Tree hollows in Tasmania A guide - CRC for Forestry

ContentsWhat is a hollow and why is it important?....................................... 4Hollow related legislation..................................................................... 5How do hollows form?......................................................................... 6What makes a good hollow? ............................................................... 7Hollow depth......................................................................................................7Entrance size.......................................................................................................7Hollow location..................................................................................................8Hollow shape......................................................................................................8Identifying hollows ...............................................................................10False hollows....................................................................................................10Poor visibility....................................................................................................10Evidence of previous use............................................................................... 11Searching for hollows..........................................................................12Selecting habitat trees.........................................................................13Tree diameter ..................................................................................................13Dead wood.......................................................................................................14Tree form..........................................................................................................15Fire damage......................................................................................................16Selecting habitat trees – summary.............................................................. 17Habitat trees..........................................................................................18Recruitment trees................................................................................21How many hollows are enough?.......................................................22Nest-boxes – a practical solution?...................................................23Practical ideas for land managers......................................................24Retention..........................................................................................................24Protection.........................................................................................................25Recruitment .....................................................................................................25Additional reading................................................................................26Appendix 1: Tasmanian hollow-using species .................................27Appendix 2: Flow diagram for identifying habitat trees...............28

What is a hollow and why is it important?4Tree hollows are holesor cavities in trees thatprovide important shelterand breeding sites for manyanimals. Hollows used byanimals in Tasmania rangefrom less than 2 cm to morethan 30 cm in entrancediameter. There are eightbat species, five arborealmarsupials (possums), about29 bird species (see Appendix1) and an unknown numberof invertebrates that usehollows to varying degrees.This includes several speciesthat are listed as threatened.Seven of these hollow-usingspecies are introduced,including serious pests suchas the common starling. Itis important to considerthese introduced specieswhen thinking about hollowdemand because theycompete with our nativehollow-users.The aim of this guide is tohelp people interested intree hollows to identify thetrees that are most likely tobe used by hollow-dependentspecies (see Appendix 1).

Hollow related legislationThe 1997 Regional Forest Agreement identified hollow-dependent faunaas priority species. Threatened hollow-using species need to beconserved under the Threatened Species Protection Act 1995.Production forest areas are subject to the Forest Practices Act 1985. Theassociated Forest Practices Code prescribes the manner in which forestpractices are to be conducted so as to provide reasonable protectionto the environment. As part of this, tree hollow management mustbe considered during planning for and implementation of all forestoperations.A mosaic of modified and retained areas where careful management of hollows is required.5

How do hollows form?In Australia, some species of animals (e.g.cockatoos) can help enlarge hollows, but nospecies build hollows from scratch. Instead,hollows are produced by environmentalprocesses that take lots of time.The onset of hollow-formation is dependenton damage to the tree, from fire, fromanimals (vertebrates or invertebrates), orfrom branches dropping. Young and healthytrees can quickly heal after damage. As treesage they become slower at healing, allowinghollow-formation to progress. Termitesand beetles can bore into the wood andhollow out extensive areas. Wounds provideentry-points for fungal spores that can leadto active decay, that eventually rot out thetree. Fire can help enlarge hollows createdby other processes (although fire can alsodestroy hollows). Trees less than 100 yearsold are unlikely to contain hollows. Treeswith hollows suitable for use by animals aregenerally more than 150 years old.6Key factors for hollow formation are:• physical damage• fire• fungi• invertebrates (e.g. termites and beetles)• time.A hollow partially formed byfire (top);fruiting fungal bodies near abasal hollow (bottom).

What makes a good hollow?(continued)HOLLOW LOCATIONHollows can be found anywhere from thebase of the tree to the small branchesnear the tree top. Branches need to belarge enough to contain a hollow bigenough to house an animal. While someanimals prefer a certain height above theground, hollows at any height may be used.HOLLOW SHAPEA wide variety of hollow shapes can potentially be used by an animal, ifthe hollow is of the right size and in the right place. Hollow shapes thatcan be used include, but are not restricted to:• large jagged openings in the dead tops of trees• small round hollows in the trunk• the broken off end of branches• big jagged tears• small cracks.8From the top of the page going clockwise: circular hollows in the trunk; hollow in a branchend; crack hollow in the trunk; hollow where a branch has torn off.

A number ofhollows known tohave been usedby fauna, showingthe range of hollowtypes and shapesthat may be used,including a slenderbranch hollow witha tree martin flyingout (top right).9

FALSE HOLLOWSIdentifying hollowsWhat can, at first, appear to be a hollow may not actually be one.Common mistakes are thinking that small stains, burns, solid branchends or shallow cavities are actually hollows.10POOR VISIBILITYIt is obviously easier to detect hollows in dry open forest than densewet forest, but hollows will be missed in all forest types. More than halfof the hollows occurring in an area may be missed during ground-basedsurveys. They may be hidden by the surrounding vegetation, by otherparts of the same tree or they may be facing upwards and so cannot beseen from the ground. The more time spent looking, the more hollowsare likely to be found.Clockwise from top left:what appears to be ahollow is actually a shallowfire scar; a spout thatis shallow and does notactually contain a hollow;a burnt branch stub thatin poor lighting looks like ahollow; a striated pardalotenest where the hollow is notvisible because it is facingupwards; the entranceto a green rosella hollowpartially obscured by foliage.

EVIDENCE OF PREVIOUS USEOne of the best ways of telling if a hollow is a good one is if it haspreviously been or is currently being used by animals. Evidence ofcurrent or previous use includes, but is not limited to:• rubbing or chewing marks around the hollow entrance• splash marks or droppings at or near the hollow entrance• extensive scratch marks leading up to the hollow• visible nesting material inside the hollow• an animal observed inside or entering the hollow!From left toright: rubbingmarks aroundthe entrance toa hollow; a nestvisible inside ahollow; splashmarks around ahollow entrance.11

Searching for hollowsHollow surveys are generally conducted near the base of the tree usingbinoculars. When doing hollow surveys it is a good idea to:• use a good pair of binoculars• do searches in good weather• move around to find the best vantage point and try to look at thetree from all angles.Hollow surveys can work well for some trees and in some forest types,but not all hollows will be found. This means that it can be just as,and in some areas more important, to assess tree attributes as well assearching for hollows.12A forest researcher usesbinoculars to search forhollows (right) and poorlight conditions make hollowspotting difficult (above).

Selecting habitat treesA habitat tree is a tree that contains features such as hollows that arelikely to be used by fauna. Observing hollows is obviously the best wayof selecting a habitat tree and of assessing hollow availability. The morehollows you can see in a tree, the more likely the tree is to contain a hollowsuitable for use by fauna.TREE DIAMETERA large diameter tree (above) and a medium diametertree (left). While either may contain a hollow, thelarger diameter tree is much more likely to do so.Given that hollows take so long to form, hollow-bearing trees aretypically very old (>150 years and often much more for trees withlarge hollows). Within a particular area or forest type, the larger a treeis around the base, the older it is likely to be and the more likely it isthat the tree will have a hollow.13

Selecting habitat treesDEAD WOODDead wood in thecanopy of a tree generallymeans that the tree hasexperienced some stress.Stressed trees do not healas readily as healthy treesand therefore hollowformation is more likely.A large, healthy tree with fewdead branches (right) and alarge tree with lots of large deadbranches which is, therefore, morelikely to contain hollows (below).Dead branches can also bedropped by the tree and ahollow may form where thebranch broke off. This meansthat the more dead branchesand the bigger the deadbranches you see in the canopyof a tree, the more likely thetree is to have a hollow.14Opposite page from top left to bottomright: trees with increasing levels ofsenescence. The crown starts out small,then grows, thins, the number and sizeof dead branches increases, the tree dies,loses its limbs and will eventually fall over.

TREE FORMAs trees age, their growth and senescence typically follows a sequencethat affects their form and potential for developing hollows. Trees withvery healthy crowns are less likely to have hollows than trees furtheralong this sequence. Dead trees may have a lot of hollows, but thesecan be lost as branches are dropped. While trees of any form maycontain hollows, the best shape for a habitat tree is one which is a livetree with dead wood and a crown of reasonable size.1 2345 678915

Selecting habitat trees (continued)FIRE DAMAGEWhile fire can destroy hollowbearingtrees and the hollowsthemselves, it can also helpform hollows. Trees that havebeen damaged by fire aremore likely to have hollows.While burnt bark indicates that a firehas passed through, it may have been a‘cool’ fire that didn’t actually damagethe tree. Fire scars, which often arefound at the base of the tree, are abetter sign that the fire may havehelped form hollows.16A hollow partially formed by fire (top); a heavily and recently burnt forest that may havehollow-bearing trees in the future (right); burnt bark on a tree indicating a fire has gonethrough but there is little evidence of fire scarring on these trees (left).

Selecting habitat trees– summaryWhile this guide outlines a number of features that should be lookedfor when selecting habitat trees, not all features will be found in an areaor on the same tree.What is required, then, is some common sense and good judgement.Look at the trees available and see which ones best fit the descriptionor have as many of the key attributes as possible.Key tree attributes indicating hollow occurrence are:• visible hollows (the more the better)• large diameter• old age• dead wood in the tree (e.g. dead branches)• fire scarring.Looking for habitat trees.17

Habitat treesOptimal habitat trees either have at least one large or several smallhollows visible, or they have several other features suggesting theyoffer prime habitat.Visible hollows, large diameter,dead wood, large canopyLarge diameter, dead wood, firescarring18Visible hollows, large diameter,dead woodVisible hollows, large diameter,dead wood

In areas where no trees have visible hollows, habitat trees should beselected from the trees available to contain as many of the featuresindicating hollow presence as possible.Very large tree, but little deadwood and no visible hollowsAdvanced tree form with deadwood, although small crownSome dead branches but theseare small, the canopy is small andthere are no visible hollowsTree shape is senescent but treecrown is very healthy and thereare no visible hollows19

Habitat trees (continued)In some areas there may be little difference between trees in their size,form or senescence. However, keeping the key attributes of habitattrees in mind can still help identify the best trees to retain, even inthese areas.Large diameter but no dead wood,no visible hollowsLarge tree but little dead wood,small canopy and no visible hollows20The largest tree with the mostbranching crown in the area,although has no dead woodFire damage and some deadwood but no visible hollows andthe tree is very small in diameter

Recruitment treesBecause tree hollows take such a long time to form, it is important tothink about how hollow availability will change in our landscape overtime. Recruitment trees are trees which may not provide a hollow now,but which will probably do so in the future if retained.Good recruitment habitat trees are those from the age cohortyounger than the hollow-bearing trees, but that are of reasonablesize and have the capacity to survive, grow and develop hollows overtime. Recruitment trees may be damaged, branchy or open-growntrees. Severely damaged or suppressed young trees are unlikely togrow further or respond to release and so are less important forconsideration as recruitment trees.Left: no hollows, medium diameter, no dead wood, large canopy.Right: habitat tree on left, recruitment tree on right. Recruitment tree is the next largest treeafter the habitat tree. No hollows, medium diameter, little dead wood, large canopy.21

How many hollows are enough?The question ‘how many hollows are enough to maintain populationsof native hollow-using species?’ is not easily answered. The numberrequired is likely to vary between areas and over time. Hollowusinganimals vary in how dependent they are on hollows, how manyhollows they will use in a year and the spacing of the trees they use.Hollow-using species live at varying densities in different forest types,and in different areas of Tasmania. Hollow demand in an area can varyfrom year to year as some species move according to food availablity.Available information currently suggests that more hollow-using speciesare found in the drier forests in eastern Tasmania than the wetterforests in the west.The number of hollows needed in an area depends on:• species diversity in the area• species abundance in the area• number of hollows used by an individual of each species• competition for hollows, e.g. with introduced species• availability of other resources (e.g. food).22Scattered hollow-bearing trees retained in a highly modified area.

Nest-boxes – a practical solution?Nest-boxes (artificial hollows) can be used to help speciesconservation efforts in some situations. However, they are not areplacement for naturally occurring hollows.While nest-boxes can be successfully used by some species, and evenhelp in some species recovery programmes, we do not understandenough about the hollow requirements of many species to ensure theirbreeding success in nest-boxes. We know animals can be very selectivein their choice of hollows. This means that choice of nest-box entrancesize, the aspect and height at which the nest-box should be placed allrequire careful consideration.The diverse requirements of our hollow-using fauna means nestboxesare generally not a practical conservation strategy. The costsinvolved in making and hanging nest-boxes, monitoring, maintaining andreplacing them for even a single species, combined with our limitedunderstanding of hollow requirements, mean that nest-boxes should beseen as a last resort for species conservation.Additional problemswith nest-boxes arethat they can potentiallyincrease populationsof introduced and pestspecies, there is a risk ofdisease to the speciesusing them and nestboxesdo not providethe other benefits thatnatural hollows provide(e.g. habitat for fungi andinvertebrates).Pygmy possum in nest box,photo Matt Webb.23

Practical ideas for land managersThe three main principles for goodhollow management in any location are:• retention• protection• recruitment.Right: firewood being obtained froma young tree rather than froman older tree containing hollows.RETENTION24Mature tree retained within a youngplantation.Older trees should be kept in an areawhenever possible.Forestry operations:Careful consideration should be takenwhen selecting trees to retain in aharvesting operation to ensure thatthe best quality trees available (interms of providing hollows) are beingkept.Other forest and land users:Instead of targeting older hollowbearingtrees for firewood collection,target younger trees (alwaysensuring enough recruitment treesare retained). Think twice beforeremoving paddock-trees.

PROTECTIONHollow-bearing trees can be structurally less-sound than trees withouthollows, making them more prone to windthrow. The chances ofwindthrow can be reduced by retaining other trees around the hollowbearingtrees.Forestry operations:Retain hollow-bearing trees in the middle of clumps or strips wheneverpossible rather than as isolated trees.Other forest and land users:Retain other trees, or plant additional trees and shrubs around hollowbearingtrees. Protect trees from livestock (e.g. by fencing).Right: stock damage preventingregeneration.RECRUITMENTBecause hollows takeso long to form, it isimportant to ensurethat new trees areestablished to providefuture hollows.Forestry operations:Retain younger recruitment trees as well as older hollow-bearing trees.This can be achieved by retaining trees in clumps and strips.Other forest and land users:In addition to retaining and protecting younger trees, planting trees orfencing around established trees can encourage regeneration.25

Additional readingCawthen, L 2007, Den use by the common brushtail possum in logged and unlogged dryforest in SE Tasmania, Honours thesis, University of Tasmania, Hobart.Commonwealth of Australia and State of Tasmania 1996, Tasmanian-Commonwealth RegionalForest Agreement. Tasmanian Government, Hobart.Duhig, N, Munks, S, Wapstra, M and Taylor, R 2000, Mortality rates of retained habitat trees instate forest coupes: a long-term monitoring project – initial report, Forestry Tasmania andthe Forest Practices Board, Hobart.Forest Practices Board 2000, Forest Practices Code, Forest Practices Board, Hobart, Tasmania.Gibbons, P and Lindenmayer, D 2002, Tree Hollows and Wildlife Conservation in Australia,CSIRO Publishing, Collingwood, Vic, Australia.Haseler, M and Taylor, R 1993, ‘Use of tree hollows by birds in sclerophyll forest in northeasternTasmania’, Tasforests 5: 51–56.Koch, AJ 2007, The abundance and type of hollows in Eucalyptus obliqua forest and the use ofthese hollows by fauna, PhD thesis, University of Tasmania, Hobart.Koch, AJ, Munks, SA and Whoeler, E 2008, ‘The hollow-using vertebrate fauna of Tasmania:distribution, hollow requirements and conservation status’, Australian Journal of Zoology56: 323–349.Koch, AJ, Munks, SA and Driscoll, D 2008, ‘The use of hollow-bearing trees by vertebratefauna in wet and dry Eucalyptus obliqua forest, Tasmania’, Wildlife Research 35: 727–746.Koch, AJ, Munks, SA, Driscoll, D and Kirkpatrick, JB 2008, ‘Does hollow occurrence vary withforest type? A case study in wet and dry Eucalyptus obliqua forest’, Forest Ecology andManagement 255: 3938–3951.Koch AJ 2008, ‘Errors associated with two methods of assessing tree hollow occurrence andabundance in Eucalyptus obliqua forest, Tasmania’, Forest Ecology and Management 255:674–685.Munks, SA, Wapstra, M, Corkrey, R, Otley, H, Miller, G and Walker, B 2007, ‘The occurrence ofpotential tree hollows in the dry eucalypt forests of south-eastern Tasmania, Australia’,Australian Zoologist 34: 22–36.Taylor, RJ and Haseler, M 1993, ‘Occurrence of potential nest trees and their use by birds insclerophyll forest in north-east Tasmania’, Australian Forestry 56: 165–171.Taylor, RJ and Savva, NM 1988, ‘Use of roost sites by four species of bats in State forest insouth-eastern Tasmania’, Australian Wildlife Research 15: 637–645.Wapstra, M and Taylor, R 1998, ‘Use of retained trees for nesting by birds in logged eucalyptforest in north-eastern Tasmania’, Australian Forestry 61: 48–52.26

Appendix 1: Tasmanian hollow-using speciesSpecies Status HollowsizeBIRDSMusk lorikeetEndemicsubspeciesSmallRainbow lorikeet Exotic SmallSwift parrot Native SmallOrange-bellied parrot Native SmallBlue-winged parrot Native SmallTree martin Native SmallWelcome swallow Native SmallAustralian owlet-nightjarEndemicsubspeciesSmallDusky robin Endemic SmallFlame robin Native SmallForty-spotted pardalote Endemic SmallStriated pardalote Native SmallDusky woodswallow Native SmallCommon starling Exotic SmallSouthern boobook owlEndemicsubspeciesMediumPacific black duck Native MediumAustralian shelduck Native MediumChestnut teal Native MediumAustralian wood duck Native MediumGrey teal Native MediumGalah Exotic MediumLong-billed corella Exotic MediumLittle corella Exotic MediumLaughing kookaburra Exotic MediumEastern rosellaEndemicsubspeciesMediumGreen rosella Endemic MediumYellow-tailed blackcockatooNativeLargeSulphur-crested cockatoo Native LargeMasked owlEndemicsubspeciesLargeSpecies Status HollowsizePOSSUMS and GLIDERSSugar glider Exotic? SmallEastern pygmy possumKeyEndemicsubspeciesStatus:Endemic = found only in TasmaniaEndemic subspecies = subspecies found onlyin TasmaniaNative = historically found in TasmaniaExotic = recently introduced to Tasmania? = classification is uncertainHollow size:(Minimum width of hollow entrance)Small = 2–5cmMedium = 6–10cmLarge = >10cmSmallLittle pygmy possum Native SmallCommon brushtailpossumCommon ringtail possumBATSEndemicsubspeciesEndemicsubspeciesMediumMediumLesser long-eared bat Native SmallGreater long-eared batEndemicsubspeciesSmallGould’s wattled bat Native SmallChocolate wattled bat Native SmallLittle forest eptesicus Native SmallKing River vespadelus Native SmallLarge forest vespadelus Native SmallTasmanian pipistrelle Native Small27