Seed bank status and dynamics of Acacia saligna at two sites in the Western Cape, South Africa

Acacia saligna is the most damaging invasive species in the coastal lowlands of the south-western Cape. The gall rust fungus, Uromycladium tepperianum, has been highly successful as a biological control agent for A. saligna populations in South Africa and has effectively reduced the density, canopy cover and seed production of the tree. However, there are still concerns about the soil-stored seed bank and knowledge of seed bank status and dynamics is crucial for effective management. This study evaluated the effectiveness of two different sampling methods in assessing the status of the seed bank, how the seed bank of A. saligna at two different sites varies over time and how these findings compare to findings of other seed bank studies of A. saligna across southern Africa. Even with the reduction in seed production caused by biological control, numbers of seeds in the soil seed bank are high enough to maintain high levels of recruitment after management or natural disturbances. Both sampling methods (grid and random sampling) attempted were effective in assessing the vertical distribution of the seed bank and estimated the size of the seed bank to be within the same order of magnitude. However, random sampling will be more effective in assessing the seed bank size as it was found that the seeds have a clumped horizontal distribution. The vertical distribution of seeds in the seed bank was found to be influenced by soil properties. The largest portion of the seed bank is situated in the upper 0-10 cm of the soil and declines in size with depth.