Characteristics of hollows and hollow-bearing trees in semi-arid river red gum woodland and potential limitations for hollow-dependent wildlife ERIN L. WESTERHUIS,* 1 CHRISTINE A. SCHLESINGER, 1 CATHERINE E. M. NANO, 2 STEPHEN R. MORTON 1 AND KEITH A. CHRISTIAN 3 1 Research Institute for the Environment and Livelihoods, Charles Darwin University, Alice Springs, Northern Territory, 0870 (Email: erin.westerhuis@cdu.edu.au); 2 Flora and Fauna, Department of Environment and Natural Resources, Alice Springs Australia ; and 3 Research Institute for the Environment and Livelihoods, Charles Darwin University, Casuarina, Northern Territory, Australia Abstract 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. Key words: cavity-dependent wildlife, ephemeral rivers, Hollow-bearing trees, river red gum, semi-arid woodland. INTRODUCTION Tree hollows are a key structural component for arboreal wildlife in forests and woodlands globally (Goldingay 2009; Seymour & Dean 2010; Voigt et al. 2014). In Australia, it is estimated that 15% of birds, 31% of mammals, 10% of reptiles and 13% of amphibians use tree hollows (Gibbons & Linden- mayer 2002). In recognition of the importance of tree hollows to many Australian animals, research has focussed on quantifying the availability of hollow- bearing trees at different spatial scales (Smith & Lin- denmayer 1988; Lindenmayer et al. 2000; Gibbons & Lindenmayer 2002; Taylor & Chisholm 2005; Koch & Baker 2011; Davis et al. 2014; Rayner et al. 2014; Ellis et al. 2015). However, most research has occurred in temperate climates, and knowledge of the characteristics and abundance of hollow-bearing trees in arid and semi-arid Australia (which repre- sents 70% of the continent) remain poor (but see Rayner et al. 2014; Ellis et al. 2015). In the arid interior of the Australian continent, the majority of hollows suitable for wildlife are confined to Eucalyptus woodlands along river channels and waterways. River red gum (Eucalyptus camaldulensis subsp. arida) woodlands occupy an estimated 7000 km of river channel and floodplain habitat (Geoscience Australia Spatial Data 2016) in central Australia. The river red gum is the only tree in the region that grows in sufficient density, and with a distribution broad enough to provide widespread, predictable, aggregated hollow resources. Other hol- low-bearing trees (e.g. ghost gum, Corymbia aparrer- inja; coolabahs, Eucalyptus spp.; bloodwoods, Corymbia spp.; and desert oak, Allocasuarina decais- neana) have a scattered distribution. In central Aus- tralia, therefore, we propose that river red gum woodlands are critical for animals that use hollows. *Corresponding author. Accepted for publication March 2019. © 2019 Ecological Society of Australia doi:10.1111/aec.12765 Austral Ecology (2019) , –