Biogeography of Oxalis (Oxalidaceae) in South Africa: A preliminary study
Oxalis L., commonly called sorrel, is a large and cosmopolitan taxon that has undergone spectacular speciation within southern Africa (± 270 taxa), and more specifically within the winter rainfall regions of the western Cape Region (CR). The main objective of this study was to analyse the geographical distribution of Oxalis in South Africa in relation to currently defined phytogeographic units. The observed patterns of biodiversity and endemism within South African members of the genus show interesting disjunctions and concentrations of species. Oxalis is one of the few CR taxa that is shared between the core Fynbos and Succulent Karoo Biomes, and this study therefore provides a novel insight into evolutionary trends across, and not only within, these phytogeographic units. The major centre for diversity for Oxalis is situated on Table Mountain and the northern areas of the Cape Peninsula (grid square 3318CD). Subsidiary centres are located in the Clanwilliam/Niewoudtville and Kamiesberg regions. The reported patterns in Western Cape suggest that Oxalis species richness has been generated and retained in areas which have been identified as core Fynbos (Table Mountain), Fynbos refugia during interglacials (Kamiesberg), and an ecotonal region which might switch between the two biome types (Clanwilliam/Niewoudtville). Presumably these three types of areas would provide interesting material for DNA-based phylogenetic work, and a test of the climate change 'species-pump' hypothesis proposed by Midgley et al. (2001).