Effets de la pâture sur la dynamique de la végétation herbacée au Sahel (Gourma, Mali): une approche par modélisation

Abstract

The vegetation of Sahel rangelands is composed of an herbaceous layer dominated by annual plants and a sparse woody plant population dominated by spiny species. Most of the forage production of these rangelands occurs between July and September. The fodder resource available during this short period decreases during the nine months of the dry season as a result of several factors, including grazing by domestic ruminants. The effects of grazing on the seasonal dynamics of the herbaceous mass have been seldom studied. The dry season dynamics of the herbaceous mass under grazing is simulated by the "Patsec" module of the STEP model. In a sensivity analysis of Patsec to grazing seasonality, intensity and to the species composition of grazing herds, the module outputs are compared to the observations made in 2010-2011 in three pastorals sites near Hombori, in Eastern Mali. The results show that the fodder intake by grazing livestock contributes little to the seasonal degradation of the herbaceous mass. However, grazing is determinant to transfer standing herbaceous mass into litter, and to bury the litter in the soil. The study also shows that, at equal stocking rate expressed in live weight units, cattle's grazing contributes less to the transfer of standing mass to litter and to the burial of litter than does sheep or goat grazing, despite the large fraction that browse may have in their diet. Patsec and more generally the STEP model are successful in predicting the seasonal dynamics of the rangeland fodder resources. They are a potential tool to prevent the pastoral crisis due to fodder shortage in the Sahelian rangelands by providing seasonal monitoring of the livestock stocking rates and the fodder resource available for grazing.