African Journal of Range & Forage Science 2015: 1–9 Printed in South Africa — All rights reserved Copyright © NISC (Pty) Ltd AFRICAN JOURNAL OF RANGE & FORAGE SCIENCE ISSN 1022-0119 EISSN 1727-9380 http://dx.doi.org/10.2989/10220119.2015.1036460 African Journal of Range & Forage Science is co-published by NISC (Pty) Ltd and Taylor & Francis This is the final version of the article that is published ahead of the print and online issue Land degradation caused by soil erosion is one of the most critical environmental issues affecting South Africa (Le Roux et al. 2007, 2008). More than 70% (854 000 km 2 ) of South Africa’s land surface has been affected, to varying intensities, by soil erosion (Le Roux et al. 2008). Much of the erosion in these areas has been facilitated by a combination of highly erodible soils and poor farming practices (Le Roux et al. 2007). In South Africa’s semi-arid region, extensive overgrazing has led to widespread soil erosion and drying up of rivers and rangeland (Milton and Dean 1995). The concomitant spread of invasive alien plants (IAPs) across the region (Milton et al. 2003) could have intensified soil erosion by enhancing deleterious effects of over-grazing on rangeland vegetation cover. Vegetation cover is an important determi- nant of rainfall infiltration, runoff and ultimately soil erosion potential (Elwell and Stocking 1976; Dunne et al. 1991; Le Roux et al. 2008; Zuazo and Pleguezuelo 2008). Plant canopy cover (i.e. the proportion of the ground surface covered by aerial plant parts) promotes infiltration and limits runoff by protecting the soil surface from raindrop impact, which causes detachment of soil particles and physical crusting (Elwell and Stocking 1976; Herrick et al. 2005a). Plant basal cover (i.e. proportion of the soil surface covered by plant bases) reduces the erosive ability of runoff by dissipating its power through deflection and obstruction (Rogers and Schumm 1991; Herrick et al. 2005a). The Nama-Karoo is the largest of the three biomes that comprise the semi-arid Karoo-Namib ecoregion of southern Africa (Palmer and Hoffman 1997). In South Africa, the biome occupies ~346 100 km 2 (28% of the country) and supports extensive commercial sheep and goat production on natural rangeland (Palmer and Hoffman 1997; Hoffman 1999; Suttie et al. 2005). Large areas of the Nama-Karoo have been invaded by an array of IAPs (Richardson and van Wilgen 2004; Henderson 2007). Notable among the IAPs are leguminous trees of the genus Prosopis, which cover at least 18 000 km 2 of the biome’s low-lying alluvial plains and seasonal watercourses (Richardson and van Wilgen 2004). Prosopis trees, which are indigenous to the Americas, were introduced into arid regions of South Africa in the late 1880s to provide shade, fodder and fuel wood (Zimmermann and Pasiecznik 2005). However, alien Prosopis trees have had serious negative impacts on the environment and human livelihoods (Richardson et al. 2000; Richardson and van Wilgen 2004; Zimmermann and Introduction Effect of Prosopis (mesquite) invasion and clearing on vegetation cover in semi-arid Nama Karoo rangeland, South Africa Thabisisani Ndhlovu 1 *, Suzanne J Milton 2 and Karen J Esler 3 1 Department of Conservation Ecology and Entomology, Stellenbosch University, Stellenbosch, South Africa 2 Percy FitzPatrick Institute of African Ornithology, DST–NRF Centre of Excellence, University of Cape Town, Cape Town, South Africa 3 Centre for Invasion Biology and Department of Conservation Ecology and Entomology, Stellenbosch University, Stellenbosch, South Africa * Corresponding author, email: thabisisani.ndhlovu@gmail.com The Nama-Karoo is the largest of the three biomes that comprise the semi-arid Karoo-Namib ecoregion of southern Africa. Large tracts of Nama-Karoo rangeland have been invaded by alien leguminous trees of the genus Prosopis. We assessed the impact of Prosopis invasion and clearing on vegetation cover in heavily grazed Nama-Karoo rangeland on two sheep farms in the Beaufort West district of the Western Cape province of South Africa. Our results suggest that, below critical threshold cover levels, Prosopis trees do not affect indigenous vegetation and add to overall rangeland vegetation cover. However, once Prosopis cover exceeds critical thresholds, the trees begin to negatively impact on indigenous vegetation (especially grasses) and overall vegetation cover. In addition, our data suggest that vegetation cover recovers after Prosopis clearing, being temporarily higher than pre-invasion levels possibly as a result of enhanced post-invasion soil fertility. Lastly, our results suggest that Prosopis trees reduce rangeland erosion potential at low cover and increase it after they exceed critical threshold cover. Information about thresholds of Prosopis impact can be used to direct clearing efforts for maximum benefit when resources are limited. Keywords: invasive plants – exotic, Nama Karoo, rehabilitation, semi-arid, soil erosion Online supplementary information: Supporting information for this paper is available as online supplementary material at http://dx.doi.org/10.2989/10220119.2015.1036460 African Journal of Range & Forage Science