Morphological responses of woody species to simulated twig-browsing in Botswana

Abstract

Increased shoot growth has recurrently been recorded following real or simulated shoot browsing, although responses may vary between different woody species. We investigated morphological responses to simulated dry-season browsing on Colophospermum mopane, Acacia tortilis, Grewia flava, Peltophorum africanum and Ziziphus mucronata, which differ in growth rate, occurrence of spines and use by mammalian herbivores. Treatment groups included a control, four clipping intensities and cutting at ground level. Responses were monitored during the following growth season on three annual shoots, located in the bottom, middle and top height interval, on each sapling. All five species showed compensatory growth responses, although no obvious relation was found between the strength of responses and growth rate or natural browsing pressure. The general pattern was an increased shoot length, shoot diameter, number of lateral shoots and shoot biomass with increased clipping intensity. These responses are probably an effect of the reduction of competing growing points and disrupted apical dominance. Growth responses were stronger in shoots with higher location on the sapling, which may reflect the pressure on saplings to attain a certain height to avoid browsing, reproduce and increase resistance to fire. We found no evidence of the number of spines being an induced defence in either of the two spinescent species studied.