Effect of <i>Prosopis</i> (mesquite) invasion and clearing on vegetation cover in semi-arid Nama Karoo rangeland, South Africa

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 <i>Prosopis</i>. We assessed the impact of <i>Prosopis</i> 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, <i>Prosopis</i> trees do not affect indigenous vegetation and add to overall rangeland vegetation cover. However, once <i>Prosopis</i> 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 <i>Prosopis</i> clearing, being temporarily higher than pre-invasion levels possibly as a result of enhanced post-invasion soil fertility. Lastly, our results suggest that <i>Prosopis</i> trees reduce rangeland erosion potential at low cover and increase it after they exceed critical threshold cover. Information about thresholds of <i>Prosopis</i> impact can be used to direct clearing efforts for maximum benefit when resources are limited.