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, Adansonia gibbosa (A.Cunn.) Guymer ex D.Baum, a tree endemic to north-western Australia, was conducted in Gregory National Park and Keep River National Park, in the Northern Territory. It was found that populations of 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 contrast, there was an absence of small size classes in a population growing on a sandstone escarpment. On limestone substrates, high densities of small size-class stems were observed at a site topographically protected from wildfire, but there was an absence of small size classes at a site exposed to wildfires. Differences in gross allocation patterns, as inferred from relationships between tree height, trunk diameter and height of the swollen trunk, were observed between alluvial, sandstone and limestone substrates. These differences suggest growth rates also differ on distinct substrates. There was a highly significant negative association between the presence of fruit and evidence of recent fire damage for stems greater than 10 cm dbh. This pattern may be related 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 plants resprout from swollen tap roots after wildfire, thus frequent fires would be required to have a long-term impact on stands of A. gibbosa heavily stocked with juveniles that possess swollen tap roots. However, it is unknown at what age juveniles develop swollen tap roots and if all juveniles with swollen tap roots resprout following fire damage. It is hypothesised that the local distribution of A. gibbosa is controlled by fire history, and that changes in fire regimes associated with cattle grazing are causing changes in the distribution of this species. More research is required to critically evaluate this conjecture.