Savannas cover approximately half of the African land surface and one fifth of the land surface of the world. They are one of the most important, but least understood terrestrial ecosystems. They are the basis of the African livestock industry and the wildlife they support is of key importance in bringing in tourists. The Nylsvley area in South Africa is one of the most intensively studied savanna regions in the world and as such it is a key source of data and theory relating to this important tropical biome. The South African Savanna Biome Programme was set up to develop the understanding necessary to predict changes in the ecosystem stability induced by both natural and man-made stresses. This book provides a synthesis of the programme's sixteen years of research at Nylsvley and aims to develop a unified vision of the ecology of the dry savanna.

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Abstract. The Kalahari sand sheet occupies 2.5 million ha in southern Africa. It is an area with relatively similar deep aeolian soils, and a strong south to north gradient in rainfall, from 200 to 1000 mm mean annual precipitation (MAP) in the region studied. This provides an excellent basis for gradient studies at the subcontinental scale. This paper briefly reviews the literature on the vegetation of the Kalahari and describes the vegetation structure and composition at 11 new sites. There is a clear gradient in woody plant biomass (as indexed by basal area) from south to north. Above the minimum level of 200 mm MAP, the woody basal area increases at a rate of ca. 2.5 m2.ha‐1 per 100 mm MAP. Mean maximum tree height also increases along the gradient, reaching 20 m at ca. 800 mm MAP. The number of species to contribute > 95% of the woody basal area increases from one at 200 mm to 16 at 1000 mm MAP. Members of the Mimosaceae (mainly Acacia) dominate the tree layer up to 400 mm MAP. They are replaced by either the Combretaceae (Combretum or Terminalia) or Colophospermum mopane of the Caesalpinaceae between 400 and 600 mm MAP, and by other representatives of the Caesalpinaceae above 600 mm MAP. The vegetation is largely deciduous up to 1000 mm MAP, except for species that apparently have access to groundwater, which may be locally dominant above about 600 mm MAP.

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