First sign of trouble - when trees shed leaves
The first sign a tree is sick is when it loses its foliage. When its trunk dries up, it means it could topple over soon, said Mr Ng Tze Peng, an arborist of 12 years, from TP Arbo Care.
Mr Ng, who was with the National Parks Board (NParks) for seven years, told The New Paper that if a tree has less than half of its foliage, it is an indication something is amiss.
"A healthy tree should not have sudden shedding of more than 5 per cent of its foliage."
The tree's roots, trunk and soil movement are also checked.
The trunk of a tree is checked for cavities, decay or wounds.
Mr Ng said: "A healthy tree has a firm and solid texture, but if there is decay, then the texture will be mushy and soft."
Fungus may grow in these cavities, or on the trunk itself. Root collars, the areas between the trunk and the roots, and anchoring roots that can be seen, are also checked for decay.
When the tree is suspected to be sick, second level checks are conducted. About 12 ultrasound sensors are placed on the trunk.
Using the soundwaves from the sensors, a digital three dimensional image is formed to display the internal cavities.
A sick tree will be cordoned off if it has dead branches or major defects and treated with fertilisers, nutrients and pruning. Ex-Singpore Botanic Gardens senior aborist Lahiru Wijedasa said there is a chance a sick tree may not show visible symptoms.
"If there are no visible symptoms, then no second level checks will be conducted. But these are rare."
NParks' group director of Streetscape Oh Cheow Sheng said first-level inspections for mature trees in the Botanic Gardens usually take up to an hour.
"When more detailed inspections are required, second-level checks may take a few hours to half a day... Trees will be thoroughly inspected for defects before we assess the next steps to take. This may include pruning. Removal of the tree will be considered only if there are safety concerns," he said.