Tree hazard assessment
The first step in assessing the safety of a tree is a ground-level visual inspection. The main hazard is of course that the tree suffers mechanical failure, whether of the trunk, the root system or an individual branch, risking damage or injury. In practice, the risk depends on (a) the seriousness of any defect in the tree and (b) on the tree's situation, for example its proximity to a road, building or footpath.
A tree that is free of visual defect is generally regarded as safe for practical purposes, although even sound trees can fail in exceptional conditions. The same applies, of course, to chimneys and roof tiles. In consequence, an appropriate balance often has to be sought between the risk and benefits of retaining a particular tree (Lonsdale, 2006).
Trees have safety factors (like any other structure), estimated at 4.5 times the normal working load when the tree is free of significant defect, and the loss of some of this safety margin is sometimes acceptable (Mattheck & Breloer, 1998).
Defects which create weak points include narrow branch forks, poorly developed root systems, decay, cracks, damage by some external agency, pruning wounds and a change in wind loading due to pruning or a loss of shelter. The tree adapts with new growth, and the new wood may have exceptional resilience to compensate for the high stresses where it develops, restoring (at least to some extent) the strength and stability of the structure.
If a visual inspection gives cause for concern further investigation may be appropriate, depending on the context. This could involve soil sampling, the identification of a pathogen or pest, the use of specialist equipment to assess the extent of decay within the tree or an aerial inspection conducted by a tree surgeon.
The extent of decay (if near ground level, as it often is), can often be estimated to some extent during the visual inspection. Open cavities or regions of the stem softened by decay can be physically probed, while internal cavities can sometimes be identified with a sounding mallet. There are two main techniques for evaluating decay more accurately, both of which require specialized equipment.
The resistograph drills a small-diameter hole into the tree while recording the resistance to the drill bit, and the way the wood softens with depth is automatically drawn on a graph. The main disadvantage of this technique is that it is invasive, itself creating a defect. The second technique, the Picus, involves strapping several sensors round the tree and inferring the extent of internal decay from the speed of transmission of impulses passing between the sensors. The main disadvantage is that it is perturbed by the flutes and buttresses at the base of a tree, in which case it can't be used very close to ground level.
Lonsdale, D. (2006). Principles of tree hazard assessment and management. Dept. Communities & Local Government and the Forestry Commission. Research for Amenity Trees No. 7. 4th Impression. The Stationery Office, London. ISBN 0-11-753355-6.
Mattheck, C. and Breloer, H. (1998). The body language of trees: A handbook for failure analysis. D.E.T.R. Research for Amenity Trees No. 4. 4th Impression. The Stationery Office, London. ISBN 0-11-753067-0.
Summary of Example Report
The … avenue… consists mainly of late-mature trees. Many have dense crowns and the trees have numerous branch wounds, largely arising from former pruning operations, which are now decayed to a variable extent. The main hazard is therefore mechanical failure of a whole tree or of individual branches, while the main target is the traffic using the drive to the Manor.
One tree is newly-dead and should be felled as soon as possible, while some trees have become structurally weakened to the extent that they should be relieved of some of their mechanical loading by a combination of crown reduction and crown thinning. Many trees have a high incidence of dead and suppressed branchwood in their crowns which should be removed to avoid it being shed in due course onto the drive.
Prescriptions are set out in the attached Schedule of Works, which can be used to solicit quotations from tree surgeons. The trees in the avenue are subject to a collective Tree Preservation Order so no work to them should be undertaken without the prior approval of the local District Council.