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

Abstract

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.

Tree cavities in forests – The broad distribution pattern of a keystone structure for biodiversity

“…Median cavity density in the 133 study sites was 16.2 ha À1 (quartile range 4.9-38.0), with a maximum of 963 cavities ha À1 (Blakely et al, 2008). The density of excavated cavities levelled off at high total densities (>10 cavities ha À1 ).…”

“…Bai et al, 2003;Boyle et al, 2008;Remm et al, 2006) and the generally detrimental effect of forestry is widely accepted (Lindenmayer and Franklin, 2002). While 'tree cavity' has no generally agreed upon definition, and individual studies have frequently omitted to define the term or to provide precise measurements (but see Blakely et al, 2008;Fan et al, 2003a,b), we use 'tree cavity' to refer to a chamber in a tree trunk or branch that is large enough (and otherwise suitable) for a vertebrate to nest or roost in. Since cavity-nesting fauna differs strongly among different regions, we mostly relied on the original interpretations and have not attempted to use a more precise definition or measurements.…”

Container-breeding mosquitoes and predator community dynamics along an urban-forest gradient: The effects of habitat type and isolation

“…However, the proportion of urban habitat was not positively related to variables that represented predator colonization rates even though these habitats are likely to have more containers and other waterfilled objects (Kittayapong & Strickman 1993). In particular, because the forest habitats in this study rarely consisted of large old trees, the number of treeholes in these habitats was likely to be very low (Fan, Shifley, Spetich, Thompson, & Larsen 2003;Blakely et al 2008). In addition, only the larval stage of mosquitoes are present in water-filled objects, while the mobile adults are dispersing through the habitat.…”

Distribution and abundance of hollow-bearing trees in urban forest fragments

“…The availability of one resource, tree hollows, generally limits the distribution and abundance of many obligate hollowbearing tree (HBT) users and general species richness of an area (Gibbons and Lindenmayer, 2002). The importance of HBTs for a large range of fauna has been demonstrated globally (Bai et al, 2003;Aitken and Martin, 2004;Monterrubio-Rico and EscalantePliego, 2006;Holloway et al, 2007;Blakely et al, 2008;Goldingay, 2009). Hollow-bearing trees are particularly important for Australian faunal communities, given the large number of species (approximately 300) which are reliant upon them as habitat trees (Gibbons and Lindenmayer, 2002;Goldingay, 2009Goldingay, , 2011.…”