Encroachment and expansion of woody species in the savanna areas of Masutlhe and Lekung in the North West Province : a case study

Abstract

Large areas of southern Africa are affected by woody plant encroachment. The increase in the tree-grass ratio in the savannas has been attributed to the replacement of indigenous herbivores by domestic grazing animals and the intense utilization of the natural vegetation by domestic livestock. The encroachment of woody species into savanna is a global phenomenon and often has an impact in the development of the herbaceous sward. It is especially in the communally managed, such as Masutlhe and Lekung and rural areas where woody plants often develop and expand at the expense herbaceous vegetation, mainly grasses. The encroachment of woody plant species was quantified at the selected sites and compared to nearby reference sites in Masutlhe and Lekung Villages. The prominent encroaching woody species included Vachellia tortilis, Grewia flava and Ziziphus mucronata. All selected sites, except the benchmark sites, had woody plant densities, exceeding 2 000 TE ha⁻¹ that will almost totally suppress grass growth. Remote Sensing techniques were used to analyse the overall trend of vegetation in the study area. High spatial resolution digital satellite images and appropriate image processing algorithms were used to facilitate monitoring of the woody encroachment. Mean Euclidean Distance Texture analysis in 3×3 moving windows enhanced woody cover. SPOT images of 2004, 2006 and 2014 were used to monitor change detection of vegetation. Land cover maps were established, comprising three classes woody vegetation, grass and bare area. Analysis of vegetation conditions trends revealed decline in grass cover with an increase in woody vegetation, especially in the villages of the study area.