Grass defoliation affecting survival and growth of seedlings of Acacia karroo, an encroaching species in southwestern Zimbabwe

Abstract

Two experiments were conducted, one in the field and the other in the greenhouse, to investigate the effects of the intensity and frequency of grass defoliation on the survival and growth of Acacia karroo seedlings. In the greenhouse, seedlings growing with heavily clipped grasses had higher biomass production than those competing with moderately clipped grasses. Root/shoot ratios were higher in treatments with unclipped grasses. There was a negative relationship between grass root production and A. karroo biomass production. The field experiment was carried out in two paddocks, one previously heavily-grazed and the other lightly-grazed. Grazing in both paddocks was simulated by artificial defoliation. Generally more A. karroo seedlings emerged under lightly defoliated treatments. Clipping frequency had a strong effect (P = 0.066) on the survival of emerged seedlings during the wet season. There were no differences in survival rate at the end of the dry season. Though grass defoliation was shown to enhance seedling growth under controlled conditions, no evidence was found to suggest that seedling establishment during the first year is influenced by the intensity of grass defoliation.

Keywords: Acacia karroo; botany; bush encroachment; grass; grass defoliation; grazing; growth; seedling establishment; seedlings; survival; Zimbabwe; biomass production; clipping; defoliation; setaria incrassata; trees

African Journal of Range & Forage Science, Vol. 15(1 & 2), pp. 41–47