The abundance, distribution and structural characteristics of tree-holes in Nothofagus forest, New Zealand

Tree-holes provide an important microhabitat that is used for feeding, roosting and breeding by numerous species around the world. Yet despite their ecological importance for many of New Zealand's endangered species, few studies have investigated the abundance or distribution of tree-holes in native forests. We used complementary ground and climbed tree surveys to determine the abundance, distribution and characteristics of tree-holes in undisturbed Nothofagus forest in the Lewis Pass, New Zealand. We found that hole-bearing trees were surprisingly abundant compared with many other studies, including Australian Eucalyptus species and American beech. In fact, we estimated as many as 3906 tree-holes per hectare, of which 963 holes per hectare were potentially large enough to provide roost sites for hole-nesting bats in New Zealand, while only eight holes per hectare were potentially suitable for specialist hole-nesting birds. This was of great interest as primary cavity-excavating animals are absent from New Zealand forests, compared with North America and Australia. Moreover, tree-hole formation in New Zealand is likely to be dominated by abiotic processes, such as branch breakage from windstorms and snow damage. As has been found in many other studies, tree-holes were not uniformly distributed throughout the forest. Tree-holes were significantly more abundant on the least abundant tree species, Nothofagus fusca, than on either N. menziesii or N. solandri. In addition to tree species, tree size was also an important factor influencing the structural characteristics of tree-holes and their abundance in this forest. Moreover, these trends were not fully evident without climbed tree surveys. Our results revealed that ground-based surveys consistently underestimated the number of tree-holes present on Nothofagus trees, and illustrate the importance of using climbed inspections where possible in tree-hole surveys. We compare our results with other studies overseas and discuss how these are linked to the biotic and abiotic processes involved in tree-hole formation. We consider the potential implications of our findings for New Zealand's hole-dwelling fauna and how stand dynamics and past and future forest management practices will influence the structural characteristics of tree-holes and their abundance in remnant forest throughout New Zealand.