I have been asked by my boss to find out information on what to do when an Oak tree forms a large cavitiy in its trunk. We are currently tring to save one from being removed, from a church ground in New Zealand, due to a large hole in the trunk of the tree - the city officers are concerned that the main branch coming off of the tree is going to fall and hurt someone walking underneath the tree one day.

We have been told that in England they used support sturctures - if anyone has any information on this subject or knows where or who can help me please email to technix@technix.co.nz

(I have no experience with erecting tree supporting structures.) Your post was absent of any information about the credentials of persons looking at your tree. It seems to me that you would benefit first by having a qualified arborist examine the tree and help you come to terms with the tree's condition. This person should be able to offer advice about what (if any) mitigation might be made for a hazardous condition.

In hazard evaluation, possible targets for injury weigh heavily. Also, the most likely point(s) of structural failure, their size, etc. come into play. The general vigor and age of the tree, the stability of its root system, flawed limb attachments, disease, decay, and previous history of branch failure all should be considered. (Also consider local wind conditions that may contribute to forces affecting tree failure, and other site conditions that affect the tree's well-being.)

There is also the relative value of the tree versus the cost of dealing with mitigating the hazards. The tree might be saved by some effort, but at what cost? Sometimes it is better to start over with a new tree, taking care to protect and support its establishment. On the other hand, a tree of large functional and aesthetic value may be well worth the cost of bringing it to a *reasonably* safe condition (if that is possible). My understanding of external (pile) support structures starts with the fact that they are expensive. Appropriately placed cables are not so expensive. Or are you talking about interior trunk cavity supports? In most cases, any supports added to a tree will be preceded by some pruning to reduce weight of branches. The support is intended to reduce the effect of wind forces on a weak point where movement might otherwise excede the structural holding ability of the tree. In some cases supports are static and are placed under a limb to counter the effect of gravity. I have seen a large oak tree completely fall apart in a great wind even with pile supports under its great low limbs.