Tree failure at Botanic Gardens unlikely due to lack of space : Former NParks arborist

Workers are seen clearing branches and cutting up the trunk of the heritage tembusu tree on Feb 12, 2017, one day after it fell.
Workers are seen clearing branches and cutting up the trunk of the heritage tembusu tree on Feb 12, 2017, one day after it fell.ST PHOTO: SEAH KWANG PENG

SINGAPORE - The 40m-tall tembusu tree that on Saturday (Feb 11) uprooted and killed a 38-year-old woman and left four others injured was unlikely to have fallen due to the lack of space, said Mr Lahiru Wijedasa, a former senior arborist at Singapore Botanic Gardens.

The 270-year-old tembusu tree had already been there when the Gardens was established in 1859, Mr Lahiru told The Straits Times.

"It has been standing in the Gardens for more than a century since the Gardens was built. If its roots had not been able to adapt to the development, we would have seen tree failure ages ago.

"If restricted rooting was the issue, the tree would have long adapted, as there hasnt been any developments around the tree except building the road 160 years ago," said Mr Lahiru, who is now pursuing a doctorate at the National University of Singapore.

Mr Lahiru, who was a senior arborist at the Gardens between 2006 and 2009, was responding to speculation that the tree had failed because it did not have enough space to spread its roots.

He urged members of the public to await the results of final investigations and not to jump to conclusions, adding that he had decided to speak up to clarify some of the speculations raised since the incident happened.


On suggestions that the tree might have been affected by the recent winds and rain, Mr Lahiru said the weather conditions could cause a "compromised" tree to be uprooted.

"But like a chair with four legs, one of which has been hollowed out by termites, there may not have been visual clues.

"The point is that even if measures are taken to reduce the risk by 99 per cent, there is always a one per cent chance of something happening," Mr Lahiru said, adding that NParks has done its due diligence to ensure tree health.

On another suggestion of having mature trees inspected monthly, Mr Lahiru said this may not help in identifying the cause of the problem, if symptoms did not manifest in other parts of the tree.

He pointed out that there is already a system in place to look out for such red flags. The National Parks Board (NParks), which manages the Singapore Botanic Gardens, does detailed, twice-yearly routine inspections, in addition to the daily visual checks by arborists patrolling the grounds, he said.

NParks Commissioner of Parks and Recreation Leong Chee Chiew said in a statement on Sunday that it adopts a systematic regime of inspection checks based on the tree care standards prescribed by the International Society of Arboriculture in the United States, which is of the highest international standards.

A 2015 document on NParks' website said such Visual Tree Assessments are a non-invasive method of evaluating diagnostic symptoms of internal defects and measuring probability of tree failure.

"Further investigations using diagnostic decay instruments will sometimes be required to quantify these defects and determine if a tree actually poses a hazard," said the document, which The Straits Times has seen.

Mr Lahiru explained that when a tree fails due to defects either in its crown, trunk, or roots, there are usually symptoms. These could include having a receding crown, leaves turning colour, or more leaves falling off than usual.

Crown or trunk damage can be assessed easily as they are above ground. If there is a termite infestation, for example, there would be track marks. Crown failure due to excessive branching is also prevented by NParks' practice of regular pruning.

But even if the roots are damaged, by rot or fungal infections, symptoms would be manifested above ground, by dying leaves, for example. This is because the roots of trees take in water, which is vital for the nutrition of the plant and survival of the whole tree, he said.

"In this case, there did not appear to have been such signs," said Mr Lahiru. NParks had determined the tree healthy during its last routine inspection in September 2016.

If a red flag had been detected, the next step in tree inspection would be to use technology such as ultrasound scans. But this is not the first line of defence for arborists, as such scans are invasive - they would require metallic sensors to be drilled into the trunk. Doing so could introduce viruses and fungus into the tree, said Mr Lahiru.

Even if no red flags are detected, there are measures in place to ensure tree health, Mr Lahiru said.

NParks has said it conducts routine mulching to supplement the regular application of fertilisers, as well as pruning techniques to improve the structure and balance of trees.

The Board has also since May 2016 taken additional steps in tree pruning, such as by undertaking crown reduction and pruning prior to periods of more severe weather conditions.