Selective grass herbivory in the absence of browsing or suppression of shade-intolerant grasses by trees are commonly cited reasons behind bush encroachment in savannas. We show that grazing may confer a competitive advantage to tree seedlings and promote bush encroachment more readily on rocky substrates. This may be due to the imposed sharing of the soil depth niche on rocky substrates, whereas possible niche separation on sandy substrates minimizes the advantage conferred by reduced competition. Keywords: Acacia mellifera, biomass, bush encroachment, Interspecific competition, nutrient status, semi-arid savanna, substrate.