Vegetation of mesas and surrounding plains in the southeastern Nama Karoo, South Africa

Abstract

Plant community composition and distribution on and off isolated mesas were investigated across Nama Karoo communities in the Eastern Cape, South Africa. Species composition and cover were measured in plots along a transect extending from the SE plains and slopes, across the plateaux, onto the NW slopes and plains of three mesas (Tafelberg, Folminkskop and Buffelskop). Data were then classified using the floristic-sociological approach, assisted by the computer package TWINSPAN. Canonical Correlation Analysis was used to analyse the relationship between vegetation and environmental patterns. Mesa communities were found to be distinctly different from plains communities, with no shared communities between the two habitats. The distribution of communities across the landscape is attributed to a soil-moisture gradient. Differences in habitat probably existed before the impact of domestic livestock, but overgrazing has likely exacerbated the differences. The potential to use mesas as sources of seeds and propagules for the surrounding degraded plains is low, since few species are shared between the two habitats. However, generalist, palatable species such as Felicia muricata, Eragrostis obtusa, Pentzia incana etc. could have some potential for future restoration. Dolerite-capped mesas such as Tafelberg and Folminkskop had a general slope community shared between the two mesas. Aspect and the expected cooler, more moist conditions on SE slopes as factors determining community composition for the dolerite mesas were overridden by soil type and associated nutrient status. In xeric sandstone mesas such as Buffelskop, soil type and associated nutrient status were overridden by aspect and slope as determinants of community composition. Mesa habitats are generally not grazed heavily by livestock due to their inaccessible nature, and are not threatened by current land use practices. Plains habitats are often degraded, a consequence of 200 years of selective grazing.