Short rotation woody crops have gained global interest as an alternative energy source to fossil fuels. The availability of this resource is, however, dependent on successful research trials and the identification and quantification of the environmental controls on rapid growth. Knowledge of willow root dynamics is required to determine belowground and aboveground growth relationships, and to provide valuable inputs for the development of a willow carbon model. The objectives of this study were to 1) determine fine root turnover, biomass, and productivity of six willow clones over two growing seasons at four locations in Saskatchewan using the minirhizotron method; 2) determine fine root biomass and fine root carbon sequestration of six willow clones over one growing season at four locations in Saskatchewan using the soil coring method; and 3) determine lateral coarse root structure of six willow clones at two locations in Saskatchewan.<p> Monthly fine root biomass and production was estimated for six willow clones in Saskatoon, Saskatchewan using repeated minirhizotron observations from May to September of 2008 and 2009. Fine root biomass increased from 0.78 Mg ha-1 in May 2008 to 25.75 Mg ha-1 in September 2009. Significant differences were seen between months throughout each growing season, but not between the clones. Mean monthly productivity reached its peak of 8.00 Mg ha-1 in July 2009. Mean turnover for all the clones was 0.96 yr-1 and mean longevity was 1.04 yr-1. The high fine root biomass estimates determined by the minirhizotron method in Saskatoon suggest that this method is not suitable for use in a Vertisolic soil. There was no significant effect of clone on fine root productivity, biomass, turnover or longevity (P < 0.05).<p> Fine root biomass estimates from the soil cores were lower than those from the minirhizotron method. The mean fine root biomass value in Saskatoon for September 2008 was 0.298 Mg ha-1. Mean fine root biomass at each site from September 2007 to September 2008 ranged from 0.022 Mg ha-1 to 0.915 Mg ha-1. Mean root carbon content ranged from 0.010 to 0.426 Mg C ha-1. Fine root biomass and root carbon content were significantly different between each site, with the exception of Saskatoon and Estevan (P < 0.05). Differences in fine root estimates between the sites are suggested to be a function of the soil texture and moisture accessibility at each site. This research indicates that willow roots in Saskatchewan find initial establishment difficult in low moisture, fine textured soils. Also, approximately 44.6 % of fine root biomass is comprised of C, and decomposes to form soil organic matter. Therefore, fine roots have potential to store substantial amounts of carbon if growing conditions are suitable.

Short rotation woody crops have gained global interest as an alternative energy source to fossil fuels. The availability of this resource is, however, dependent on successful research trials and the identification and quantification of the environmental controls on rapid growth. Knowledge of willow root dynamics is required to determine belowground and aboveground growth relationships, and to provide valuable inputs for the development of a willow carbon model. The objectives of this study were to 1) determine fine root turnover, biomass, and productivity of six willow clones over two growing seasons at four locations in Saskatchewan using the minirhizotron method; 2) determine fine root biomass and fine root carbon sequestration of six willow clones over one growing season at four locations in Saskatchewan using the soil coring method; and 3) determine lateral coarse root structure of six willow clones at two locations in Saskatchewan.<p> Monthly fine root biomass and production was estimated for six willow clones in Saskatoon, Saskatchewan using repeated minirhizotron observations from May to September of 2008 and 2009. Fine root biomass increased from 0.78 Mg ha-1 in May 2008 to 25.75 Mg ha-1 in September 2009. Significant differences were seen between months throughout each growing season, but not between the clones. Mean monthly productivity reached its peak of 8.00 Mg ha-1 in July 2009. Mean turnover for all the clones was 0.96 yr-1 and mean longevity was 1.04 yr-1. The high fine root biomass estimates determined by the minirhizotron method in Saskatoon suggest that this method is not suitable for use in a Vertisolic soil. There was no significant effect of clone on fine root productivity, biomass, turnover or longevity (P < 0.05).<p> Fine root biomass estimates from the soil cores were lower than those from the minirhizotron method. The mean fine root biomass value in Saskatoon for September 2008 was 0.298 Mg ha-1. Mean fine root biomass at each site from September 2007 to September 2008 ranged from 0.022 Mg ha-1 to 0.915 Mg ha-1. Mean root carbon content ranged from 0.010 to 0.426 Mg C ha-1. Fine root biomass and root carbon content were significantly different between each site, with the exception of Saskatoon and Estevan (P < 0.05). Differences in fine root estimates between the sites are suggested to be a function of the soil texture and moisture accessibility at each site. This research indicates that willow roots in Saskatchewan find initial establishment difficult in low moisture, fine textured soils. Also, approximately 44.6 % of fine root biomass is comprised of C, and decomposes to form soil organic matter. Therefore, fine roots have potential to store substantial amounts of carbon if growing conditions are suitable.

博士(理学) / Doctor of Philosophy in Science / 同志社大学 / Doshisha University

Il est généralement admis que la végétation peut stabiliser les pentes naturelles et artificielles contre les glissements de terrain superficiel. Par rapport au rôle mécanique, les racines renforcent le sol d'une pente en fournissant une cohésion additionnelle (cr). La quantification des cr est une étape essentielle pour évaluer la stabilité des pentes, quantifiée par le facteur de sécurité (FoS, défini par le ratio entre les forces de la résistance et les forces motrices sur une pente). La plupart des modèles prédictifs de cr existants ne prennent pas en compte la dynamique racinaire à l'échelle spatiale et temporelle qui peut conduire à une hétérogénéité de renforcement des racines. Ainsi, cette thèse vise à caractériser, quantifier et modéliser la répartition spatiale et temporelle de la dynamique racinaire ainsi que son impact sur la cr estimée. La distribution, croissance et mortalité racinaire ont été échantillonnées à l'aide de monolithes et de rhizotrons à deux altitudes dans des forêts mixtes et naturellement régénérées dans les Alpes françaises, composées d'îlots et de trouées. Avec les méthodes de modélisation statistique, une série de facteurs abiotiques et biotiques affectant la dynamique racinaire ont été étudiés. Pour quantifier les cr, une méta-analyse a été effectuée et les divers algorithmes de modélisation ont été employés et leurs résultats comparés. Cette étude a montré que: (i) dans un écosystème à espèces mixtes, la densité racinaire influence davantage les cr que les propriétés de la qualité racinaire; (ii) tous les facteurs abiotiques (altitude, paysage écologique, profondeur du sol et mois) peuvent faire varier la densité racinaire selon des conditions différentes du sol ; (iii) lors de l'observation de 1,5 ans à l'aide de rhizotrons, le cr augmente continuellement, rapidement dans la saison active et lentement pendant la saison dormante, mais cette augmentation est dépendante de la profondeur du sol, de l'altitude et du patch écologique ; (iv) malgré le fait que les racines les plus fines (]0, 1] mm en diamètre) soient les plus actives dans le cycle de nutritions et de carbone (selon des publications précédentes), elles contribuent peu au renforcement mécanique de terrain. Cette étude a permis d'élargir et d'approfondir nos connaissances sur le rôle des racines dans l'éco-ingénierie. / It is largely recognized that vegetation can stabilize artificial and natural slopes against shallow landslides. Mechanically, plant roots reinforce soil on a slope by providing an additional cohesion (cr). Quantification of cr is a key step to estimate the stability of a given slope, usually quantified by the Factor of Safety (FoS, defined as the ratio between resisting forces and the driving forces on a slope). Most existing cr predictive models do not take into consideration spatial and temporal root dynamics which result in heterogeneous root reinforcement along a vegetated slope. Therefore, this thesis aims to characterize, quantify and model the spatial and temporal patterns in root dynamics and their impact on the estimation of cr. Root distribution, growth and mortality were measured using monoliths and rhizotrons installed at two altitudes in naturally regenerated mixed forests in the French Alps. These forests are composed of trees growing in groups (tree islands) with large gaps between the islands. Using statistical modeling approaches, abiotic and biotic factors affecting root dynamics were investigated. For quantifying cr, a meta-analysis was performed and different modeling algorithms were employed and results compared. Based on these studies, the following conclusions were made: (i) in a mixed, mature forest ecosystem root density influenced cr more than root mechanical properties; (ii) all abiotic factors (altitude, type of vegetation patch, soil depth and month) significantly affected root quantity to different degrees, depending on soil conditions; (iii) during the 1.5 years' observations in rhizotron, cr increased rapidly during the growing season and more slowly in the dormant season but the increment increase was largely dependent on soil depth, altitude and vegetation patch. (iv) The finest roots (]0, 1] mm in diameter), which are considered the most important for nutrient and carbon cycling, contributed little to mechanical reinforcement of the soil. Results are discussed with regard to ecological engineering strategies for unstable slopes.

Combustion instabilities, induced by the resonant coupling of acoustics and combustion occur in many practical systems such as domestic boilers, gas turbine and rocket engines. They produce pressure and heat release fluctuations that in some extreme cases can provoke mechanical failure or catastrophic damage. These phenomena have been extensively studied in the past, and the basic driving and coupling mechanisms have already been identified. However, it is well known that most systems behave differently at cold start and in the permanent regime and the coupling between the temperature of the solid material and combustion instabilities still remains unclear. The aim of this thesis is to study this mechanism. This work presents an experimental investigation of combustion instabilities for a laminar premixed flame stabilized on a slot burner with controlled wall temperature. For certain operating conditions, the system exhibits a combustion instability locked on the Helmholtz mode of the burner. It is shown that this instability can be controlled and even suppressed by changing solely the temperature of the burner rim. A linear stability analysis is used to identify the parameters playing a role in the resonant coupling and retrieves the features observed experimentally. Detailed experimental studies of the different elementary processes involved in the thermo-acoustic coupling are used to evaluate the sensitivity of these parameters to the wall temperature. Finally a theoretical model of unsteady heat transfer from the flame root to the burner-rim and detailed experimental measurements permit to establish the physical mechanism for the temperature dependance on the flame response.

Predicting the interactions between climate change and ecosystems remains a core problem in global change research; tropical forest ecosystems are of particular importance because of their disproportionate role in global carbon and water cycling. Amazonia is unique among tropical forest ecosystems, exhibiting a high degree of coupling with its regional hydrometeorology, such that the stability of the entire forest-climate system is dependent on the functioning of its component parts. Belowground ecohydrological interactions between soil moisture environments and the roots which permeate them initiate the water transport pathway to leaf stomata, yet despite the disproportionate role they play in vegetation-atmosphere coupling in Amazonian forest ecosystems, the impacts of climate variability on the belowground environment remain understudied. The research which follows is designed to address critical knowledge gaps in our understanding of root functioning in Amazonian tropical forests as it relates to seasonality and extremes in belowground moisture regime as well as discerning which ecohydrological mechanisms govern ecosystem-level processes of carbon and water flux. A secondary research theme is the evaluation and use of models of ecosystem function as applied to Amazonia - these models are the "knowledge boxes" which build in the ecohydrological hypotheses (some testable than others) deemed to be most important for the forest ecosystems of Amazonia. In what follows, I investigate (i) which mechanisms of water supply (from the soil environment) and water demand (by vegetation) regulate the magnitude and seasonality of evapotranspiration across broad environmental gradients of Amazonia, (ii) how specific hypotheses of root function are or are not corroborated by soil moisture measurements conducted under normal seasonal and experimentally-induced extreme drought conditions, and (iii) the linkage between an extreme drought event with associated impacts on root zone soil moisture, the inferred response of root water uptake, and the observed impacts on ecosystem carbon and water flux in an east central Amazonian forest.

The Southern Appalachian hardwood forests contain a wide diversity of flora and fauna. Understanding processes that affect nutrient availability in these forests is essential for sound forest management. Three interconnected research projects regarding soil resource heterogeneity were designed to increase our understanding of this ecosystem. The objective of these projects were as follows: 1) to examine and quantify the role of decaying stumps in regards to total carbon (C) and nitrogen (N) pools and fine-root dynamics, 2) compare and contrast the use of ground-penetrating radar (GPR) vs. a soil auger for estimating soil depth and site quality and 3) to evaluate how eastern red-backed salamanders (Plethodon cinereus) affect N-availability. For the stump study, results show that decomposing stumps occupy approximately 1.2% of the total soil volume and constitute 4% and 10% of total soil N and C pools. Significant differences in N (p = 0.0114), C (p = 0.0172), microbial biomass C (p = 0.0004), potentially mineralizable N (p = 0.0042), and extractable NH4+ (p = 0.0312) concentrations were observed when compared to mineral soil horizons. In particular, potentially mineralizable N was 2.5 times greater in stump soil than the A-horizon (103 vs. 39 mg kg-1), 2.7 times greater for extractable NH4+ (16 vs. 6 mg kg-1) and almost 4 times greater for MBC (1528 vs. 397 mg kg-1). These measured properties suggest higher N-availability, organic matter turnover and N uptake in stump soil versus the bulk soil. 19% of the total fine root length and 14% of fine root surface area also occurred in the stump soil. The increased fine root length suggests higher concentrations of labile nutrient in the stumps since roots often proliferate in areas with higher nutrient availability. Significant differences occurred in N and C concentrations between all four decay classes and the A-horizon, which validated the use of this system and the need to calculate weighted averages based on the frequency and soil volume influenced by each decay class. In the GPR Study, depth estimations were shallower using a soil auger compared to estimates obtained using GPR across all plots (p = 0.0002; Figure 3.4). On a soil volume basis, this was equivalent to about 3500 m3 of soil per hectare unaccounted for using traditional methods. In regards to using soil depth as a predictor for site quality, no significant relationships were observed with soil depth estimations obtained from the auger (Table 3.3). On the other hand, depth measurements from GPR explained significant amounts of variation across all sites and by physiographic region. Across all sites, soil depth estimates from GPR explained 45.5% of the residual variation (p = 0.001; Table 3.3). When the data were stratified by physiographic region, a higher amount of variation was explained by the regression equations; 85% for the Cumberland Plateau (p = 0.009), 86.7% for the Allegheny Plateau (0.007) and 66.7% for the Ridge and Valley (p = 0.013), respectively (Table 4.2). Results from this study demonstrate how inaccurate current methods can be for estimating soil depth rocky forests soils. Furthermore, depth estimations from GPR can be used to increase the accuracy of site quality in the southern Appalachians. In the salamander study, no significant salamander density treatment or treatment by time effects were observed over the entire study period (p < 0.05). However, when the data were separated by individual sampling periods a few significant treatment by time interactions occurred: 1) during August 2006 for available NH4+ under the forest floor (i.e. horizontal cation membranes; p = 0.001), 2) August and 3) September 2006 for available NH4+ in the A-horizon (p = 0.026), and 4) May 2007 for available NO3- under the forest floor (p = 0.011). As a result of these trends, an index of cumulative N-availability (i.e. NH4+ and NO3-) under the forest floor and in the A-horizon was examined through the entire study period. Cumulative N-availability under the forest floor was consistently higher in the low- and medium-density salamander treatments compared to the high-density treatment. For cumulative N-availability in the A-horizon, a gradient of high to low N-availability existed as salamander density increased. Factors such as a prolonged drought in 2007 may have affected our ability to accurately assess the effects of salamanders on N-availability. We concluded that higher salamander densities do not increase N-availability. Implementing methodologies that accurately account for soil nutrient pools such as stump soil, physical properties such as depth and fauna such as salamanders, increase our understanding of factors that regulate site productivity in these ecosystems. As a result, landscape-level and stand-level management decisions can be conducted more effectively. / Ph. D.

As substâncias húmicas (SH) podem alterar o crescimento das plantas bem como estimular a absorção de nutrientes. A ureia é o fertilizante nitrogenado mais utilizado na agricultura, porém, apresenta baixa eficiência de uso. Sabe-se que as substâncias húmicas podem aumentar a eficiência de uso da ureia por meio de efeito bioestimulante no crescimento radicular. Somado a isso, as SH atuam nas bombas de próton das raízes estimulando a absorção de nitrogênio e também alteram a dinâmica de hidrólise e mineralização do N-ureia. O objetivo deste trabalho foi verificar se as SH extraídas de turfa, associadas ou não ao N-ureia, alteram as transformações do N-ureia no solo, assim como o crescimento radicular e da parte aérea no desenvolvimento inicial da cana-planta. Nesse sentido, foram instalados dois experimentos em casa de vegetação, um com a presença de plantas para aferir parâmetros morfológicos e nutricionais e outro, sem plantas, para verificar a dinâmica de hidrólise e mineralização do N-ureia. Os experimentos foram planejados em delineamento experimental inteiramente casualizado com 16 tratamentos e cinco repetições, correspondendo a um esquema fatorial completo com quatro doses de nitrogênio na forma de ureia: 0; 100; 200 e 300 mg dm-3 e quatro doses de SH: 0; 13,8; 27,6; 41,7 mg dm-3 de C, e o experimento sem plantas utilizando o mesmo delineamento porém com 3 repetições e análises feitas em duplicatas. As substâncias húmicas aumentaram a hidrólise da ureia elevando a concentração de NH4+ no solo. O processo de nitrificação da ureia foi reduzido na presença das substâncias húmicas. As aplicações isoladas de N e SHs estimularam o crescimento radicular e da parte aérea da cana-de-açúcar, bem como modificaram as características morfológicas das raízes e parte aérea da cana-planta. A associação dos produtos resultou em decréscimos no crescimento vegetal devido às altas concentrações de amônio causadas pela maior hidrólise da ureia e também atuaram na redução do acúmulo e teor de nitrogênio na planta. Tanto a ureia quanto as SHs se mostraram eficientes no estímulo do crescimento inicial da cana-de-açúcar, no entanto, devido às altas doses de N-ureia utilizadas, o possível efeito sinérgico dos produtos não foi verificado. / Humic substances (HS) can alter plant growth and stimulate the absorption of nutrients. Urea is the most widely used nitrogen fertilizer in agriculture, however, has low use efficiency. It is known that humic substances can increase the efficiency of urea through bio-stimulant effect on root growth. Added to this, the SH proton pumps operate in the roots stimulating the uptake of nitrogen and also alter the dynamics of urea\'s hydrolysis and mineralization. The aim of this study was to verify whether the HS extracted from peat, associated or not with N-urea, alter the dynamics of transformation of urea-N in the soil and the growth of roots and shoots in the cane-plant cropcycle. In this direction, two experiments were carried out in a greenhouse, with the presence of plants to assess morphological and nutritional parameters and one without plants, to verify the dynamics of hydrolysis and mineralization of N-urea. The experiments were designed in a completely randomized design with 16 treatments and five replications, corresponding to a full factorial design with four rates of nitrogen as urea: 0, 100, 200 and 300 mg dm-3 and four rates of HS: 0, 13.8, 27.6, 41.7 mg dm-3 C, and experiment without plants using the same design but with three replications and analyzes performed in duplicates. The humic substances increased hydrolysis of urea by raising the concentration of NH4+ in soil. The nitrification of urea was reduced in the presence of humic substances. Individual applications of N and SHs stimulated root and shoot of sugar cane growth and modified the morphological characteristics of roots and shoots in the crop cycle of the cane-plant. The association of the product resulted in decreases in plant growth due to high ammonium concentrations caused by increased hydrolysis of urea and also reduced accumulation and nitrogen content in the plant. Both urea and the SHs were effective in stimulating early growth of sugar cane , however , due to high doses of N-urea used, the synergistic effect of the products has not been verified.

Les instabilités de combustion induites par le couplage combustion-acoustique se produisent dans de nombreux systèmes industriels et domestiques tels que les chaudières, les turbines à gaz et les moteurs de fusée. Ces instabilités se traduisent par des fluctuations de pression et un dégagement de chaleur qui peuvent provoquer une défaillance mécanique ou des dégâts désastreux dans certains cas extrêmes. Ces phénomènes ont été largement étudiés par le passé, et les mécanismes responsables du couplage ont déjà été identifiés. Cependant, il apparaît que la plupart des systèmes se comportent différemment lors du démarrage à froid ou en régime permanent. Le couplage entre la température des parois et les instabilités de combustion reste encore méconnu et n’a pas été étudié en détail jusqu’à présent. Dans le cadre de ces travaux de thèse, on s’intéresse à ce mécanisme. Ces travaux présentent une étude expérimentale des instabilités de combustion pour une flamme laminaire de pré-mélange stabilisée sur un brûleur à fente. Pour certaines conditions de fonctionnement, le système présente un mode instable autour du mode de Helmholtz du brûleur. Il est démontré que l’instabilité peut être contrôlée, et même supprimée, en changeant uniquement la température de la surface du brûleur. Une analyse de stabilité linéaire peut être mise en œuvre afin d’identifier les paramètres jouant un rôle dans les mécanismes d’instabilité, et il est possible de modéliser analytiquement les phénomènes observés expérimentalement. Des études expérimentales détaillées de différents processus élémentaires impliqués dans le couplage thermo-acoustique ont été menées pour évaluer la sensibilité de ces paramètres à la température de la paroi. Enfin un modèle théorique du couplage entre le transfert de chaleur instationnaire à la paroi et la fluctuation du pied de flamme a été proposé. Par ailleurs, d’autres mesures expérimentales ont permis de comprendre les mécanismes physiques responsables de la dépendance de la réponse de la flamme à la température de paroi. / Combustion instabilities, induced by the resonant coupling of acoustics and combustion occur in many practical systems such as domestic boilers, gas turbine and rocket engines. They produce pressure and heat release fluctuations that in some extreme cases can provoke mechanical failure or catastrophic damage. These phenomena have been extensively studied in the past, and the basic driving and coupling mechanisms have already been identified. However, it is well known that most systems behave differently at cold start and in the permanent regime and the coupling between the temperature of the solid material and combustion instabilities still remains unclear. The aim of this thesis is to study this mechanism. This work presents an experimental investigation of combustion instabilities for a laminar premixed flame stabilized on a slot burner with controlled wall temperature. For certain operating conditions, the system exhibits a combustion instability locked on the Helmholtz mode of the burner. It is shown that this instability can be controlled and even suppressed by changing solely the temperature of the burner rim. A linear stability analysis is used to identify the parameters playing a role in the resonant coupling and retrieves the features observed experimentally. Detailed experimental studies of the different elementary processes involved in the thermo-acoustic coupling are used to evaluate the sensitivity of these parameters to the wall temperature. Finally a theoretical model of unsteady heat transfer from the flame root to the burner-rim and detailed experimental measurements permit to establish the physical mechanism for the temperature dependance on the flame response.

Le système racinaire, lien essentiel entre la plante et le sol quoi qu'essentiellement invisible, reste encore aujourd'hui peu étudié dans son ensemble. Les plantes étant ancrées au sol via leur système racinaire, leurs facultés d'adaptation et de survie sont donc très dépendantes de leurs capacités à savoir tirer profit de l'espace qui les entoure, notamment par l'intermédiaire de leur système racinaire. Certains arbres font néanmoins partie des plus grands et plus anciens êtres vivants de la planète, preuve s'il en est de leur capacité d'adaptation au milieu dans lequel ils poussent et aux changements environnementaux pouvant survenir dans le temps. Dans le cas des plantations, les contraintes anthropiques imposées se heurtent parfois aux limites de cette formidable capacité d'adaptation. Les travaux développés dans la première partie de cette thèse font l'objet de deux articles publiés et se concentrent sur une revue bibliographique des connaissances sur l'enracinement profond. Le rôle vital du système racinaire pour la plante est ainsi mis en avant ainsi que ses fonctions dans le continuum sol-plante-atmosphère. Son rôle essentiel, souvent sous-estimé dans le stockage du carbone, est également abordé. Les différentes techniques permettant d'accéder aux systèmes racinaires in situ sont passées en revue et nous présentons la technique du puits d'accès que nous avons utilisé pour observer les racines jusqu'à 5 mètres de profondeur. Nous avons également développé nos propres outils de prise de vue par l'intermédiaire de scanner à plat ainsi qu'un outil d'analyse d'images ( IJ_Rizo) aujourd'hui disponible en ligne.Dans la deuxième partie du mémoire, nous nous consacrons à l'étude du système racinaire de l'hévéa adulte (Hevea brasiliensis) dans le nord-est de la Thaïlande. Premier producteur mondial de caoutchouc naturel, la Thaïlande a fortement développé ses plantations d'hévéas allant jusqu'à étendre son exploitation au-delà de sa zone climatique naturelle, dans des zones à fortes contraintes hydriques. Dans ce contexte, nous avons étudié la saisonnalité et la dynamique des racines fines pendant trois ans ainsi que leur contribution au cycle du carbone. Nous avons pu mettre en évidence la continuité de la dynamique racinaire indépendamment des périodes de défoliation mais en lien avec la saisonnalité de la pluviométrie. Nous avons également pu caractériser la faible différenciation de la dynamique racinaire de 0 à 4,5 mètres de profondeur dans un tel contexte. La troisième partie de la thèse concerne l'étude du teck (tectona grandis), espèce endémique d'Asie du Sud-Est, et notamment du Laos dans la région de Luang Prabang. Le développement de sa culture sous forme de plantations mono-spécifiques se poursuit depuis les années 80. En appliquant la technique d'exclusion de pluie pendant 2 ans sur des arbres d'une plantation d'une vingtaine d'années, nous avons observé l'influence du régime de précipitations sur l'enracinement. Un arrêt pratiquement total de croissance des racines en surface et en profondeur a ainsi été mis en évidence. Nous concluons, dans le cas d'espèces à une forte dépendance à la ressource en eau de surface, à un impact direct sur l'état physiologique et une stagnation de la croissance des troncs. Certains individus ont démontré une capacité d'adaptation en modifiant leur cycle de foliation / défoliation accompagné d'une reprise de la croissance des troncs lors de la deuxième année du régime de sécheresse imposé. Nous avons également démontré l'importance de l'enracinement fin pour le stockage du carbone qui, dans ce contexte, représente plus de 45 % du carbone total, soit environ le double des données publiées dans la littérature.Les données obtenues pour les espèces étudiées dans ce mémoire pourront être utilisées pour la modélisation de scenarios simulant le changement climatique et le changement d'usage des terres. / The root system is essential but essentially invisible. Plants are anchored to the soil through their root system; their adaptation and survival abilities are highly dependent on their ability to learn to take advantage of the space that surrounds them. Certain trees are nevertheless some of the largest and oldest living species on the planet- evidence of their ability to adapt to changes in their environment. Within managed anthropogenic constraints, particular species are vulnerable. It is clear that a wide range of parameters are likely to influence the root system and its operation, which offers many entry points to improve our understanding of a root's capacity for expansion, its dynamics, its role within the plant itself and within the soil plant atmosphere continuum. Available data on the extent and dynamics of plant roots includes several thousand references. However, the vast majority of these observations were made within the first meter of the soil profile. Data acquisition for the fine and/or deep roots is currently limited by constraints of time and financial resources. To overcome this lack of information, while trying to assess root dynamics under different environmental conditions, many models have been developed. However, it is still difficult to describe the complexity of root development in the community and to integrate its "plasticity".To understand such a complex environment, we must work toward establishing a definition of objectives and the tools necessary to develop and implement them. The work developed in the first part of this thesis is the subject of two articles and focuses on a literature review about deep roots. The vital role of the root system for the plant is well highlighted, as is its impact as a link within the atmosphere. We discuss the role of deep roots in carbon storage, which is critical and often underestimated. The different techniques for accessing the root systems in situ are also considered, and we present our access technique for observing the roots down to depths of five meters. We have also developed tools for shooting through a flatbed scanner and image analysis ( IJ_Rizo ) now available online.In the second part of the thesis, we focus on the study of root systems of adult rubber trees (Hevea brasiliensis L. RRIM 600) in the northeast of Thailand. Thailand has greatly expanded its acreage to extend its operations beyond its natural climate zone, into areas of high water stress. In this context, we studied the seasonality and dynamics of fine roots for three years and their contribution to the carbon cycle. We were able to highlight the continuity of root dynamics during independent periods of defoliation, yet link these to the seasonality of rainfall. We were also able to characterize the low differentiation of root dynamics at 0 to 4.5 m of depth in this context.The third part of the thesis concerns the study of teak particular to the region of Luang Prabang, Laos. By applying the rain exclusion technique for 2 years on a plantation of twenty-year-old trees, we observed the influence of precipitation patterns. During the period of water stress, an almost total cessation of root growth, at the surface and significant depths, has been highlighted. We conclude that in the case of species with a high dependence on water resources there is a direct impact on the physiological state and stagnated trunk growth. Some individuals have demonstrated an ability to adapt by changing their foliation/defoliation cycle, accompanied by a resumption of trunk growth during the second year of imposed drought. We also demonstrated the importance of rooting as a means of carbon storage, which in this context represents more than 45% of total carbon- roughly double the amount published in other literature .The data obtained for the species studied in this paper can be used for modelling scenarios simulating climate change and changing land use.