The 40m-tall tembusu tree that fell and killed a woman at the Botanic Gardens in February was decaying from the inside, but signs of the rot had not been visible to inspectors, making it hard to predict that it would topple.
This was the evidence presented by tree expert Derek Yap yesterday at the coroner's inquiry into the death of 38-year-old Indian national Radhika Angara.
The arborist, who had examined the 270-year-old heritage tree after the incident, told the court that the tree was about 70 per cent decayed at the section where it broke off 2m above ground.
The decay would have affected the tree's structural integrity, said Mr Yap, who runs a private consultancy for trees and had previously been with the National Parks Board (NParks) for a decade.
Ms Angara, a regional digital marketing head at MasterCard, was with her family to attend an outdoor concert when the tree fell on her. She died in hospital from traumatic asphyxia and broken ribs. Her husband and twin children, and a Singaporean woman were injured.
Mr Yap suggested that the rot could have started with the roots, and raised the possibility that this could have set in as far back as 1859, when the roots were last cut. That was the year the Botanic Gardens was founded.
"Most likely, the root cut caused decay in the root which propagated in the trunk and worked itself upwards," he said.
The tree was last checked on Sept 29 last year.
Mr Yap said that typically, he would look for external signs suggesting tree decay, such as open cavities or the presence of mushrooms. "Once we see mushrooms, we would investigate further. Cracks may suggest that the tree was under abnormal stress in the trunk and may also indicate internal decay," he said.
But the tembusu tree that fell lacked these telltale signs. Mr Yap also said that roots below soil level could not be assessed. "It is my opinion that the tree failure in this case was unpredictable," he said.
State Counsel Kumaresan Gohulabalan asked Mr Yap that had he inspected the tree a day before it fell, would he have passed it?
Mr Yap said most likely he would have. He added that he would have asked for some pruning to reduce the risk of branch failure rather than to reduce the risk of tree failure.
Mr Yap said that when the tree fell, there was most likely localised increase in wind speed. Together with the asymmetrical canopy, it added to the load on the tree. "And... it increased to a point where it failed the trunk and caused the root to lift up and eventually caused the total uprooting of the tree."
Ms Angara's sister, father and husband Jerome Rouch-Sirech, a 39-year-old Frenchman, attended the inquiry and were represented by Senior Counsel Chelva Retnam Rajah.
Mr Yap agreed with Mr Rajah that if further, more invasive tests, which usually take place if there are outward signs of decay, were conducted, the unfortunate accident might not have happened.
Another witness to take the stand was arborist Richard Gordon Thomas, who has more than 30 years' experience in the industry. He said it was not known what caused the root decay. A slew of factors could have caused the tree to fall, such as strong wind, root decay and soil condition, he added.
The inquiry was adjourned to another day.
After the incident, NParks said its inspections were in line with global standards. The tembusu heritage tree that collapsed was inspected twice a year as it was located in an area with high human traffic, more frequently than most heritage trees, which are checked once a year.