IMPACT OF TOPOGRAPHIC AND ANTHROPIC GRADIENTS ON THE DIVERSITY OF SUDANIAN WOODLANDS

The Sudanian landscape is gradually changing as its woodland formations become more Sahelian. The aim of this study of Sudanian ligneous vegetation was to assess the isolated effects of land use and topographic position on the one hand, and the effects of their interactions on the diversity of plant formations on the other hand. To do so, phyto-sociological surveys were conducted in three types of land use units of varying extent (classified forest, fallow lands and fields) along a topographic gradient (glacis, valley). The effect of topography and of the land use gradient on plant diversity was assessed through two-way variance analyses. The statistical analyses showed that protected lands are significantly more diverse than fallow lands and fields. The high plant diversity observed in protected lands can be explained by the conservation measures applied there. There is a high degree of similarity between the vegetation in fallow lands and fields due to continually shortening fallow periods. Moreover, and independently from land use, plant formations established on glacis are more diverse than in valleys because these ecosystems are less selective and their soil conditions are favourable to the establishment and development of a range of species. The interaction between land use patterns and topographic units does not affect the diversity of Sudanian woody plants because human impacts in the different habitats on the diversity of woody vegetation are not related to topographic position. Topographic conditions thus have a more determining influence on the diversity of woody vegetation than on land use patterns in this study site.