Topographic controls on the invasion of Pteronia incana (Blue bush) onto hillslopes in Ngqushwa (formerly Peddie) district, Eastern Cape, South Africa

Abstract

The role of topographic factors in the invasion of hillslopes by Pteronia incana, an unpalatable shrub, was investigated. The study combined field observations with image analysis based on high-resolution infrared imagery. A Digital Elevation Model (DEM) of 20 m spatial resolution was used to derive terrain parameters. The Topographic Wetness Index (WI), a component of the TOPMODEL, was derived from the DEM and its relationship with the spatial distribution of P. incana was explored. The absence/presence of P. incana was noted to be strongly influenced by slope angle and aspect. The probability for P. incana occurrence increased with slope steepness and southerly slope orientation. Abandoned and grazing lands were identified as the main invasion hotspots on hillslopes. The combined influence of slope gradient and aspect, and land use was noted to have promoted the invasion. This is borne out by the concentration of the invasion on abandoned steep slopes with a southerly orientation. The WI confirmed the bearing local topographic variations have on P. incana spatial distribution such that, P. incana was associated with the low WI values of convexities. The coupling between local topography and soil surface crusting underpins soil moisture variability. This in turn determines the competition between the patchy P. incana and grass species and the eventual replacement of the latter by the former. Restoration efforts of the invaded lands should focus on trapping of sediment and litter, and moisture retention on the inter-patch bare areas.