A conceptual model of vegetation dynamics in the semiarid Highland savanna of Namibia, with particular reference to bush thickening by Acacia mellifera

Namibian rangelands are encroached with Acacia mellifera, partially resulting from a poor understanding of vegetation dynamics. A conceptual state-and-transition model of vegetation dynamics in the semiarid Highland savanna in central Namibia, emphasising bush thickening by A. mellifera, is described. Two main states, a grassy and a bushy state, are identified. These are further subdivided, and 11 transitions are identified. The key transition initiating a change from grassy to bushy state can be termed a “leap” (an occasional, infrequent mass recruitment event) following a long “sleep” (no or little change in A. mellifera density). It is rare because it requires three consecutive years of above-average rainfall for seedling establishment. Fire, coinciding with seedling establishment, can interrupt it, while a low biomass grass sward facilitates it. The phenology and physiology of the encroaching species, seed predation and sapling herbivory influence this transition. The model proposes opportunistic management interventions, particularly the use of fire, to minimise the risk of further landscape-scale transitions to a bushy state. It highlights areas where understanding of vegetation dynamics is lacking and recommends crucial research foci. Conceptual models of bush-thickening processes need to account for differences in climate and phenological details of encroaching species.