Effects of water, grass and N on responses of Acacia karroo seedlings to early wet season simulated browsing: Aboveground growth and biomass allocation

Acacia species are important components of bush encroachment and browse production in Africa. Goats are both abundant and regarded as useful biological agents of woody plant control, but compensatory regrowth responses can reduce the impacts of herbivory. Compensatory/overcompensatory regrowth of woody plants following herbivory is postulated to be most likely to occur when resources are not limiting, but the responses of Acacia seedlings to resource–browsing interactions have not been studied. A split-plot experiment was conducted to investigate effects of irrigation, N fertiliser and grass cover on growth and biomass allocation of Acacia karroo seedlings after early wet season clipping in a semi-arid savanna. Responses were estimated at the end of the wet season. Compensatory/overcompensatory regrowth occurred when water was added without grass cover. Clipping increased the mean leaf:shoot mass ratio and had either neutral or positive effects on spine:leaf and spine:shoot ratios that appeared to be unrelated to resources. We postulate that in conditions of heavy grazing that reduce grass biomass, it is more difficult to control Acacia seedlings with goats in wet than in dry years. This is important to consider for bush control because Acacia seedling recruitment is believed to be high in wet years, when control by goats may be most difficult.