Vegetation changes (1949-71) in a semi-arid, grassy dwarf shrubland in the Karoo, South Africa: influence of rainfall variability and grazing by sheep.
The influence of annual and seasonal (summer vs. winter) rainfall variability and of different seasons and systems of sheep grazing, including complete protection, on the botanical species composition of a community characterized by dwarf shrubs, annual and perennial grasses, and ephemeral dicotyledons, was investigated for 23 years in the Karoo, South Africa. Ordination and bivariate techniques revealed dramatic changes in response to interannual rainfall variability, mostly associated with annuals or short-lived perennials. Annual rainfall showed a pronounced temporal pattern being wetter in the 1950s and drier in the 1960s, and with annual rainfall correlated with summer rainfall. Annual grasses and ephemeral dicotyledons irrupted in certain years of suitable rainfall to an extent that was influenced by grazing treatments. Perennial grasses and shrubs were strongly influenced by rainfall variation, with grazing treatments further influencing the rate and extent of change. Only perennial grasses were almost eliminated by grazing during the summer. Community change was mostly driven by rainfall variation, but the influence of grazing treatments on longer-lived plants became more important over a longer time period. The community was therefore a matrix of longer-lived perennial grasses and dwarf shrubs within which dramatic variations in the abundance of annual grasses and ephemeral forbs took place. The need is discussed for long-term data sets and appropriate monitoring approaches, as aids to a more complete understanding or the forces determining community structure in semiarid environments.