Singapore, the Garden City, is home to more than two million trees.
With climate change causing extremities in weather, and a lack of rainfall depleting water levels in the Linggiu Reservoir in Malaysia - which helps to meet Singapore's water needs - it is little wonder the Government is encouraging more prudent use of water. This also extends to the care of greenery here.
The National Parks Board (NParks), which manages trees in Singapore's streets, parks and state lands, has been planting trees which are more drought-tolerant all over Singapore.
Over the years, it has drastically cut down on watering. It now waters only newly planted saplings and more sensitive trees, such as heritage trees, during periods of drought.
Some of the more drought-tolerant trees here include the yellow flame (Peltophorum pterocarpum) and illawarra flame tree (Brachychiton acerifolius).
"During the unprecedented dry spell in 2014, we observed that despite being subjected to great stress, most of our roadside plants and trees held up quite well," said Mr Oh Cheow Sheng, group director of Streetscape at NParks.
Water is a valuable natural resource in Singapore. Introducing drought- tolerant trees definitely contributes to water savings by reducing at least half of the irrigation water used for roadside landscape.
ASSISTANT PROFESSOR CHEN ZHONG from the National Institute of Education's Natural Sciences and Science Education Academic Group.
"The trees that were unable to weather the dry spell have been gradually replaced by hardier species." There are about 800 tree species across Singapore.
Assistant Professor Chen Zhong, from the National Institute of Education's Natural Sciences and Science Education Academic Group, said plants have adapted themselves to better withstand dry weather over the years.
Some of these adaptations include having a more advanced root system to increase the uptake of water as well as decreasing water loss by having small or curled leaves, or leaves covered with wax.
"Drought-tolerant roadside trees can keep alive for 30 to 60 days without water," said Prof Chen.
The yellow flame, illawarra flame and seashore mempari trees, for instance, need only one-third to half the amount of water required by trees that are sensitive to drought.
"Water is a valuable natural resource in Singapore. Introducing drought-tolerant trees definitely contributes to water savings by reducing at least half of the irrigation water used for the roadside landscape," added Prof Chen.
But while such trees can save water, it does not mean more is better, Prof Chen noted, and a balance must be struck between water conservation and preserving biodiversity.
"Imagine an extreme case... If one day, the whole of Singapore has only one type of the most drought-resistant trees, we will save much water but we will lose the majority of insects, birds and small mammals," he pointed out.
"And if there is a tree disease outbreak, we will lose all of them. I believe that NParks is cautious on this matter."
Besides drought tolerance, NParks also looks at a variety of characteristics when selecting species to be planted, said Mr Oh.
For instance, NParks replicates characteristics of natural ecosystems in urban Singapore, such as the structure of natural forests, as seen in Nature Ways.
Nature Ways are routes planted with specific trees and shrubs to facilitate the movement of animals like birds and butterflies between two green spaces. According to NParks, these routes also connect areas of high biodiversity to urban communities.
Illawarra flame tree (Brachychiton acerifolius)
A prolonged period of dry weather may result in the whole tree being covered with flowers. According to Assistant Professor Chen Zhong from the National Institute of Education's Natural Sciences and Science Education Academic Group, flowers not only serve as an aesthetic pleasure but are also a survival mechanism.
Lack of water poses environmental stress to plants and they may grow more flowers, for instance, to cope with the stress.
"Producing more flowers, and in turn more seeds, is one way that keeps the species better sustained over generations as seeds are well known for their durability over drought," he added.
The flowers of the Illawarra flame tree do not have petals but can bear fruit and turn a stunning scarlet.
They can be found in Commonwealth Avenue and Tampines Road.
Yellow flame (Peltophorum pterocarpum)
The fast-growing tree grows to 15-25m in height. The critically endangered tree, native to Singapore, is particularly fragrant at night. Trees will flower after four years and flowering usually occurs from March till May. They can be found in Cranwell Road, Loyang Avenue and Simei Road.
Seashore mempari (Pongamia pinnata)
Growing to 15-25m, this tree has medicinal uses. The seeds produce pongam oil, which has therapeutic properties. The juice of the leaves, shoots and bark is used as traditional medicine. In addition, its bark can be used to make string and rope, while the seeds and roots are used as fish poison.
They can be found in Science Park Road and Jalan Buroh.
Sources: NParks Flora and Fauna Web, Assistant Professor Chen Zhong from NIE's Natural Sciences and Science Education Academic Group