Drought and poor soil fertility are often cited as major constraints to rainfed lowland rice production in Laos, particularly in the central and southern parts of the country, where uncertainty in the growing season is common, due mainly to a combination of unreliable rainfall and coarse textured soils with a low water holding capacity and high rates of deep percolation. The soil conditions, together with fluctuations in rainfall distribution, are regarded as the most serious constraints to achieving high and stable grain yields in the rainfed lowland rice ecosystem in this region. Improvements in rainfed lowland rice productivity depend, in part, on improved predictions of water availability, as well as better understanding of soil nutrient and water stress limitations to grain yield. The use of a soil water balance model, in conjunction with the quantification of soil nutrient availability, can help in estimating free water levels in the rice fields, thereby helping determine the duration of the growing period, as well as, helping with predictions of potential yield reduction due to water stress and soil nutrient limitations. Linking the simulated results with Geographic Information System (GIS) can help quantify the spatial pattern of these attributes at a provincial or regional scale. This study was aimed at quantifying the spatial distribution of water availability, including the frequency and severity of water stress development during the growing period, and to estimate the effects of soil fertility and water stress on rice productivity in Savannakhet province in southern Lao PDR. The current status in relation to the variability in field water availability, soil fertility, farm crop management practices and grain yield throughout Savannakhet province were quantified, first by collecting farm data from 53 and 48 farmers’ fields in the 2007 and 2008 cropping seasons, respectively, These farms were selected to be representative of a wide range of soil and climate conditions within the province. The results show that rainfall distribution pattern, soil type and toposequence position of paddy fields, are crucial factors contributing to the temporal variation in field water availability. The overall yield loss due to water stress associated with late season drought was estimated to be rather small (10%, 5% and 3% for the top, middle and bottom toposequence positions of rice fields, respectively) in the 2008 wet season. On the other hand, application of chemical fertilizer has a large effect on final grain yield, with 50 kg N ha-1 and 30 kg P2O5 ha-1 increasing yield by 600 to 800 kg and 800 to 1000kg ha-1 respectively, in the 2007 and 2008 cropping seasons. A new soil water balance (SWB) model that incorporates the effect of low soil clay content on deep percolation, was developed to quantify field water availability and the length of growing period (LGP) for various rainfall lowland rice cropping environments. The model estimates the amount of water stored in a soil profile, the profile being divided into two layers: Layer 1 (0-200 mm) consists of standing water and the topsoil layer, while Layer 2 (200-1000 mm) is the subsoil layer. The SWB model was validated with field experimental data obtained in the 2002 and 2008 cropping seasons. The simulated free water levels were close to those recorded for the observed field data, with a small mean average error, lower root mean square error, and significant correlation coefficient and index of agreement over all sites across the three toposequence positions of paddy fields. Maps of the length, start and end of growing period (LGP, SGP and EGP, respectively) for rainfed lowland rice in Savannakhet province, were developed using the SWB model, with inputs of median weekly climatic data and soil water characteristics. The province was delineated into three main LGP zones with a short LGP zone (less than 21 weeks) in the east, northwest and some rice fields in the south-western corner of the province; an intermediate LGP zone (21 to 24 weeks) was defined in the central and western part of the province; and a long LGP zone (greater than 24 weeks) for the south and for some rice fields in the western part of the province. The variation in the SGP from year-to-year was due largely to the variation in rainfall early in the wet-season (e.g. April), while EGP was strongly dependent upon the clay content of the soils being cropped. The SWB model was combined with other models that estimate yield potential, soil nutrient supply and yield reduction by low soil water level, to characterize and map the suitability zones for rainfed lowland rice in Savannakhet province. The overall results of the model performance on yield estimates were satisfactorily, with a significant correlation coefficient (0.54**) and high index of agreement (0.68) over the 2007 and 2008 seasons. The model classified three main rice agro-ecological zones according to the suitability of climate and soil conditions. The majority of the lowland rice growing areas are classified as moderately suitable to marginally suitable, while the potential area classified as being high suitable is very small. A large potential response of rice yields to fertilizer inputs is predicted for most of rice growing areas in the province. The best sowing time for achieving a high yield, as evaluated by the model, is the first half of June. Appropriate crop phenology and increasing fertilizer use efficiency that matches with water availability and soil conditions in each rice agro-ecological zone, are important in achieving improvements in rice productivity, as substantial improvements in rice fields cannot be achieved by improving water availability alone, where paddy fields are dominated by soils with low level of indigenous fertility. Although the model is capable of quantifying field water availability and crop yield due to the limitations associated with low levels of soil nutrients and water stress, the model has the potential for further improvements in two areas. First, the estimates of water loss need to be modified by incorporating variable factors such as slope of paddy field, which can affects lateral water movement and hence free water level. Second, the model should incorporate some key agronomic variables, such as internal efficiencies and recovery efficiencies of applied fertilizer, which depend on variety, crop management and climatic conditions, and these factors can be modelled.

Drought and poor soil fertility are often cited as major constraints to rainfed lowland rice production in Laos, particularly in the central and southern parts of the country, where uncertainty in the growing season is common, due mainly to a combination of unreliable rainfall and coarse textured soils with a low water holding capacity and high rates of deep percolation. The soil conditions, together with fluctuations in rainfall distribution, are regarded as the most serious constraints to achieving high and stable grain yields in the rainfed lowland rice ecosystem in this region. Improvements in rainfed lowland rice productivity depend, in part, on improved predictions of water availability, as well as better understanding of soil nutrient and water stress limitations to grain yield. The use of a soil water balance model, in conjunction with the quantification of soil nutrient availability, can help in estimating free water levels in the rice fields, thereby helping determine the duration of the growing period, as well as, helping with predictions of potential yield reduction due to water stress and soil nutrient limitations. Linking the simulated results with Geographic Information System (GIS) can help quantify the spatial pattern of these attributes at a provincial or regional scale. This study was aimed at quantifying the spatial distribution of water availability, including the frequency and severity of water stress development during the growing period, and to estimate the effects of soil fertility and water stress on rice productivity in Savannakhet province in southern Lao PDR. The current status in relation to the variability in field water availability, soil fertility, farm crop management practices and grain yield throughout Savannakhet province were quantified, first by collecting farm data from 53 and 48 farmers’ fields in the 2007 and 2008 cropping seasons, respectively, These farms were selected to be representative of a wide range of soil and climate conditions within the province. The results show that rainfall distribution pattern, soil type and toposequence position of paddy fields, are crucial factors contributing to the temporal variation in field water availability. The overall yield loss due to water stress associated with late season drought was estimated to be rather small (10%, 5% and 3% for the top, middle and bottom toposequence positions of rice fields, respectively) in the 2008 wet season. On the other hand, application of chemical fertilizer has a large effect on final grain yield, with 50 kg N ha-1 and 30 kg P2O5 ha-1 increasing yield by 600 to 800 kg and 800 to 1000kg ha-1 respectively, in the 2007 and 2008 cropping seasons. A new soil water balance (SWB) model that incorporates the effect of low soil clay content on deep percolation, was developed to quantify field water availability and the length of growing period (LGP) for various rainfall lowland rice cropping environments. The model estimates the amount of water stored in a soil profile, the profile being divided into two layers: Layer 1 (0-200 mm) consists of standing water and the topsoil layer, while Layer 2 (200-1000 mm) is the subsoil layer. The SWB model was validated with field experimental data obtained in the 2002 and 2008 cropping seasons. The simulated free water levels were close to those recorded for the observed field data, with a small mean average error, lower root mean square error, and significant correlation coefficient and index of agreement over all sites across the three toposequence positions of paddy fields. Maps of the length, start and end of growing period (LGP, SGP and EGP, respectively) for rainfed lowland rice in Savannakhet province, were developed using the SWB model, with inputs of median weekly climatic data and soil water characteristics. The province was delineated into three main LGP zones with a short LGP zone (less than 21 weeks) in the east, northwest and some rice fields in the south-western corner of the province; an intermediate LGP zone (21 to 24 weeks) was defined in the central and western part of the province; and a long LGP zone (greater than 24 weeks) for the south and for some rice fields in the western part of the province. The variation in the SGP from year-to-year was due largely to the variation in rainfall early in the wet-season (e.g. April), while EGP was strongly dependent upon the clay content of the soils being cropped. The SWB model was combined with other models that estimate yield potential, soil nutrient supply and yield reduction by low soil water level, to characterize and map the suitability zones for rainfed lowland rice in Savannakhet province. The overall results of the model performance on yield estimates were satisfactorily, with a significant correlation coefficient (0.54**) and high index of agreement (0.68) over the 2007 and 2008 seasons. The model classified three main rice agro-ecological zones according to the suitability of climate and soil conditions. The majority of the lowland rice growing areas are classified as moderately suitable to marginally suitable, while the potential area classified as being high suitable is very small. A large potential response of rice yields to fertilizer inputs is predicted for most of rice growing areas in the province. The best sowing time for achieving a high yield, as evaluated by the model, is the first half of June. Appropriate crop phenology and increasing fertilizer use efficiency that matches with water availability and soil conditions in each rice agro-ecological zone, are important in achieving improvements in rice productivity, as substantial improvements in rice fields cannot be achieved by improving water availability alone, where paddy fields are dominated by soils with low level of indigenous fertility. Although the model is capable of quantifying field water availability and crop yield due to the limitations associated with low levels of soil nutrients and water stress, the model has the potential for further improvements in two areas. First, the estimates of water loss need to be modified by incorporating variable factors such as slope of paddy field, which can affects lateral water movement and hence free water level. Second, the model should incorporate some key agronomic variables, such as internal efficiencies and recovery efficiencies of applied fertilizer, which depend on variety, crop management and climatic conditions, and these factors can be modelled.

Les derniers scénarios du changement climatique, prévoient une élévation de température (Europe, +2 à +4°C en moyenne en 2099, IPCC, 2007) associée à des épisodes extrêmes, sécheresses sévères par exemple. Connaître les potentialités d'adaptation des espèces forestières s'avère crucial afin de comprendre leurs réponses et le devenir des écosystèmes forestiers, dans un futur proche. Dans ce cadre, nous nous sommes intéressés à un complexe d'espèces du genre Fraxinus, (frêne, Oléacées). En France F. excelsior L., et F. angustifolia, Vahl, sont des espèces autochtones présentant une plasticité phénotypique et écologique remarquable. L'hybridation, suspectée depuis longtemps a été prouvée en conditions contrôlées et naturelles. Les principales zones documentées sont la vallée de la Saône et de la Loire. Cette hybridation entre les deux espèces de frênes européens, pourrait favoriser l'apparition d'individus (génotypes) plus aptes que les espèces parentales à faire face à un environnement changeant. Notre objectif est de caractériser les potentialités d'adaptations de différentes populations de frêne (espèces parentales et de statut hybride) sous une contrainte abiotique (contrainte hydrique). Pour répondre à cet objectif, nous avons testé les réponses à la fois écophysiologiques et génétique de jeunes plants à une contrainte légère (-0,9 MPa). Une seconde expérimentation, centré sur l'écophysiologie a eu pour objet de mesurer la perte de conductivité hydraulique des frênes, sous une forte contrainte (-4 MPa). Le principal résultat de ces travaux est le comportement souvent intermédiaire et très variable des populations de frênes hybrides testés dans ces 2 expérimentations (A, gs, WUEi, PLC), que ce soit en conditions avec ou sans contrainte hydrique. Ce comportement intermédiaire est en lien avec le degré d'introgression respectif des hybrides de frênes (plus proche de F.excelsior ou de F.angustifolia). Ces arbres hybrides pourraient servir de ressources et d'assurance contre des évènements de dépérissement catastrophiques pour les forestiers pour un environnement climatique futur. / The latest climate change scenarios predict a rise in mean temperature in Europe of 2 to 4°C for 2099 (IPCC, 2007), associated with extreme climatic events such as severe droughts. Knowing adaptation capabilities of tree species is crucial for understanding their responses and forest ecosystem fate in the near future. Our study object is a species complex inside the Fraxinus genus (ash, Oleaceae). In France, F. excelsior and F. angustifolia are autochthonous, form natural hybrid populations and show remarkable phenotypic and ecological plasticity. This could promote the emergence of new individuals (genotypes) more able to deal with fluctuating environments. Our objective is to characterise the capability of adaptation of different Fraxinus populations, representing the three statuses (F.excelsior, F.angustifolia and hybrids) under abiotic constraints (water constraint). To solve this issue, we examine in a low water constraint experiment (-0.9 MPa) ecophysiological and genetic response, using saplings. A second and more severe water constraint experiment (-4 MPa) was used to investigate ash response to the loss of hydraulic conductivity. The most noticeable result was an intermediate and highly variable behaviour of hybrid ash populations in the two experiments (A, gs, WUEi, PLC) linked with they respective introgression degree (closer to F.excelsior or F.angustifolia). This hybrid trees could be used for foresters as a resource and insurance against catastrophic forest stand decline, for a future climate.