Rooting out sick trees

Samantha Boh, The Straits Times AsiaOne 16 Feb 17;

Trees in Singapore are inspected in accordance with global standards and at times, the inspections are even more stringent, said National Parks Board (NParks) yesterday in the wake of two separate incidents involving fallen trees.

A 40m-tall tembusu heritage tree in the Botanic Gardens fell last Saturday killing one and injuring several others, and on Monday, a tree fell in a Yuan Ching Road carpark, sending a woman to the intensive care unit. The tree in Yuan Ching Road comes under the purview of the Singapore Land Authority, while the one in the Botanic Gardens was monitored by NParks.

NParks said checks on trees in high-traffic sites, such as expressways and major roads, are done once every six to 12 months. This is more frequent than the International Society of Arboriculture's guideline of checking "high-risk" sites once every one to two years.

Trees which are located in areas with high human traffic could also be inspected more frequently than once a year because high activity can cause the soil in their root zone to get compacted, which could in turn impede root growth.

There are 500 certified arborists in Singapore, of which 200 are from NParks.

Mr Oh Cheow Sheng, group director of streetscape at NParks, yesterday explained its inspection regime at the Singapore Botanic Gardens. Inspections start with "first-level" visual checks - where NParks arborists look out for things like leaf-shedding, cracked branches and slanted trunks in order to assess if more checks are needed on the tree.

If internal decay, for instance, is suspected, a "second-level" check is conducted using diagnostic tools such as a resistograph to confirm or contradict the suspicion. The resistograph is used to drill into the tree's trunk at a constant speed, and the resistance the drill meets is recorded and analysed. Decayed wood would offer less resistance.

NParks said it is developing modelling techniques to better understand the structural behaviour of trees under environmental conditions like rain, wind and soil quality.

Said Mr Oh: "Bear in mind that trees are living organisms, they are not engineered structures, they will react to changes in environmental conditions, site conditions and soil conditions." He added that "healthy trees can still be affected by strong wind gusts and heavy rainfall".

NParks declined to comment on the fatal incident as investigations are ongoing.

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