Richness, composition and relationships of the floras of selected forests in southern Africa

Abstract

Species lists of 14 widely separated forests representing particular geographic regions in southern Africa were used to study the size and composition of the individual floras, the similarities between them, and possible determinants of the observed patterns. The forests contain 1 438 species which belong to 155 families and 661 genera. The growth form spectra show specific patterns amongst the individual forests such as an abundance of ferns in montane forests, and of woody plants and vines in coastal forests. The richness of a forest flora increases with increasing altitudinal range within the forest. Significant linear species-area relationships exist for both woody and herbaceous species, but explain only 30% and 38% of the variation respectively in the size of the floras. In a multiple regression model the number of dispersal corridors, the proximity to other forests and mean altitude explained 81% of the variation in the number of woody species. The number of landscape types and of dispersal corridors explained 75% of the variation in number of herbaceous species. Several other factors contribute to the disproportionately large floras of relatively small forests such as at Umtamvuna, Sabie and Richards Bay. A high proportion of unique taxa are present (30% woody and 42% herbaceous species). The shared taxa show definite trends of the southward attenuation of species and the presence of elements of the Afromontane and Indian Ocean Coastal Regions.In conclusion, it is suggested that the southern Cape forests have been isolated from forests along the escarpment and mountains to the east since at least the Pliocene due to the Sundays River valley which stretches from the coast to the escarpment in the arid interior.