Artificial formation of tree cavities – part 2
A manual of techniques to create simulated natural cavities in Slender Cypress Pine (Callitris gracilis murrayensis): for use by Major Mitchell’s Cockatoo (Lophochroa leadbeateri leadbeateri).
Cavity creation techniques
As the decision matrix illustrates there are several techniques available depending upon the size of the tree and the state of any cavities in the tree. Each of the four techniques for creating a cavity ready to use by MMC are described in full detail with images to illustrate each of the key steps in sequence.
Excavation involves the creation of an entirely new cavity. This technique is generally only suited to dead wood as it involves removal of a face plate which would cause extensive damage to sapwood if undertaken on a living tree. Also in some dead trees it may be necessary to remove some of the canopy to reduce wind drag in order to increase the longevity of the remaining trunk and cavity (Figure 9). In some cases a natural scar may already exist following the loss of a branch exposing the heartwood. The resulting scar may be used as an entry point to excavate a new cavity.
The first step is to use a chainsaw to cut out a face plate. This is best done by first cutting two cross cuts ~70 cm apart and not more than ¼ the diameter of the tree at the height of the cuts. Then make two longitudinal cuts with the chainsaw bar held at a 900 angle for each on either side of the face plate starting one at a time from just below one end of the top cut and continuing down to the lower cut. Repeat this on the other side. The angling and depth of the longitudinal cuts should allow the cuts to meet in the middle of the trunk all the way along the cuts (Figure 10). The face plate should naturally fall loose once the final cut is complete. The face plate should be set aside for further work later.
Once the face plate is removed, use a chainsaw to make a series of long, deep vertical, parallel cuts into the exposed heartwood (Figure 11). The cavity walls being created should be ≥10 cm thick. Furthermore, check the required blade depth to ensure that no holes are made through to the other side of the tree trunk. This is critical as large parrots, such as Galahs, will chew-out and enlarge even the smallest of holes until they have created a further entrance-sized opening (Hurley, 2009).
Following the chainsaw cuts the resulting slabs can either be prised-out with a pinch bar or horizontal cross cuts can be made to further speed up the removal of the bulk of the trunk’s heartwood (Figure 12). Then use the Arbortech© to clean out the trunk cavity and form contours to the desired dimensions. It is important to leave some rough surfaces or small grooves in the walls of the cavity to provide climbing holds for birds to easily enter and exit the cavity.
If the height of the hole left by the face plate does not match the desired cavity depth it is relatively straightforward to use the Arbortech© to deepen the cavity in the tree trunk. The measurements provided for this dimension should be regarded as a minimum as cavities up to 1.8m deep have been used successfully by MMC at Pine Plains (Hurley, 2006a).
Figure 3: Decision matrix for the selection of the appropriate technique in simulated cavity formation in Slender Callitris Pine (Callitris gracilis murrayensis). Diamonds are decision points and light teal boxes are actions. The dark decision strips represent recording and monitoring activities
It is critical that the proposed nest chamber floor is relatively flat with minimum diameters of 18cm x 19cm. In addition to cavity depth, this is the single most important aspect of the SNC construction (Korpimäki & Higgins, 1985).
Smaller nest chambers will either deter MMC from nesting in them or limit the clutch size laid. A nest chamber with a larger diameter may be used by MMC and may be excavated if the trunk diameter is of sufficient size to accommodate it.
Now, the inside face of the face plate must be carved-out so it has internal concave contours to match the curvature of the cavity being carved from the inside of the tree trunk. The cavity entrance can also be carved from the face plate (Figure 13-14). These carving tasks are best done with an Arbortech© blade attached to an angle grinder.
Refitting the face plate is a critical stage for long-term viability of SNCs. Gaps and/or air flow between the remaining trunk cavity walls and the face plate must be eliminated. The kerf width from the chainsaw creates a 1cm gap between the face plate and cavity walls and requires in-filling. Use plywood strips cut to suit or Callitris lengths custom made to fit each joining surface of the face plate.
Regardless of which timber is used, these infill or caulking strips should be glued and tacked to the joint edges of the trunk and then trimmed with the Arbortech© so that they sit flush with both the external and internal contours of the trunk and cavity. The contouring of theses strips will help to camouflage them from the prying beaks of Galahs and other cockatoos. A standard builder’s glue can be used to bulk up any minor cracks between the jointed surfaces. Tech screws may also be used to strengthen the join between face plate and tree.
Remember to use a standard angle grinder blade to grind off or disfigure the tech screw heads, then paint them matt black and grey. This last action is recommended so as to not to attract undue attention from people or predators. The external surfaces of the caulking strips must also be painted a mottled matt dark-grey and black to protect from the weather. Alternatively thick strips of bark may be attached using silicone glue to cover the caulking strips. Once the face plate has been reattached, fine Callitris wood chips must be placed in the base of the new cavity. One or two handfuls is enough (Figure 15). Following this the face plate should be camouflaged with bark from the same tree the work is being done. Entrances should look as natural as possible Initially allow a half to one full day (4-8 hours) for the excavation of new cavities. With experience a team of two chainsaw operators will be able to an average excavate two cavities per day.
Cavity formation is normally associated with older mature or senescent trees (Gibbons & Lindenmayer, 2002). It is estimated that Slender Cypress Pine do not reach a sufficient size to support a cavity suitable for MMC (i.e. 50cm DBH) until trees are >80 years of age (Gibson et al. , 2008). A variety of methods involving the exposure of heartwood and manipulation of tree health have been proposed to accelerate cavity development (Gibbons & Lindenmayer, 2002).
The exposure of heartwood provides an entrance for fungi, bacteria and saprophytic insects which cause decay and, in conjunction with the trees adaptive growth response, leads to cavity formation (Mattheck & Breloer, 1997; and Lonsdale, 1999; and Taylor et al., 2002). The rate at which wounding techniques are able to accelerate the processes of natural cavity formation is on a temporal scale unsuited to the rapid provision of cavities for threatened species recovery (i.e. formation will take place over decades). Cavity augmentation involves accelerating the formation of a cavity or improving the function of an existing cavity. Accelerating cavity formation for this purpose is based on many of the techniques required to excavate a cavity in a tree without an existing cavity.
In addition to accelerating cavity formation, cavity augmentation also involves making improvements to an existing cavity. This may be as simple as enlarging the entrance to an existing cavity or may involve more extensive works to enlarge the internal dimensions of a small cavity. Furthermore, augmentation often is in the form of inserting carved timber sections to deflect rainwater from flowing into a cavity or repairing cracks in cavity walls.
The primary augmentation process involves mechanically removing the decay and heartwood associated with wounds caused by natural processes. A pre-existing scar, revealing advanced rotting of the heartwood is slightly enlarged to allow use of Arbortech blades to excavate the rotten timber (Figure 18A and B). Once the rot has been removed to sufficient dimensions for an SNC a face plate is carved from a single piece of timber and glued and screwed in place (Figure 18C).
Augmentation can also include modifications to an existing cavity to improve cavity function. These may be in the form of simply enlarging the entrance or other features of a cavity to suitable dimensions (Figure 19). In some cases augmentation may involve strategically placing a plug of timber to block water from flooding into a cavity(Figure 20 and Figure 21).
Ironbark Environmental Arboriculture Pty Ltd is an interdisciplinary company with expertise in arboriculture, ecology and wildlife conservation. In 2014, in partnership with DELWP, they pioneered the use of chainsaw-carved hollows for threatened species conservation with their work on the Major Mitchell’s Cockatoo. Since this time they have completed a range of targeted hollow creation projects in urban and forest environments, including Musk Lorikeets in the City of Melbourne and Brush-tailed Phascogales in Box-Ironbark forests of Central Victoria.
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Hurley, V.G. & Harris, G.J., (2015) A manual of techniques to create simulated natural cavities in Slender Cypress Pine (Callitris gracilis murrayensis) for use by Major Mitchell’s Cockatoo (Lophochroa leadbeateri leadbeateri). A report to the Department of Environment, Land Water and Planning, Melbourne.