The influence of rainfall, competition and predation on seed production, germination and establishment of an encroaching Acacia in an arid Namibian savanna

Seed production and seedling survival are under-researched in savannas. We investigated these in a population of a major thickening species, Acacia mellifera, in an arid Namibian savanna over a nine year period (late 1998–early 2007) We asked the following questions: (i) How does viable seed production vary with rainfall and tree size, (ii) when does seed germination occur, (iii) is the seed bank of A. mellifera persistent, and (iv) how do competitive interactions with established trees influence recruitment of A. mellifera seedlings? Seed production was highly correlated with annual rainfall. In dry years, there was no viable seed production. En masse seed production only occurred in exceptionally high rainfall years, and was strongly correlated with size among trees >2 m tall. Seed predation was low. Seedlings only emerged directly after en masse seed production, suggesting ephemeral seed banks. Three times more seedlings emerged per m2, but seedling survival was five times less, under trees than away from trees, indicating strong competition for water with established trees. Seed production is a recruitment bottleneck in this species. Recruitment requires at least two consecutive seasons of favourable rainfall, and is highly episodic in arid savannas.

► We studied a population of Acacia mellifera shrubs in an arid savanna over 10 years.
► Seed production was highly correlated with rainfall and tree size.
► Trees only produced seeds en masse in exceptionally high rainfall years.
► Seeds germinate only in seasons when seeds are produced.
►  Consequently establishment requires protracted periods of high rainfall events.