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In this study the contribution of climatic factors and phylogenetic relationships affecting the geographical distribution of C3 and C4 genera of the Cyperaceae in South Africa was investigated. The δ13C values of herbarium specimens of 68 southern African species from 22 genera and eight tribes were used to assign the species to either the C3 or C4 photosynthetic pathway. Geographical distribution data for the Cyperaceae were used to investigate relationships between climatic factors and the number of species and proportional abundance of C4 species per region. The number of Cyperaceae species per 2° × 2° square across South Africa varied from less than five in the north-western regions to more than 15 in the south-western and north-eastern regions of South Africa where rainfall exceeds 800 mm y-1. Of the 68 species investigated, 28 had C4 photosynthesis and these were scattered among nine genera of four tribes (Cypereae, Scirpeae, Abildgaardieae and Rhyncosporeae). The proportional abundance of C4 species ranged from 14% in the winter rainfall regions of the south-west of South Africa to 67% in the summer rainfall areas of the north-east. The geographical distribution of species was related to their phylogenetic position such that the distributions of C3 and C4 species in Cypereae, Scirpeae and Schoeneae was quite distinct. Linear regression analysis showed that the transition temperatures (equal C3 and C4 species numbers) for the Cyperaceae were different to those obtained for the Poaceae from the same region. No strong relationships were found between the proportional abundance of C4 species and other climate factors such as altitude and rainfall. Our analysis of the current geographical distribution of C4 Cyperaceae in southern Africa in a phylogenetic context suggests that the ecological advantages conferred by the C4 pathway differ amongst the different plant groups. [ABSTRACT FROM AUTHOR]