General part

Abstract

Distribution and phytochorological association. The distribution of the genus Duvalia is disjunct with one partial distribution area in southern Africa (sect. Duvalia) and another in northwestern Africa and southwestern Arabia (sect. Arabica, Fig. 1). The distribution gap starts in Somalia, spans nearly 3000 km and ends with the occurrence of D. polita (sect. Duvalia) in Malawi, Zambia, Angola, etc. This allopatric species is restricted to the “Sudano-Sambesian Region” (sensu Werger 1978). All the other species of sect. Duvalia occur in the Cape Provinces of South Africa, from where few species radiate into the southern Namib or the Free State (Figs. 1, 2). One of the two centres of diversification is the Little Karoo of the Western Cape with 4 species /100 km² (Fig. 2). The Little Karoo is part of the “Succulent Karoo Region” (Jürgens 1991), which is known for its richness in endemic species, as well as for its extraordinary diversity. In the south this region merges into the “Cape Floristic Region” (=Capensis sensu White 1983), where only two Duvalia species can be found. Both regions form the “Greater Cape Flora” sensu Jürgens (1991), which receives mostly winter rainfall. In the southern and eastern Cape Provinces, however, there is an increasing influence of summer rainfall along the transition into the “Nama-Karoo Region” (Jürgens 1991). Here, around 32° S 24° E, the occurrence of Duvalia increases considerably and reaches the highest concentration with 6 species/100 km² (Fig. 2).