SINGAPORE - The inspection of trees in Singapore are in line with global standards, said National Parks Board (NParks) on Wednesday (Feb 15) in the wake of two separate incidents involving fallen trees.
Trees at high traffic sites, such as expressways and major roads, are inspected once every six to 12 months, more frequent than the International Society of Arboriculture's (ISA) guideline of 'high-risk' sites once every one to two years.
The 40m-tall tembusu heritage tree which collapsed last Saturday killing one was also inspected twice a year as it was located in an area with high human traffic, more frequently than most heritage trees which are inspected annually.
High human traffic on a tree's root zone could cause the soil to become compacted, and impede its the growth of its roots, said Mr Oh Cheow Sheng, group director of Streetscape at NParks, at a media briefing held at the Singapore Botanic Gardens.
The tembusu tree killed Indian national Radhika Angara, 38, after it collapsed, bringing down nearby palm trees along with it. Her French husband, Mr Jerome Rouch-Sirech, 39, and their one-year-old twin children - a boy and a girl - were also injured. A Singaporean woman, Ms Tay Pei Lei, 26, was also hurt.
NParks however said it could not comment on what cause it to fall as investigations were ongoing.
Giving a run through of NParks' inspection regime Mr Oh said inspections starts off with 'first level' visual checks where things like leaf-shedding and slanted trunks are looked out for and assessed.
If the arborist conducting the checks suspects internal decay, a 'second level' check where diagnostic tools such as a resistograph and sonic tomographs are used to confirm if this is so.
A resistograph is a tool used to drill into the tree's trunk. The resistance the drill meets is an indication of whether there is decay - decayed wood would offer less resistance.
A sonic tomography measures the speed sound travels through the wood. Sound travels slower through decayed wood than healthy wood.
The agency has assured the public that it has a comprehensive tree management programme that is also designed to cope with unpredictable and severe weather conditions.
Tree pruning is done before periods of heavy wind and rain such as squalls and during the north-east monsoon period.
NParks also trims the crowns of tall trees with dense crowns to improve their ability to withstand bad weather.
Mr Oh said NParks continuously refines its tree care programme.
It is for instance developing modelling techniques to better understand the structural behaviour of trees under varying environmental conditions like rain, wind and soil.
Though he noted: "But 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".
On Monday, a tree that fell in a Yuan Ching Road carpark sent a woman, Ms Cher Hwee Hwang, 48, to the Intensive Care Unit. The tree on the premises of student apartments was not managed by NParks.
