Extent of woody plant invasion along a riparian zone of the Molopo River, North West Province, South Africa

Abstract

River systems play a pivotal role in the functioning of the ecosystem. The frequent and intense disturbances of these systems create problems for conservation of biodiversity and ecosystem functioning. As in most parts of the world, the riparian zones of the Molopo River are highly modified. Changes are caused by alien plant invaders such as Prosopis velutina and human induced overgrazing and other human activities such as cutting down of indigenous trees and creating boreholes along and within the Molopo River system. The extent of woody plant invasion was quantified at selected sites and reference sites along the riparian zones of the Molopo River and up-stream, in-land in the vicinity of the villages of the study area. The prominent species identified in the selected sites included Prosopis velutina, Senegalia mellifera and Vachellia tortilis. The other woody species observed included Vache Ilia erioloba, V. hebeclada, Grewia jlava, Ziziphus mucronata and Tarchonanthus camphoratus. The exotic, non-woody cactus, Opuntia ficus-indica was also present. All the selected sites, except the benchmark sites, had woody plant densities, exceeding 2 000 TE/ha that will almost totally suppress grass growth. Remote sensing techniques were used to analyse the overall trend of vegetation in the study area. Landsat TM images of 1988, 1996 and 2013, were used to monitor change detection of vegetation. Land cover maps were established, comprising five classes of land cover, viz. intact vegetation, degraded vegetation, grassland, bare surface and water body. The classification of the images was achieved using the supervised K-nearest neighbour algorithm. Analysis of vegetation condition trends revealed a decline in intact vegetation with an increase in degraded vegetation especially in the vicinity of the villages in the study area. However, it was further observed that water as an important source of life was also decreasing.