Only 2,000 hectares of forest area remains in Singapore and much of it is found in the Central Catchment Nature Reserve and the Bukit Timah Nature Reserve. This amounts to only 3% of primary rainforest left, which is a far cry from the lush vegetation we once had in 1819. Our tiny island was once covered by tropical rainforest which was later replaced by cash crop plantations in 1834 when the agriculture scene started to take off. Large regions of forests were cleared to make way for plantations and to meet domestic needs for timber, charcoal and fuel. Consequences of the deforestation were severe, an estimated 95% of natural habitats were lost and 28% of natural species in the rainforest went extinct.
Image source: Gambier plantation (left), Pepper plantation(right)
Gambier and pepper plantations were the most common during that time due to high demand by dyeing and tanning industries. By the late 1840s Singapore had over 400 of these types of plantations.However these plantations had a limited lifespan of 20 years after which, the land would be infertile and thus rendered useless.
It is difficult to replace the loss of primary rainforest as these types of forests are untouched by mankind and have complex natural structures. There are multiple layers and these layers sometimes overlap each other, forming an intricate web of vegetation. The unique dynamics of these layers produces microclimates which allow a wide diversity of plant species to co-exist. 
Layers in a rainforest: Source
However just because it is tricky doesn’t mean we wont try! NParks has played an active role in reforestation of parks and nature areas in Singapore since 1991.Reforestation refers to the act of planting trees in regions which used to be a forest. Through its habitat enhancement efforts, NParks seek to restore damaged lands into a second forest. The planting of secondary forests could help to replace some of the biodiversity that has been lost and preserve what is left of our natural heritage.
The objective of NParks reforestation program is to restore secondary forests with a substantial aspects of primary rainforest integrated within it. One of the techniques employed is to stagger the planting of primary forests species alongside fast-growing native species. This ensures a forest canopy cover is established which creates a microclimate that is conducive for shade-loving species to grow, thus contributing to a wide diversity of plant species. Fruit trees are also introduced to attract insect and animal pollinators.
These are some of the species used in reforestation! 
Source
The NParks habitat enhancement efforts seeks to improve the quality of our local parks and almost 18 parks have undergone this reforestation program. You may be familiar with some of these parks:
- Kent Ridge park
- Telok Blangah Hill park
- Fort Canning Park
- Punggol Park
- East Coast
View the full list here!
These parks were selected to act as buffers to protect and strengthen core regions of nature reserves which lie in close proximity. The parks also serve as stepping stones to promote ecological connectivity. NParks regularly carries out replanting efforts by collaborating with Non-governmental agencies, private corporations, students and members of the public. These volunteers get a chance to plant their own trees, assist with clearing weeds and concurrently gain greater insight about the importance of conserving forested areas.
Forest Restoration Action Plan
Just this year, on January 25, NParks unveiled the new 10 year plan that aims to restore the secondary forests buffering Bukit Timah Nature reserve and Central catchment Nature Reserve. This Forest Restoration Action Plan seeks to regenerate damaged patches in these two major nature reserves that houses 40% of our native plant species. NParks hopes to boost the maturity of our forest to firstly enhance its biodiversity and secondly to protect it against climate change. Some strategies that will be implemented are:
