Dynamique de la strate herbacee des paturages de la zone sud-sahelienne

The present state of the Sahelian rangelands and their overexploitation requires control of their use and the development of other management practices. This is the more so, because of the devastating effects of the droughts during the last 15 years. For this purpose description and evaluation of the species composition of the herbage vegetation is needed, which is especially difficult because the predominance of annuals contributes to large differences from year and from place to place.The present thesis concerns research on the interaction between the properties of species, rainfall, substrate and way of exploitation, as reflected in the dynamics of the rangeland vegetation. This research was executed in Niono in Mali in the South of the Sahel at an average rainfall of 600 mm per year. Experiments were done on sand, loam and clay soils. Natural rainfall was varied by shielding against rain and additional sprinkling.The main annual species were studied throughout the growing season from germination to seed production. Auto-ecological experiments in the field and under controlled conditions provided additional information about the species response to environmental differences. The observed changes in the absolute and relative contribution of the species in terms of number and biomass clarified the role of various plant properties in conjunction with rainfall amount and distribution, substrate properties and management.Important properties of the species that govern the dynamics of the vegetation are germination rate, drought resistance of the young seedlings, type of photosynthesis (C 3 or C 4 ), day length, length of the growing cycle, degree in which the plants continue vegetative growth after the onset of flowering and the production of seeds. The main effect of the substrate is its influence on the rain-dependent availability of water throughout the growing season, as affected by physical soil properties and topography. Exploitation by grazing adversely affects this rain-dependent availability of water, but also has a direct effect on height, length of the growing cycle and seed production of the species. Rapid germination, drought resistance of seedlings, C 4 -photosynthesis, a long growing cycle and extension in height are in general advantageous properties to survive, but plants can neither have it all ways. Moreover, slow germination and a short growing cycle are apparently advantageous in long periods of drought, whether these are due to irregular rainfall as such or to substrate properties or overexploitation.Vegetation surveys to support the improvement of rangeland use are of more lasting value, if the dynamics of the herbage cover is interpreted against the background of the results that are presented in this study.