An evaluation of the differences in the herbaceous vegetation of sites dominated by different encroacher bush species in the Kalahari Sands Plateau of Namibia

Abstract

Bush encroachment is a form of rangeland degradation that reduces the grazing capacity, livestock and wildlife productivity. This study evaluated how sites dominated by encroaching bush species Senegalia mellifera, Dichrostachys cinerea and Terminalia sericea varied in terms of their herbaceous vegetation structure. The herbaceous vegetation data were collected from 42 fixed points per plot, and a total of 27 plots were surveyed. The results showed that Eragrostis trichophora and Stipagrostis uniplumis were the dominant grass species in the three farms, and 48% of the grass species were annuals. The principal component analysis (PCA) showed that S. mellifera sites were associated with Cenchrus ciliaris, Eragrostis lehmanniana and Brachiaria euriciformis. In contrast, D. cinerea sites corresponded with Aristida stipitata, Urochloa brachyura and Enneapogon cenchroides, while herb species and Pogonarthria squarrosa were more closely linked to T. sericea sites. The herbaceous biomass yield of sites dominated by S. mellifera (410 kg DM ha^–1) was higher compared to the sites dominated by D. cinerea (292 kg DM ha^–1) and T. sericea (231 kg DM ha^–1). The study concluded that field sites dominated by different encroacher bush species had varying herbaceous layers, which ecologically implies that woody species exert species-specific effects on understory vegetation.