Characteristics of hollows and hollow-bearing trees in semi-arid river red gum woodland and potential limitations for hollow-dependent wildlife

Up to 37 species of the birds and microbats inhabiting inland Australia are dependent on tree cavities for breeding or roosting. The river red gum (Eucalyptus camaldulensis), a well-known hollow-bearing tree species, occurs in linear semi-arid woodland along thousands of kilometres of ephemeral river channels and is the only tree species that provides widespread, aggregated hollow resources across a landscape otherwise dominated by shrublands. Here we assess the type and quantity of hollows available along ephemeral rivers of the MacDonnell Ranges bioregion in central Australia and determine which characteristics of river red gums best predict the abundance and characteristics of different tree hollows, as first steps towards assessing the current availability of hollows in the region. Approximately a third of all river red gums sampled were hollow-bearing, but individual trees with abundant hollows were rare. Further, 36% of hollows had an entrance ≤ 5 cm, and 37% had entrances which were 6–10 cm in diameter, whereas only 13% of hollows had an entrance diameter > 20 cm suitable for larger hollow-using species. Large and high hollows only occurred on trees that did not display post-disturbance resprouting. Trees with multiple and diverse hollows were rare and tended to be in advanced stages of senescence and had larger stems (82.3 ± 3.33 cm) and were taller (14.4 ± 0.53 m) compared to non-hollow-bearing trees (23.44 ± 1.68 cm, 8.0 ± 0.34 m). Further research is required to establish whether the current abundance of hollows and diversity of hollow types are limiting to cavity-dependent wildlife, and to identify any threats to availability of hollows.