Tree hollows are cavities formed in the trunk or branches of a living or dead tree. Hollows are usually more characteristic of older, mature to over mature trees. Hollows may develop in the trunk and branches of trees as a result of wind breakage, lighting strikes, fire and/or following the consumption and decay of internal heartwood by fungi and invertebrates, primarily termites. Hollow entrances are more common in larger trunks and branches because damage is less likely to be covered by growth of external sapwood.
Hollows occur primarily in old eucalypts trees, and are uncommon in many other native and introduced species such as wattle (Acacia), cypress pine (Callitris), she-oak (Allocasuarina) and pine (Pinus). The presence, abundance and size of hollows is positively correlated with tree trunk diameter, which is an index of age. Hollows with large internal dimensions are the rarest and occur predominantly in large old trees, which are rarely less than 220 years old. Larger, older trees also provide a greater density of hollows per tree. As such, large old hollow-bearing trees are relatively more valuable to hollow-using fauna than younger hollow-bearing trees. The latter are important as a future resource.
Mature and old hollow-bearing trees offer other valuable resources. Mature trees provide more flowers, nectar, fruit and seeds than younger trees, and a complex substrate that supplies diverse habitats for invertebrate populations. When hollow-bearing trees collapse or shed limbs they also provide hollow logs that serve as important foraging substrates and shelter sites.
The distribution of hollow-bearing trees depends on tree species composition, site conditions, competition, tree health and past management activities. Hollows occur at varying densities; undisturbed woodlands typically contain 7–17 hollow-bearing trees per hectare, undisturbed temperate forests 13–27 per hectare and old-growth wet and dry sclerophyll forest of south-east Queensland typically contains 35 and 37 hollow-bearing trees per hectare. On a landscape basis, dead trees often account for 20–50% of the total number of hollow-bearing trees. However they are far more prone to collapse or incineration than live trees and are selectively harvested for firewood.
Occupancy of hollow-bearing trees is also related to their position in the landscape. Some species prefer hollows near riparian habitat or foraging areas, although more mobile species may travel long distances from roost. Breeding behaviour can also govern the suitability of hollows, with birds that nest colonially (e.g. Superb Parrot) or in clusters across the landscape (e.g. Glossy Black-cockatoo) requiring a local abundance of hollow-bearing trees. Conversely, strongly territorial species that prevent other individuals from nesting nearby require an even distribution of hollow-bearing trees if all pairs are to breed.
Many vertebrates are known to select hollows with specific characteristics, indicating that suitable hollows represent a fraction of the total hollow resource. Preference is typically shown for entrance dimensions that approximate body size, presumably to exclude larger competitors and predators. Small animals that roost communally or raise large litters require hollows with small entrances but large internal dimensions. The use of hollows with suboptimal characteristics can adversely affect survival and reproductive success.
The density of hollow-bearing trees required to sustain viable populations of vertebrates is controlled by the diversity of competing fauna species at a site, population densities, number of hollows required by each individual over the long-term, and the number of hollows with suitable characteristics occurring in each tree. The presence, abundance and species richness of hollow-using fauna are correlated with the density of hollow-bearing trees; suggesting that the availability of hollows is often a limiting environmental factor. In some instances it is the prey species of a threatened predator that is limited by hollow availability. In habitats with dense mid-storey the Common Ringtail Possum (Pseudocheirus peregrinus) builds stick nests, but in dry open forest it is dependent on large hollows and only abundant in areas with large trees. As this species is an important prey item of the Powerful Owl (Ninox strenua), loss of hollows indirectly hinders recovery efforts for the predator.
Experimental supplementation of hollows using nest boxes demonstrates that hollow availability can limit the density of bats, arboreal mammals and breeding birds. Box occupancy is greatest at sites of low natural hollow densities and a higher proportion of bird populations breed when nest boxes are provided. Conversely, experimental reductions in hollow density can lead to a decline in the number of nesting birds.
The distribution and abundance of hollow-bearing trees in NSW has been reduced and fragmented by extensive clearing of native vegetation during the past two centuries, primarily for agriculture. For example it has been estimated approximately 70% of native vegetation has been cleared from the NSW wheat-sheep belt, the tablelands of the Great Divide and the coastal plain. Clearing in NSW has continued since 1995 at an estimated rate of over 30 000 ha per annum. Clearing has occurred at a greater intensity on flatter and more fertile landscapes, which typically support the highest densities of hollow-using fauna and most remnant vegetation now exists on rugged and infertile landscapes.
A range of direct and indirect processes contribute to the ongoing loss of hollow-bearing trees, the relative importance of these processes varies according to past and current land management regimes. Owing to the slow process of hollow development, and the particular value provided by large old trees, adverse effects from the continuing loss of old hollow-bearing trees will take centuries to fix.
In agricultural landscapes hollow-bearing trees typically persist as isolated mature individuals in cleared paddocks or in small fragmented vegetation remnants. Such trees frequently suffer from poor health (e.g. 'dieback') and have a shorter lifespan than in forested landscapes. Eventual loss of current hollow-bearing trees, and a lack of recruitment of younger trees to replace them, will result in a large decrease in the hollow resource over the wide geographic area covered by agricultural landscapes in the medium term.
Road reserves and Travelling Stock Reserves (TSRs) provide hollow-bearing trees within cleared agricultural landscapes. However, their availability in road reserves and TSRs is reduced because of habitat fragmentation and competition among hollow-dependent species. In urban and rural residential landscapes hollow-bearing trees persist in parks, small reserves, yards and road corridors, although hollow density varies greatly. Clearing of vegetation for urban expansion and other development, including the creation of asset protection zones against wildfire, contributes significantly to the ongoing loss of hollow-bearing trees. Concern over the risk to humans from falling branches, and the potential for litigation, has increasingly led to removal or pruning of hollow-bearing trees.
In forests managed for timber and firewood production, silvicultural practices have greatly reduced the density of hollow-bearing trees, especially where repeated harvesting events have occurred. In some forest types there has been a gradual shift in the relative composition of tree species toward those desired for timber. Among trees grown for silvicultural purposes, current rotation intervals between harvesting events – typically 30 to 90 years – are insufficient to allow for hollow development. There have previously been limited requirements for retention of hollow-bearing trees on private property managed for silviculture, although prescriptions are currently being developed.
Even when trees are retained during harvest they are susceptible to damage from logging operations and post-harvest burning, or can suffer poor health owing to changes in abiotic conditions. Consequently, retained trees are prone to early mortality, especially with repeated exposure to harvesting events over their lifespan. In addition, the average age of hollow-bearing trees in harvested areas will continue to decrease as the few remaining very old trees die. Trees are also retained in areas excluded from harvesting, such as along drainage lines, with the aim of creating a matrix of harvested and non-harvested areas.
The density of hollow-bearing trees in conservation reserves that have previously been logged should gradually increase until reaching an equilibrium of recruitment and loss, albeit with a long time lag in some areas. Wildfire may temporarily disrupt the age structure of these forests but in the long term can also promote hollow formation in standing trees. Wildfire is a particular threat at sites where the hollow resource is restricted to large, senescent hollow-bearing trees that are susceptible to incineration. Where feral species and unusually abundant native species (e.g. Galah) occur, competition for hollows limits their availability to other species. This is more common in smaller reserves. One widespread competitor is the introduced Honeybee Apis mellifera, which typically builds hives in large cavities with small entrances.