PhD Dissertation

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


Oumorou Madjidou (2003)
PhD, Université Libre de Bruxelles

ABSTRACT

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.