Sunday, November 11, 2007

ISA HK/China --- HK Tree News (Pruning public trees)

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Dear Station Members,

It is all familiar that our public frequently complained about the so-called 'incorrect pruning' in our territory. When the attached newspaper cuttings were sent in, this Station decided to respond with an Arborist's viewpoint on events described as such.

In the first set of news, two Bauhinas were pruned for safety reason after branch breakage in strong wind in Ap Lei Chau. Someone wrote in to accuse ' wanton chopping' by a Govt Dept to disturb the greenery development in our city. The second news suggested planting trees in Tsim Sha Tsui created more liability than asset due to Tree Selection.

Before saying anything as such, an Arborist would need to look into the evidence presented for analysis. First, we would need to study the Design & Tree Selection in both cases as a beginning. In the Ap Lei Chau case, the trees selected were Bauhinias which are known not to develop a good Central Leader naturally, but would rather give crooked bends along the stem to branch out in unpredictable manner, which would pose a nightmare for anyone trying to structurally prune. If the Nursery Stocks were not initially selected to have good Central Leader, 60% Live Crown Ratio, Strong Branch Attachment, Balanced Canopy & Taper before installation, even the best pruning operator in the world would have a hard time to correct them back to good Structure & Health afterwards. Poor Nursery Stocks are a bad start & correction will take time & efforts. In reality, some trees may never be able to be pruned back to requirement at all.

Next, we shall need to understand who were doing the pruning? It was said to be the contractor. While it may be easy to teach a contractor to prune at the correct angle from Branch Bark Ridge to Branch Collar in a branch, it would be much harder to teach them what parts to remove & how much to remove. Pruning will lead to responsive growth in a tree. Incorrect pruning will lead to incorrect future growth. To be expert in pruning, the operator will need to understand Tree Biology for tissue response, & also Tree Biomechanics for wind stability. Would it not be too much to ask for a contractor to understand in such details of what they were asked to do? After all, they were only a contractor. It should really be the responsibility of an experienced Arborist to design & supervise the work, & to instruct the contractor on where & how much to cut, & for what objectives.

If an experienced Arborist is to be brought in to design & supervise the pruning, cost will be involved. According to the current Professional Fees Guidelines for CA to provide this kind of supervision for a day, the cost will be HK$6,000.00 each time. Can the tree owner provide this cost? Even if such cost can be provided to employ the right person to carry out the Structural Pruning, there may not be that many qualified Arborists available in our territory to perform the required work at present, because it is rather difficult to teach Crown Thinning, Crown Reduction & Crown Restoration in a classroom. For tall trees, Tree Climbing may also be required to get up inside the canopy to select the branches. Bucket Truck can not penetrate inside tree canopy safely but rather stay on the outside, & usually can not get on top to perform Crown Reduction. Then how many of our CA in HK can climb at present to do this kind of work?

The case in Tsim Sha Tsui followed similar principle of apparently selecting fast growing & wide spreading species without adequate room for future growth, which then led to continuous trimming back to reduce size. This appears to be a classic case of 'tree liability' upon improper Tree Selection in our territory.

The problems with our urban trees appear to be historical. The public also seem to have a lack of understanding of trees. To blame on the maintenance party alone would be like asking Cinderella to dance with an oversized shoe. Would that be fair? On the other hand, if it is the design & tree selection which are causing the problem, there is usually not much that the maintenance party can do to correct it afterwards. Sometimes, a complete renovation may be the only solution. Then the public may outcry for ' tree murder'. Isn't it easy to throw out criticism, but what about finding a workable solution?

Tree Pruning can be compared to human surgery on a person in many ways. No sane person would like his/her body to be operated upon by a meat butcher. Then why should thing happened to our trees ?? Time may be required to train our Arborists to perform quality pruning (with Tree Biology, Tree Biomechanics & perhaps Tree Climbing on real trees), & the road ahead will be long & winding. Maybe an immediate solution is to employ experienced Arborist to design & supervise pruning for certain categories of trees (OVT's, Mature Trees over a certain height/girth, Rare & Endangered Species, Champion Species, etc), & then train up the interested Arborists soonest (Join TCHK?). There appears to be no quick fix on it right away.

Has anybody any better solution?

best regards,

Sammy Au
Station Manager

The ISA Mission - Through research, technology, and education, promote the professional practice of arboriculture and foster a greater public awareness of the benefits of trees. (

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