Using aerial photographs to remotely assess tree hollow availability

Tree hollows (cavities) are important habitat features and their conservation is challenging for forest managers worldwide. Required by many faunal species for successful breeding (Martin and Eadie 1999; Gibbons and Lindenmayer 2002), the availability of hollow bearing trees can be an important predictor of the presence and abundance of hollow-using wildlife (Lindenmayer et al. 1991a, b; Aitken and Martin 2008). Hollows occur more often in older trees (Gibbons et al. 2000; Koch et al. 2008b; Temesgen et al. 2008) and hollow availability is declining in many areas, primarily due to land clearance and timber harvesting (Gibbons and Lindenmayer 2002). In some areas of the world, the majority of hollows are formed by primary hollow excavators such as woodpeckers, which are selective in their choice of tree (Martin et al. 2004; Ojeda et al. 2007). Hollows can also be formed by stochastic processes such as wind, fire, termites and fungi, and these non-excavated hollows provide important habitat in areas both with and without excavators (Gibbons and Lindenmayer 2002; Wesolowski 2007). Difficulties in obtaining stand- and landscape-level information on hollow density and distribution hinder effective management of hollows and hollow-dependent fauna