Severely degraded dunes of the southern Kalahari: Local extinction, persistence and natural re-establishment of plants
This study aimed to quantify and understand the impact of severe land degradation on plant attributes and diversity on dunes of the southern Kalahari. Heavy grazing pressure in particular resulted in a significant decline of canopy cover and species number in annual and perennial life forms; forb and graminoid growth forms; erect and prostrate habits; and leafy stem, tussock and stoloniferous architectures. However, no significant change was found in shrub and tree forms which persisted without apparent new recruitment. Under these conditions, species diversity dropped sharply and a number of species, mainly graminoids, became apparently locally extinct. The perennial shrub, Crotalaria cf. spartioides, showed the converse with a high frequency of establishing seedlings on the degraded dunes. Changes in relative dominance show that the extreme treatment favours perennial over annual, woody over graminoid and forb, erect over prostrate and leafy stem over stoloniferous and tussock. Some of these results and certain species and soil responses differ from those reported from grazing studies elsewhere, and are possibly no longer directly related to the impact of the primary grazing pressure but to the secondary effect of subsequent instability of the dunes. © 2009 Blackwell Publishing Ltd.