Arid savannas have a high incidence of spinescence, a feature almost absent in moist savannas. Although spinescence has been related to water economy there is little physiological evidence in support of the suggestion. The absence of strongly spinescent savannas, and past or present high densities of indigenous herbivores in Australia adds to the evidence favoring herbivores browsers as being the causal factor in the evolution of spinescent savannas. It is of interest that heavily overgrazed base rich savannas in Queensland soon become dominated by Maytenus, Canthium, Carissa and other spinescent taxa. The low nutrient value of moist savanna herbagem and hence low herbivore biomass, accounts for the absence or rairity of spinescent forms in this system - such as the cerrado of Brazil, the miombo of Africa and the tropical moist savannas of Northern Territory, Australia.
Succulents are common in arid savanna but rare or absent in moist savannas.
These woody plants comprise massive underground axes with short-lived foliar shoots above ground. They occur in great abundance on the margins of dambo grasslands and on the vast, treeless, seasonally waterlogged Kalahari sands of the upper Zambezi Basin. The growth form is absent or extremely rare in arid savannas. Their distribution is strongly correlated with extremely oliogotrophic, seasonally waterlogged sand subject to frequent burning. The advantage of this growth form in such edaphic conditions has yet to be satisfactorily explained.