Hollows in the main stem either occur in the trunk or a short section of residual branch that connects to a pipe in the main stem (Figure 4.4A). Hollows of this type accounted for 32%
of all hollows in eucalypts from tall, open forest, in East Gippsland and south-eastern NSW (Gibbons 1999). In six species of eucalypts from open forest, hollows in the main stem represented 21-47% of all hollows (Lindenmayer et al. 2000c). These figures contrast with woodland species in which hollows in the main stem are rare (Ambrose 1982). In a study of River Red Gum, hollows in the main stem accounted for only 9% of all hollows (Newton-John 1992).
Most hollows in eucalypts occur in branches (Figure 4.4B). In four species of eucalypts from tall, open forest in East Gippsland and south-eastern NSW, hollows in living and dead branches of the crown accounted for 65% of all observed hollows (Gibbons 1999). In six eucalypt species from open forest in southern NSW, 49-69% of all hollows occurred in branches (Lindenmayer et al. 2000c). Trees from woodlands contain a larger proportion of hollows in branches than trees from forests. In woodland dominated by River Red Gum, 91 % of all hollows occurred in branches of the crown (Newton-John 1992). Woodland trees typically have a proportionally larger crown area in which to support branch hollows than trees from forests.
The hollows in living and dead branches located in the crown typically form when existing heartwood decay is exposed as branches are shed. The branches of eucalypts are continually shed as the crown expands. There are many theories as to why this is the case — competition between branches for light (Jacobs 1955); geotropism occurring as branches extend and lean downwards (Jacobs 1955); the structural limitations of extending branches (Mackowski 1987); and the need for a tree to maintain a constant proportion of branches in its vascular networks (West et al. 1997). Branches can also be shed by physical damage associated with fires and windstorms. When the end, or leaf-bearing section, of a branch begins to lose vigour (for one or more of the reasons above), the flow of auxin is inhibited and epicormic shoots (sprouting branches appearing after injury) begin to develop along the length of the branch (Jacobs 1955) (Figure 4.5). The length of the branch beyond this point then dies. Hollows
can form either in the dead section of branch beyond the dominant shoot, or in the heart-wood exposed when the dead section of branch breaks, or is shed. Eucalypts continually shed branches in this way and in the majority of cases the wound where the branch has been shed is occluded. However, branches that break and expose heartwood are less likely to be occluded if the heartwood contains an existing column of decay. This is because the sap-wood, which can grow callus tissue around the injury, has no base on which to gain purchase and therefore grows around the hollow rim rather than across it. If the sapwood layer is thin, and the hole large, this reduces the chance that callus tissue will cover a large enough area to occlude the entire lesion. Hollows in branches of the crown therefore will typically occur only in trees with relatively large branches and where some branch-shedding has occurred (i.e. no leaf-bearing units occur on primary branches). This is why branch hollows rarely occur in young trees. Hollows in the main stem form via a similar process to hollows in branches of the crown, except they occur only when primary branches are shed, or snap at the base.
Hollows in branches of the crown and the main stem also can form because decay has been initiated where a branch has broken. In eucalypts from East Gippsland and south-eastern NSW, most of the hollows in the main stem and branches of the crown were associated with a hollow pipe and attendant termite galleries in the main stem However, 29% of trees with hollows did not contain a column of decay in the main stem (measured at the base of the tree), indicating that hollow development can be initiated from damage that occurs in the crown. Hymenomycetous (decay-causing) fungi can gain access to heartwood via partially occluded branches in the crown (Jacobs 1955, Wilkes 1982a, Marks et al. 1986). At certain times of the year this is evident from the development of 'exit structures' (Rayner and Boddy 1988), or fungal fruiting bodies, at these sites.
The diameter of a branch that is shed influences the probability of hollow formation. The capacity of trees to successfully occlude branch stubs decreases with the diameter of branches. In studies of Mountain Ash (.Eucalyptus regnans), branches 5-1 Omm in diameter had about a 65% chance of successful occlusion, whereas branches 20-25mmin diameter had only a 25% chance of successful occlusion (Marks et al. 1986). The occlusion of branches is aided by the formation of a 'brittle zone' at their base (Jacobs 1955). This brittle zone does not extend into the heartwood (Marks etal. 1986). The ratio of heartwood to sap-wood increases with branch diameter (Florence 1996), which may therefore inhibit the occlusion of larger branches. In trees with a small diameter, hollows in the main stem can be relatively common compared with hollows in branches of the crown (Gibbons 1999, Linden-mayer etal. 2000c; but see Mackowski 1987) (Figure 4.6). This is because it is generally only primary branches in small, living trees that are large enough to create a wound of sufficient size to prevent successful occlusion. However, the negative relationship observed between tree diameter and the occurrence of main-stem hollows (Figure 4.6) suggests that either young trees in which main-stem hollows form do not persist; or main-stem hollows that occur in young trees are usually occluded over time.
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