Eco-geomorphological processes within grasslands, shrublands and badlands in the semi-arid Karoo, South Africa

Abstract

Vegetation type and cover play an important role in the operation of geomorphological processes by controlling runoff and sediment dynamics. In drylands, land degradation is particularly sensitive to these eco-geomorphic interactions. Although many geomorphological studies of land degradation focus on the change in hydrological response as a function of vegetation cover, few have investigated how the autogenic response of plants may influence the susceptibility of soil to erosion through a change of soil resources. This study investigates the hypothesis that shrub communities possess greater soil parameter heterogeneity compared with grasslands and assesses how these different scales of heterogeneity can influence the susceptibility of soil to erosion.  Soil samples were taken from seven 60 m × 60 m plots within grasslands, shrublands and badlands situated in the Sneeuberg uplands of the central Karoo. One hundred and eight samples per plot were analysed for bulk density, organic matter, pH, conductivity and available sodium, calcium, magnesium, potassium and phosphorus. Geostatistical analyses determined that the grassland landscape was largely homogenous in its distribution of soil parameters, whereas shrublands demonstrated an increase in heterogeneity. Periodicity in the semi-variograms indicated that regular patterns across the landscape were evident for all parameters and thus likely to represent the differences between shrub and intershrub regions, areas of high and low erodibility. More pronounced patterns were identified in the badlands. This indicates that, if the conditions are right, changes in plant-soil interactions caused by soil parameter redistribution in shrubland landscapes can exacerbate erosion, leading to further degradation in the form of badlands.