Visual Tree Assessment:
Tree risk assessment is a systematic process for identifying analysing and evaluating risk ( Dunster et al 2013)
Tree risk assessors are often educated the ways and theories and concepts of Mattheck et al Visual Tree Assessment. In Mattheck’s numerous publications these theories have been developed, amended and progressed over time, have the backing of internationally published experts (Fink,2012) and have been used in many court cases.
The purpose of hazard assessment in trees must be considered before any analysis of systems can be made:
“Predict the potential of hazard that trees pose to the public and private safety”….arguably not for the benefit of society as a whole but to “reduce the liability of the tree owner” (Fink,141,2012) Nobody wants to be responsible for a potential incident and the law will examine if the case of tree failure was “foreseeable”
The requirement; to observe defect systems in trees that can be predicted…if no action is taken then it could be argued as an “act of negligence in court”
Foreseeable is important…if the damage/failure could not be predicted then it’s an “act of god” and as most failure in healthy trees showing few defeat comes from excessive wind load and the fact that even the healthiest trees will finally fail, thus is cannot be the tree owners liability (Fink,143,2012)
As a consequence the development of tree risk assessment systems, such as VTA have been developed. This really is the construction of rule book for arborists to follow, a guiding set of principles to help identify risk and an attempted standardise the approach of tree surveying.
Mattheck introduces a number of rules that the tree surveyor must adhere to, such as the Axiom of Uniform Stress and the VTA –t/R > 0.32 rule. These are open to challenge and the t/R > 0.32 rule has been the subject of many studies before Mattheck and since (Kane et al,2001, Smiley & Fraedrich 1993 Coder,1989)
Some of the concepts presented such as the Axiom of uniform stress, however is an area that needs to be carefully considered by the practicing arborist, if not discounted completely (Slater,2017) These weaknesses in Mattheck’s publications devalue his proposals and leave his theories open to criticisms such as that presented by Gruber(2008) who argued that Mattheck was simply wrong. Gruber’s argument however, is weak and offered little in way of an alternative thus leaving the practicing arborist with Mattheck’s ideas as a guiding principles.
The creditable work done by Mattheck on VTA is identifying potential areas of tree failure and highlighting these in easy to read publications. This I would argue is where Mattheck’s genius lies in the way the content is presented!
A significant failing of VTA; in an attempt to standardise tree risk assessment, through implementation of VTA, is open to interpretation and anyone can have a go. Simply read through a “Field guide for Visual Tree Assessment” combine this with the little appetite to standardise formally with a British Standard (Slater,2018) VTA is open to wide ranging interpretation and conclusions. The below table shows some Advantages and disadvantages of the system:
| Advantages | Disadvantages |
| Easy to carryout | Prognosis based qualitative interpretation rather than quantitative fact |
| Low cost – carry VTA many trees in a day little cost per tree to assess at level 1 | VTA theories based upon research from forestry trees ( We have felled all our urban trees!) |
| Accepted in the courts as best practice | No agreed minimum standard! We have a BSi for slab laying but not tree inspecting |
| The calculative elements are research backed (Mattheck, various) | Limited research on urban trees to found/confirm theory ( we chopped them all down) |
| Easy for a client to understand the principles and concepts within VTA | Mitigations are often made before a full cost benefit analysis is undertaken |
| Tree statistics are applied to make decisions | Unqualified people can carryout |
| Monitors trees to keep people and property safe | No frequency of survey is advised |
| Some areas of the research are questionable and have been challenged |
VTA has been categorised by the ISA and described a systematic process of identifying, analysing and evaluating risk. (Dunster, Smiley, Matheny & Lilly 2017) It is systematic, a series of steps need to be followed to successfully complete a VTA, yet it’s evaluating factors are done by human assessment that are influenced by risk perception & acceptance, professional bias and even potentially the clients. Therefore VTA despite its attempts to quantify with mathematical calculations and systems (QTRA) t/R ratios etc. will always be a qualitative and inconsistent form of tree hazard assessment. SEE CONCLUSIONS It must be improved by approving a British standard of VTA minimum expectations to ensure consistency.
IML Resistograph
The IML Resistograph, this has become the most popular method, used in a situation when significant decay is detected in trees.
A good tree manager will take into account site conditions, tree age, shoot growth and if the tree is able to compartmentalise the existing decay, building up sufficient compensatory radial increments (Rinn,2016,204)
Before the micro-drilling, the option to incrementally bore the tree was used. This had a number of failings, the identification and efficiency of compartmentalisation was difficult to determine (Rinn,2016,204) and the wounding that such an instrument inflected upon the tree provided a pathway for fungal penetration.
Before evaluating the benefits of such a tool it’s important to consider the criticism associated with its usage.
- The device penetrates into breech reaction zones (Shigo, 1977) that have surrounded and acted to limit pre-existing zones of decay.
- The penetration provides a channel for the development of a decay fungus, possibly latent, that had been previously protected by the trees defences (Lonsdale,1999)
- Drill wounds allow infection into the tree as the cambium is breeched.
- If drilling equipment has not be sterilised it could pass infections on from tree to tree as particles would be stored on the drill bit.
- It’s expensive to purchase and maintain – thus operating costs may outweigh the benefits
- The device is heavy and the results are open to interpretation
- It only provides a small snapshot of the horizontal decay pattern of the stem/root
Many of those criticisms have been challenged by Kersten & Schwarze who concluded “fear of casually transferring fungal propagules from one tree to another is unfounded” (2012,178) but also remarked “ it was clear that the IML Resistograph drill holes provided better conditions for the outgrowth of the fungus from pre-existing decaling columns than the increment borer holes” (2012,176)
Careful consideration before the use of such a tool, awareness of negative implications. The tool they use must be reliable, a number of systems are on the market, providing differing results (Finn,2016) the user must be trained correctly to interpret the results, knowledge of the tree species ability to compartmentalise and the invasiveness of the fungi must be considered. The machine must be well maintained. Annual calibration is important and costly. If the tree is deemed of high amenity value then it may be worth further investigation. “any risks of harming trees by the use of invasive diagnostic methods are far less serious than the harm that is caused by the kinds of injury that typically allow the development of fungal decay” (Schwarze,2012,179)
Improvements:
As a user of the IML resistograph I would suggest that improvements to the training and guidance with the system could be provided. It has taken me a few years of use, practice and interpretation to feel confident of result interpretation. The system is delivered and an explanation of how to use it, but this is provided from a mechanical perspective only. A detailed handbook of examples of readings and the corresponding cross sectional photographs of drilled wood, would be of benefit to the inexperienced user. A database of such information on a website would be a wonderful resource that would support the standardisation of result interpretation. I personally drilled lots of different types of wood that had been felled. I photographed the results and have built up a collection that I can refer to if I am unsure, for example Horse chestnut decay I have discovered, can be difficult to interpret sometimes.
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