Effect of seasonal rainfall pattern on seedling emergence and establishment of grasses in a savanna in south-eastern Botswana

The emergence and spatial distribution of grass seedlings were studied in a degraded savanna in south-eastern Botswana. When the rainy season started early, several separate establishment opportunities occurred, but with late rains only one germination wave was observed. Small rainfall events of 10-12 mm were unable to trigger germination. The phenology of emergence confirms the presence of a drought avoidance syndrome in this semi-arid climate. Seedling densities varied from a few in perennial Eragrostis rigidior grassland to 3000 m(-2) in annual Tragus berteronianus grassland influenced by runoff processes. Seedlings emerged mainly from the top 1 cm of the soil. Vertical distribution of seeds showed a concentration in the top 1 cm of soil and the litter layer. Grass seedling survival was positively correlated with seed size. The importance of seedlings for the regeneration of grasslands is discussed. (C) 1996 Academic Press Limited