Tropical and subtropical savanna-woodland systems are the dominant ecotype between the arid Sahara Desert and the lush rainforests of the Congo. The most famous savanna system in this ecotype is the Sahel, the transition zone from arid to semi-arid savanna directly bordering the southern Sahara Desert. These savanna-woodlands are dominated by low tree species, such as Acacia spp and grass species including Andropogon, Aristida, Stipagrostis and Chrysopogon spp. The area is characterized by summer rains with erratic distributions and drastic inter-seasonal temperature regimes. Semi-desert savanna vegetation of the Sahel predominantly exists on sandy soils that grade to oxisols in the more humid border with the Congo rainforests.