Population dynamics of the shrub Acacia suaveolens (Sm.) Willd.: Fire and the transition to seedlings

Fire, through soil heating effects, causes flushes of seed germination in Acacia suaveolens. Optimal temperatures for germination are between 60 and 80°C for any duration, or up to 100°C for durations less than 1 h. Exposure to temperatures less than 60°C leaves seeds dormant and viable, whilst seed death occurs in increasing proportions with increasing exposure to temperatures greater than 80°C. A field study of temperatures in the soil under simulated burns showed that the innate seed dormancy in A. suaveolens would only be broken for seeds up to a depth of 1 cm in ‘cool’ or 4 cm in ‘hot’ burns. In the hot burns some of the seeds in the top 1 cm of the soil were killed by excessive heating. These simulated burns most resemble cool and moderate/high intensity wildfires, respectively. Seeds can emerge from depths up to 8 cm and, for any seeds buried deeper than this, the probability of emergence is progressively reduced down to nil at 14 cm. Seeds buried between 5 and 10 cm will be heated sufficiently to break their dormancy only in a very high intensity wildfire. Seeds buried between 5 and 10 cm deep mostly occur in nests of an ant, Pheidole sp. Field observations of emergent seedlings confirm that post-fire emergence is concentrated over a small range of soil depths directly related to the intensity and duration of heating that occurs, whilst occasional seedlings may appear from greater or lesser depths largely dependent upon the spatial heterogeneity of soil heating in natural fires.