Distinct edaphic habitats are occupied by discrete legume assemblages with unique indicator species in the Cape Peninsula of South Africa

The Cape Peninsula is a small area (471 km²) situated on the south-westernmost tip of the Core Cape Subregion (CCR) of South Africa. Within the Cape Peninsula, Fabaceae are the third most species-rich plant family (162 species) and they have the second highest number of endemic species after the Ericaceae. However, legumes are not the dominant taxa in the vegetation. They tend to show patchy distributions within the landscape and different species assemblages usually occupy particular niches at any given locality. The present study undertook to establish if edaphic factors influence legume species distribution in the Cape Peninsula and to determine the key indicator species for the different assemblages. Soils from 27 legume sites, spanning all major geological substrates of the Cape Peninsula, were analysed for 31 chemical and physical properties. Legume species present at each site were recorded and a presence/absence matrix was generated. Cluster analysis and discriminant function analysis (DFA) were run to group the sites based on overall similarity in edaphic characteristics and to identify the soil parameters contributing towards discriminating the groups. Canonical correspondence analysis (CCA) was used to test for a correlation between legume species compositions and edaphic factors. The strength of the association between legume species and site groupings based on edaphic properties was assessed using indicator species analysis. Based on similarity in overall soil characteristics, the sites formed three clusters: one comprising sites of sandstone geology, one with dune sand sites and the third cluster comprising sites of both shale and granite geologies (hereafter referred to as soil types). The DFA confirmed the distinctness of these clusters and the CCA showed a significant correlation between legume species composition and edaphic factors. The key edaphic parameters were clay content, iron (Fe), potassium (K), sulphur (S) and zinc (Zn). These findings reveal that the Cape Peninsula is edaphically heterogeneous and edaphically distinct habitats contain discrete legume species assemblages that can be distinguished by unique indicator species. Furthermore, multiple soil parameters, rather than a single parameter, are involved. Therefore, edaphic factors play a significant role in driving the distribution of legume species in the Cape Peninsula and discrete legume species assemblages occupy distinct habitats.