Ultramafic substrates and floristic patterns in Sekhukhuneland, South Africa

Abstract

Sekhukhuneland is an area of diverse geology and rich phytodiversity. This region harbours the world's largest reserves of chromium and platinum ore. A literature review of heavy metal soils is presented as introduction and motivation for this dissertation. Special attention was given to the effects of chromium on plants, due to its abundance in the Sekhukhuneland study area. Floristic elements of the Sekhukhuneland study area were determined by means of field work, herbarium studies and literature surveys. Approximately 50 endemic and 70 near-endemic taxa were identified for this region. Families such as Euphorbiaceae, Fabaceae, Liliaceae and Vitaceae proved the most prolific in endemic taxa. Distribution ranges for each of the floristic elements were mapped and analysed with a Geographical Information System. Plant distributions showed distinct floristic patterns within Sekhukhuneland, and links with other mountainous regions of the Northern Provinces of South Africa. Percentage surface cover of all the various geological substrates (predominantly Rustenburg Layered Suite and Transvaal Sequence) were calculated for each quarter degree grid in the study area. A correlation was found between the region's percentage endemism and percentage surface cover of ultramafic substrates. The ultramafic substrates, norite, anorthosite and pyroxenite, showed a significant positive correlation with percentage endemism. The high number of endemics and the endemic-ultramafic correlation in certain quarter degree grids, enabled the identification of focal areas of endemism in the study area. These focal areas of endemism support the status of Sekhukhuneland as an important Centre of Endemism.