Tree species such as the Albizia (left) and the Andira inermis are kept on a special watch by NParks. Albizias have soft branches that are likely to snap. Branches of the Andira inermis tree are prone to breaking at old pruning wounds. -- PHOTOS: NPARKS & UNIVERSITY OF PUERTO RICOThere are trees and there are trees, with some species more prone to 'stress' and others more resilient. That is why the National Parks Board (NParks) keeps a special watch on species like the Albizia and Andira inermis.
On the other hand, species like the raintree have stood the test of time - as well as freak weather - and there is even a 'succession plan' to ensure that the older hardy trees have a chip off the old block, so to speak.
Regular inspection is NParks mantra, so that old and weak trees or those that are prone to falling are replaced or removed to minimise such occurrences. Weak trees are felled, with the wood and branches made into woodchips for mulch.
A team of 120 arborists carries out the inspections. 'Our officers would be able to recommend the point at which we fell some of the trees,' said NParks' director of streetscape, Mr Simon Longman.
Visual inspection and the use of technology like a resistograph, which detects internal defects in wood, help the arborists in making these decisions.
While NParks is careful to plant trees that are suitable for Singapore's environment, it still has to do regular work to look for and replace old and weak trees.
NParks maintains more than a million roadside trees.
The Albizia poses a particular problem as it grows wild and is not a species that NParks plants. There is an ongoing culling programme to remove this species.
NParks works with other agencies to identify and remove trees in poor condition, or in close proximity to areas with public access, Mr Longman explained.
Albizias have fast-growing soft wood, with the branches likely to snap. About 650 Albizias have been removed in the past three years.
In 2007, a 43-year-old woman was crushed to death by a falling Albizia tree at Bukit Batok Nature Park.
The Singapore Land Authority and NParks had earmarked 63 Albizia trees in South Buona Vista Road to be cut down after the incident, but it sparked an intense lobbying effort by tree lovers.
Eventually only 13 were felled.
NParks has also culled almost all Andira inermis trees over the last six years due to problems of branches snapping at old pruning wounds.
Hardier species such as the raintree are NParks' preferred choice of trees.
The mahogany and Yellow Flame are also resilient, Mr Longman said.
'Trees here have to be able to withstand tough conditions, the traffic running by, all the fumes and the heat building up into the branches of the tree.
'We choose trees that can withstand those kinds of conditions well,' he said.
Mr Longman also elaborated on the succession programme for trees.
'Our process now is to have something already growing in between these (older) trees, so that when the time comes to have to fell them because they're declining, those new interplanted trees will be ready to provide the greenery on the roads.
It's a succession situation,' he said.
Mr Veera Sekaran of landscape firm Greenology agreed with NParks' choice of trees, saying that 'they've stood the test of time'.
'These trees have been around for many years and they were selected because studies have shown them to be safer.'