Thursday, February 19, 2009

ISA HK/China --- Decay Detection Device (DDD)

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Dear xxx,

Thank you for your 30 minute phone call on your enquiry of Decay Detection Devices (DDD) for Tree Inspection. I hope I have given you a satisfactory answer with my best ability & I hope you do not mind I am repeating some of my reply to you in an open Station Mail to let our Station Members understand our conversation without disclosing any detail of yours, since I think this exchange may be useful to them too.

From my research, communication with international Arborists & observation at seminars & conferences, it is known to me that DDD can only supply a cross sectional analysis of decay to some degree of accuracy, with possibly the Micro-drill Method (Resistograph?) to be more precise than the Sonic Method (Tomograph?), if used correctly. The micro-drill method was initially used to detect wood rot in lamp post in the utilities industry, adapting to detecting wood decay by the Tree Care Industry later on.

Prof Schwarz of Albert-Luwigs-Universidad of Switzerland has given research on the preferential fungal development in micro-dill wounds rather than in normal size drill holes in Tree Inspection, due to smaller hole harbours the fungi better, at the ISA Brisbane Conference in 2008. This would mean that using micro-drill in Tree inspection may lead to a higher chance of fungal development in a tree, i.e. the more holes drilled, the more internal decay would happen as a result. This is something to watch out for in choosing micro-frill as a DDD.

No matter whether a micro-drill or a sonic device is to be used for decay detection, a longitudinal examination of decay development in a tree is not possible, with the existing DDD available in the market. It is a known fact that any cavity in a tree would not be arranged in a 2-D fashion, but rather in a 3-D configuration. It is also not possible to increment the current DDD to be at a practicable distance to map out the 3-D configuration of a cavity in a tree. Therefore, the result of such DDD would have to become a source of reference only, & not a direct determination of whether a tree would stand or fall.

I personally have seen Tomograph result of a limb before cutting, to be completely out of proportion to the actual decay distribution of the same limb after removal. This could be due to incorrect use of the instrument, poor technique in configuration or others.

Even with a high accuracy of any DDD result, it still has to be interpreted by an experienced Arborist to imply his/her knowledge to whether this tree would stand or fall under a certain condition, e.g. wind pressure, wood structural strength, leaning severity & root stability. There are many mathematical formulas to predict tree failure, & it appears that international Arborists can not agree on any one to be acceptable in a universal basis. Most would agree that it would still depend on the judgment & skills of the Arborist in concern to make out the guess work. I am attaching 2 previous Station Mail on this subject for you to refresh your memory.

Therefore, my advice in your case would be that do not take the DDD to be the Holy Grail in making a Tree Failure Analysis. The DDD must combine with the knowledge & experience of the Arborist to help him/her to make an educated judgment backed up with research.

Please let me know if you have further queries on this complicated subject. I shall try my best to answer with my limited ability.

best regards,

Sammy Au
Station Manager

"Trees are good for the community. Trees need care like human beings. Arborists are the Tree Care Professionals."