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We assessed the impact of Prosopis invasion and clearing on the grazing capacity of heavily grazed Nama Karoo rangeland in the Beaufort West district of the Western Cape province of South Africa. Invasion (c. 15% Prosopis canopy cover) reduced grazing capacity by 34%, whereas clearing improved it by 110% within six years. Much of the loss in grazing capacity during invasion was due to the displacement of the annual grass Aristida adscensionis that dominated herbaceous forage production at the study site. Improvement in rangeland grazing capacity after Prosopis clearing was due to increases in abundance of A. adscensionis, the perennial grass Cynodon dactylon and the establishment of the perennial grasses Eragrostis obtusa and Eragrostis lehmanniana. Grazing capacity in cleared rangeland was 39% higher than in uninvaded rangeland due to a higher abundance of the annual grasses A. adscensionis, Chloris virgata, Setaria verticillata and Tragus berteronianus and the perennial grasses E. obtusa, E. lehmanniana and C. dactylon. Our results indicate that Prosopis invasion (13% mean canopy cover) can lower grazing capacity in Nama Karoo rangeland, whereas clearing Prosopis from such rangeland can, even under heavy grazing, substantially improve grazing capacity within 4-6 years.