Cavity Trees in Healthy Woodland

Cavity trees are very scarce in British woodland, the consequence of centuries of removing older trees. Their loss is very much to the detriment of the forest. The nest boxes we provide compensate for a while, but not adequately, for inevitably our interest flags and the boxes become neglected. What is really required is a long term strategy that contributes to the self-sufficiency of the woodland ecosystem.
There is very little guidance available on cavity trees (see References at the foot of this webpage). So I should be pleased to hear from you with additional information.

How do we Define a Cavity Tree?

Associated with older trees; having one or more cavities, of various sizes, crevices, loose bark, decay from broken limbs, lightning strike, etc.  Hollow trees are especially valuable.  This is quite distinct from wildlife hotels well endowed with creepers (ivy and honeysuckle, etc), forked branches, etc of any age.  Cavities have a microclimate which add to biodiversity and provide a niche for specialised forms of life such as fungi, ferns and plants that require dark and humid conditions to flourish.

A Vital Source of Food, Shelter, Refuse, and Nesting

In Britain cavity trees are used by owls and small birds; starlings, various tits, nuthatches, flycatchers, tree creeper, redstarts, jackdaws, etc, as well as dormice, bats, diverse insects and others. For nesting each species has its own specific requirements, evolved hand in hand with nature. In addition, refuge sites provide protection from the weather and predators. Bugs, larvae of wood-boring beetles, tree sap, etc, provide a source of food.
Woodpeckers are our one species that do not use ready-made cavities but rather excavate their own. They select trees that decay quickly; like ash (Fraxinus), birch (Betula), alder (Alnus) and beech (Fagus).

How do Cavities Form?

The following has been stimulated by Alex Shigo’s book Modern Arboriculture.

Natural Cavities usually form where the branch joins the tree trunk. You’ll see rounded bulges where the loss of a branch has successfully healed, over, known as callusing. The branch is supported by a collar (Shigo: Section 64) and typically, as the branch slowly decays it prevents the collar from callusing over and thus a cavity begins to form.

Man-Induced Cavities: Cavities can also form by our incorrect cutting of the branch thus damaging the collar (Shigo: Section 72), or if we cut the branch so that it is left protruding from the collar. Shigo states that if cavities are wanted for birds or small mammals (Shigo: Section 73), the branch should be cut as show on the sketch (left). We are trying this cut on our experimental trees to gage its success and will report here).

Tree Surgeons are trained to cut the branch so that the cut callus’s over. They are not however taught to cut the branch to encourage the formation of wildlife cavities. Natural Forest Practice requires both types of cut, the aim being to form natural cavities as well as clean trees for timber.

Chainsaw Cavities should only be made by an experienced tree surgeon (see photos). This requires skill to cut them just right. I advise that a practice cavity is cut at ground level and approved by the owner before ascending the tree (Appendix 1). The trouble in making cavities can be worth the effort if correctly done and located.

Experimental Cavity Trees

In my woodland we are identifying trees with suitable branches on which we can experiment. We will modify the branch and collar and observe how the tree responds over some decades. Some cavities will be formed by nature, others by our cutting a branch short and also by making a ‘wildlife cut’ across the collar (Shigo: Section 73). Others will be clean cut to callus over as a control. We’ll choose a variety of branch sizes and observe how they progress and report back on this page.

Artificial old trees: We are also trying to replicate old trees by using a clean drum filled with woodland waste- woodchips, bark, twigs, leaves and the occasional dead mammal. This has apparently been successful elsewhere but it is too early to gauge whether ours work.
We have successfully been making Beetle Houses (see Insects Make the Forest Work)

Actions

* There must be a greatly increased AWARENESS OF the importance of cavity trees;

* Cavity trees are important for Healthy Woodland Communities;

* Tree Surgeons must be trained to make both types of cut - clean cuts and wildlife cuts and use them as appropriate;

* Techniques for making cavities using a chainsaw should be developed;

* Man-made nest boxes provide a short term solution to the absence of cavity trees.  A long term strategy is required to provide food, shelter, and refuge

* Tree surgeons must be trained in making and fixing large nest boxes, the design of which must accommodate
the roundness of tree trunk and branch;

* Aim for at least six Cavity Trees per hectare.

Have you observed good cavity trees?  Please send a photo. Your experience and comment will be greatly appreciated. 
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Bat boxes for roosting and nesting being fixed in place to compensate for absence of cavity trees

APPENDIX 1 - Benefiting from our Experience of Creating Cavities Using a Chainsaw
by Paul Goetzee - Tree Surgeon

DISCLAIMER:- These materials have been prepared for information purposes only. They are not intended to be nor do they constitute safety advice. The author accepts no responsibilty for their implementation.

1. Only qualified tree surgeons should carry out this work wearing full safety clothing and well maintained equipment;
2. Use a short narrow bar for ease and to prevent/ reduce kick back with a new or almost new chain that is very sharp;
3. First cut a sample cavity in a log on the ground to gain an understanding of what is involved (photo on right);
4. Climb to the height required and use climbing irons and if possible a lifeline and anchor point to reduce strain on back and legs while working (photo- bottom right);
5. Make the cavity facing sunrise or sunset, not mid-day;
6. Bare a vertical slot at an angle into the tree and then bore into the same slot at an equal and opposite angle (photo- bottom left);
7. Make several more bore cuts through the same slot;
8. Then gouge out the ‘walls’ created by moving the bar laterally;
9. Ensure the floor of the cavity gently drains out.
   

Paul, tree surgeon cutting demonstration cavity. Left- shows angle of cut made from narrow entrance