Effects of water, grass and N on responses of Acacia karroo seedlings to early wet season simulated browsing: Leaf N, fibre and tannin concentrations

Bush encroachment is a phenomenon that affects semi-arid savannas world-wide. To apply effective browsing to control trees, the browse–browser interaction must be well understood. An important aspect is feedbacks between browsers and trees because browsers influence the nutritional value of plants. When resources are abundant, tannins, which are assumed to negatively affect browsers, are predicted to decrease following herbivory because defoliation is assumed to reduce C relative to N. In opposite conditions, defoliation is expected to increase tannins by increasing C relative to N. A split-plot experiment was conducted to investigate the effects of irrigation, fertiliser, grass cover and clipping on leaf N, neutral detergent fibre (NDF) and condensed tannin (CT) concentrations of Acacia karroo seedlings in a semi-arid savanna. Clipping reduced N of irrigated plants, and increased N of unirrigated plants, in the absence of grass cover. By reducing N, clipping in relatively high resource conditions resulted in reduced potential value of the seedlings as fodder. The reverse occurred in relatively low resource conditions through N elevation. The development of negative feedback in conditions that promote woody seedling encroachment would render bush control by browsing difficult.