Composition and population responses of an African savanna grassland to rainfall and grazing.
Abstract
The influence of inter-annual rainfall variability and cattle grazing on the abundance (density, basal area), population dynamics (recruitment, growth, survival) and population structure of the dominant perennial grasses of an African savanna (Gazankulu, South Africa) was investigated from 1984 to 1989. The unpalatable Aristida bipartita and the palatable Heteropogon contortus and Themeda triandra reproduce by seed; Bothriochloa insculpta reproduces mainly by seed, but sometimes by stolons; Digitaria eriantha can be stoloniferous; Setaria incrassata has rhizomatous growth. Species abundance was more responsive to rainfall variability than to grazing. Density increased in all species in response to several successive dry years because of recruitment and tuft fragmentation, but eventually decreased in a severe final drought year because of plant mortality. The response of basal cover differed between species and sites, although all species (except Digitaria) declined in the final drought year. Seedling recruitment, plant basal area, and mortality were likewise more strongly influenced by rainfall variability than by grazing. Differences in the population dynamics of species were related to their life history attributes. Aristida, a prolific seed producer, was favoured by grazing because high levels of seedling recruitment could be maintained. Seedling recruitment of palatable species (Heteropogon, Themeda) and of Bothriochloa was initially increased under heavy grazing because openings were produced within the vegetation, but this was reversed by the end of the study because of a decline in available seed. There was limited seedling recruitment of Digitaria and Setaria, but recruitment of Digitaria from stoloniferous growth increased population size, although there was no expansion of Setaria clones under the conditions of this study. Themeda and Aristida seedlings attained greater size under heavy than under light grazing. Mortality of all species increased over time, and was generally highest in the final severe drought year. Almost all mortality occurred in small individuals (<2.5 cm diameter) which had been recently recruited or which had fragmented from existing tufts. Fragments of Aristida or Heteropogon were more likely to die than young tufts of the same size. The direction of the effect of grazing on mortality depended on year. Mean tuft size of most species decreased over time due to seedling recruitment and tuft fragmentation. The fraction of basal area after 5 years comprising individuals present at the start of the study, an index of population turnover, was greater than half for seed reproducing species and Bothriochloa, and almost unity for Setaria (>98%) and Digitaria (94%). This fraction for seed-reproducing species was slightly less under heavy grazing. Thus, the pattern of community change was determined by the unique responses of recruitment, growth and mortality to rainfall variability and grazing among species. Heteropogon was almost eliminated and Themeda was markedly reduced by the combination of heavy grazing and drought. This supports the idea that palatable, seed-reproducing species may be prone to local extinction under such conditions because of increased mortality of mature plants and the diminishing availability of seed leading to diminished recruitment. The unpalatable Aristida showed irruptive behaviour under the same conditions because it could maintain seedling recruitment.