Vegetation of rock outcrops in Guinea: granite inselbergs, sandstone table mountains and ferricretes — remarks on species numbers and endemism

Abstract

Summary A survey of characteristic plant communities and species occurring on granite inselbergs, sandstone outcrops, and ferricretes in Guinea is given. Examples of species restricted to these insular biota are Pitcairnia feliciana (Bromeliaceae) and rodracoides squamosus (Cyperaceae) . Rock outcrops harbour characteristic habitat types: cryptogamic crusts on rocks, drainage channels, crevices, rock pools, shallow depressions, Afrotrilepis pilosa -mats, ephemeral and wet flush vegetation. Sandstone outcrops of the Fouta Djalon are rich in species and endemics probably due to combined effects of vertical differentiation, large area, long-term climatic stability and isolation. Granite inselbergs and ferricretes have no local endemics and a lower number of species. Floristic homogeneity of granite inselbergs indicates the importance of deterministic processes for controlling diversity. The vegetation of Guinean inselbergs was compared with similar localities in other parts of tropical Africa. Species diversity of inselbergs in Guinean rain forests is higher than on inselbergs in Ivorian rain forests due to higher numbers of growing sites (e.g. rocky slopes of mountain ranges) available for typical inselberg species. Principally, species richness of rock outcrops is positively correlated with the existence of otherwise suitable resources. Supposedly the most diverse inselberg vegetation of tropical Africa is found in Tanzania, Malawi, Mozambique, Zambia, Zimbabwe and Angola.