ISA HK/China --- Latest development in Tree Stability Assessment

Dear Station Members,

ISA arboriculture is an advancing profession, & even more so after the possession of relevant credentials. This is what making ISA Arborists around the world the leaders in their field. ISA Arborists simply do not rest on their laurels.

For those who are now studying for the Certified Arborist (CA) exam, p.138 of the Arborists' Certification Study Guide on Tree Assessment & Risk Management section states that:

"There are a number of formulas to estimate the amount of a trunk that is decayed. Most formulas are based on determining the proportion of sound wood relative to stem/branch diameter. While the formulas vary, most experts agree that a threshold of 30 to 35 percent loss requires that some action be taken. If there are large cavity openings or other aggravating factors, the threshold drops to 20 to 25 percent."

It is possible that the above statement would come from the research of Mattheck & Breloer (1994) which is widely adopted in America by arboricultural practitioners, in which:

t / R > 0.3

t = radial thickness of sound wood
R = radius of the stem

However, in p. 31 of the Arborist News Dec 2006 edition, Jerry bond of Davey Tree Expert has challenged this formula with additional statistics provided by the German research of Wessolly & others, & claimed that :

"The ratio t / R can no longer be used by itself as an index of trunk failure potential" in p.33 of this article.

As a Practising Arborist, one of our important job is to assess the failure potential of a tree to advise our clients. This latest research has prompted the Station Manager to think twice in his future assessment by just quoting the t / R ratio alone for consideration & recommendation. Since Drilling & Ultrasonic Tests merely describe the extent of the hollowness (decay?) inside a tree primarily, but the arborist would need to draw conclusion from the results obtained & to make a recommendation. The simple ratio of t / R > 0.3 may apply on its own no more....

In the conclusion of Jerry Bond's article, he stated that (p.34) " The practitioner in front of a tree with a centred cavity should use t / R in conjunction with evaluation of other factors that contribute to failure: wind load, exposure, crown architecture & species". This then comes back in line with the lower half of the same paragraph in the Arborists' Certification Study Guide (p. 138) that "It is difficult to determine how much sound wood is required to support a given tree. Variables include the strength of the wood, the canopy size & configuration, defects, & any number of environmental factors."

Therefore, as can be seen once again, mathematical models & equipment should be used as guidelines only in Tree Stability Assessment. It seems to be that they can not replace completely the experience & observation of the Practising Arborist. Trees are living creature & quantifying their behaviour by simple mechanical means may lead to unexpected outcome. We simply have to respect our brain & Mother Nature.

As for those taking the next CA exam, there is no need to panic over this issue. As Chief Proctor in our region, the Station Manger has not yet received any instruction on amendment of content of the CA exam over this particular subject. If so, the latest information will be released on Station Mail immediately. Therefore, work hard, be humble & best of luck!

best regards,

Sammy Au
Station Manager