Response of three mesic South African perennial grassland forbs to defoliation and competition

The importance of maintaining biodiversity in natural grasslands under livestock production systems has been recognised. Forbs contribute more to species richness than grasses in South African grasslands. However, little is known about the impact of grazing and grass competition on the population dynamics of forbs. The aim of this study was to determine the response of three mesic grassland perennial forb species to simulated intensive defoliation and interspecific competition from a neighbouring dominant grass, Themeda triandra, in a field experiment. Two of the selected forb species are sensitive to defoliation (Afroaster hispida and Gerbera ambigua) and one is potentially resistant to defoliation (Hypoxis hemerocallidea). Defoliation resulted in smaller plants of all three species following regrowth. Only A. hispida showed an interaction response to defoliation and competition. Thus, a competitive release was observed for A. hispida in that the height of defoliated plants was reduced by 45% compared with undefoliated plants under full competition, but there was no difference under partial or no competition. Gerbera ambigua was unaffected by competition. Hypoxis hemerocallidea was sensitive to competition irrespective of being defoliated or undefoliated. Findings of this study highlighted an individual response such that a general response cannot be predicted.