Herbaceous vegetation change in variable rangeland environments: The relative contribution of grazing and climatic variability
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abstract
A 44-yr record of herbaceous vegetation change was analysed for three contrasting grazing regimes within a semi-arid savanna to evaluate the relative contribution of confined livestock grazing and climatic variability as agents of vegetation change. Grazing intensity had a significant, directional effect on the relative composition of short- and mid-grass response groups; their composition was significantly correlated with time since the grazing regimes were established. Interannual precipitation was not significantly correlated with response group composition. However, interannual precipitation was significantly correlated with total plant basal area while time since imposition of grazing regimes was not, but both interannual precipitation and time since the grazing regimes were established were significantly correlated with total plant density. Vegetation change was reversible even though the herbaceous community had been maintained in an altered state for ca. 60 yr by intensive livestock grazing. However, ca. 25 yr were required for the mid-grass response group to recover following the elimination of grazing and recovery occurred intermittently. The increase in mid-grass composition was associated with a significant decrease in total plant density and an increase in mean individual plant basal area. Therefore, we failed to reject the hypotheses based on the proportional change in relative response group composition with grazing intensity and the distinct effects of grazing and climatic variability on response group composition, total basal area and plant density. Long-term vegetation change indicates that grazing intensity established the long-term directional change in response group composition, but that episodic climate events defined the short-term rate and trajectory of this change and determines the upper limit on total basal area. The occurrence of both directional and non-directional vegetation responses were largely a function of (1) the unique responses of the various community attributes monitored and (2) the distinct temporal responses of these community attributes to grazing and climatic variation. This interpretation supports previous conclusions that individual ecosystems may exist in equilibrial and non-equilibrial states at various temporal and spatial scales.
