Population dynamics of the shrub Acacia suaveolens (Sm.) Willd.: Survivorship throughout the life cycle,a synthesis

Survivorship in Acacia suaveolens was assessed through seedling and adult stages. Moisture stress was found to be the critical factor limiting early seedling survival. Both seedling and adult populations were characterized by periods of low mortality interspersed with pulses of high mortality. A composite survivorship curve for A. suaveolens based on nine sites predicts that some 20–25 years after afire, established plants should disappear from the above-ground flora if another fire does not occur. Fecundity and survivorship data were used to estimate the flux of seed in the soil over time in a hypothetical A. suaveolens population. From this it was predicted that, following establishment of plants after a fire, the seed-bank would rapidly reach a maximum after 6 years and thereafter slowly decline, until after 60 years there would be only as many seeds as there were original parental plants. The situation would vary with predispersal seed predation, seed predation on the soil surface, seed dispersal by ants to ‘unsafe sites’ and the size of the initial seed-bank prior to establishment. Only after a very long inter-fire period would A. suaveolens be eliminated from a site. Elimination of the species is also possible under very frequent fires. A 2–5 year fire-free period is needed for plants to reach maturity and another 6 years are needed to maximize seed input into the soil seed-bank. In addition, seedling recruitment following cool burns is low to non-existent as dormancy is not broken for most seeds in the soil during such burns.