sentences:  The transition zone that lies between the southern Sahel and northern Sudan–Guinea is also a seasonal wetland.The Sudan Savanna zone mostly consists of Sudanian woodland with intermittent vegetation of edaphic grassland and "acacia".Lake Chad, lying on the country's western edge and extending across the border into Niger, Nigeria and Cameroon, is one of the richest areas of flora in the country, although it has rapidly reduced in size during the last century.