Spatial and temporal patterns in the emergence and survival of seedlings in arid Karoo shrubland.
The influence of seed availability, weather and microsite on natural 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. Small-scale clearing and herbivore exclusion experiments were conducted to test the hypotheses that seedling survival and growth were unaffected by competition and herbivory. In undisturbed vegetation 20-800 seedlings m-2 emerged in the relatively 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 varied from 5% in 1989 and 1990 to 25% in 1991, and appeared to be limited by the quantity of rain in winter and spring. Vegetation clearing increased the amplitude of soil moisture oscillations at 50 mm below the soil surface, but reduced the rate of soil-moisture depletion at 150 mm below the surface. The species composition of perennial seedlings in cleared vegetation was influenced by the original plant cover and the presence of plant litter on the soil surface. Established plants 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. Grazing by sheep and wild animals reduced the size, but not the survival, of perennial seedlings palatable to mammalian herbivores. Annual plants were larger and more abundant in clearings than in undisturbed shrubland.