SINGAPORE - The two trees that fell in Sembawang Park on Thursday night (Dec 20) which led to 14 people being injured were last inspected in May last year and found to be healthy, said the National Parks Board (NParks) on Friday.

The uprooted trees appeared to have fallen due to heavy winds, based on the direction of falls and their large sizes, said Mr Chuah Hock Seong, NParks' group director of parks, in a statement, adding that investigations are ongoing.

On Thursday, two mature trees were uprooted at around 5.30pm during an intense thunderstorm with gusty winds.

One of the trees, which was 21m tall, fell on a shelter in the park, injuring a group of SMRT workers who were having a Christmas gathering.

The group of 17 friends had sought refuge there after it started raining heavily, and 14 of them were taken to Khoo Teck Puat Hospital after the shelter collapsed.

As of last night, most of them had been discharged or treated for light injuries, such as cuts and head wounds requiring stitches.

Meteorological Service Singapore said that moderate to heavy thundery showers that fell in the Sembawang area on Thursday was the most intense between 5.20pm and 5.50pm.

The strongest recorded wind gust was 63kmh at Seletar station, about 6.5km away from Sembawang Park, it added in a statement on Friday.

For the year to date, the strongest recorded wind gust was 133.3kmh at Tengah on March 30.

The tree that fell on the shelter was a Erythrophleum suaveolens, commonly known as ordeal tree, which was about 3.1m in girth.

The other tree was a Tabebuia rosea, also known as trumpet tree, measuring about 18m in height and 3.6m in girth.

The area affected by the tree falls at the park has been cordoned off to facilitate tree clearance works, and the rest of the park is still open to visitors, said Mr Chuah.

NParks is the central agency for the maintenance of public greenery.

It manages about two million trees along streets, in parks and state lands.

Detailing the tree management measures it has taken, Mr Chuah said NParks has a "comprehensive" programme in place since the early 2000s, which includes "a rigorous regime of inspections and pruning, the use of specialised equipment, and professional certification of staff".

Tree inspections are carried out based on tree care guidelines by the International Society of Arboriculture.

Since November 2016, NParks has carried out advanced inspections of trees more than 4m in girth, and is currently developing modelling tools to better understand the behaviour of trees in different environmental conditions, said Mr Chuah.

"With the strengthening of our tree management regime over the years, the annual number of tree incidents has fallen by about 85 per cent from 3,000 in 2001 to about 400 as of December 2018, despite more intensive rain and stormy weather," he added.

He said that in addition to pruning and inspection of trees, NParks officers carry out checks when they do their routine rounds along streets and in parks.

"These checks are intensified during periods of adverse weather conditions to pick out trees for targeted tree pruning and crown reduction, or more detailed inspection," he said, adding that officers also check affected sites after any storm event to identify storm-damaged trees and take necessary mitigation measures.

He also said frequency of inspection and pruning ranges between one and two years, and varies with a number of factors, such as location, species, age and the condition of trees, with higher frequency for some trees and places.

He noted that this frequency is more stringent than the recommendations by the International Society of Arboriculture.

When The Straits Times visited the park on Friday morning, workers were seen clearing the tree and damaged shelter.

Mr Jeremiah Tan, 22, who is organising a youth camp for the Ambassador Baptist Church at the park for about 100 participants aged between three and 19, said the group will move indoors when it rains rather than seek shelter within the park.

The camp started on Tuesday and will end on Sunday. The group is staying at The Boys' Brigade and The Girls' Brigade campsite next to the park.

"We have been conducting our camps here every year for the past five to six years without trouble," said the National University of Singapore undergraduate.

"But after Thursday's incident, we will move indoors when the weather turns bad, as it seems the shelters might be unsafe too, especially for the children."

NParks said measures suggested in a coroner's report into the death of a woman who was hit by a falling 40m-tall Tembusu tree at the Botanic Gardens last year have been implemented.

This includes the taking of photographs during all inspections of significant trees, such as old and large ones.

Coroner Marvin Bay had said in April this year that such images would be useful in investigations "as an archive of contemporaneously taken photographs of the same tree can allow a more cogent analysis of the actual baseline health of the tree".