Saturday, April 3, 2010

ISA HK/China --- HK Tree News (Homantin Tree Toppling and Wong Tai Sin Leaning Araucaria)

*** Our weekly Station Mail is for the information of Station Members only, but Station Mail has given up copyright & can be freely circulated. For administrative reasons, comments from outsiders are usually not entertained & may be circulated within our system locally & overseas. Please note Station Mail is sometimes given in Blog at , although images are usually not attached due to size. ***

Dear Station Members,

Attached are two sets of news sent in by Station Members recently:

(A) Homantin Ficus Toppling

A 30 year old Ficus toppled around midnight on 30.3.10 to cause property damage in a Homantin housing estate. No one was hurt this time. The incident was blamed on the construction work nearby to cause root damage, which is yet to be verified.

From the photo shown in the newspaper cutting, very little anchoring roots can be seen torn up around the base of the tree after the failure. This may suggest that root development had been poor all the way for this tree to stand up against Wind Loading. The lack of extensive root anchorage could have been caused by the soil compaction of the pavings around. This is a common planting practice in our territory.

Tree does not commit suicide. As long as water uptake & photosynthesis can be maintained by the minimum amount of absorbing roots available, tree canopy will expand despite not growing much anchoring roots. Then the tree may become top-heavy to stand in wind. When the public are applauding to the greening effect, the tree may topple in wind.

It is hard to detect & predict how a tree would stand up against Wind Loading with the common Evaluation Methods. Most Strength Formulae focus on stem breakage, not root friction. Profound knowledge in Wind Mechanics & Tree Biology may be the key for making decision.

How a tree spreads it roots is always hard to tell. Some might say deploying Root Detecting Machines may give the answers. However, most of these machines are designed to map roots in a forest setting, i.e. natural soil without compaction, not paved areas. In urban setting where paving is the identity of a city, radio waves may have difficulties in penetrating through hard surface & compacted layers. Besides, the underground utilities may confuse the readings between roots & conduits. Most Root Detecting Machines can not detect roots smaller than 1/2 inch diameter either.

Sonic Tomographs may not help for root detection either. They are designed to detect trunk & branch decay to a limited accuracy. However, most Tree Failures recorded by ISA HK/China appeared to happen with the roots , not trunk or branch. Planting large tree in a small hole appears to be the culprit.

Experience & knowledge of an Arborist may be the key to judge such a situation. Again, even an experienced Arborist does not have a crystal ball to tell when a tree would fail in what condition of Wind Loading. It remains an educated guess.

Most trees in public area of HK appear to be growing in paved environment. More Tree Failure as a result of it may arise. On prevention, proper Tree Selection & proper Aftercare (including proper pruning) may be the key. Trees should be in the hand of Arborists to stay sound.

(B) Wong Tai Sin leaning Araucaria

If compared to the Maryknoll Ghost Pine, this Wong Tai Sin Araucaria is younger as shown in the photo, although more leaning.

A resident complained to the media that this tree is becoming a Time Bomb, & action should be taken by the tree owner immediately. The politician commented by putting Public Safety as a priority, but treatment should also be given without delay if the tree has any conservation value. From these words, no clear & positive protocols are given as to what to do with the tree.

A scenario may arrive at this. If the tree is retained but fails later, the tree owner would get the blame. If the tree is removed for this & that reason right now, the tree owner may be labeled a Tree Murderer.

Then what can the tree owner do to stay proper ??

best regards,

Sammy Au
Station Manager

"If a tree is not designed, installed, maintained & inspected properly, it is likely to become a liability rather than an asset."

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

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