SINGAPORE - The National Parks Board (NParks) carries out rigorous tree inspections every six to 24 months, with trees that are close to areas with high pedestrian or traffic activity receiving more frequent checks.

The tree inspection regime follows the best management practices set by the International Society of Arboriculture (ISA), Minister for National Development Desmond Lee told Parliament on Wednesday (March 3), after a woman was killed by a tree that fell in Marsiling Park last month.

"During a tree inspection, NParks' inspectors conduct a comprehensive visual examination of the tree crown, branches, trunk and roots to assess the tree's health and stability," said the minister.

"Trees that are found to have possible defects are subjected to an additional in-depth inspection, which involves the use of diagnostic equipment to assess the internal condition of the tree."

Mr Lee added that since November 2016, such in-depth inspections are also conducted annually for trees of more than 4m in girth, as a precautionary measure in response to changing weather conditions.

This goes beyond the ISA's prescribed best management practices, he noted.

The minister was responding to questions by Ms Hany Soh (Marsiling-Yew Tee GRC) and Mr Shawn Huang (Jurong GRC) on NParks' tree inspection regime, after a woman was killed by a 20m-tall tree that fell on her in Marsiling Park on Feb 18. The 20-year-old tree was found to be healthy when it was last inspected in April last year.

The minister said NParks has other measures in place to reduce the risk of such incidents, including targeted arboriculture treatments to improve general tree health and their ability to withstand severe weather conditions. Trees also receive regular pruning to remove weak, dry or overgrown branches, which improves their structure and balance.

NParks is also progressively replacing storm-vulnerable tree species with hardier variants and using technology to analyse risks and improve inspection processes.

It also offers training for NParks staff as well as tree care professionals in the private sector and other public agencies that manage trees.

Training is conducted by international and local arboriculture experts.

Mr Lee said training raises the standards of arborists here, who are also deployed under town councils and other agencies.

Staff of town councils and managing agents administering landscaping contracts can also attend training organised by NParks on plant health and other topics.

There are currently more than 620 arborists in Singapore who are accredited with the ISA. NParks also requires its tree maintenance contractors to have certified arborists in their teams, Mr Lee said.

While the number of tree failure incidents has been reduced almost ninefold over the last two decades, from 3,100 in 2000 to 339 in 2020, Mr Lee noted that it is not possible to completely prevent tree-related incidents.

"Trees are living organisms that can be affected by pests and diseases, as well as environmental conditions. Even healthy and structurally sound trees can fail during storm events due to exceptionally strong winds or heavy rainfall."

Ms Soh had also asked if NParks would consider measures like tree restraints and ensuring trees are placed in a way that would ensure they fall away from human traffic in the event of tree failure.

Mr Lee said NParks already uses tree support systems for trees that have reached maturity or have known structural weaknesses.

He gave the example of a lychee tree sitting atop Mount Rosie in the Novena area. The tree, which is over 100 years old, is supported by a permanent galvanised steel structure built to withstand elements.

But such systems can also constrain the growth of young or semi-mature trees or restrict their swaying movements during strong winds and may inevitably weaken the tree's structure, Mr Lee noted.

"NParks will study carefully what more situations we can use it (tree support system) in, but we also have to recognise that these structures can also pose safety issues if used without a care in park settings."

Mr Yip Hon Weng (Yio Chu Kang) asked how NParks works with town councils on tree inspections.

Mr Lee noted that NParks manages about 6 million trees in Singapore, while about a million are in areas under town council management. He said there are also trees managed by private property owners and other agencies.

"In relation to trees under the town councils, NParks works closely with agencies that manage trees, town councils and land owners to advise them on tree care standards."