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

Fire, through soil heating effects, causes flushes of seed gertrtination in Acacia suaveolens. Optitnal temperatures for gertrtination are between 60 and 80°C for atiy duration, or up to lOO'C for durations less than 1 h. Exposure to tetnperatures less than 60°C leaves seeds dorinatit and viable, whilst seed death occurs in increasing proportions with increasing exposure to tetnperatures greater than 80°C. A field study of temperatures in the soil under sitnulated burns showed that the innate seed dormancy in A. suaveolens would only be broken for seeds up to a depth of! cm in 'cool' or 4 ctn in 'hot' burns. In the hot burns sotne of the seeds in the top 1 cm of the soil were killed by excessive heating. These simulated burns tnost resemble cool atid moderate/high intensity wildfires, respectively. Seeds can emerge from depths up to 8 ctn 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 atid 10 cm will be heated sufficiently to break their dortrtancy only in a very high intensity wildfire. Seeds buried between 5 and 10 ctn deep mostly occur in nests of an ant, Pheidole sp. Field observations of emergent seedlitigs confirtn that post-fire etnergettce 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 frotn greater or lesser depths largely dependent upon the spatial heterogeneity of soil heating in natural fires.