Transformation of a savanna grassland by drought and grazing

Abstract

The relative effects of drought and heavy grazing on the floristic composition, population size and structure, and basal cover of an African savanna grassland were differentiated by comparing changes over eight years, which included a severe drought year, across a gradient of grazing history. Drought had an overriding effect on community change, but grazing history had an additional effect. Severe drought in combination with a history of severe grazing transformed grassland of predominantly palatable, perennial grasses (Themeda triandra, Setaria incrassata, Heteropogon contortus) to grassland dominated by the unpalatable perennial Aristida bipartita, annual grasses and forbs. Palatable species were almost eliminated from the sward between tree canopies, but residual populations were protected beneath certain woody species. The most lightly grazed grassland maintained its character of palatable, perennial grass species but was changed considerably in the relative proportion of these species. After the drought, tuft size of palatable species was smaller than any previously recorded but recovered thereafter. A number of annual and perennial grass species were recorded for the first tune subsequent to the drought. Consideration of the life history attributes of species was useful for predicting species’ responses. Grazing management during drought episodes would seem critical for determining the direction of community change. The notion that savanna grasslands are insensitive to grazing because of their disequilibrium behaviour was rejected.