Forage quality, twig diameter, and growth habit of woody plants browsed by black rhinoceros in semi-arid subtropical thicket, South Africa

Rapid growth of the black rhino (Diceros bicornis) population in the Great Fish River Reserve (GFRR), South Africa, has potential to negatively impact the semi-arid thicket vegetation that occurs in the reserve. Woody plants presenting nutritious twigs to twig-biting browsers are assumed to be potentially at greater risk of losing biomass than those that do not and should be considered in management protocols. We investigated plant growth habits, twig diameter and forage quality of woody species commonly browsed by black rhino in the GFRR to see if any fit a high risk profile. Samples of twigs 2, 6, 10 and 14 mm in diameter, and the leaves thereon, were collected during the wet season to determine the dry mass (DM), crude protein (CP) and neutral detergent fibre (NDF) concentrations of leaves and wood, as well as leaf:stem ratios (LSR). CP and LSR decreased while NDF and DM increased with twig diameter. Rates of change appeared to be greater for evergreen than deciduous species. Black rhinos foraging in the GFRR are therefore expected to inflict greatest biomass losses on deciduous species, especially ones with many long shoots. We recommend that relationships among forage quality and growth habits of woody species need to be better understood for the management of black rhinos in semi-arid subtropical thicket.