Boundary of treeless grassland in relation to nutrient content of soils on the Highveld of South Africa

The scarcity of indigenous trees on the Highveld of eastern South Africa is usually attributed to frost, fire and drainage, regardless of whether the soils are nutrient-poor (sourveld grasslands) or nutrient-rich (sweetveld grasslands). However, soil physicochemical properties—such as nutrient availability—are likely to affect vegetation structure by influencing competitive outcomes between herbaceous plants and tree seedlings. Woody cover in western southern Africa has been shown to be greatest on soils of intermediate nutrient status, decreasing on both nutrient-poor and nutrient-rich soils. We predicted – based on a theory of catabolic nutrient demand – that grassland–savanna boundaries on the Highveld would at least partly reflect soil properties. The soil contents of 22 elements, as well as pH and EC, were consequently analysed across 14 such boundaries in 6 provinces. Multivariate analysis showed that site location had the greatest influence on elemental content (r2>0.75, P