Diversity, gene flow and conservation of the serpentine flora on the Witwatersrand

Abstract

Serpentine soils and their floras are extremely variable in soil composition, age, elevation and climate and thus make generalizations difficult. The stressful and highly selective nature of the serpentine habitat, a consequence of the interplay of physical, chemical and biotic factors, defines the serpentine syndrome. Soils derived from serpentinites (serpentine soils) usually have very high magnesium to calcium ratios and high levels of toxic trace metals such as nickel and chromium. Regions containing serpentine soils are often considered to be living laboratories where the processes of evolution can be studied in situ because plants respond in various ways to elevated levels of toxic metals and the lowered nutrient status of serpentine-rich soils and often have characteristic vegetation growing on them, usually with few species or with stunted plants. In some species ecotypic races have evolved and survive on these less than optimum areas. Other species are endemic to these soils, yet others appear to be indifferent to these toxic soils (bodenvag) and are found growing equally well on and off serpentine soils. Botanists have been aware of Greenstone outcrops on the Witwatersrand, but, until now, little work had been undertaken on the Witwatersrand. The aim of this study was to gain an understanding of the relationship between the serpentine flora and its associated soils, between the serpentine flora and the adjacent non serpentine flora on the Witwatersrand, to investigate life histories, breeding biology and gene flow patterns in selected bodenvag taxa at specific sites and thus observe any possible processes involved in the formation of ecotypic races and to determine the implications of the results of this study for the conservation of serpentine flora on the Witwatersrand. This study found that the serpentine areas on the Witwatersrand were not very toxic and did not support a unique flora and possessed no endemic taxa. Species richness and diversity between the serpentine and non serpentine sites was also not significantly different indicating that soil factors, such as the concentrations of nickel, magnesium and iron, do not significantly influence diversity. Reproductive and life history data did not separate the study taxa into ecotypes implying that the bodenvag serpentine taxa investigated were either pre-adapted for the serpentine condition or, because the serpentine conditions are similar to that of the Highveld grasslands, there was no selection pressure for the formation of ecotypes. The AFLP results indicated that gene flow occurred amongst the sub populations growing on and off serpentine soils and that these sub populations were genetically similar. It can thus be concluded that other factors, such as the harsh climatic conditions in the form of severe Highveld winters, being part of a fire dependent grassland and occurring at a high elevation, exert a stronger influence on these plants than does a relatively mild serpentine condition thus resulting in a flora that is similar to the non serpentine flora on the Witwatersrand. Currently three of the nine study sites receive some protection as municipal parks or nature reserves and one as a conservancy; but none are nationally proclaimed parks, so the future of these sites is not assured. However, if the ridge development policy guidelines are strictly implemented, then the rocky serpentine outcrops may remain safe from development and contribute towards the richness of the flora of the Witwatersrand as a whole and act as corridors and refuges of local biodiversity.