SPATIAL AND TEMPORAL PATTERNS IN THE EMERGENCE AND SURVIVAL OF SEEDLINGS IN ARID KAROO SHRUBLAND

1. The influence of seed availability, weather and microsite on natura l emergence and survival of seedlings was investigated over 38 months in arid shrubland in the southern Karoo, South Africa, developed under <200 mm annual precipitation. 2. Small-scale clearing and herbivore e xclusion experiments were conducted to test the hypotheses that seedli ng survival and growth were unaffected by competition and herbivory. 3 . In undisturbed vegetation 20-800 seedlings m(-2) emerged in the rela tively cool and humid austral autumn. Densities of emerging seedlings were greater in microsites influenced by grazing, termitaria and plant litter than in undisturbed or exposed sites. The species composition of seedling assemblages resembled that of the vegetation. Survival var ied from 5% in 1989 and 1990 to 25% in 1991, and appeared to be limite d by the quantity of rain in winter and spring.4. Vegetation clearing increased the amplitude of soil moisture oscillations at 50 mm below t he soil surface, but reduced the rate of soil-moisture depletion at 15 0 mm below the surface. 5. The species composition of perennial seedli ngs in cleared vegetation was influenced by the original plant cover a nd the presence of plant litter on the soil surface. 6. Established pl ants had more influence on seedling survival than did grazing by wild animals or sheep. Proximity to mature perennials reduced both survival and growth of Karoo annuals and perennials. 7. Grazing by sheep and w ild animals reduced the size, but not the survival, of perennial seedl ings palatable to mammalian herbivores. Annual plants were larger and more abundant in clearings than in undisturbed shrubland.