Effects of herbivore exclosures on variation in quality and quantity of plants among management and habitat types in a semiarid savanna

Abstract

The effects of grazing on plant biomass, plant quality, species evenness, species diversity and species composition were determined among management types (communal, commercial and game) and among habitat types (open savanna, rocky and pan) in a semiarid savanna in South Africa. Over three growing seasons we compared fenced and unfenced plots among all management and habitat types. Fenced plots had greater mean plant height as well as higher crude protein (CP) and phosphorus (P) yields (= concentration ¥ biomass). The only significant difference in species composition between fenced and unfenced plots was observed in the pan habitat type during the third growing season. Negative effects of grazing were more pronounced in the commercially managed area than under other management types, although the higher abundance of poisonous plants under communal management is of considerable concern. The open savanna habitat had the highest plant biomass and lowest CP and P levels, while the reverse was true for the pan habitat type. Most parameters assessed showed significant effects between fenced and unfenced plots towards the end of the study only because species composition in fenced plots was altered by competition between species normally suppressed by grazing.