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Listening to trees to care for them better: The work of an NParks arborist

About 200 National Parks Board (NParks) staff look after some two million trees in public places around Singapore, including along roads and streets, fields and over 300 parks. Here’s a look at how they carry out their work.

Listening to trees to care for them better: The work of an NParks arborist

NParks arborists Robin Ong and Clarice Xue carry out advanced-level inspections on a "small number of trees" that require it in Singapore to keep them in the pink of health. (Photo: Marcus Ramos)

SINGAPORE: “A lot of trees hold stories that they can’t tell you,” said National Parks Board (NParks) arborist Robin Ong.

“For example, 'I’ve been hit by a car 20 years ago and therefore I have this defect at my feet',” he added.

The 33-year-old is one of 200 arborists or tree doctors employed by NParks to manage about two million trees growing in public spaces around Singapore.

“They can’t speak (so) we need to come in and speak on their behalf to ensure that they have all the care they require,” Mr Ong told Channel NewsAsia at a field along Evans Road where several towering rain trees are being checked.

Finding out the story of a tree is a two-step process. First, a basic tree assessment followed by an advanced assessment if there are signs of damage.

A basic tree assessment is like a routine body check-up. Aside from a visual assessment at ground level, Mr Ong would record the vital statistics of the tree such as its girth and height.

“There are certain things that we look out for like dead branches because dead branches will decay and possibly fall during a high wind event,” said Mr Ong.

A tablet with a drop-down checklist helps him with the basic tree assessment. It also contains a database of the tree's history and condition when it was previously inspected.

They also use basic tools like binoculars, cangkul, a tree probe and measuring tape to get a better understanding of the tree. (Photo: Marcus Ramos)

The inspection takes about about 15 to 20 minutes.

“If you multiply this by 500 (trees) a month, that is what our colleagues in certain divisions have to do. 15 to 20 minutes, this is a concise version,” he said.

Trees that do not require advanced-level inspections are checked every 12 to 24 months. This is more frequent than what is recommended by the International Society of Arboriculture, which is every one to five years.

A hole was discovered near the base of the tree during the inspection. Using a changkul, Mr Ong exposed the hole for a closer look to determine if action is needed.

“So it’s a 40-cm hole. The tree is about more than 2 metres in diameter. Comparing 40 centimetres to 2 metres, it really isn’t a lot,” he said.

In these instances, the arborists will use advanced assessment tools such as a resistograph to check for decay.

A needle is drilled into the tree and the amount of resistance felt is recorded. This tells him how dense the wood is. When there is decay, fungi would colonise the wood and make it less solid leading to cavities.

“They paint a fuller picture of what’s beyond the surface,” Mr Ong added.

“Speaking from my own experience, we spare no effort … if there are any defects, we assess it and give it a certain likelihood of failure. If this likelihood of failure is beyond the threshold that we are comfortable with or beyond the management of the tree, you have to mitigate it sooner,” he said.

“Like a tree in the middle of Orchard Road, obviously, you wouldn’t want a dead branch hanging over there for five weeks,” he added.


Arborists also use sound wave tomography to check for cavities within the tree trunk. It is similar to how an ultrasound scan works.

Mr Ong sets up a detection system around the circumference of the rain tree's trunk that will work as “ears” to listen out for soundwaves. His colleague strikes a part of the tree with a mallet to send sound waves through its trunk.

By striking the trunk with a hammer, sound waves will travel through the trunk to be picked up by devices monitoring for activity. If it travels slowly, it could be due to the presence of a cavity. (Photo: Marcus Ramos)

The sound waves travel faster if the trunk is dense and solid. Soundwaves travelling slower through parts of the trunk suggest there could be a cavity within.

Mr Ong said he is not too concerned of the tree's condition but suggests a more frequent inspection cycle.

“Every year, this guy will probably get two inspections minimum which is by certain standards excessive,” he said.

To get to nooks and crannies that are hard to get to, NParks arborists fly small drone cameras that will allow them to look at the spot real-time. (Photos: Marcus Ramos)

“As you would imagine, data alone can only give you this much. It’s up to you to interpret it and tell the story for the tree and whether or not based on the site conditions, targets, based on many many other things whether to retain the tree or not,” he said.

Sometimes Mr Ong and his colleagues like Ms Clarice Xue scale trees to get a closer look to help them understand what is going on.

Nothing beats getting “up close and personal with the tree to understand what is going on”, said Mr Ong. In fact, he said many within the arboriculture industry in Singapore are certified tree climbers. 

They undergo a tree-climbing course that teaches them to scale trees without hurting the tree or the climber. 


Mr Oh Cheow Sheng, group director of streetscapes at NParks said that one of the challenges unique to Singapore is changing weather patterns.

“We are getting more severe and intense storm events. A combination of heavy rainfall and strong wind gusts can cause damage to our trees,” Mr Oh said.

Last year, there were several instances of trees falling in a storm. An 89-year-old man was killed by a falling tree branch at North Bridge Road. 

READ: 14 taken to hospital after 2 trees uprooted at Sembawang Park

READ: Two cars damaged by fallen tree near Dawson

READ: Fallen trees on TPE block traffic on Saturday morning

Since 2016, NParks has switched its approach to help trees stand a better chance against adverse weather. It makes sure that tree canopies are pruned before the onset of monsoonal periods to make them less top-heavy.

At the same time, trees with girths exceeding 4 metres are given advanced checks on an annual basis.

According to Mr Oh, this reduced falling tree incidents by 85 per cent from 3,000 cases in 2001 to about 400 cases in 2018.

“Most trees are healthy and they do not require advanced checks. … It is a very small number of trees that require that kind of attention,” he said.

“Trees that require advanced level checks simply means that they have some kind of defect that we want to monitor. It does not mean that the tree is not safe,” he added.

With the changing weather patterns, Mr Ong said that his workload in the last few years has been “ridiculous”. Weekends are sometimes spent writing reports and visiting sites to monitor the human traffic around the tree for a better gauge of the potential impact if the tree were to fall, he said. 

“If you consider the urban environment, especially in highly-populated places here there are a lot of buildings and narrow channels, wind picks up. Any tree, given the right amount of wind load is just going to be picked up and toppled,” he said.​​​​​​​

"If a tree falls somewhere then our workload suddenly spikes up because of reactions by the public. It has really been ridiculous for the past couple of years. A lot, a lot of work (due to the changing weather conditions)," Mr Ong added. 

Aside from the weather, there are other factors beyond the control of the arborists.

"When a development happens right next to the tree, the pH value of the soil becomes more and more alkaline. These are changes that you are not able to mitigate because it's soil. You can't just remove all the soil around the tree and replace it," he said. 

“Trees and construction, it’s a very real thing in Singapore especially because of space constraints and there is construction everywhere,” he added.

Alkaline soil conditions could affect tree health.

Given the "ridiculous" workload, Mr Ong wishes that more people would be interested in the vocation. 

“It’s not an easy science. It’s never enough. We need more people interested in outdoor work. Right now, Singaporeans don’t seem to be very keen on working outdoors,” he said.

Source: CNA/fs


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