Seasonal vegetation changes in the Hoanib River catchment, north-western Namibia: a study of a non-equilibrium system

摘要

The numbers of livestock and wildlife that can be supported in an arid environment such as the Hoanib River catchment, north-western Namibia, are determined by annual and seasonal availability of grazing and browsing. The availability and abundance of vegetation was investigated in eight focus-study areas across the catchment. The vegetation of the focus area was observed to behave in a non-equilibrial manner in that it was dependent on annual rainfall rather than intensity of landuse. The Zurich-Montpellier method of vegetation assessment was used for the evaluation of vegetation plots. The amount of dry season grazing was also dependent on the previous season's rainfall, with 'dead grass' and perennial grass only available after an 'above average' rainfall season. Vegetation communities were dominated by mixed Colosphosermum mopane woodlands in the 100-350 mm rainfall zone, while perennial grasses dominated the 50-100 mm rainfall zone. While the bulk rangeland appeared not to be disturbed by landuse, a 2km 'sacrifice zone' around water sources was found to have changes in vegetation species and abundance. During below-average rainfall seasons and drought, when there is limited or no grazing, browse becomes the most important source of nutrition for domestic stock and wildlife.