Introduced Cavities And Hollows

Our team of arborists and ecologists can assist on any canopy access project from arboreal research to the installation of species targeted cavities and hollows.

Natural hollows, introduced hollows, nest boxes

natural tree cavity
Natural cavity with nesting material

Many animals use hollows, and lots of those are obligate hollow users – meaning they need access to hollows to survive and/or breed. There is much that is not known about hollows, their formation and the relationship with the fauna that use them. It is very clear however that as we remove, or fail to protect habitat, fauna is adversely impacted.

Of primary importance should be the protection and rehabilitation of existing habitat, however in any regenerating site there will exist a significant shortfall of larger mature trees and therefore in these areas there will be a dearth of natural hollows. Where natural hollows are scarce nest boxes and introduced cavities can (and have) been employed to assist re-establish populations of hollow using fauna.

Treetec has worked on a range of projects using both nest boxes and introduced cavities (hollows or cavities cut into trees), installing, maintaining and monitoring those sites, sometimes side by side. There are pro’s and con’s with both approaches, unfortunately there is very little published data comparing the methods. In our experience, in general, we have found that introduced cavities provide better outcomes on a number of levels however are more expensive initially to establish.

There have been a number of projects undertaken exploring various strategies to accelerate or introduce hollow formations in trees, these include fungal inoculation, fire, explosive charges and various mechanical methods such as power tools and chainsaws.

Nest boxes

nest in man made hollow
Full nest visible within a man made or ‘introduced’ cavity, condensation on the nest indicative of occupancy.
This project included the installation of temperature data loggers.

Nest boxes provide a relatively quick solution where hollows are needed, they can be constructed off-site to suit the target fauna species and are quickly installed. Unfortunately there are a few drawbacks e.g. wooden construction boxes last, on average, only 8-9 years and the thermal properties are quite different from natural cavities. Some of the negative attributes around nest boxes can be mitigated with good design, siting and installation.

Man made hollows / cavities in trees

Creating a hollow in a tree that is both suited to a specific fauna species and does not result in the loss of the tree (tree failure) appears to be very achievable, although it is early days in terms of scientific evidence.

Treetec has created some hundreds of various cavities in a range of forest types and urban settings for environmental and research purposes, the uptake by animals has been very high, though not consistently by the target species. There is clearly demand for hollows across various environments. In contrast we have seen a project with the installation of ~300 nest boxes where the uptake was very low (<5% after 1 year), this poor result almost certainly related to the fixing method and aspect of the boxes which highlights the importance of good design and planning.

Introduced hollow installation
Installation of a hollow into a living tree.

Introduced cavities can be configured to suit a range of criteria including access for research, placement of sensors, target species, safety, longevity.

Over time the Treetec team has developed innovations around introduced hollows as we’ve learnt how the tree and animals  respond. An introduced hollow within a large old tree with  decay is very different from an introduced hollow, in a semi-mature tree. Additionally the ability to monitor requires the design to be adapted; this may be facility for electronic installations or access.

Often a man made cavity will be created within the main trunk of the tree however this may not suit some trees and settings in which case limbs can be used. If perceived risk is an issue a limb can be cut (lopped) back to a stub and the hollow placed within that stub thereby effectively eliminating the risk of failure that might be attributable to the hollow.

Live 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. Unfortunately they will eventually fail and if located in a higher use area will need monitoring and eventually removal.

Living trees, we believe, are a significantly under-used resource for habitat creation, there do exist issues around safety and perceived safety (yes, these are quite different issues) that need to be considered but otherwise a living tree provides a site that can 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 hundreds of introduced cavities we have observed just three related failures and they were predictable given the extent of the work done in comparison to the host trees size and existing condition.

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 key-hole style cavity.

Tree hazards and risk

Every tree presents a hazard, the actual risk associated with that hazard is, on average, infinitesimally small. The benefits of trees far outweigh the risks except in a very tiny percentage of cases. Tree related risk is determined by a range of factors:

  • Location – busy urban setting – parkland – or forest
  • Species – some trees are indestructible, others are highly susceptible to decay and failure
  • Age – Old trees are less able to compartmentalise decay than a vigorous semi-mature tree
  • Size – a failed limb from 30 metres is quite different to one from 4 metres
  • Local environment – edge effect, wind tunneling, soil, nearby works or interference
  • Weather – waterlogged soil in combination with wind immediately alters risk

It is true that cutting into a tree does reduce it’s structural integrity however the actual risk appears to be lower than one might commonly think.

In summary: a small number of trees in specific settings are, due to risk, not well suited to introduced / man made hollows. However some urban trees and most parks, bushland settings and forests can support introduced cavities without any significant increase in risk to people. These introduced cavities can be designed to be similar in many ways to a natural hollow. They can be accessed for research and have some significant advantages over nest boxes if well designed. Key advantages being that they more closely resemble natural cavities and are cheaper over the medium to longer term.

Please contact us to discuss your project.

Melbourne Arborist