Matthew F. Adkins 107

A burning issue: using fire to accelerate hollow formation in Eucalyptus species

Matthew F. Adkins1,2

1School of Science and Engineering, Centre for Environmental Management, University of Ballarat, Mt Helen, Victoria 3350, Australia 2Email: [email protected]

Revised manuscript received 22 December 2005

Summary for the natural development of hollows. In order to ensure economic viability of the resource, are most The importance of hollows to many species of arboreal mammals commonly felled on a 40–120 y rotation (Gibbons and and birds in Australia has been clearly established, as has the Lindenmayer 1997a), considerably less than the 120–180 y significance of large, old trees in providing hollows. Since required for trees to develop hollows suitable for hollow-using European settlement, considerable areas of eucalypt and fauna, and well short of the 220 y or more needed to develop woodlands have been cleared, resulting in significant loss of large hollows (Gibbons and Lindenmayer 2002). habitat, including hollow-bearing trees. Recruiting and retaining large, old trees is important to the ongoing survival of hollow- One strategy that is used to reduce the loss of hollows in wood dependent fauna. However, in wood production areas maintaining production areas is to exclude from a number of hollow- sufficient numbers of hollows is problematic, since the age at bearing trees, and to identify and conserve future hollow-bearing which trees become economically mature is considerably less trees (Loyn et al. 1980; Loyn 1985; SFNSW 1995; Gibbons and than that at which they become hollow-bearing. Management Lindenmayer 1997a; NRE 1997). In some managed forests, strategies aimed at retaining some hollow-bearing trees can assist, however, very few hollow-bearing trees exist, and trees retained but many forests and woodlands are immature and contain too as future habitat trees are unlikely to develop hollows in the near few of these trees. The ability to accelerate hollow formation future. Reasons include a long history of utilisation, relatively where hollow-bearing trees are lacking is crucial, but cost- recent use of tree retention prescriptions, and the inadequacies effective methods suitable for use on a large scale are seemingly and vagueness of some prescriptions (Gibbons and Lindenmayer few. The use of fire to accelerate hollow formation in Eucalyptus 1997a). The managed box-ironbark forests of central Victoria trees is one option. are an example of a heavily utilised that is lacking in hollows. These forests are predominantly (79%) made up of trees Keywords: fauna; habitats; nests; hollow stem; ; age; <20 cm diameter at breast height over bark (dbhob) of which fire; termites; fungi; Eucalyptus 0.1% are likely to be hollow-bearing (Soderquist 1999). In fact most stems are unlikely to contain habitable hollows for several Introduction decades. In contrast, prior to European settlement these forests were reported to contain mostly large (>60 cm dbhob), evenly The importance of tree hollows in providing vital habitat for spaced trees. This issue is not confined to wood production Australian vertebrate fauna was first recognised over 30 y ago forests; recruitment of hollow-bearing trees in some urban through the work of Cowley (1971), Tyndale-Biscoe and Calaby remnant forests is also unlikely for a considerable time (Harper (1975) and Ambrose (1979). The protection from the weather et al. 2005). In situations such as this, management techniques and predators provided by hollows makes them attractive sites that can accelerate the formation of hollows are vital to sustaining for breeding and roosting (Ambrose 1982). Recent estimates of hollow-dependent species over the next century. hollow usage in Australia indicate that 15% of all terrestrial vertebrates use hollows, including 114 species of birds and 83 The aim of this paper is to review the literature on natural hollow species of mammals (Gibbons and Lindenmayer 2002). However, formation and, based on that knowledge, to discuss the feasibility as a result of European settlement, extensive areas of forest and and practicality of using fire to accelerate hollow formation in woodland habitat have been cleared or modified across Australia. wood-production forests. Consequently, hollow availability is vastly reduced in many regions, which poses serious problems for many fauna species Hollow formation (Saunders et al. 1982). Hollow formation is a slow process reliant on several abiotic Species that exist in forests and woodlands used for wood and biotic events (Fig. 1). It occurs when areas of susceptible production are at particular risk of having inadequate numbers tissue (particularly heartwood) within the tree become accessible of hollows (McIlroy 1978). The lack of hollows in these forests to decay agents such as fungi and termites. Exposure of the is due to the cyclic of trees at intervals that do not allow

Australian 2006 Vol. 69 No. 2 pp. 107–113 108 Accelerating hollow formation in Eucalyptus susceptible tissue to these organisms occurs through the creation Sapwood contains living tissue where active conduction of water of wounds associated with dead branches and roots, branch stubs, takes place. It contains several types of cells including paren- or scars caused by senescence, fire, wind or logging equipment. chyma and sclerenchyma. It is also known as secondary xylem Other forms of physiological weakness besides senescence, e.g. (Raven et al. 1992). Sapwood is able to actively respond to decay competition (Greaves and Florence 1966), can predispose trees organisms and greatly reduce the spread of infection by several to decay. mechanisms, including containment of pathogens through balloon-like outgrowths or tyloses from parenchyma cells, and Termite and fungal decay through production of antibiotics (Wilkes 1985c). However, sapwood tissue that is killed at the time of an injury is extremely Hollows are created via the degradation and excavation of wood susceptible to both fungal decay and termite attack (McCaw 1983; tissue by decay-causing fungi and termites (Saunders et al. 1982). Wilkes 1985a) and is the route by which termites can penetrate Actual excavation of wood tissue by fungi is negligible fungus-affected heartwood (Perry et al. 1985). The susceptibility (Mackowski 1984), but in some instances termite attack occurs of dead sapwood is presumably related to the high levels of stored or is accelerated only where fungal decay of wood tissue has nutrients within the dead cells and the lack of fungitoxic previously occurred (French 1978; Ruyooka and Edwards 1980; extractives (Wilkes 1985a). McCaw (1983) showed that where Perry et al. 1985). Fungi in Australia enter trees through basal damaged, regions of included (grown-over) sapwood were injuries (scars), branch stubs and insect bole wounds (Wilkes particularly susceptible to fungal decay. 1985a). Termite excavation of Eucalyptus species usually begins in the main stem, typically at the base, and works it way up into Heartwood is the non-living, innermost area of a tree and has no the larger branches of the tree (Mackowski 1984), forming a active response to decay organisms (Fig. 2.). Patterns of fungal central pipe through the tree. Some termite species access the decay in several Eucalyptus species show that most attack occurs base through the roots (Greaves et al. 1962b). Hollows become within the heartwood (Wilkes 1985a). Termites show a similar suitable for habitation when branch shed or injuries expose areas pattern of attack (Greaves 1962a; Elliot and Bashford 1984). decayed or excavated by fungi and termites (Mackowski 1984). Heartwood is formed when sapwood, after ageing and slowly losing its conductive ability, eventually dies. At this stage it Fungal decay and termite attack occur in the wood section of contains some passive resistance to decay organisms in the form Eucalyptus trees (Greaves and Florence 1966; Elliot and Bashford of residual chemicals. Several changes occur before sapwood 1984; Mackowski 1984; Wilkes 1985a). The wood section occurs cells die; food reserves are lost and substances, including tannins, on the inside of the vascular cambium (Raven et al. 1992) and is infiltrate the wood (Raven et al. 1992). The infiltration of these generally classified as either sapwood or heartwood (Fig. 2). substances, commonly known as extractives (Hillis 1972), plays an important role in the resistance of heartwood to fungal and termite decay. Extractives are the non-structural constituents of plants of which the most common components are polyphenols Climatic Factors (Hillis 1971). The amount of extractives formed at the heartwood Light boundary is influenced by the amount of carbohydrate available Temperature at the time when sapwood dies (Hillis et al. 1962). Moisture Season

Bark

Sapwood Tree Characteristics Tree species Damage Agents Growth form Branch shedding Tree age Wind Tree size Fire Wood properties Logging operations Vascular cambium Site Characteristics Decay Organisms Latitude Soil type Bacteria Slope Aspect Fungi Exposure Soil nutrients Termites Heartwood

Figure 1. Factors that may influence the hollow formation process Figure 2. Typical cross-section of a tree bole showing the internal (derived with permission from Gibbons and Lindenmayer (1997b) features relevant to hollow formation Chapter 3 — Hollow Formation, Figure 1)

Australian Forestry 2006 Vol. 69 No. 2 pp. 107–113 Matthew F. Adkins 109

The influence of tree age on hollow formation commonly excavated. Greaves (1962a) also showed that the central part of a tree was most likely to be eaten out by termites, Various studies have shown a positive correlation between tree and Wormington and Lamb (1999) showed that piping caused age or size (diameter), the proportion of trees containing hollows, by termites was most common in the centre of cut stumps. and the number of hollows per tree and/or hollow size (Mackowski 1984, 1987; Newton-John 1992; Lindenmayer et Another influence of tree age and size on the hollow formation al. 1993, 2000; Taylor and Haseler 1993; Bennet et al. 1994; process relates to branch composition. When branches die they Williams and Faunt 1997; Ross 1999; Soderquist 1999; become important in hollow formation in three ways: (i) they Wormington and Lamb 1999; Whitford 2002; Whitford and provide entry points for decay agents, i.e. the start of the hollow Williams 2002). Since tree size (diameter) generally increases formation process (Wilkes 1985a), (ii) when eventually shed they with age (Stoneman et al. 1997), large mature trees within potentially expose areas of decay and thus create a hollow suitable woodlands and forests are more highly valued as hollow for fauna (Gibbons et al. 2000a), or (iii) they contain within them resources. a hollow suitable for occupation by fauna. The larger a tree, the greater the number of dead branches, and the greater the range Tree age influences the hollow formation process in several ways. of branch sizes there will be (Adkins et al. 2005). The larger the The physiological condition (Gibbons et al. 2000a), chemical branch, the greater will be its penetration into the main stem and composition (Rudman 1966) and branching characteristics therefore its penetration into inner heartwood (Jacobs 1955). This (Jacobs 1955) associated with older trees are all conducive to will influence the probability of decay agents entering the tree hollow formation; and the older a tree, the greater the probability and their ability to expand into other areas. Another influence of it has been exposed to stochastic events that cause damage, which tree age and size on hollow formation is the persistence of increases the likelihood of hollow formation. branches after they have died. Small dead branches (<2.5 cm) are shed from Eucalyptus stems through the formation of a ‘brittle First let us consider the influence of tree ageing on the zone’ (Jacobs 1955), a mechanism that allows for the quick and physiological condition of a Eucalyptus tree and how it relates efficient ejection of branches. The ‘brittle zone’ forms only in to hollow formation. Both the sapwood and heartwood become sapwood, so the larger the branch, the less likely it is to be shed accessible to decay organisms when conduits are created through cleanly because of an increase in the ratio of heartwood to branch shedding or some form of injury. Dead branches are shed sapwood (Jacobs 1955). Large dead branches will tend to splinter at all stages of a tree’s life, but its ability to successfully shed rather than break cleanly, thereby prolonging persistence (Jacobs branches gradually decreases with age. The diminished vigour 1955). Wilkes (1985a) showed that for four Eucalyptus species, of mature trees reduces their ability to occlude branch stubs and larger branch stubs commonly provided an entry source for decay scars (Jacobs 1955; Wilkes 1982); lower vigour results in greater organisms. Once established within a tree, fungal and termite infection potential (Shigo and Hillis 1973). Trees in poor attack will extend into the branches containing the oldest heart- physiological health are more susceptible to damage and will wood, i.e. the larger branches. Thus the branching characteristics occlude branch stubs and wounds more slowly than healthy trees. of older, larger trees are more conducive to the formation of Gibbons et al. (2000a) showed that ‘for all values of diameter, hollows than young, small trees because of the greater number an unhealthy tree is more likely to contain hollows than a tree of of larger branches and the associated influence of branch size. the same diameter and species, but in better health’.

Another aspect of the ageing process is the decline in the The influence of fire in hollow formation resistance of eucalypt heartwood to fungi and termites. Decay in the heartwood occurs when the abundance of resistance chemicals Since dead branches, scarring and reduced tree vigour facilitate within the tree tissue is sufficiently low to facilitate attack fungal and termite attack, natural or artificial events that enhance (Rudman 1964b; Ruyooka and Edwards 1980; Ruyooka and these conditions could accelerate the process of hollow formation. Griffin 1980). Within the heartwood there is a radial gradient in Perry et al. (1985) and Wilkes (1985a) both found that fire attack susceptibility, with the inner-most heartwood (oldest tissue) damage (fire scarring) and mechanical injury greatly facilitate usually being more susceptible than the outer heartwood (younger fungal and termite degradation of heartwood. Rose (1993) found tissue) (Rudman 1964a; Da Costa and Osborne 1967; Rudman that, along with tree age, the main factor influencing hollow and Gay 1967; Ruyooka and Edwards 1980; Ruyooka and Griffin occurrence in Eucalyptus wandoo and E. salmonophloia was bole 1980; Ruyooka and Groves 1980; Wilkes 1985a,b). This gradient damage, to which fire was a major contributor. Through is correlated with the amount of extractives found within the enlargement of pre-existing hollows and scar creation, fire can heartwood. Greater amounts of extractives are found in outer reduce the time taken for trees to become hollow-bearing by about heartwood than in inner heartwood (Hillis 1971); this is attributed 100 y (Inions et al. 1989). Timber loss caused by degradation of to the progressive breakdown of these inhibitory chemicals over heartwood by fungi and termites can be reduced in wood time (Rudman 1964a, 1965). Thus fungal and termite attack will production forests if the incidence of fire is minimised (Greaves not occur until the heartwood has sufficiently ‘aged’, in part et al. 1965). Putting this finding in an ecological perspective, explaining the tendency for hollows to occur in older trees. All hollow formation can be enhanced through damage caused by of these findings were based on test blocks cut from fallen trees, fire. A fire scar is typically an exposed area of dead cambium but similar patterns of attack occur within live standing trees. and sapwood. However, McCaw (1983) found that termites Elliot and Bashford (1984) showed that damage caused by attacked included fire scars in E. marginata while leaving termites in two Tasmanian eucalypt forests increased directly with adjoining heartwood untouched. The study showed that fire scars diameter; the centre heartwood of the tree was the area most not only provide access to heartwood but, where included, are

Australian Forestry 2006 Vol. 69 No. 2 pp. 107–113 110 Accelerating hollow formation in Eucalyptus also susceptible to termite attack. The thickness of bark that grows resistance. However, the degree to which fire can influence this back over a fire scar is typically less than that of bark on process will be most likely influenced by the intensity and duration undamaged areas (Gill 1974). Thus occluded fire scars are more of the fire event. Several authors have suggested that fire could susceptible to damage from subsequent fires, potentially be used to alleviate the loss of hollow-bearing trees (Rose 1993; increasing the area of scarring (Gill 1974). Williams and Faunt 1997; Whitford 2002).

Basal fire scars have been correlated with the occurrence of - tops in giant sequoia trees of North America (Rundel 1962). It is Other methods for accelerating hollow formation therefore feasible that Eucalyptus trees suffering from severe The emphasis of this review is the acceleration of hollow damage to the main stem after fire may respond in a similar way, formation in live, standing trees. Although death can produce experiencing increased physiological stress and providing greater more hollows in younger, smaller diameter stems than occur in numbers of dead branches, both of which influence hollow live trees of comparable size (Gibbons 1999; Ross 1999) and formation. Many Eucalyptus species respond to fire by hollows within them are preferred by some hollow-using species developing epicormic shoots (Jacobs 1955). Infection by decay (Adam-Gates 1996; Rowston 1998), their reduced standing life organisms can occur when epicormic branches are shed from the (Bull and Partridge 1986; Lindenmayer et al. 1997) has led main stem and the knob from which they grew is occluded (Jacobs Gibbons and Lindenmayer (2002) to conclude that ‘dead trees 1955). Rundel (1962) showed that the functional loss a tree suffers should not be viewed as a replacement to live standing trees for from major damage to the sapwood manifests as branch death providing hollows’. within the crown. A similar effect could occur within the root zone of a tree, resulting in dead roots which may facilitate the While several techniques for accelerating hollow formation have entrance of termites into a tree. been assessed in North American forests, there is a dearth of such studies in Australia. Techniques studied in North America Depending on the temperature and duration of exposure, fire may include the inoculation of routed live trees (i.e. making a cavity lead to the breakdown of anti-fungal and anti-termitic properties by drilling of the wood tissue) to accelerate hollow formation within the heartwood. Ruyooka (1978) showed that the resistance (Carey and Sanderson 1981). The method though was generally of E. acmenioides, E. camaldulensis, E. delegatensis and E. regnans unsuccessful and led the author to conclude that it may be more heartwood to attack by termite species, Nasutitermes exitiosus and appropriate to construct cavities that are suitable for immediate Coptotermes lacteus, was reduced if exposed to temperatures use (Carey and Sanderson 1981). Several different pieces of above 100°C. Hillis (1975) found that the polyphenolic equipment could be used to create hollows in such a manner, extractives within heartwood are broken down in high- including chainsaws and various attachments. As unhealthy trees temperature drying of wood samples. Interestingly, McCaw’s are more likely to have hollows (Gibbons et al. 2000a), techniques (1983) study of wood defects and fire scars in E. marginata that damage trees, such as sub-lethal poisoning or girdling, may showed a correlation between fire scar size and the occurrence be beneficial in accelerating hollow formation (Gibbons and of decay within the heartwood region. In most instances, stem Lindenmayer 2002). Research into the effectiveness of these 2 with wounds <1100 cm in area showed no alteration to the techniques to provide hollows suitable for fauna is required. heartwood, yet all wounds >1100 cm2 did. Greater levels of decay in large scars may be simply due to the greater surface area Strategies that can be applied at a landscape level may be more available for infection, or to either higher temperature and/or economically appropriate (Gibbons 1999). For this reason the prolonged exposure, causing the breakdown of decay-resistant focus of this review is on fire, as it is most likely to be cost chemicals within the heartwood. effective on a large scale and also because fuel-reduction burns are common in many forests and woodlands. As such, there is Increased heartwood susceptibility through exposure to heat may potential for integration of the two strategies. also occur through the net movement of resistant chemicals via osmosis from the inner region of the tree towards the outer region. Vines (1968) demonstrated that a vertical movement of fluid in Using fire to damage young trees — trees acted as a cooling mechanism when areas of a tree were is there any point? exposed to heat. Where the entire of a tree was exposed to Damaging young Eucalyptus trees, particularly with fire, may heat, it was postulated that while vertical movement of fluid would be useful in providing a catalyst for hollow formation. While not provide effective cooling, horizontal flow may occur (Vines damage may allow termite and fungal activity within a tree earlier 1968). Horizontal flow of fluid could result in a net movement than would naturally occur, a smaller proportion of the heartwood of water and water-soluble chemicals including deterrent may be susceptible to attack by organisms in young trees than in extractives. older, larger trees, since the breakdown of resistance chemicals In summary, various studies have shown that fire can be a major appears to be a function of time (Rudman 1965, 1966). Addition- influence in the hollow formation process by: (i) creating sites, ally, while heat has been shown to degrade termite/fungal-resistant e.g. scars or dead branches, that allow for the entrance of decay- heartwood chemicals (Ruyooka and Groves 1980), it is unclear causing organisms, and/or (ii) increasing the area of tissue whether sufficiently high temperatures can be achieved in a live susceptible to termite or fungal attack, by lowering the resistance standing tree during a fire. As included fire-killed sapwood is of heartwood or creating areas of included dead sapwood. prone to attack by termites, there is potential for an internal hollow Increasing the area of susceptible tissue would be extremely pipe extending from the base to the crown to develop where advantageous in young forest where the susceptibility of trees is extensive scarring has occurred. However, significant mortality relatively low because of the factors associated with tree of retained habitat trees following high-intensity burning has been

Australian Forestry 2006 Vol. 69 No. 2 pp. 107–113 Matthew F. Adkins 111 recorded (Gibbons et al. 2000b). Thus, while fire may assist in There may be opportunities to integrate hollow management with creating large areas of sapwood vulnerable to termite attack within fire management through fuel reduction burns (Gibbons and a tree, it may also — through premature death — reduce its Lindenmayer 2002). However, to promote hollow formation, a lifespan as a habitat tree. The structural integrity of a tree may fire will, in most cases, need to be of high intensity, but burning also be compromised, causing it to collapse, if an area close to an area of forest for hollow recruitment by creating and its periphery becomes hollow. ‘managing’ a high-intensity fire may not be feasible. The risk of losing control of a high-intensity fire and the harm it could cause Although fire may be useful in initiating hollow formation in to faunal species, including those hollowing-using species younger trees, the size of resultant hollows is still dependent on targeted for conservation, outweigh any benefits. Therefore novel initial branch size. Gibbons (1999) found that ‘the majority of approaches will be required: manual construction of fuel piles hollows (95%) formed within broken or shed branches, and that around selected trees, though relatively labour intensive, may the size of a hollow was a reflection of the size of the previously produce adequate scarring during a fuel reduction burn to shed branch’. As branch size increases with tree diameter, larger- accelerate hollow formation and minimise any damage to trees diameter trees will contain larger hollow entrances than smaller retained for economic purposes and harm to fauna. Solutions at trees (Gibbons et al. 2000a). Other studies have also documented a site or landscape level could include deliberately allowing fuel positive correlations between the size of hollow entrances and to build up within small areas of forest, while maintaining lower tree diameter (Bennet et al. 1994; Soderquist 1999; Wormington fuel loads in surrounding areas (containment patches). This would and Lamb 1999; Gibbons et al. 2000a; Wormington et al. 2003). create patches of forest that burn intensely during a controlled Although internal hollowing within a tree might be achieved burn which is manageable when it reaches containment patches. through fire damage, the size of the resultant hollow appears Using fire of the highest manageable intensity will be of greatest dependent on the size of the branches. Adkins et al. (2005) assistance. showed that because box-ironbark forest trees in north-eastern Victoria < 60 cm dbhob were unlikely to contain large or very large dead branches, recruitment of large and very large hollows Conclusion may be negligible in such forests if trees are predominantly <60 cm The need to accelerate hollow formation in some forest types, dbhob. For this reason Gibbons et al. (2000a) recommended that especially those that have a long history of utilisation, is urgent. attempts to accelerate hollow formation should occur only in trees Fire is a potentially useful tool to accelerate hollow formation with relatively large diameter. by acting as an agent for predisposing damage and as a primary If fire can accelerate the occurrence of internal hollowing within excavation mechanism in Eucalyptus species. However, methods trees, the restrictive influence branch size has on the size of entrances suitable for applying these approaches are still in their infancy. to hollows in young, small trees could be overcome by drilling into Burning small trees may help create hollows in them that might their trunk and artificially creating entrances of appropriate size. otherwise take considerably longer to form. However, even if However, as most naturally-occurring hollows are situated in the the process starts earlier, the internal and external size of the crown of trees, some fauna may not use these hollows, depending hollows is likely to be influenced more by tree size than by the on individual habitat requirements. Many species that use hollows effect of fire. Therefore fire may be useful in accelerating the do so at considerable height within the tree. For example, the powerful process of hollow formation, but only in forests and woodlands owl prefers nests sites at least 9 m above the ground (Debus and that contain trees large enough to provide suitable openings and Chafer 1994). Similarly, Menkhorst (1984) found that most arboreal internal cavities for most hollow-dependent fauna. The vertebrates in the Gippsland forest prefer high nest boxes as a recruitment and retention of large trees will continue to be the mechanism of predator avoidance. Other species have varying hollow most vital management option for providing habitat for hollow- requirements (Oldroyd et al. 1994; Garnett 1999).Therefore, where dependent fauna. trees contain an internal pipe and drilling is considered an option, Two hundred years of European settlement has severely reduced holes should be drilled in a manner that best mimics naturally-formed the availability of hollow habitat, a trend which continues today hollows. This may involve drilling at considerable heights along the in many of the Eucalyptus wood-production forests and bole and in the crown. This approach would be quite time consuming, woodlands of Australia. Without proper management and the and is yet to be tested. Re-burning trees after a period of time may ability to accelerate hollow formation, the future survival of many help to create hollow openings to internal piping, but achieving a hollow-dependent fauna is threatened. The potential of fire to balance between creating a scar large enough to provide an opening, accelerate hollow formation has long been acknowledged but and minimising damage to the tree, is problematic. Repeated burning not assessed. It is time to determine whether fire can be used as to create hollow openings may reduce the overall standing life of a management tool to assist in promoting hollow formation. a tree.

It is important to consider that the use of fire to predispose trees Acknowledgements to hollow formation in production forests will potentially have negative effects on economic values. It is normal practice to I would like to thank my supervisors Martin Westbrooke and minimise intense fires in wood-production forests to reduce Singarayer Florentine (University of Ballarat) for their damage to the heartwood of trees by fungi and termites (Greaves encouragement to produce this review and to Peter Spooner et al. 1965). There is therefore a conflict between using fire to (Charles Sturt University) and Janet Leversha (Centre for promote hollow formation for fauna conservation and minimising Environmental Management) for comments and editing. I am also fire to promote timber values. grateful to the referees for their comments and suggestions.

Australian Forestry 2006 Vol. 69 No. 2 pp. 107–113 112 Accelerating hollow formation in Eucalyptus

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