Late 20th century landscape-wide expansion of Allosyncarpia ternata (Myrtaceae) forests in Kakadu National Park, northern Australia

Abstract

Allosyncarpia ternata S.T.Blake is a large tree endemic to the rugged western edge of the Arnhem Land Plateau, northern Australia, with most of the species conserved in Kakadu National Park (KNP). A. ternata stems suffer substantial mortality following wildfire but the species resprouts prolifically from root stocks. Nonetheless, there is concern about the persistence of A. ternata rainforest patches following breakdown of traditional Aboriginal landscape burning that generated a mosaic of burnt and unburnt areas. Generalised linear modelling was used to identify the landscape features associated with the fragmentary distribution of A. ternata rainforest. We randomly sampled 12 areas that together made up 12.6% of the total coverage of A. ternata in KNP (12 191 ha) that spanned the geographic range of this vegetation type within the Park. The modelling of these data showed that A. ternata forests were most likely to occur at sites with fire protection, as inferred from the small number of fire scars apparent on sequences of satellite imagery, steep slope angles and proximity to drainage lines. Analysis of historical aerial photography revealed that, despite considerable negative and positive variation, there has been a 21% expansion of A. ternata forests over the last 50 years. Expansion occurred by incremental growth from existing forest boundaries and not by nucleation, reflecting the poor seed dispersal of the tree. The forest expansion was negatively correlated with fire activity. A regionally wetter climate since the mid-20th century may be an important cause of the expansion despite currently unfavourable fire regimes.