Grasslands and herbaceous fringes on inselbergs in Atlantic central Africa
Parmentier, Ingrid; Müller, Jonas V.
Inselbergs are rock outcrops forming "xeric islands" in the rain forest. Plants on inselbergs endure very harsh edaphic and microclimatic conditions. Monocotyledonous mats develop on the rock surface, and grasslands are present on the shallow soils. The inselberg-rain forest ecocline is characterized by the spatial transition from an herbaceous fringe to a shrubby fringe, a forest fringe and a saxicolous forest. We describe here the plant communities of the inselberg grasslands and herbaceous fringes of 24 sites in Southern Cameroon, Equatorial Guinea and Gabon. The plant communities defined from the classification of 177 phytosociological relevés are compared to the existing phytosociological literature and the correlation between the floristic data and the environmental and spatial variables is analyzed. We define a new phytosociological order: the Sclerio melanotrichae-Habenarietalia procerae, two new alliances and four new associations. Herbaceous fringes and grasslands differ both in their flora and in their life form spectra, but the main differentiating factor in the floristic data set is the geographical location of the relevés rather than the plant formation. Three ecogeographical groups of inselbergs are identified.Weeds are an important component of the inselberg flora. They are well adapted to the harsh inselberg habitat by their ability to cope with unpredictable environmental fluctuations (e.g. drought). Grazing and trampling by wild herds of buffaloes contribute to create suitable habitats for these species.Epiphytes are present in the herbaceous fringes. This is explained by the similarity of the environmental conditions on the rock and on the inselberg rain forest ecocline with the environmental conditions encountered by epiphytes in the upper part of the forest canopy.The inselberg grasslands and herbaceous fringes contribute to the regional biodiversity in three ways:a) fourteen species are restricted to the inselberg habitat and to the Guineo-Congolian region,b) fourteen species are only present on the inselbergs in the rain forest zone, their main distribution being in the savannah or at higher altitudes,c) the inselberg grassland and herbaceous fringe plant communities do not exist outside the inselberg habitat.