OBSERVATIONS ON THE DEMOGRAPHY OF THE AUSTRALIAN BOAB (ADANSONIA-GIBBOSA) IN THE NORTH-WEST OF THE NORTHERN-TERRITORY, AUSTRALIA

A study of the size-class distributions of the Australian boab, Adanso nia gibbosa (A.Cunn.) Guymer ex D.Baum, a tree endemic to north-wester n Australia, was conducted in Gregory National Park and Keep River Nat ional Park, in the Northern Territory. It was found that populations o f A. gibbosa on alluvial plains had high densities of small, immature stems < 10 cm dbh (diameter at breast height), and that the density of immature stems decreased with increasing distance from a river. By co ntrast, there was an absence of small size classes in a population gro wing on a sandstone escarpment. On limestone substrates, high densitie s of small size-class stems were observed at a site topographically pr otected from wildfire, but there was an absence of small size classes at a site exposed to wildfires. Differences in gross allocation patter ns, as inferred from relationships between tree height, trunk diameter and height of the swollen trunk, were observed between alluvial, sand stone and limestone substrates. These differences suggest growth rates also differ on distinct substrates. There was a highly significant ne gative association between the presence of fruit and evidence of recen t fire damage for stems greater than 10 cm dbh. This pattern may be re lated to the destruction of reproductive structures by late dry season fire. However, additional factors appear to control fruit production, as 36% of the unburnt stems > 10 cm dbh also lacked fruit. Juvenile p lants resprout from swollen tap roots after wildfire, thus frequent fi res would be required to have a long-term impact on stands of A. gibbo sa heavily stocked with juveniles that possess swollen tap roots. Howe ver, it is unknown at what age juveniles develop swollen tap roots and if all juveniles with swollen tap roots resprout following fire damag e. It is hypothesised that the local distribution of A. gibbosa is con trolled by fire history, and that changes in fire regimes associated w ith cattle grazing are causing changes in the distribution of this spe cies. More research is required to critically evaluate this conjecture .