<p>Northern boreal peatlands represent a globally significant carbon pool that are at risk of drying through land-use change and projected future climate change. The current ecohydrological conceptualization of peatland response to persistent water table (WT) drawdown is largely based on short-term manipulation experiments, but where the long-term response may be mediated by vegetation and microtopography dynamics. The objective of this thesis is to examine the changes to peatland evapotranspiration, soil physical properties, and moisture stress in response to a long-term WT manipulation. The energy balance, hydrology, vegetation, and soil properties were examined at three adjacent peatland sites in the southern sub-boreal region which were subjected to WT manipulations on the order of ±10 cm at two treatment sites (WET, and DRY) compared to the reference site (INT) as a result of berm construction in the 1950s.</p> <p>Sites with an increasing depth to WT were found to have greater microtopographic variation and proportion of the surface covered by raised hummocks. While total abundance of the major plant functional groups was altered, species composition and dominant species of vascular and non-vascular species within microforms was unaltered. Changes in vegetation and microtopography lead to differences in albedo, surface roughness, and surface moisture variability. However, total ET was only significantly different at the WET site. Transpiration losses accounted for the majority of ET, where LAI best explained differences in total ET between sites. Surface moisture availability did not appear to be limiting on moss evaporation, where lab results showed similar moisture retention capacity between microforms and sites, and where low surface bulk density was shown to be a strong controlling factor. Modelling results further suggested that, despite dry surface conditions, surface moisture availability for evaporation was often not limited based on several different parameterizations of peat hydraulic structure with depth.</p> / Doctor of Philosophy (PhD)

Global warming and extreme weather events have impacted the ability of Earth’s forest ecosystems to sequester atmospheric carbon dioxide. The full effects of these events on forest productivity, vulnerability, and the carbon cycle have not yet been fully assessed. One potentially fruitful approach is to explore past climate and forest growth patterns through tree ring records. These records may be used to explore how past environmental events may have impacted tree growth and provide insight into the functioning of forest ecosystems in the future. The stable isotope ratios (e.g. 13C to 12C) of tree ring material also provide additional information about tree growth trajectories and environmental stressors that may not be recognized in radial growth. In this study, tree ring and stable isotope records were measured and constructed to explore the dynamics of growth over the lifespan of plantation pine stands in southern Ontario. Tree ring growth records were used to determine the effects of climate and other environmental changes on radial growth. These records were constructed from two white pine (Pinus strobus L.) plantations established in 1939 (TP39) and 1974 (TP74) and one red pine plantation established in 1931 (TP31). Air temperature, precipitation, and drought indices were analyzed at monthly combinations to determine controls on growth. Temperature was consistently negatively correlated to growth, while precipitation and Palmer Drought Severity Index (PDSI) were consistently positively correlated to growth. The effectiveness of each climate variable to control ring growth differed between sites which may be related to stand age, stand density, and management factors. In both white pine plantations, inter-annual eddy-flux quantifications of gross ecosystem productivity (GEP) was found to be significantly related to tree ring growth over the overlapping period from 2003 to 2017. These relationships enabled an inter-annual estimate of GEP to be constructed for both growth chronologies over the period 1942 to 2017 for TP39 and 1981 to 2017 for TP74). Additionally, growth rings from three specimens in two different-age (14- and 77-year old) white pine plantation forests were analyzed for stable carbon isotope ratios to identify both short- and long-term variations in the physiological response to changing environmental conditions. Variations in δ13C time series from whole wood samples provided a potential record of intrinsic water use efficiency (iWUE) for these three trees. These iWUE records were compared to climate records and inter-annual eddy-flux quantifications of GEP and evapotranspiration (ET). Long-term iWUE was found to increase by 50 μmol mol–1 yr–1, with nearly all of the increase occurring as the tree shifted into active homeostasis of stomatal control in the late 1960s. Changes in time series of internal and external concentration of CO2 (ratio) also displayed a significant shift from first increasing and then decreasing trend. In the three wood samples, air temperature, ET, and GEP were found to be significantly, but inconsistently related to iWUE. The work of this thesis shows that tree ring properties are strongly related to key environmental variables such as temperature and drought stress in pine plantation forests in southern Ontario, Canada. Results also suggest that dendrochronology and isotope tracers are useful tools to be used to evaluate historical environmental impacts on growth in these different-age plantation stands. The background knowledge of climate drivers acting on tree ring growth and ring isotopic compositions over the forests’ history may be used to make informed management decisions to promote tree productivity in a changing climate in Eastern North America. / Thesis / Doctor of Philosophy (PhD) / The full effect of water availability and environmental factors on forest productivity, vulnerability, and the carbon cycle has not been fully assessed. Tree ring chronologies offer one approach to explore past climate and forest growth patterns. These records may be used to identify past environmental events may have impacted tree growth and provide insight into the functioning of forest ecosystems in the future. Additionally, stable carbon isotope ratios (δ13C, or 13C to 12C) of tree ring material provide information about tree intrinsic water use efficiency (iWUE) which is not captured in radial width measurements. Lastly, inter-annual eddy-flux quantifications record stand-level dynamics of ecosystem productivity. In this study, tree ring, stable isotopes, and eddy-flux records were measured and constructed to explore the dynamics of growth over the lifespan of plantation pine stands in southern Ontario. In all three techniques, records were constructed from three white pine (Pinus strobus L.) plantations established in 1939 (TP39), 1974 (TP74) and 2002 (TP02). Air temperature, precipitation, and drought indices were analyzed at monthly resolution to determine controls on water use and productivity. Temperature was consistently negatively correlated to growth, while precipitation and PDSI were consistently positively correlated to growth. Variations in the δ13C time series from whole wood samples also provided a record of iWUE. Long-term iWUE was found to increase by 50 μmol mol–1 yr–1, with nearly all of the increase occurring as the tree shifted into active homeostasis of stomatal control in the late 1960s. In all three white pine plantations, inter-annual eddy-flux quantifications ecosystem productivity were found to be significantly related to tree ring growth over the overlapping period from 2003 to 2017. These relationships enabled an inter-annual estimate of tree ring-inferred fluxes to be constructed for all three growth chronologies. These results suggest that dendrochronology and isotope tracers are useful tools to be used to evaluate historical environmental impacts on growth in these different-age plantation stands. The interrelationships of tree ring growth, ring isotopic compositions, and eddy-flux quantifications found here serve as useful background knowledge on which to base additional studies of forest climate change impacts.

Humans have extensively altered terrestrial surfaces through land-use and land-cover change. This change has resulted in increased food, fiber, fuel, and wood that is provisioned by ecosystems to support the human population. Unfortunately, the change has also altered climate through carbon emissions and changes in the surface energy balance. Consequently, maximizing both the provisioning and climate regulation services provided by terrestrial ecosystems is a grand challenge facing a growing global population living in a changing climate. The planting of pine forests for timber and carbon storage and switchgrass fields for bioenergy are two land-cover types that can potentially be used for climate mitigation. Importantly, both are highly productive systems representing contrasts in albedo (grass are brighter than pines) and vegetation height (pines are taller than the grass) along with unknown differences in carbon and water balance that influence local to global climate. Here I use eddy-covariance data to investigate how a transition from a perennial bioenergy crop (switchgrass) to a planted pine plantation alters the local surface temperature, global carbon dioxide concentrations, and global energy balance. First, I found that switchgrass and pine ecosystems have very similar local surface temperatures, especially during the grass growing season. After the switchgrass is harvested, surface temperature in the pine forest is much lower than switchgrass because no vegetation is present to facilitate the evaporation of water. The surface temperature in a bare-ground system (a recent clear-cut) was also high relative to the pine and pre-harvest switchgrass ecosystems. This illustrates the importance of maintaining vegetation cover to reduce local surface temperature. Second, I found that the 30-year mean change in global energy balance (i.e., radiative forcing) from planting a pine ecosystem rather than a switchgrass field was positive (pine warms climate) when considering changes in albedo and carbon measured using eddy-covariance systems. When including harvested carbon, pine and switchgrass can have similar global radiative forcing if all harvested pine carbon is stored, but harvested switchgrass carbon is burned. However, no scenarios I explored resulted in a strong negative radiative forcing by the pine ecosystem relative to the switchgrass field. These results show that afforestation or reforestation in the eastern United States may not result in any climate benefit over planting a switchgrass field. However, the presence of vegetation in both ecosystem types offers a clear benefit by cooling local surface temperatures. / Doctor of Philosophy / Humans are changing the Earth's climate by using oil and gas as fuel that emits greenhouse gases, mainly carbon dioxide, into the atmosphere. Planting trees to reestablish forests is a natural solution for climate change because forests absorb carbon dioxide from the air, but reforestation also changes the Earth's climate in other ways. For example, forests are generally darker than crops and grasses and absorb more sunlight, which traps energy in the atmosphere that can warm global temperature. These non-carbon effects can potentially offset the climate benefit from absorbed carbon dioxide. An alternative natural climate solution is to replace oil and gas with fuels derived from plants, known as bioenergy. Here I compared the local and global climate influence of a tree plantation (loblolly pine) to a bioenergy crop (switchgrass). I found that the local temperature of pine and switchgrass were similar in the summer when the grass was growing, and both were cooler than bare-ground, which was unable to evaporate and transpire water to the atmosphere. Over 30 years, I found that pine and switchgrass absorb similar amounts of carbon. The pine forest absorbs more carbon than switchgrass when it is fully grown but releases carbon during the first five years of growth. As a switchgrass field is brighter than a pine forest, planting a pine forest instead of a switchgrass field warms the Earth's climate. However, assuming no carbon from the harvested trees is released to the atmosphere, the pine and switchgrass have the same influence on global climate. My findings show that a pine plantation and a bioenergy crop can have similar climate benefits when carbon is stored in forests.

L'impact des modifications climatiques et de l'augmentation de la démographie sur le cycle de l'eau et de l'énergie dans la région ouest africaine passe par la quantification des échanges entre les différents couverts de la surface continentale et l'atmosphère. Toutefois, la compréhension du rôle des interactions entre la surface et l'atmosphère dans la dynamique de la mousson ouest africaine est limitée par le manque d'observations dans cette région. Cette thèse porte sur l'étude des flux turbulents, en particulier l'évapotranspiration réelle, en climat soudanien. Les deux couverts étudiés sont une forêt claire (site de Bellefoungou) et une mosaïque de culture/jachère (site de Nalohou), situés dans la région du Nord – Bénin. On dispose de quatre années de mesures (2007 à 2010). Les sites d'étude font partie de l'observatoire hydro – météorologique AMMA – CATCH.Les données de flux turbulents de l'atmosphère ont été mesurées avec la technique d'eddy covariance. La partition énergétique des flux a été examinée à travers la fraction évaporative (EF) et le rapport de Bowen (β) aux échelles diurne, saisonnière et inter-annuelle. Des caractéristiques de surface (conductance de surface et aérodynamique) et le coefficient de découplage ont été calculés pour interpréter la dynamique de l'évapotranspiration réelle.L'analyse des résultats est basée sur un découpage du cycle saisonnier suivant quatre phases du cycle de la mousson : la saison sèche, la saison humide, les phases d'humidification et d'assèchement de l'atmosphère. Aux échelles diurne et saisonnière, on montre que le taux d'évapotranspiration réelle de la forêt est toujours supérieur à celui de la mosaïque de culture/jachère quelle que soit la saison. L'évapotranspiration réelle demeure non nulle en saison sèche sur le site de Nalohou malgré les conditions de surface peu favorables à ce processus. En saison humide, après le saut de mousson, la partition énergétique des flux atteint un régime stationnaire avec une moyenne égale à 0,75 à Bellefoungou et 0,70 à Nalohou pour les 4 années étudiées. Le rapport de Bowen pris dans le même ordre est environ de 0,4 et 0,6 traduisant ainsi, en dépit des conditions humides, la part non négligeable du flux de chaleur sensible sur les deux couverts végétaux. La différence de rugosité entre les deux couverts végétaux entraîne une conductance aérodynamique nettement supérieure à Bellefoungou par rapport à Nalohou. On montre également que la végétation du site de Nalohou est plus efficace en transpiration pendant la saison humide que celle du site de Bellefoungou. A l'échelle inter-annuelle, on n'a pas pu mettre en évidence une relation entre flux de chaleur latente et pluviométrie pour les quatre années étudiées qui sont toutes des années excédentaires. Cependant nous avons observé que le rayonnement net explique la majeure partie de la variabilité inter-annuelle des flux turbulents.Enfin, nous avons également montré avec le coefficient de découplage que la surface soudanienne et l'atmosphère restent couplées toute l'année. Ce fonctionnement de l'interface surface – atmosphère reflète le rôle majeur que jouent les conditions de surface dans la variabilité saisonnière de l'évapotranspiration réelle. Les résultats issus de cette étude donnent une première estimation des flux de chaleur latente et de chaleur sensible sur une forêt claire et une mosaïque de culture/jachère en climat soudanien. Ils sont d'une importance capitale pour la paramétrisation et la validation des modèles de surface ainsi que pour la quantification robuste de la ressource en eau disponible en surface pour l'agriculture, principale activité génératrice de revenus des populations locales. / Assessing the impact of climate and anthropic changes on the water and energy cycles, mainly rely on the quantification of the transfer between the various land covers and the atmosphere. Nevertheless the land – atmosphere interactions in the West African monsoon dynamic is not yet well understood because of the lack of observations in this region. This thesis focuses on the analysis of the sensible and latent heat fluxes under Sudanian climate. The two studied land covers are a clear forest (Bellefoungou) and a cultivated area (Nalohou), located in northern Benin, during four years (2007-2010). The study sites are a part of the hydro – meteorological AMMA – CATCH observatory.Turbulent fluxes were measured with the eddy covariance technique.The flux partitioning was investigated through the evaporative fraction (EF) and the Bowen ratio (β) at diurnal, seasonal and inter-annual scales. Surface characteristics (surface and aerodynamical conductance) and the decoupling factor were calculated to interpret the dynamic of the actual evapotranspiration.The analysis was performed according to four different stages of the monsoon cycle: dry and wet seasons drying and moistening intermediate stages. At diurnal and seasonal scales, actual evapotranspiration was always higher on the forest than on the cultivated area. It remained non zero during the dry season at Nalohou despite surface conditions which were not favorable to this process. During the wet season, after the monsoon onset, EF remained steady with a mean seasonal value of 0.75 at Bellefoungou and 0.70 at Nalohou for the four studied years. The Bowen ratio was 0.4 and 0.6 respectively, thus the sensible heat flux was significant on the two contrasted vegetation covers during the wet season. The contrasted roughness length of the two vegetation covers led to a highest aerodynamic conductance at the clear forest site. The mixed of crop/fallow was shown to be more efficient than the clear forest regarding wet season transpiration. At the inter-annual scale, no relationship can be evidenced between evapotranspiration and annual rainfall for the studied period (2007-2010), which was rather rainy. Nevertheless, the net radiation explains the main part of turbulent fluxes inter-annual variation.Finally, complete surface atmosphere decoupling was never observed. This property of the surface – atmosphere interface underlines the key role of the surface conditions in the actual evapotranspiration. Our results provide a first estimate of the latent and sensible heat fluxes over a clear forest and a mixed crop/fallow under sudanian climate. They are relevant to land surface models parametrisation or evaluation and to a robust quantification of the water resources for agriculture, the main economic activity in this region.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste estudo, o fluxo de calor sensível (H), fluxo de calor latente (Le), evapotranspiração (ET), foram investigados utilizando o método de covariância de vórtices durante o período de cultivo de trigo em 2011. A estimativa usando a equação de Penman-Monteith também foi realizada. A influência dos diferentes sistemas de cultivo, convencional e direto, na fenologia de trigo foi realizada utilizando os dados de índice de área foliar (IAF) e produtividade.O experimento foi realizado no município de Cruz Alta-RS, em uma área agrícola localizada na Cooperativa Central Gaúcha Ltda Tecnologia. Durante o período de cultivo do trigo foram coletados dados fenométricos e micrometeorológicos. O fluxo de energia na forma de calor latente (Le) foi responsável pela utilização da maior parte da energia disponível, principalmente nos períodos em que o índice de área foliar (IAF) do trigo é maior. Os valores médios diários para o período de cultivo do trigo para H é de 7,8 Wm-2, para Le de 73,17 Wm-2, para G de -1,72 Wm-2 e para Rn de 70,95 Wm-2. Além do subsolo aquecer a superficie, também houve uma advecção de mais de 15 Wm-2 no período para esta região. A evapotranspiração experimental acumulada foi de 256 mm, enquanto que a precipitação total foi de 799 mm. Contudo, a evapotranspiração acumulada obtida utilizando a equação de Penman-Monteith foi de 282,72 mm. As médias para as resistência aerodinâmica e de superfície foram 39,54 sm-1 e 378,98 sm-1, respectivamente. Com relação à fenologia da cultura agrícola, para o sistema de cultivo convencional o trigo obteve maior produtividade, além de maior IAF. / Neste estudo, o fluxo de calor sensível (H), fluxo de calor latente (Le), evapotranspiração (ET), foram investigados utilizando o método de covariância de vórtices durante o período de cultivo de trigo em 2011. A estimativa usando a equação de Penman-Monteith também foi realizada. A influência dos diferentes sistemas de cultivo, convencional e direto, na fenologia de trigo foi realizada utilizando os dados de índice de área foliar (IAF) e produtividade.O experimento foi realizado no município de Cruz Alta-RS, em uma área agrícola localizada na Cooperativa Central Gaúcha Ltda Tecnologia. Durante o período de cultivo do trigo foram coletados dados fenométricos e micrometeorológicos. O fluxo de energia na forma de calor latente (Le) foi responsável pela utilização da maior parte da energia disponível, principalmente nos períodos em que o índice de área foliar (IAF) do trigo é maior. Os valores médios diários para o período de cultivo do trigo para H é de 7,8 Wm-2, para Le de 73,17 Wm-2, para G de -1,72 Wm-2 e para Rn de 70,95 Wm-2. Além do subsolo aquecer a superficie, também houve uma advecção de mais de 15 Wm-2 no período para esta região. A evapotranspiração experimental acumulada foi de 256 mm, enquanto que a precipitação total foi de 799 mm. Contudo, a evapotranspiração acumulada obtida utilizando a equação de Penman-Monteith foi de 282,72 mm. As médias para as resistência aerodinâmica e de superfície foram 39,54 sm-1 e 378,98 sm-1, respectivamente. Com relação à fenologia da cultura agrícola, para o sistema de cultivo convencional o trigo obteve maior produtividade, além de maior IAF.

Les sols agricoles sont la principale source du gaz à effet de serre N₂O. Ces émissions sont caractérisées par une variabilité spatiale et temporelle considérable, ce qui rend très difficile leur quantification. L’UR SOLS étudie depuis 2008 les émissions de N₂O dans une zone agricole du Centre de la France. Spécifiquement, nous avons étudié au laboratoire l’effet de la température sur ces émissions et développé une méthode permettant l’estimation des émissions de N₂O à l’échelle du paysage. De façon surprenante, nous avons observé que les émissions de N₂O n’augmentent pas systématiquement avec la température. L’indicateur Q₁₀ est apparu, pour les émissions de N₂O, variable avec le temps. L’utilisation de l’acétylène, inhibiteur de la réduction de N2O, a révélé que les processus biologiques de production et de consommation de N₂O répondent différemment à la température. Les émissions de N₂O mesurées au champ à l’aide de différentes techniques ont permis d’obtenir des résultats cohérents, avec des moyennes de 43 μg N- N₂O m⁻² h⁻¹ pour la méthode par eddy covariance, 37 μg N- N₂O m⁻² h⁻¹ pour la méthode de fast-box et 71 μg N- N₂O m⁻² h⁻¹ pour la méthode des chambres automatiques sur un blé fertilisé. Des méthodes d’attribution des flux ont été développées pour déterminer de façon exhaustive les variations spatiales et temporelles des émissions de N₂O avec élaboration de cartes originales d’émissions à l’échelle du paysage. L’ensemble de ces résultats pourra être utilisé pour le développement de modèles de fonctionnement des écosystèmes. Ils vont contribuer à quantifier les émissions de N₂O aux échelles adaptées pour les inventaires et les stratégies d’atténuation. / The greenhouse gas N₂O is mainly emitted by soils. Soil emissions are characterized by considerable spatial and temporal variabilities that make their quantification very difficult. While soil N₂O emissions are studied on an agricultural area in the Central France by the UR SOLS since 2008, we specifically studied in the laboratory the effect of temperature on these emissions and also developed a method for upscaling N₂O emissions from the plot to the landscape scales. Surprisingly, N₂O emissions were observed not to increase with temperature. Q₁₀ values, describing N₂O emission sensitivity to temperature, were observed to change over time. The use of acetylene for inhibiting N₂O reduction has revealed that the biological processes involved in the N₂O production and its consumption respond differently to temperature variations. N2O fluxes measured in the field using several methods covering different scales of the landscape gave consistent results. The mean measured N₂O fluxes were 43 μg N- N₂O m⁻² h⁻¹ for the eddy covariance mast, 37 μg N- N₂O m⁻² h⁻¹ for the fast-box over a similar area, while it was 71 μg N- N₂O m⁻² h⁻¹ by the automatic chambers over a fertilized wheat field. Flux attribution methods were developed to determine both the spatial and temporal variability of the N₂O flux over a 1-km landscape, resulting in original maps of N₂O emissions at the landscape scale. All these results could be further used for developing ecosystem models. Both these ecosystems models and the methodologies hereby proposed for upscaling N₂O emissions will help in soil N₂O emission quantification at large scales, relevant to the inventories and mitigation strategies.

While it is not the most abundant greenhouse gas, a significant portion of the greenhouse effect is caused by methane. The amount of methane in the atmosphere is increasing, indicating that there is a continuous source of methane to the atmosphere. One significant source of methane is freshwater lakes, even though they cover only a small portion of the Earth’s surface. Because of this, it is important to monitor methane fluxes from lakes in order to understand the processes which affect the magnitude of these fluxes. Methane is produced in the sediment at the bottom of the lake, and transported through the water by ebullition, diffusive flux, storage flux, or plant mediated emission. This study looked to examine the amount of methane transmitted to the atmosphere by these processes on Lake Erken in eastern Sweden. Using the eddy covariance method, we can study the methane flux with good spatial and temporal resolution. Regular sampling of lake water, both from the surface and depths of 5 and 10 meters, also helps us to understand the amount of methane dissolved in the lake. These measurements can help us to better understand the transfer velocity, or the efficiency of the exchange between water and air, as well as the amount of methane transported from lakes to the atmosphere. Water sampling showed that there is very little variation in methane concentration between different parts of the lake. Concentrations at four surface locations are nearly identical. These surface measurements are also similar to concentrations at different depths. Over time, the concentrations generally stayed the same, with isolated high and low concentration events. The amount of methane emitted by the lake was studied with the lake divided into a shallow water area, and a deep water area. The magnitude of fluxes from both areas was very similar, but the area of shallow water had a higher total flux. The fluxes were well correlated with wind speed; higher fluxes coming during times with higher wind speed. This relates well to the transfer velocity theory, and the bulk flux approximation. However, there was no clear diurnal cycle in methane fluxes. The fluxes during the night were similar to daytime fluxes. Atmospheric pressure also had an impact on fluxes, with greater fluxes coming at times of lower pressure. A large seasonal variation was clear. More methane escaped the water in autumn and winter than in spring or summer. This is due in part to the fluxes from when the lake freezes over/thaws and the water in the lake turns over, bringing methane rich water from the lake’s bottom to the surface. As expected, the waterside concentration of methane also had a strong correlation with the fluxes. The main conclusions of this study are: 1) Methane fluxes are variable with wind speed, waterside concentrations, and the seasons 2) Water depth and diurnal cycles do not affect methane fluxes as strongly. Keywords: methane, transfer velocity, flux, waterside concentration, eddy covariance

Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases. Recent studies have shown that lakes, although they cover a small area of the globe, can be very important natural sources of atmospheric CH4 and CO2. It is therefore important to monitor the fluxes of these gases between lakes and the atmosphere in order to understand the processes that govern the exchange. By using the eddy covariance method for lake flux studies, the resolution in time and in space of the fluxes is increased, which gives more information on the governing processes. Eddy covariance measurements at a Swedish lake revealed a diel cycle in the fluxes of both CH4 and CO2, with higher fluxes during nighttime than daytime. The high nighttime CO2 fluxes could to a large extent be explained with enhanced transfer velocities due to waterside convection. For the diel cycle of CH4 flux it was suggested that waterside convection could enhance the transfer velocity, transport CH4 rich water to the surface, as well as trigger ebullition. Simultaneous flux measurements of CH4 and CO2 have been presented using both the eddy covariance method and the floating chambers method of which the latter is the traditional measuring method for lake fluxes. For CO2 the two methods agreed well during some periods but differed considerably during others. Disagreement between the methods might be due to horizontal heterogeneity in partial pressure of CO2 in the lake. The methods agreed better for the CH4 flux measurements. However, it is clear that due to the discontinuous nature of the floating chambers, this method will likely miss important high flux events. The main conclusions of this thesis are: 1) the two gas flux methods are not directly comparable and should be seen as supplementary to each other 2) waterside convection enhances the fluxes of both CH4 and CO2 over the water-air surface. If gas flux measurements are not conducted during nighttime, potential high flux periods might be missed and estimates of the total amount of gas released from lakes to the atmosphere may be biased.

Compte tenu de la vulnérabilité de la population rurale de la région sahélienne aux aléas pluviométriques, et devant les ambitions de certains acteurs d’utiliser le levier de l’usage des terres pour contribuer à l’atténuation du changement climatique, il est important de comprendre les facteurs contribuant à la variabilité de la couverture végétale au Sahel.Une synthèse de la littérature expliquant l’évolution récente de la végétation au Sahel est donc d’abord présentée. Les études s’intéressant au paradigme qui souligne l’impact de l’usage des terres sur les précipitations en Afrique de l’Ouest évaluent principalement ces effets par le couplage de modèles dynamiques globaux de végétation – DGVM – avec des modèles de circulation générale. C’est à l’amélioration d’un tel DGVM, ORCHIDEE, développé à l’Institut Pierre Simon Laplace, que le reste du travail cherche à contribuer.Comme d’autres études ont montré qu’il était possible d’utiliser en première approximation les steppes pâturées et les jachères pour décrire le comportement global de la surface sahélienne, les écarts entre modèle et mesures sont caractérisés pour une jachère située à proximité de Wankama (Niger). Plus précisément, les forces et faiblesses de la paramétrisation et de la structure par défaut du modèle sont diagnostiqués, et l’importance de la réduction d’erreur permise par l’optimisation de certains des paramètres est donnée. En particulier, l’emploi d’une résolution aux différences finies de la diffusion de l’eau dans la colonne de sol est évalué, dans la mesure où cela permet de mieux simuler la réponse rapide du flux évaporatoire aux événements pluvieux que le schéma conceptuel utilisé par défaut dans ORCHIDEE.Le réalisme du modèle est également mesuré à l’échelle régionale, par la comparaison d’observations de NDVI GIMMS_3G à la couverture végétale simulée par le modèle en réponse à différents forçages climatiques . Si les modifications introduites au cours du travail ne permettent pas de mieux décrire les tendances de la végétation au cours des dernières décennies, tirer partie des leçons du présent travail pourra se révéler utile. Il en est de même des conclusions de l’étude de la transitivité des biais conditionnels du modèle réalisée avec Tao Wang et présentée en annexe B. / The evolution of the land-surface conditions is often assessed through the use of “dynamic global vegetation models”, as is shown in a review of the current understanding of the factors of variability and of the recent evolution of the vegetation cover in the Sahel. Such models are also coupled to atmospheric general circulation models to evaluate the land feedback on precipitation in monsoonal climates.Thus, the improvement of the skills of such surface models to simulate the radiative and turbulent fluxes between the land of surface and the atmosphere in the Sahel over a range of scales from hourly to multi-annual has a potential to have significant implications. This is especially true considering the vulnerability of the rural population of the region, which largely relies on rainfed agriculture and the interest on the evolution of the carbon stocks of ecosystems in the context of climate change. Such a work on the ORCHIDEE model is presented here. In complement to croplands, rangelands and fallows represent a large share of the sahelian landscapes and have intermediate characteristics between erosion glacis and acacia bushes. As such, their evolution (in terms of albedo, roughness length,…) may be used to study the Sahel ecosystem behaviour as a first approximation. Differences between model outputs and field observations are quantified for a fallow close to Wankama (Niger). More precisely, some of the drawbacks of the standard parametrisation and structure of the model are diagnosed, and the range of reduction of the model-observation mismatch that results from optimizing some of the parameters are given (plant phenology,…). In particular, the use of a finite difference resolution of the soil water diffusion is considered as it enables to better simulate the fast response of evaporative fluxes to rainfall than the conceptual scheme routinely used in ORCHIDEE. The benefits of the use of such a “physical” hydrological scheme on the different outputs of the surface scheme is evaluated.The realism of the model is also measured at the regional scale, through a comparison with GIMMS_3G NDVI time series over West Africa. If the modifications that have been introduced in the model don’t improve its ability to describe the vegetation cover trends over the last decades in the region, several lessons can be kept from the analysis that has been realised, especially from the work on the transitivity of state-dependant model biases that has been conducted with Tao Wang and which is presented in annex B.

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