Loss of hollow-bearing trees - key threatening process listing

The Scientific Committee, established by the Threatened Species Conservation Act, has made a Final Determination to list the Loss of Hollow-bearing Trees as a KEY THREATENING PROCESS in Schedule 3 of the Act. Listing of key threatening processes is provided for by Part 2 of the Act.

NSW Scientific Committee - final determination

The Scientific Committee has found that:

1. In NSW, terrestrial vertebrate species that are reliant on tree hollows for shelter and nests include at least 46 mammals, 81 birds, 31 reptiles and 16 frogs (Gibbons and Lindenmayer 1997, Gibbons and Lindenmayer 2002). Of these, 40 species are listed as threatened on Schedule 1 and Schedule 2 of the Threatened Species Conservation Act:

2. Hollows develop in the trunk and branches of trees following consumption and decay of internal heartwood by fungi and invertebrates, primarily termites (Wilkes 1982, Mackowski 1984). In Eucalyptus species, decomposition of heartwood typically begins in the stem at an early age and extends slowly into the larger branches of mature trees (Greaves and Florence 1966, Mackowski 1984). Internal cavities develop after this decayed material collapses, and access is provided by breakage of branches or the stem. Hollow entrances are more common in larger trunks and branches because damage is less likely to be occluded by growth of external sapwood (Marks  et al. 1986). Fire damage can also expose heartwood to decay or create cavities (Inions  et al. 1989). In Australia, active excavation by vertebrates is not an important process in hollow formation.

3. Hollows are common in old myrtaceous trees (primarily eucalypts), but 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 are positively correlated with tree basal diameter, which is an index of age (Lindenmayer  et al. 1991a, Bennett  et al. 1994, Ross 1999, Soderquist 1999, Gibbons  et al. 2000, Shelly 2005). Tree diameter at breast height (DBH) is, in turn, a strong predictor of occupancy by vertebrate fauna (Mackowski 1984, Saunders  et al. 1982, Smith and Lindenmayer 1988, Gibbons  et al. 2002, Kalcounis-Rüppell  et al. 2006). The minimum size-class at which trees consistently (>50% of trees) contain hollows varies depending on the species and environmental conditions, yet is always skewed toward the larger, more mature trees (Table 1).

Table 1. Size class and estimated age at which 50% or more of trees measured are hollow-bearing. Trends found in areas outside NSW are applicable to similar forest types in this state.

4. Hollows with large internal dimensions are the rarest and occur predominantly in large old trees (e.g. Soderquist 1999, Wormington and Lamb 1999). Eucalypts containing large hollows are rarely less than 220 years old (Gibbons and Lindenmayer 2002). Larger, older trees also provide a greater density of hollows per tree (e.g. Bennett et al. 1994, Gibbons  et al. 2000, Lindenmayer  et al. 2000, Shelly 2005). 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.

5. The distribution of hollow-bearing trees is uneven across the landscape, depending on tree species composition, site conditions, competition, tree health and past management activities (Bennett et al. 1994, Harper  et al. 2005). In their summary of inventories, Gibbons and Lindenmayer (2002) documented 7–17 hollow-bearing trees per hectare in relatively undisturbed woodlands, and 13–27 per hectare in undisturbed temperate forests. In old-growth wet and dry sclerophyll forest of south-east Queensland hollow-bearing trees numbered 35 and 37 per hectare, respectively (Wormington and Lamb 1999). Woodland remnants of northern Victoria that have not been systematically logged retain a density of hollow-bearing trees ranging from 17–32 per hectare (Bennett  et al. 1994, van der Ree  et al. 2001).

6. On a landscape basis, dead trees often account for 20–50% of the total number of hollow-bearing trees and typically contain hollows when at a smaller DBH than live trees (Bennett et al. 1994, Gibbons 1999, Soderquist 1999, Ross 1999, Harper  et al. 2005). Although dead trees are sometimes preferentially selected as roost sites by certain species (e.g. Taylor and Savva 1988, Lumsden  et al. 2002) they are far more prone to collapse or incineration than live trees (e.g. Ross 1999) and are selectively harvested for firewood. 'Removal of dead wood and dead trees' is listed as a Key Threatening Process under the NSW Threatened Species Conservation Act, with the loss of hollows in dead trees exacerbating the currently limited resource in live trees.

7. Although large hollow-bearing trees are numerically rare, vertebrate species strongly select for them as nest and roost sites. Of 228 hollow-bearing trees examined after felling in East Gippsland, Victoria, the mean DBH of trees used by vertebrates was 151±9 (SD) cm for E. fastigata, 125±7 cm for  E. cypellocarpa, 114±8 cm for  E. obliqua and 92±11 cm for  E. croajingolensis (Gibbons  et al. 2002). About 80% of trees used by Glossy Black-cockatoos for nesting in box-ironbark forest are greater than 60 cm DBH (Cameron 2006). A review of roost selection by bats demonstrated consistent selectivity for large hollow-bearing trees (Kalcounis-Rüppell  et al. 2006).

8. Occupancy of hollow-bearing trees is also related to their position and spatial configuration in the landscape. Some species prefer hollows near riparian habitat (Law and Anderson 2000, Kalcounis-Rüppell et al. 2006) or foraging areas (Eyre and Smith 1997, Kavanagh and Wheeler 2004), although more mobile species may travel long distances from roost sites (e.g. Lumsden  et al. 2002). Breeding behaviour can also govern the spatial 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 (Gibbons and Lindenmayer 2002, Cameron 2006). Conversely, strongly territorial species that prevent conspecifics from nesting nearby require an even distribution of hollow-bearing trees if all pairs are to breed (Rowley and Chapman 1991).

9. Apart from nesting birds, which use a single hollow during breeding, many species move between hollows over time. For example, Australian Owlet-nightjars (Aegotheles cristatus) shelter in up to six hollows over several months (Brigham  et al. 1998). Maternity colonies of bat species switch between a network of hollow-bearing trees every few days (Law and Anderson 2000, Lumsden  et al. 2002, Kunz and Lumsden 2003, Rhodes  et al. 2006). The elapid snake,  Hoplocephalus stephensi, uses 20 to 30 arboreal shelters, including hollows, in each home range (Fitzgerald  et al. 2002). Individual Brush-tailed Phascogales use up to 38 hollows over a year (Rhind 1998). Frequent movements between hollows may serve to reduce parasite infestation, minimise risk of predation, provide appropriate thermal microclimates and allow energy-efficient access to foraging areas (e.g. Lewis 1995, Reckardt and Kerth 2006).

10. Many vertebrates are known to select hollows with specific characteristics, indicating that suitable hollows represent a fraction of the total hollow resource (Newton 1994, Gibbons et al. 2002, Kalcounis-Rüppell  et al. 2006). Preference is typically shown for entrance dimensions that approximate body size, presumably to exclude larger competitors and predators (Tiddeman and Flavel 1987, Dickman 1991, Harley 2004). Small animals that roost communally or raise large litters require hollows with small entrances but large internal dimensions (e.g. Soderquist 1993, Kunz and Lumsden 2003). Some vertebrates only use vertically oriented hollows (Newton 1994, Lumsden  et al. 2002), which usually occur in stems. Other characteristics that are often important to hollow selection include: height (Newton 1994, Kunz and Lumsden 2003, Cameron 2006), exposure to solar radiation (Kerth  et al. 2001), thermal insulation (Sedgeley 2001), foliage mass near the hollow (Willis and Brigham 2005), and whether the hollow is surrounded by dead or live wood (Gibbons and Lindenmayer 2002). The use of hollows with suboptimal characteristics can adversely affect survival and reproductive success.

11. The density of hollow-bearing trees required to sustain viable populations of vertebrates is a function of 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. These factors vary spatially among habitats and temporally throughout the year with, for example, the demand for nests increasing greatly during the spring breeding season (Calder et al. 1983). Accurately estimating hollow requirements in a given habitat is currently difficult due to the lack of this baseline information, and attempts at modelling have been limited. However, there is much circumstantial and some experimental evidence, as presented below, to indicate that hollows are a limiting resource, and that recovery of threatened hollow-using fauna would benefit from their greater availability.

12. The presence, abundance and species richness of hollow-using fauna are correlated with the density of hollow-bearing trees in a wide range of studies (e.g. McIlroy 1978, Meredith 1984, Smith and Lindenmayer 1988, Lindenmayer et al. 1991b, Traill 1991, Newton 1994, Smith  et al. 1994, Eyre and Smith 1997, Kavanagh and Stanton 1998, Alexander  et al. 2002, Gibbons and Lindenmayer 2002, Kavanagh and Wheeler 2004). Other evidence that hollows are a limited resource include fighting for hollow possession within and among species, successive use of the same hollow by different pairs of breeding birds, and progressively greater use of poor quality hollows as population density increases or hollow availability decreases (Nilsson 1984, Newton 1994, Gibbons and Lindenmayer 2002, Heinsohn  et al. 2003). 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 (Soderquist  et al. 1999). 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 (Newton 1994, Gibbons and Lindenmayer 2002, Beyer and Goldingay 2006). 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.

13. 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 (e.g. Recher 1996). When hollow-bearing trees collapse or shed limbs they also provide hollow logs that serve as important foraging substrates and shelter sites (e.g. Mac Nally et al. 2001).

14. 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, Walker et al. (1993) estimated that six billion trees (60% of the pre-European population) were cleared in the Murray-Darling Basin. Approximately 70% of native vegetation has been cleared from the NSW wheat-sheep belt, the tablelands of the Great Divide and the coastal plain (Benson 1999, EPA 2003). Many of the trees would have been hollow-bearing. Clearing in NSW has continued since 1995 at an estimated rate of over 30 000 ha per annum (Benson 1999, Cox  et al. 2001, EPA 2003, Audit Office 2006). Clearing has occurred at a greater intensity on flatter and more fertile landscapes, which typically support the highest densities of hollow-using fauna (e.g. Lindenmayer  et al. 1991a) and most remnant vegetation now exists on rugged and infertile landscapes. Clearing of native vegetation is listed as a Key Threatening Process under the Threatened Species Conservation Act.

15. 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 (Table 2). 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.

16. In agricultural landscapes hollow-bearing trees typically persist as isolated mature individuals in cleared paddocks or in small fragmented vegetation remnants (Bennett et al. 1994, Gibbons and Boak 2002). Such trees frequently suffer from poor health (e.g. 'dieback') and have a shorter lifespan than in forested landscapes (Yates and Hobbs 1997, Saunders  et al. 2003, Reid and Lansberg 2000). 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 (Saunders  et al. 2003, Vesk and Mac Nally 2006).

17. 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. These trees are further threatened by health decline and early senescence ('dieback'), firewood collection, clearing during road improvements and, in some areas, a lack of recruitment.

18. In urban and rural residential landscapes hollow-bearing trees persist in parks, small reserves, yards and road corridors, although hollow density varies greatly (Harper et al. 2005). 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.

19. 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 (Lindenmayer et al. 1991a, Smith  et al. 1994, Ross 1999). Culling of mature trees to reduce competition with younger, production trees has specifically targeted large hollow-bearing trees. In some forest types there has been a gradual shift in the relative composition of tree species toward those desired for timber. For example, the  Eucalyptus- Callitris woodlands of central NSW have changed from dominance by  Eucalyptus species (78% of basal area) to dominance by  Callitris (74%) under silvicultural management (Lunt  et al. 2006). As  Callitris rarely develops hollows, this has considerably reduced the density of hollow-bearing trees and contributed to faunal declines (e.g. Paull and Kerle 2004).

Table 2. Processes contributing to the loss of hollow-bearing trees or affecting their value to fauna under various management regimes. Multiple processes typically operate across different spatial scales at the same time. 'C' denotes areas where processes operate on a relatively consistent basis; 'V' denotes the process is of variable intensity depending on local site conditions and management.

Among trees grown for silvicultural purposes, current rotation intervals between harvesting events – typically 30 to 90 years – are insufficient to allow for hollow development. As a result, the persistence of hollow-bearing trees is primarily a function of the number and fate of retained trees. Prescriptions for forestry operations aim to ensure the perpetuation of a minimum density of hollow-bearing trees in harvested areas (e.g. 2–5 trees per hectare, although variable across forest types and management regimes). The diversity of hollow-using fauna, dynamics of hollow use and specificity in hollow requirements indicate that these minimal densities will have a large impact on the population viability of some hollow-dependent fauna. There have previously been limited requirements for retention of hollow-bearing trees on private property managed for silviculture, although prescriptions are currently being developed.

Trees retained during harvest are susceptible to damage from logging operations and post-harvest burning, or can suffer poor health owing to changes in abiotic conditions (Gibbons and Lindenmayer 2002). Consequently, retained trees are prone to early mortality, especially with repeated exposure to harvesting events over their lifespan. Prescriptions for forestry operations also stipulate that young trees are retained for long-term replacement of hollow-bearing trees, typically with one recruit for every hollow-bearing tree. The age structure in natural forests, where recruitment and loss of mature trees is at equilibrium, indicates that only a small proportion of younger trees survive to reach maturity. A ratio of one-to-one will be inadequate in itself to sustain the stipulated minimum densities of hollow-bearing trees in harvested areas. 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. In the longer term as trees mature in exclusion zones they will help to provide hollows across production forest landscapes, yet their usefulness to fauna is affected by the reduction in hollows and the quality of foraging habitat in the surrounding forest.

A model of the fate of hollow-bearing trees in production forest in East Gippsland and south-eastern NSW predicts a long-term reduction in densities due to post-harvest mortality of retained trees and an insufficient duration between harvest events for hollow-development (Gibbons 1999). Forests NSW have modelled the fate of hollows using inventory data for mixed age regrowth forest on the north coast of NSW. In currently designated harvest exclusion zones, which constitute about half of the overall area of public commercial forests, the gradual maturation of trees is predicted to result in a threefold increase in the hollow resource in these areas over 200 years (DPI, in  litt.). This model suggests that current hollow densities in harvested areas will decrease by 25% over the next 25 years, then gradually recover to near current levels by 2100, with a further 50% increase by 2200.

20. 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 (Lindenmayer et al. 1991b, Inions  et al. 1989). The frequency, extent and intensity of wildfire are likely to increase in many areas in the future owing to climate change (Hennessy  et al. 2005). 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  Cacatua roseicapilla) 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 (Oldroyd  et al. 1994). 'Competition from feral honey bees  Apis mellifera' is listed as a Key Threatening Process under the Threatened Species Conservation Act.

21. The Loss of Hollow-bearing Trees is eligible to be listed as a key threatening process as, in the opinion of the Scientific Committee:

(a) it adversely affects threatened species, populations or ecological communities, or
(b) could cause species, populations or ecological communities that are not threatened to become threatened.

Professor Lesley Hughes
Chairperson
Scientific Committee

Proposed Gazettal date: 05/10/07
Exhibition period: 05/10/07 - 30/11/07

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