evolution of dispersal: (1) negative co-variation of dispersal-related traits with peak cell density and cell survival; (2) the absence of a link between short- and long-distance dispersal. The results in this thesis contribute to our understanding of condition-dependent dispersal, dispersal syndromes, links with the general movement ecology and introduce several methodological advances widely applicable in experimental laboratory systems., (SC - Sciences) -- UCL, 2014

Mountain forests are covering 9 million km² of the Earth surface and represent a quarter of the Earth’s total forest cover. Their present structure, composition, and function are the consequence of environmental changes caused by natural and human disturbances. The aim of the thesis is to enhance our understanding of human-environment interactions in mountain landscapes by analysing impact and feedback loops between forest cover changes and the provision of ecosystem services. The thesis addresses human-environment interactions in three different mountain landscapes: the Ecuadorian Andes, Bhutanese Himalayas and Ethiopian Highlands. A methodological framework is developed that allows accurate forest cover, forest cover change and ecosystem service mapping based on high resolution multispectral satellite imagery. In mountain areas, soil erosion regulation and natural hazard control are two ecosystem services of primary importance. This study therefore first focuses on the interactions between forest cover change, erosion regulation and landslide hazards. Based on homogenized land cover data spanning a 50 years period, an innovative method for deriving a complete set of mountain ecosystem services is applied. Our results show that the ecosystem response to forest cover change not only depends on the magnitude but also on the direction of change. Afforestation is not necessarily leading to environmental and ecological benefits, as shown by the strong decrease in ecosystem service provision following grassland afforestation in South America., (SC - Sciences) -- UCL, 2014

This thesis presents a theoretical modelling approach for the key components in the socio-ecological system of Lyme borreliosis, the most important tick-borne zoonotic disease in Europe. These key components were organised and modelled from two perspectives, namely, the natural “hazard” on the spatial dynamics of ticks, and the human “exposure” to infected tick bites. Concerning the natural “hazard”, a cellular automata model was built upon empirical investigations and expert knowledge on the habitat suitability of ticks and the ecology of host community and movement. The model was applied to predict the impacts of landscape fragmentation and wildlife management practices on the spatial dynamics of ticks. Concerning the human “exposure”, an agent-based model was developed to predict the daily leisure visitation pattern in Walloon woodlands. The model integrated two empirical functions of individual travel distance and woodland attractiveness. These functions were built on individual travel behaviours, local woodland manager’s opinions, and attributes of the built environment and woodland destinations. In conclusion, this thesis shows how complex socio-ecological models for the spatial risk of vector-borne zoonoses can be adapted for decision support, allowing policy makers to assess the potential effects of changes in environmental conditions and socio-economic policies., (SC - Sciences) -- UCL, 2014

At high latitudes, a significant part of the oceans is covered by sea ice that forms when seawater freezes. Sea ice is a major player in the global climate system. It is also a primary indicator of climate change, as it responds rapidly to variations in surface air temperatures and wind regimes. Since years now, scientists have developed numerical models to assist them in better understanding the sea ice system. These models simulate the evolution of the main sea ice characteristics at high spatial resolution and temporal frequency, which makes them valuable tools to enlarge the picture given by incomplete observations. In addition, models are used for their predictive skills, from seasonal to centennial horizons. However, in spite of their increasing complexity, substantial uncertainties still persist in model reconstructions and predictions of Arctic and Antarctic sea ice. In this doctoral thesis, we have developed the tools to properly identify the possible sources of these uncertainties. We have highlighted the important role of model physics and initial conditions for the climate simulations of Arctic sea ice. We have also implemented statistical methods to optimally constrain the models given measurements, a field of research known as data assimilation. In the framework of sea ice data assimilation, we have been able to propose a multi-decadal reconstruction of Antarctic sea ice thickness, which is not possible from observational data only. Sea ice models become more comprehensive. At the same time, more and more observations are available to evaluate, constrain and improve them. To address the sensible questions about the future of sea ice and of climate in general, an optimal use of these two sources of information will be required. This thesis illustrates some steps to move forward in this direction., (SC - Sciences) -- UCL, 2014

The Arctic is one of the regions that experienced the fastest and highest warming of the planet during the recent decades, and changes in its environment had worldwide implications. Studying climate changes during the past is essential for a comprehensive understanding of the climate system, particularly to provide insights into the processes involved in the recent Arctic warming, in order to assess future global climate changes. Information about past climate evolution can be extracted from “proxy” data, while climate models can be used to interpret the observed changes. In this doctoral thesis, we have taken advantage of the complementarity of model results and proxy data, through data assimilation, to provide reliable simulations of the Arctic climate over the past millennium that are in agreement with all the possible sources of information: proxies, physics of the model and external forcings. Those simulations have then been used to analyse the role played by different forcing elements in the past Arctic climate changes. We have highlighted, for instance, the dominant contribution of volcanic eruptions in the simulated temperature trends and of the astronomical forcing in the differences in seasonal trends. The role of the atmospheric circulation in explaining some warm periods has been underlined in our simulations assimilating sparse temperature reconstructions based on proxy data. We have also demonstrated the need for a precise estimation of proxy data uncertainties to avoid inconsistencies between those data and the model physics. This study, being among the first to apply a data assimilation method in palaeoclimatology, provides a powerful tool for investigating past climate and some challenging perspectives., (SC - Sciences) -- UCL, 2014

The Antarctic sea ice widely impacts the Earth climate through its interactions with the ocean and the atmosphere. Unlike its Arctic counterpart, the sea ice surrounding Antarctica has been slightly expanding since 1979. A possible explanation for this observed increase relies on the internal variability of the Antarctic sea ice, particularly large at multi-decadal timescales. Unfortunately, reliable observations of sea ice do not span a long enough time period to properly examine how the sea ice varies between decades. The goal of this doctoral thesis is first to investigate the mean state and the internal variability of the Antarctic sea ice simulated by general circulation models (GCMs). This allows checking if these models reproduce reasonably well the observations and if the observed positive trend in sea ice extent is compatible with the internal variability of the models. Second, we assess the impact of the initialisation of a simulation using various data assimilation methods on the skill of the predictions of the Antarctic sea ice at decadal timescales. According to our results, the observed increase in Antarctic sea ice extent is compatible with the internal variability simulated by current GCMs, although it is a relatively rare event. Unfortunately, these models generally display systematic biases in the way they reproduce the mean state and the internal variability of the Antarctic sea ice. In an idealised framework, we show that the interannual variations of the Antarctic sea ice are not predictable beyond three years. Besides, an adequate initialisation clearly improves the skill of the prediction of the multi-decadal trends in sea ice extent, in both idealised and realistic conditions., (SC - Sciences) -- UCL, 2014