Dmjs. Bowman, OBSERVATIONS ON THE DEMOGRAPHY OF THE AUSTRALIAN BOAB (ADANSONIA-GIBBOSA) IN THE NORTH-WEST OF THE NORTHERN-TERRITORY, AUSTRALIA, Australian Journal of Botany, 45(5), 1997, pp. 893-904
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
.