Too many variables to assign risk value to trees

Workers clearing away the 21m-tall tree at Sembawang Park after an incident which left 14 people injured.
Workers clearing away the 21m-tall tree at Sembawang Park after an incident which left 14 people injured.ST PHOTO: CHONG JUN LIANG

I do not think it is justified to spend resources on risk assessing and labelling trees for instrumentation monitoring (SMRT group taking refuge from rain when tree fell on shelter, Dec 21; and Think out of the box to tackle tree-falling problem, Dec 27, 2018).

Planting the right trees in the right place, not transplanting trees which are too big or planting trees too close together, coupled with relevant pruning measures should be budget-realistic and sufficient preventive measures.

Unlike buildings, plants and trees grow and adapt both above and below the soil.

In the urban setting, where trees and plants are installed by people, it is difficult to assess risk factors and assign a risk value to trees.

While one tree may not grow well on slopes, another may adapt well with its root structures even if above the ground it may look lopsided and structurally dangerous. Also, even with more arborists, we cannot predict how much stress trees can take in extreme weather events.

While we can apply assessment standards and testing protocols to building structures which are man-made, there are still many unknown variables to decide whether a healthy-looking tree is going to fall, and when it might.

According to papers on the Erythrophleum suaveolens - the species of the tree that fell in Sembawang park - a coastal, wet area is not the native habitat of this species. Rather, it thrives in the dry forest zones of West Africa, while its close cousin Erythrophleum ivorense is widely found in coastal wet evergreen forests. Perhaps, it was not suitable to have planted the tree too close to the coast.

The roots of an acidic-tolerant tropical forest tree species may not grow too well if the tree is planted in more alkaline soil, which may be the case of the soil next to the sea.

For long-term planning, the National Parks Board may want to take into account climate change forces along the coasts and the extreme weather events that are expected from time to time, and carry out a survey of the tree species that were planted by humans more than 20 to 30 years ago in man-made parks along the coasts.

A shared database on uprooted trees will also be useful for all coastal cities, so as to better plan the planting of trees.

May Liu (Ms)

A version of this article appeared in the print edition of The Straits Times on January 21, 2019, with the headline 'Too many variables to assign risk value to trees'. Subscribe