Do edaphic factors influence the biogeography of legumes species in the Cape Floristic Region of South Africa?

The species composition of plant communities in the Cape Floristic Region (CFR) of South Africa is associated with the parent rock, and the resultant overlaying soil that has varied soil nutrient levels.Legume species occur in distinct populations in varying habitats ranging from water seeps, river valleys and mountain slopes to quite arid areas.For example, species of Psoralea occupy wet areas whilst those of Otholobium occur in dry sites.We hypothesized that soils of the legume habitats are more fertile than those of non-legume areas in the CFR, despite variation in soil nutrient composition between different legume species.The chemical properties of soils from legume and adjacent non-legume areas from two sandstone (Bainskloof and Silvermine) and granite sites (Jonkershoek and Camps Bay), and from 12 sites occupied by legumes (Psoralea spp.and Otholobium spp.) were determined.Soils derived from sandstone were less fertile than granitic soils.The pH of legume soil at Bainskloof (2.98) and Silvermine (3.35) was significantly (P b 0.05) lower than non-legume soils, but did not differ at the granitic Jonkershoek and Camps bay sites.In contrast, the concentrations of C, K, Ca, Mg or Na were significantly (P b 0.05) greater in legume soils relative to the non-legume soils, probably due to increased rates of plant biomass turn-over associated with stands of legumes.Despite these differences between legume and non-legume sites, considerable variation exists between the soils occupied by different legume species.For example, the soils in areas occupied by Psoralea had higher concentrations of NH 4 + and C than those occupied by Otholobium.We conclude that the legume soils, especially of the infertile sandstone sites, contained more nutrient elements than the non-legume soil, and that soil levels of NH 4 + , C, and to some extent total P influence the distribution of Psoralea and Otholobium species in the CFR.