THE INFLUENCE OF GRAZING ON SEED PRODUCTION AND SEED BANKS OF SOME AFRICAN SAVANNA GRASSLANDS

1. Species composition, population structure and the seed banks of the perennial grasses Aristida bipartita, Bothriochloa insculpta, Heteropogon contortus, Themeda triandra and Urochloa mosambicensis, were described for nine sites in mesic savanna grasslands (seed banks for five) on a similar soil type ranging in grazing history from sustained heavy grazing (> 20 years) to light grazing. The influence of two seasons of grazing on the seed production and the seed bank of Themeda, Setaria incrassata, Heteropogon, Aristida, Bothriochloa and Digitaria eriantha, was estimated at two other sites. 2. Compositional variation reflected grazing history. Lightly grazed sites were characterized by the longer-lived, palatable perennials Themeda, Bothriochloa, Heteropogon and Digitaria, and heavily grazed sites by the shorter-lived perennials Urochloa, Sporobolus nitens, Chloris virgata, the unpalatable Aristida, and some forb species. Grass tuft size declined with increasing grazing pressure. Sustained heavy grazing reduced or eliminated Themeda, Bothriochloa and Heteropogon from the seed bank, but increased the abundance of Aristida and Urochloa spp. 3. Rainfall influenced the seasonal pattern of seed production, which decreased with intra-seasonal drought. The number of seeds produced m-2 of tuft basal area varied from 21 000 for Setaria to 1.9 million for Aristida. Annual seed production m-2 of ground with light grazing was less-than-or-equal-to 400 for Themeda, for Heteropogon, 1300-4000 for Setaria, > 10 000 for Aristida, and 300 for Bothriochloa. 4. Seed viability varied from zero for Digitaria to 68% for Setaria, and varied within a season for Themeda (24-49%) and Setaria (26-68%). The seeds of all species tested possessed a period of innate dormancy of < 1 year. 5. Seven to ten grass and six to seven forb species were found in the seed banks at two sites. Perennial grasses constituted 85-95% of the seed bank, with > 90% of all seeds occurring in the top 2 cm of soil. Seed density differed markedly between sites and grazing treatments. 6. A simple model of the seed bank dynamics of the perennial grasses showed that because their seeds germinate readily and have a limited survival when in secondary dormancy, the seed bank size is determined primarily by the abundance of the species in the vegetation. Thus, the seed banks of Themeda and Heteropogon, which produce small numbers of seeds, can easily be eliminated by sustained grazing. This was also indicated by the distribution of these two species along the gradient of grazing history.