Planning introduced cavities & hollows in trees

Horses and carts

Firstly, our team would like to reiterate that protecting and rehabilitating the natural environment should be our highest priority.  Initiatives such as introducing man-made hollows should be a secondary measure.  Chainsaw hollows can be a helpful tool but not a solution to clearing and environmental degradation.

Introduced hollows & cavities

Terminology

  • Hollow vs Cavity – same thing
  • Man-made or introduced – means it was placed in a tree rather than forming naturally
  • Chainsaw hollow – generic term being used broadly to describe an introduced cavity regardless of tooling
  • Treetec or Narrow Door – one methodology for creating an introduced cavity, progressed by our team over recent years to overcome a range of problems
  • HollowHog – one of the tools we use, designed with a wood boring component and a vacuum system
  • Faceplate method – versatile but problematic method of creating introduced cavities

Planning and project design – tree hollows

No two hollow projects will be the same, they will range from a single small hollow for birds in a back yard, to Sugar Glider cavities in suburban parkland through to large scale cavity research in regenerating forests.

It’s imperative to think through all aspects of a project prior to commencing.  Some years ago after a bushfire a team was engaged on short notice to install ~300 nest boxes in a NSW National Park, unfortunately the funding was rushed and the project not well designed.  Through poor planning the nest boxes were attached using an inferior system and often in poorly selected locations.  The result was almost zero occupancy of those boxes and a lot of money wasted that could have been so much better applied to the problem.

Once materials are purchased and decisions made it’s very difficult to change a project even if some learning happens or good advice becomes available along the way.

Costings

Preparing costings around habitat work is complex due to variables such as site access, terrain, tree type, target species etc.

We recommend you determine a budget and the types of work you would ideally like done, then the contractor can assess and give you a guide on what is feasible.

It is unusual to work on a ‘per cavity’ or ‘per nest box’ price due to the high variability around logistics.

Tree hollows & introduced cavities research

When Treetec undertook our first literature review on hollows and tree failure in 2014 there was almost no research data available, there is now a significant and growing level of interest in the field of introduced cavities however the data is sparse.

Treetec has installed over 2000 cavities and we are learning however we have a relatively short timeframe of observations when thinking in ‘Tree Time’ and therefore there is much still to learn.

If you’re interested in undertaking research in this field, partnering or accessing our data please get in contact.  We’ve learnt enough to know how much we haven’t learnt….

Chainsaw hollows – tree structure and risk

Usually the first questions around introducing cavities into trees relate to tree response and risk:  will it fail?  Will the man made hollow cause a structural failure of the tree and if so what are the consequences, particularly in urban / parkland settings?

The data isn’t available yet but the short answer appears to be:  The host tree almost never fails if the cavity is well designed, carefully positioned and conservatively sized, and the hollow is placed in a suitable tree.

The host trees appear to be quickly (think ‘tree time’)  adapting or self optimising, to the wound: putting on wound wood and compartmentalising the decay.  These tree responses are both good and bad in terms of the usability of that chainsaw hollow going forwards.

Observations to date suggest that, if properly designed and executed, introduced cavity projects are beneficial and well suited to urban settings and parklands.

Tree size

If an introduced chainsaw cavity / hollow is too large in relation to the section of tree then of course the tree or limb will fail, maybe not today but probably in the next big storm.

What’s the maximum cavity size vs tree?  that depends on tree species, tree form, tree health, existing defects, wind exposure, location, any other mitigation works and the project aims.

Species

There’s a range of tree species related issues, an example being suitability to the target fauna – for example Major Mitchell’s Cockatoo (Lophochroa leadbeateri) uses the partly decayed timber from the walls inside a hollow to create nests for breeding, they tear the wood into little matchstick like sections.  It’s possible that the species of tree and the type of timber is a contributor to successful nest building and breeding.

Generally speaking, fast growing trees decay more quickly, some trees such as Silver Birch (Betula pendula) and many Wattles (Acacia spp) are poor at managing wounds, consequently, decay spreads more easily even in healthy trees.  Whilst usually slow growing trees decay more slowly,  there are exceptions to this rule.

When introducing a Treetec Door cavity do you want the tree to decay or not?  Environmentally speaking we want decay but if it’s a feature tree in an urban park then definitely not!

Tree age

Young vigorous trees will respond to a chainsaw cavity very differently to a large old tree that’s heading into senescence.

It’s important to think through the desired outcomes and ongoing management of a cavity when choosing suitable trees.

Younger trees have more resources available, relative to their mass, to use for wound response and self-optimization.  As with all factors listed below; choosing the correct age class of tree for a hollow project will rely on the project aims.

A wound in an older tree is more likely to see spreading decay and slower wound wood formation, potentially accelerating tree failure.

Younger trees, depending on species and health, are more likely to close over the opening, compartmentalise the wound reducing decay and create a swimming pool for native fauna.  Yes, drainage is a significant consideration.

Health

As with tree age, health will determine how a tree will response to any wounding related to an introduced hollow.

When a tree is wounded the response from the tree is to try and lock down that area, like a torpedoed ship, and compartmentalise the damage.  If a tree is healthy then it can more readily contain the spread of decay.  See the ‘Species’ section above for more on this.

Additionally a tree will grow structural wound wood to mitigate the damage – the healthier the tree and the more resources available, the more quickly this response can occur.

Leaning trees

Just don’t,

or target a branch.

When introducing cavities / chainsaw hollows you generally want the tree to remain standing and when working with leaning trees there are too many variables:  angle of lean, size, height, soil type, form, exposure, tree taper and the dynamics of tension and compression wood.

So, unless you are researching tree failures then stick to tree trunks with minimal lean.

However, placing a Hollow Hog cavity into a branch is usually unrelated to the lean of that trunk.  Cavities placed in branches will have no impact on the weight bias of the tree and are therefore well suited to leaning trees.

Targets

Don’t install cavities or hollows in trees that overhang, or lean towards, high use areas such as paths, roads, picnic areas or playgrounds.  If a tree can’t hit anything of value then the risk is almost zero.

Tree risk mitigation & hollows

Any tree can fail, a single branch or the whole thing.  Creating any type of wound in a tree including a chainsaw or HollowHog cavity or faceplate hollow will increase risk of failure, however measuring the degree of increased risk is very difficult.

If a project is well designed the risk associated with man-made hollows in trees is almost zero.

  • Don’t place cavities in trees near targets
  • Ensure any hollow is suited to the tree – not too large
  • Keep the width of the entrance wound narrow
  • Protect, so far as possible, the integrity of the outer shell of the tree
  • Select trees with minimal lean if targeting the trunk
  • Weight reduce the tree where appropriate – keeping in mind that each pruning cut is another wound and removing foliage reduces energy resources available to the tree.

Tree selection for introduced hollows

There are many trees not suited to introduced hollow projects – carefully consider what trees are on site before planning and be prepared to walk away from some of those trees once the arborist is up in the canopy and can inspect the tree properly.

Cultural / amenity values

Man-made hollows, their impacts on fauna, the host tree and the site more broadly are not yet fully understood, there is a lot of research needed.

Don’t place cavities in culturally important trees or high amenity trees, we don’t yet know with certainty what the long-term implications will be.

We would suggest any of these trees should not be used for habitat placement projects:

  • Scar trees or any other culturally important trees
  • Large old trees, particularly remnant trees pre-dating European settlement
  • Trees of historical importance such as those planted as a marker or part of a ceremony.

Consult with traditional owners prior to developing a habitat / hollow project in native forests, they bring an important lens to this work and everyone, as well as the environment, benefits.

Tree location

Choose trees situated away from noise, artificial light, and people

Avoid, if possible, edge trees – those trees on the edge of bushland / forest areas

Select locations that suit the target species – for example Powerful Owls prefer to nest in very close proximity to water and with some dense mid-story foliage nearby & below.

Don’t install cavities or hollows in trees that overhang, or lean towards, high use areas such as paths, roads, picnic areas or playgrounds.

Living or dead trees

artificial hollow
Using a faceplate construction method to create a hollow within a branch stub.

Dead trees provide a great opportunity for introduced habitat, they are often, particularly in urban settings, considered not useful (erroneously), however with only moderate effort & investment they can support multiple cavities for multiple species and can remain safely in situ for years.

All trees in urban settings should be considered from a risk perspective, if a large dead stag is situated beside a high use area eventually it may need removal for safety reasons.

Living trees, we believe, are a significantly under-used resource for habitat creation, the evidence to date suggests that well designed cavities can be introduced into appropriately chosen, living trees without significantly increasing risk.  Risk and perceived risk may increase and this needs to be managed, but otherwise a living tree can provide a nesting site that could last, potentially for hundreds of years.

Hollows and tree structure / failure – risk

There is a commonly held opinion that if you carve a cavity into a trunk or limb then that section of the tree is highly likely to fail as a result. We have undertaken a literature review and continue to see and collect data which suggests that this is not necessarily the case. It appears that trees can support introduced hollows long term without failure if the installation is well designed, however there is a caveat, and that is time – as yet there is not enough data over enough time.  For this reason we advocate caution, avoiding trees in high use areas and targeting parks, bushland and forest trees with minimal targets nearby.

Trees are extraordinary structures, for many reasons, and we understand very little about how they operate. They do however clearly exhibit a function of ‘self optimisation’ – that is they adapt and grow to deal with defects or weaknesses. This reactionary behaviour is both physical and chemical and can be relatively quick. We are seeing trees adapting to significant wounding and hollowing, that remain standing, even when exposed to major wind events. Consequently, as time progresses, trees appear to overcome or adapt to the introduced defect (hollow).

Across thousands of introduced cavities we have observed just four related failures and they were predictable given the extent of the work done in comparison to the host trees size and existing condition.  The failures were directly attributable to poor planning and/or workmanship.

The type of wound, location of entry on the tree, tree condition, tree size and cavity size directly impact the likelihood of related tree failure. These factors are key to setting parameters around the design of a man made hollow, as an example use of a standard faceplate design within the main trunk is much more likely to result in a tree failure than a narrow ‘Treetec door’ style cavity.

Disturbing nesting fauna

Do not disturb, in any way, nesting fauna.  Do not damage existing habitat or nests.  Any projects that could impact native fauna requires ethical approval, Treetec can assist with this process.

Context

Think about the broader environment, is the tree part of a corridor?  Could the chainsaw hollow help to bridge nearby fauna communities and would therefore be better placed in a tree a bit to the left?

Is this the best size and design of cavity given how many trees are available – if you have just the one larger tree near water then that’s your Powerful Owl tree, put the possum cavities (the supermarket for Owls) in those other less valuable trees.

Introduced cavities and hollows – the ‘how to’ details

The information below is an overview of some key learnings to date – some of it has verifiable research behind it, some doesn’t.  If you have comments, questions or information to add please let us know.

Target species

If you place a cavity in a tree, it’s highly likely an animal will move in but if you carefully design the cavity, the opening, the position of the opening relative to the cavity, the aspect, the height and consider context then it GREATLY increases the chance of attracting the target species…. and excluding unwanted tenants.

Microclimate / ventilation

Having inspected many hundreds of cavities, many of them multiple times over the years it’s apparent that the microclimate could be a significant factor.

We know that chainsaw hollows exhibit very similar temperature profiles to natural hollows, this is in stark contrast to most nest boxes which are usually colder in the cold and hotter in the hot than natural cavities.  However, we don’t yet understand much about ventilation and humidity particularly when fauna are actively nesting in a chainsaw hollow.

In the central highlands of Victoria (Damp Sclerophyll Forest) we’ve seen occupied cavities (Leadbeater’s Possum) with conditions that become very damp, we’ve seen nests no longer being used and, on occasion, fungi growing inside those hollows.

We wonder if an introduced cavity with its freshly cut, green timber and small, neat opening provides a different environment to a naturally formed hollow with various levels of wood decay, chimneys and avenues for ventilation.  If it is a different environment then is that difference significant to occupant species?

The Treetec team has adjusted our practices to now facilitate better ventilation, we do this a few different ways but suspect this approach will better mimic natural cavities.

Treetec has partnered on a pilot project using iButtons to measure humidity in these Leadbeater cavities.

Drainage from tree hollows

Timber boats float, and timber hollows can become swimming pools.

Ensure your cavity/hollow project considers this issue – long term.  The wound wood that forms around an introduced cavity (Treetec door / HollowHog / faceplate etc) can retain water.  It is very possible that a cavity could be installed:  5 years later the wound wood closes over a poorly designed drainage solution, then a bird in residence bears young only to have them drowned in a single storm event.

If you’re using a spout does it direct water into, or out of the cavity?  Will the spout that directs water away rot and fall away leaving a water catching problem?

Drainage in man-made hollows is a significant long term issue and needs to be addressed during the planning phase.

Wound wood

Wound wood is that reactionary tissue that forms on trees as a response to a wound or structural stress.  This growth occurs near hollows and cavities and is the trees direct response, self-optimizing, to mitigate against failure.

With tree hollows we are seeing two types of wound wood:

  1. Creeping in from wherever the cambium was cut, often as circular occlusion, thereby closing over the wound (wall 4 in the CODIT model) potentially narrowing or closing the cavity opening.

Some animals will chew the reactionary wood to maintain an opening, many others will not.

  1. Sometimes wound wood develops as two ribs down each side of the man made hollow exactly as you might see a human leg supported when being stretchered from the field of battle.

Design your tree cavity project so that the wound wood quickly provides the structural strengthening required to avoid failure but doesn’t close over the opening or create a swimming pool.

Monitoring hollows and introduced cavities

Monitoring and data collection are important to the effectiveness of man-made hollows going forwards.

Monitoring can be very low impact such as stag watching, automated cameras or using hand-held thermal imaging cameras from the ground.  However monitoring can be more intrusive such as accessing the cavity directly with cameras / heat sensors etc.

Treetec have a range of equipment from pole cameras, automated infrared heat cameras, audio recorders and remotely set connected cameras that transmit back vision and alerts of any activity.

Keep in mind that any monitoring that may disturb animals will require ethics approval and a research permit / Access Agreement.  This includes when pushing a camera and/or torch into a hollow.

Data collection

We would strongly urge project managers to ensure data is collected for introduced hollows, there are so many variables and unanswered questions in this field.  Great value will come from a robust data set.  We use ProofSafe to collect data on all cavities installed.

ProofSafe is an Australian company providing tools for field-based research / surveys and compliance.  The ProofSafe App enables anyone, on any project, to collect any data, anywhere.  That data is then uploaded and available to the project manager.

ProofSafe has been supporting Citizen Science projects and environmental initiatives for the last 5 years, often free of charge. They have developed forms specifically for nest boxes, introduced cavities, chainsaw hollows etc.  The data collection tools support both the establishment and maintenance of these installations but also the monitoring of occupancy.

www.ProofSafe.com.au

Connectivity and cover near tree hollows

When selecting trees for introduced hollows prioritise areas with suitable vegetation, some animals prefer a clear view of their surrounds, others prefer the cavity they use to have cover from predators and immediate access or bridging into denser vegetation.

We’ve seen some man-made hollows with absolutely no obvious visitation after 5 years, the only apparent reason being the lack of nearby vegetation connectivity.  It could be something else but we think it’s a connectivity problem.

Occupation by pest species

European bees, starlings and other pest species may use introduced cavities however to date we’ve seen only a few instances of this, an unexpectedly low number. We don’t know why.

Mudguts

A large project comparing log boxes, natural cavities and introduced chainsaw hollows near the Murray River delivered a number of insights, firstly it’s worth considering how much value is provided introducing new cavities into an area with many pre-existing natural cavities.  Secondly termites will eat seasoned dry hardwood so any introduced cavities are better done without adding timber doors, and thirdly – trying to establish a new cavity in a tree full of Mudguts is a bad idea.

‘Mudguts’ is the clay like material that builds up within trees inhabited by termites.  You can create a cavity, however the mud will continue to push down from above filling that void.

Multiple chainsaw cavities in one tree

One of the most expensive components of this work is getting an arborist with all the right gear into the tree and ready to start.  Therefore if you can place multiple cavities in a single tree it becomes much more efficient.  Additionally, the research suggests that if a hollow in a tree is occupied then other hollows in that same tree are more likely to also be occupied, based on this one might speculate that two or three holes are better than one.

On many trees we have installed a larger cavity suited to a Glider or larger bird as well as smaller slot type cavities designed for micro bats, we don’t yet have any useful results on this.

Chainsaw bar oil

Chainsaws require a lubricant to protect the cutter bar and chain, this oil continuously flies off the end of the bar.  There are two types of bar oil, the standard petroleum-based oils and biodegradable vegetable oil versions.  When working on chainsaw cavities or anywhere near water it’s essential that the biodegradable oil is used. The vegetable oil versions provide comparable protection to the saw, present less risk to the operator and much less damage to the environment.  For context, in Austria, ALL petroleum-based chain saw oils are banned.


Methods used for creating introduced hollows / chainsaw cavities

Man made hollows vs nest boxes

Please contact us to discuss your project.

Melbourne Arborist