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.

Leaf area and stomata are important parameters in studies of taxonomic classification since both parameters might reflect the plant adaptation mechanisms to different environmental conditions. This study analyzed Eurycoma longifolia Jack leaf traits in different ecological regions (moisture zone: A Luoi, Bach Ma, Nam Dong; dry zone: Phong Dien) and leaves of seedlings in the nursery. The study used scanners and Images to measure leaf area; imagej and the AxioVision SE64 softwares were used to evaluate stomatal density by separating the lower cuticle through a clear nail polish impression. Our results showed that the leaf area of mature trees from the dry zone is smaller (355.7 and 484.1 cm2/leaf), however, the stomata density is higher than that of the humid area (284.4 and 137.9 stomata/mm2). Under the same nursery conditions, the seedlings' leaf area was similar while seedlings' stomatal density, originated from the dry zone, was lower. Results indicate that the stomatal density changes as an adaptation to changing habitat conditions. This study gives indicates that E. longifolia is a tree species with high adaptability given the leaf morphology changes under a changing climate. Therefore, this tree species should be a priority for planting in order to increase biodiversity in different ecological regions. / Đặc điểm diện tích lá và mật độ khí khổng là những thông số quan trọng trong các nghiên cứu phân loại học nhằm phản ánh cơ chế thích nghi của thực vật với điều kiện sống. Vì thế nghiên cứu này đã tiến hành phân tích các đặc điểm của lá cây Bách bệnh (Eurycoma longifolia Jack) từ các vùng khác nhau (A Lưới, Bạch Mã, Nam Đông: vùng ẩm và Phong Điền: vùng khô) và lá cây con của chúng ở vườn ươm. Nghiên cứu đã sử dụng máy quét và ImageJ để đo diện tích lá và phần mềm AxioVision SE64 và ImageJ để đánh giá mật độ khí khổng thông qua tách lớp biểu bì phía dưới lá qua lớp sơn mỏng Kết quả nghiên cứu cho thấy, diện tích lá của cây trưởng thành từ vùng khô nhỏ hơn (355.7 and 484.1 cm2/leaf) nhưng mật độ khí khổng lại lớn hơn so với vùng ẩm (284.4 and 137.9 stomata/mm2). Tuy nhiên, cùng điều kiện sống ở vườn ươm, diện tích lá của cây con tương tự nhau trong khi mật độ khí khổng của cây con có nguồn gốc từ vùng khô lại thấp hơn. Kết quả này chỉ ra rằng sự thay đổi của mật độ khí khổng như là một sự thích nghi với thay đổi của điều kiện môi trường sống. Từ đó nghiên cứu này cho thấy cây Bách bệnh là một loài cây gỗ với khả năng thích nghi cao nhờ sự thay đổi về đặc điểm hình thái lá khi thay đổi khí hậu cho nên loài cây này nên được ưu tiên trong việc gây trồng nhằm tăng cường tính đa dạng cho các vùng sinh thái khác nhau.

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