Thèse de doctorat

Études écologique, floristique, phytogéographique et phytosociologique des inselbergs du Bénin

Oumorou Madjidou (2003)

PhD, Université Libre de Bruxelles

Ecological, floristic and phytosociological studies of the inselbergs were undertaken in Benin. At first, 286 phytosociological relevés totalising 354 species, which constitute the basis of this work, were divided into two main groups : the forest vegetation and grassland/grass-plot vegetation. The relevés were done following the Braun-Blanquet (1932) method. Detrented Correspondence Analysis (DCA) of the relevés permitted the individualization of seven associations. These communities, recognized as new syntaxa, are gathered into 7 alliances, 6 orders and 6 classes. The distribution of these syntaxa is as follows : under the class of Oleo-Podocarpetea Schmitz 1988 was placed the Cisso-Diospyretum abyssinicae ass. nov. ; the Anchomano welwitschii Lonchocarpodetum sericei ass. nov. was placed under the class of Strombosio-Parinarietea Lebrun and Gilbert 1954 ; under the class of Erythrophleetea africani was placed the Isoberlinio dokae-Pterocarpodetum erinacei ass. nov ; within the class of Hyparrhenietea Schmitz 1963 were two associations which are Cochlospermo planchonii-Loudetietea arundinaceae ass.nov. and Sopubio lanatae-Andropogonetum africani ass. nov.; under the class of Microchloetea indicae Schmitz (1971) 1988 was placed the Cyanoto lanatae-Afrotrilepidetum pilosae ass. nov. ; within the class of Drosero-Xyridetea Schmitz 1988 was placed the Drosero indicae-Xyridetum stramineae ass. nov. Species richness varied from 14 to 193 species. The Shannon-Weaver index ranged within 2.93 and 5.44 bits while the equitability and the reciprocal index of Simpson varied respectively from 0.64 to 0.84 and 0.69 to 0.94. In an ecological and biological perspective, the determining factor on inselbergs is the edaphic dryness due to impossibility of forming hydrous reserves. The plants, and more specifically the herbaceous component, were thus subjected to an extreme aridity. The organic matter content was particularly high within Afrotrilepis pilosa and Cyanotis lanata association. The pH is slightly acid within woody communities and highly acid within grassland or grass-plot communities. The characteristic species of the inselbergs was Afrotrilepis pilosa. The ephemerophytes (annual tiny plants) which are able to reproduce in a one season interval were most favoured. The majority of the species were not specific of inselbergs and come from surrounding formations. From the dynamic viewpoint, the communities recognized in this work represented various stages in progressive or regressive series. Afrotrilepis pilosa constitutes a pioneer species colonizing naked rock. According to the phytogeographical distribution, the sorting of flora components reveals a low floristic individuality of the soudanian base-element and a preponderance of species that are largely distributed on the African continent. No endemic species was recorded among the 395 species collected.