Cavity Trees: From Living Dens to Dead Snags

I like to think that it’s a long-established rule that we all try to avoid creating holes in any living being, to which I personally put people at the top of the list. And that’s generally a trustable rule-of-thumb when working with nature as well, since I have a reasonable confidence that most of everything in this world would appreciate that basic practice. And in relation to todays topic, it’s rather unsurprising that trees would rather not have holes in them either.

But unfortunately for trees they are just really, really accommodating locations to have holes in, no matter where the hole is on the body. So setting aside a trees personal feelings, these holes act as cavities (holes, hollows, cracks, fissures etc) to host animals, insects, lichens, fungi, and mosses, all of which are drawn to these cavities as vital safe shelter, an easy resource of foodstuff, and if well-established over the years, a promising resource of materials to express various behaviours.

So by encouraging or introducing a cavity tree into the local habitat, we can provide a increasingly rare habitat for wildlife that fulfils many functions.

Before we get to the how-to’s, you may have noticed I have yet to mention either Dens or Snags, and how they relate to Cavity Trees. Unsurprisingly, the terms Snag and Den both refer to trees with cavities, hence both are “Cavity Trees”, but any further than that and we begin to see that they are technically two different habitat types. Den trees are alive & persistent despite the damage a cavity would bring, having had holes formed within their trunks via decay, local wildlife, or mechanical wounding. Snags on the other hand are dying or dead trees created through the same process as Den Trees, yet pay the price and fail to ultimately survive the process through resulting illnesses, dire injury or instead, regular old age. Whilst the two seem similar, it’s the end result that means all the difference.

Both are alike in that they are important for many species of birds, mammals, reptiles, and the aforementioned fungi & mosses, creating a natural and long-lasting reserve throughout the year for nesting & protection from the weather. But it’s a Den’s ability to stay alive that allows wildlife to make long-term use of their features, as the Den will continue to live for perhaps hundreds of years to come and provide co-beneficial relationships to all parties involved. Alternatively Snags are important in that they fulfil all these characteristics whilst providing a stronger medium for fungi, mosses & lichen to grow, which in turn attracts & invertebrates within their aged & rotting cavities and dead wood limbs. This rather diverse offering shows itself as further foraging opportunities for birds & mammals.

The presence of Cavity Trees diversifies the pre-existing habitat by providing an area that is different from soil-bound habitats, yet doesn’t rely on the canopies & local geology to provide height. As with every new habitat, it is about the unique opportunities provided and the species that are able to make the most of them that create a functioning & interesting ecosystem.

And finally, in your benefit, is that you can introduce a cavity into a tree with only the initial investment of effort, as it is self-sustaining afterwards, to which afterwards offers a strong feature for your garden as the cavity is explored & overcome with wildlife.

Perhaps this rare habitat will give you a prime observation area to see unique & interesting species. So, shall we see what we have to do to get started?

Before you begin, the Considerations:

As you look about your local area, it’s the general thought that a total of six den trees and/or snag trees per acre (With trunks between 12-18 inches thick) is considered a good result for wildlife. If you think your area is lacking such cavity trees and you’d like to go the extra step, examine live trees with the right thickness (12-18 inches) for any defects that could lead to cavity formation.

If you’re looking at converting a tree into a cavity tree, thinking about the species of tree can be helpful in the long run. Species of White Pine, Red Spruce, Eastern Hemlock, Sugar Maple, Beech, Yellow Birch, Elm and Oaks will all stand for long periods of time, even as dead trees. Other tree’s will do well as well, but it may be worth doing a little research to see how they respond to damage, and signs of illness.

Encouraging Cavity Tree Formation, and also, why you?

The earliest tree-felling practices recorded indicate that our ancestors cut down only the healthiest, optimally developed trees for their material needs, and this approach was mostly due to the intensive time & energy it took to fell a tree back at that time. This practice left an abundance of cavity trees for Wildlife as a side-effect, as the ignored veteran trees are more inclined to cavity formation, as the slow natural process of cavity formation & microhabitat development takes time to establish itself.

In recent year we are able to see the worth in encouraging cavity formation, and so wildlife does not have to wait for natural causes to slowly occur.

With large, old trees being the staple for cavity formation, conservation efforts aim to accelerate ecological processes to mimic the features of a mature tree and it’s connection to respective woodland. The restoration and creation of both forests and wooded pastures poses a unique challenge, as it take a long time for trees to reach a acceptable age to develop, establish and accumulate the captive microclimates

This is why if you decided to create your own Snag or Den it would be an meaningful effort, since after all a large proportion of aged trees are present within gardens, having been left untouched for years.

Even more so as our tree harvesting evolved with time, our more recent ancestors began actively improving tree ‘quality’ to meet wood material demands, which resulted in repressing cavity formation.

In modern times, practised has evolved into a myriad of situational & factorial decision making balancing between ecological and economic pressures. And when it comes to rare species and habitat, the underlying preference is to conserve, restore and encourage. And in lieu of this, people are encouraged to retain their cavity trees rather than see them as a concern for trees health.

So I won’t keep you waiting any further, here, the 3 baselines that form a rough outline of the type of cavity tree that would provide the most benefit to wildlife, the tree you want to pick.

  1. Use a large diameter (tree trunk ‘width’) cavity tree where available, this being greater that 25cm, though preferably greater than 40cm.
  2. Potential for cavity formation in the uppermost trunk are more valuable than those in the lower trunk.
  3. Present with at least ten other cavity trees upon one hectare of land to produce the most productive wildlife benefits.

Hopefully you’ll be able to recognise a tree in your garden that would fit these parameters. From here, let us look at how to create your own cavity tree.

Whilst cavities in nature are the result of decay following injury and/or disease, it’s now a feasible action to create man-made cavities. One benefit to this is the instant result, though it may be rough around the edges so to speak. But if you take a less direct approach the hollows that form in trunks & branches can be formed years after the event, forming a naturalised microhabitat. It is up to you to decide how you would like to make use of your chosen tree, as wildlife will still put it to use, but you may prefer a particular outcome. The methods listed below may result in cavities in the long run. Perhaps you have a tree that you are torn on keeping, so instead of removing it you may decide to experiment with it. Of course keep in mind damaging any tree may result in long-term damage, disease & death, but if you’re confident in your choice then it is a perfect time to try your hand;

  • Remove a branch with a pruning cut made too close to the trunk.
  • Remove a large, major branch to a similar specification.
  • Breakage of a large, major branch. This damage can occur semi-naturally.
  • Topping [removing portion from the upper trunk] the trunk and/or branches.
  • Cutting a large, major root – typically results in ground level cavities.
  • Cutting through a group of roots – similar to previous.
  • Considerable damage to the trunk – this most likely occurs naturally, but you may be able to reproduce the effect.

Consider the outcome of each approach, taking the height, size, positioning, and ease of doing so as factors in deciding your next course of action.

A more direct approach would be drilling or cutting into the tree’s trunk to develop a cavity. This is a very intrusive process that can result in large damage to the tree if you go at it without a plan, but the results are instant. If you have the option, perhaps first attempt this with any dead-standing tree stumps, or a tree that has had it’s limbs removed and leaves only the trunk behind. If you decide you are confident, use caution and plan the design of the cavity beforehand.


You may be inclined to add additional features or alterations to the cavity, but be aware there are some pitfalls to be wary of.

  1. Old practices of “Tree Care” included filling cavities in an effort to strengthen the trunk. You may decide you no longer care for a cavity, and want to fill it in. Certain materials are inflexible and abrasive, and as the tree growths & sways in the wind, the abrasion enables decay to enter into the living wood.
  2. Don’t be too tempted to add further details after the initial cut, such as adding a channel for water to drain out of. Further damage puts Trees at risk, especially if stagnant water is present at the site of damage.
  3. Whilst it is good to have a cavity tree in your garden, don’t feel the urge to do so to all your trees. Tree’s avoid cavity formation where possible, so whilst it is good to offer rare habitat to your wildlife, trees support wildlife in a great majority of other ways as long as they are able & healthy. That being said, if you decide your garden is able to support more than one cavity tree, select differing tree species as they will offer unique niches.

Overall Benefits to Wildlife

We have a tendency to remove older trees due to their safety risks, but the consequence is a greater rarity of naturally appearing cavity trees, at most appearing in 10% of standing trees. Their loss is noticeable, as these are the natural nest boxes for many bird species, and provide microclimates for an abundance of species. These microclimates provide opportunities for biodiversity, a niche for specialised life of plants, fungi, and the various species that desire dark & humid conditions to flourish.

When cavities appear in older trees they often appear in numbers, especially as time goes on as the tree ages and accommodates species in abundance. Depending on the location of it, it may vary in size & design, possibly being a crevice, loose bark, decayed & broken limbs, burns from lighting strikes and so on. Even more so than all of these though are hollow trees, an especially valuable resource, and they often come endowed with ivy & honeysuckle, becoming heavily carpeted and insulated from outside factors, a true microhabitat.

No matter it Snag or Den, a tide of species make use of them. Owls, starlings, nuthatches, flycatchers, tree creepers, redstarts, jackdaws, and various small woodland birds all make use of these cavities, as do bats, mice, and a wide selection of insects. Internal nesting is individually designed to suit the species unique requirements, but it is the cavity that provides a safe refuge from weather and predators alike to do so. The abundant presence of wood-boring & loving insect, as well as tree saps, fungi, and mosses, provide a rich source of food in their own home.

Interestingly it is a useful feature of both Snags and Dens that once they fall naturally or are felled they are already primed to immediately provide benefits to wildlife as a new habitat niche, allowing wildlife to continue to express both new & different behaviours.

Snags especially, as they deteriorate, provide various substrates on which insects, plants, mosses, lichens, liverworts & fungi make refuge of, and in turn are prey to birds & bark-gleaning species. This platform upon which provides many foraging opportunities within it’s small but complex structure.

And naturally to take advantage of this promising offering, there are even species that create their own cavities, the well-known Woodpeckers are one of them, and they choose ideal tree species that decay quickly to do so, these being Ash, Birch, Alder, and Beech, as this suits their unique requirements.

In this theme, of the Birds and Mammals species that “use” cavities in trees, there are two distinct groups they are divided into. ‘Primary’ cavity users have the ability to create & excavate their own cavities, in contrast to ‘Secondary’ cavity users which rely on pre-existing cavities. The primary species list is far shorter than the extensive list of secondary users, but over time cavities are created and abandoned, creating space to host many species of ‘secondary’ cavity users. It is this that makes the presence of the ‘primary’ species all the more important as their presence will create further habitats for a wide variety of species in the absence of naturally occurring cavities. Otherwise, it is up to humans to create further cavities.

As you can see from this brief, deep delve into the background of a deceptively simple, easily overlooked fragment of a whole scale habitat, it is rather stunning what is happening within the confined spaces of Snags & Dens without us being able to see inside their often lofty heights.

So what I would take away from all this is that it’s also what’s on the inside that counts!

Benefits for You

This is one of those times where I’ll tell you, and possibly understandable in this case, that I can think of little direct benefit for yourselves from this conversation practice, aside from being able to appreciate nature to a greater extent in your garden. I could tell you that having a rich amount of species residing in your garden will bring a host of positives, but I personally count these results as passive. Passively, it’s presence would increase the natural productivity of your garden by including more species within the natural cycle, with the cavity’s microhabitat supporting a veritable host of species that in turn supports insects, then their predators, and so on. Respectively each animal would provide a service to their surroundings, which could be nutrient redistribution, population stabilisation, pollination, re-seeding, behavioural expression and you get the idea. Their presence is noteworthy for wildlife, and your garden, as a second-hand effect.

But you would have to be creative for you to create decisive & direct self-benefitting return from a cavity tree.

Of course as previously stated, the easiest method is if you are in the same mind of thought as me, appreciative of both the environmental and aesthetical results that it will result in. Simple things.

But as an Economic or other personal benefit, there is not much to be done, and instead, it will be up to you to determine a practical use or value. Look to see what occupies your Snags & Dens, as a method of benefitting may arise from species that often interact with this new microhabitat. A species that would eat pest species of certain crops & flowers perhaps, or provides a seasonal presence who’s behaviour indicates the occurrence of a certain event.

Often times there are many creative methods of benefitting from the world around us, from ancestral practices to the newly discovered, but it is out there somewhere yet forgotten or unknown.

Maybe you will find an answer?

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