Effect du couert ligneux sur la structure de la végétation herbacée de jachères Soudaniennes Léonard-Élie Akpo, Michel Grouzis, Fidèle Bada, Roger Pontanier and Christian Floret

The specific compostion of the herbaceous layer was studied within and outside of the fallow tree cover in Casamance, a subhumid SudanoGuinean savanna region in Southern Senegal. These fallow plots were arranged in n classes of different ages, i.e. J1, J3, J5, J10 & J17, respectively corresponding to 1,3,5,10 and 17 years after cultivation was abadoned. The floristic data were processed by multivariate analysis methods (CoA, diversity indexes). CoA made a clear distinction between survey data obtained in shady biotopes and those from open areas and the extent of discrimination index varied with the age of the fallows. There was therefore a tree effect on the herbaceous layer. A characteristic flora list was drawn up for each different biotope. The species richness and diversity indexes were substantially higher under the cannopy, indicating that the environmental conditons were better than in open areas. the fallow herbaceous vegetation layer evolved in two directions according to the sciophyullous and heliophilic communities