Flora Singapura
 

 
 
 
 
 
 

 
 
 

 
 

 
 
 

Freshwater Swamp Forest
Swampy stream in the Central Catchment Nature Reserve 
Freshwater Swamp Forest habitat is typically found on the low lying coastal alluvial plains and associated with river and stream systems. These Fresh Water Swamp Forests covered significant portions of lowland Sumatra, Malaysia, Singapore and Borneo and have suffered significant depletion through logging, sand mining, conversion to palm oil plantations and reclamation for other commercial uses.
Fresh Water Swamp Forests come about due to flat low lying land close to the coast becoming inundated due to rainfall runoff, inflow from surrounding elevated terrain and the tidal effects restricting outflow of water from the streams. In swamp forests, the water table is typically very close to the surface. This continuous inundation gives rise to a habitat that is floristically distinct from the surrounding Lowland Dipterocarp Forest that is characteristic of the South East Asian forest cover. 
Eleiodoxa conferta 
Over long periods of time swamp forests may transition into Peat Swamp Forests where the accumulation of organic materials gives rise to high levels of acidity which in turn limits their decomposition. The peat which is made up of partly decayed vegetative material will build up over a long period of time to form a mound several metres thick with its highest point in the centre and tapering off near the edges which are usually delimited by significant streams or rivers along with the characteristic riverside vegetation.
Stilt roots of Myristica spp

The Fresh Water Swamp Forest habitat is hostile to humans – the ground is soft and will not support your weight, the stilt and pneumatophore roots will deny you navigation and the armed Rattans and Pandan plants will deny you passage. The humidity brought about by the wet environment and tropical conditions will suck the energy out of you.
The trees that reside in the swamp forest exhibit different strategies for survival in the soft and water logged conditions, typical tricks include growth of buttresses, stilt roots as well as different types of pneumatophores. These vegetative characteristics can be used as an aid in the identification of species in the field. For example to reliably distinguish Alstonia spatulata from A. pneumatophora, one must look to the buttress form and presence or absence of pneumatophore roots in addition to the leaf characteristics which taken alone can be ambiguous. Different species variously produce none, one or up to all three of these specialized structures. The number of trees that produce all three - buttress, stilt and pneumatophore - are however a small proportion of the total and more typically you would expect one or two of these characters for most species. Once example of a species that produces all three structures is Syzygium papillosum. Just to throw a spanner in the works, some swamp or river allied trees will continue to produce stilt roots in dry hillside conditions, good examples of this include Dillenia grandifolia and Dillenia reticulata .

Swamp Forest Systems

In 1928 EJH Corner, a botanist specializing in fungus came to Singapore to take up the position of Assistant Director of the Botanic Gardens. Corner was a very enthusiastic botanist and he soon became interested in the dynamics and flora of the Fresh Water Swamp Forests in this region, ultimately spending almost all of his spare time studying the habitat in Singapore and Johor until the Japanese occupation put an end to his activities.
Upon retirement many years later he documented his findings and this work was published by the Botanic Gardens in 1978 as a supplement to the Gardens Bulletin under the title “The Fresh Water Swamp-forest of South Johore and Singapore”. It makes for a very interesting read, and though it is out of print now, the Botanic Gardens Library should have copies in their reference section.
As a result of studying the Sedili river swamp forests from the mouth at Jasons Bay up to the foothills of Gunung Panti (Peninsular Malaysia), Corner observed that the flora of the swamp and riverside forests were influenced by tidal effects, salinity, siltation, river mouth accretion, flood profile, illumination etc.  Corner distinguished the river side vegetation as distinct from the surrounding floodplain vegetation and identified eight categories of riverside vegetation naming them after predominant species that characterized these zones. As the zones proceed up-stream the diversity of plants increased as one would intuitively expect:
  1. Mangrove zone (Rhizopheraceae species) at the mouth of the river where salinity would be highest
  2. Nipa zone (Nypa fruiticans) on mud banks inside the river estuary - the water brackish
  3. Putat zone (Barringtonia conoidea) establishing in the current on submerged mud banks
  4. Rassau zone ( Pandanus helicopus)
  5. Mempisang zone (Polyathia sclerophylla, Elaeocarpus macrocerus, Myristica irya) on more or less permanent mud banks in tidal freshwater conditions.
  6. Jejawi zone (ficus micropcarpa)
  7. Tristania Banks (Tristania sumatranus) starting in the tidal region and extending up-river beyong the tidal region, the tristania banks tended to be steep river banks and over 100 individual species of plant were identified by Corner in this zone.
  8. Saraca Streams Zone (Saraca bijuga), dipterocarps and many other species along the banks of streams joining the river
Corner made the observation that as the sediments were deposited at the mouth of the river system - resulting in accretion - the zones defined by dominant or typical species also moved downstream at apparently the same rate. The various species of trees and shrubs growing in conditions that best suited them and dying out up-stream as optimal conditions reduce.
Corner then distinguishes the Fresh Water Swamp Forest as that which populates the floodplains adjoining the rivers and is subject to some extent to tidal influences as well as rain floods. Certainly during high tides the streams would be backed up and will overflow into the surrounding forest thus generating the conditions that make the habitat unique.
So it has been found that  Fresh Water Swamp Forest is not just a simple case of low lying ground becoming water logged – rather it is a dynamic system with many influencing parameters. In our quest for commercial exploitation of land resources, many of the influencing factors are altered, for example land clearing upstream will change the dynamics of flooding, siltation and nutrients deposited into the swamp system. Construction of water storage dams at the mouths of  rivers takes away the tidal influence. The effect of these changes will reduce the diversity of conditions and subsequently we can expect changes or worse reductions in the diversity of flora in this special habitat.
 

Swamp Forest in Singapore

Prior to significant commercial and residential development that Singapore has undergone since the early 1900’s, there would have been Fresh Water Swamp Forest systems associated with every significant river system in Singapore. These include Sungai Pandan, Sungai Kranji, Sungai Pongal and Sungai Seletar. Of these only a small amount of Swamp Forest remains protected within the Central Catchment Nature Reserve on a branch of the original Sungai Seletar system. The remainder has been either been inundated by the construction of municipal water storage reservoirs, or cleared for agriculture such as pineapple plantations then subsequently developed for housing and industrial uses.

Syzygium papillosum next to swamp stream
The Sungai Seletar drainage system was probably a typical river system in Singapore with catchment and head waters located in small hills covered in Dipterocarp forest. The river passes into the lowland area starting in the vicinity of the old Nee Soon village, an area where back mangrove species have been sighted, then onto the muddy estuary that would have hosted mangrove species. The Fresh Water Swamp forest would have followed Sg Seletar and its tributaries upstream from the Nee Soon village and been delimited by contour somewhere between 5 and 10 meters above sea level.
This river system is today dominated by the Upper and Lower Seletar Storage Reservoirs so not only has most of the original swamp forest been cleared and inundated, the tidal effects have been totally eliminated. the remaining swamp forest - now known as the Nee Soon Swamp Forest includes a small patch on a tributary of the Seletar river that has been protected by virtue of its inclusion within the Central Catchment Nature Reserve.

Eleiodoxa conferta  and Pometia pinnata on right
Today the Nee Soon Swamp Forest covers a number of sub-tributaries each flowing west to east and separated by higher ground hosting secondary as well as primary forest. These fingers of swamp forest are all that remains of this habitat in Singapore and as such most if not all of the species that reside exclusively in swamp forest are classified with a conservation status of Critically Endangered even though some may be considered common within the swamp forest habitat, examples of this would be Syzygium paillosum and Kopsia singaporensis.
There is great concern for the swamp forest habitat in Singapore due to the fact that several of the seed dispersal agents are present only in low numbers or have become extinct, this along with that the elimination of tidal influences due to downstream reservoir construction (Lower Seletar Reservoir)  and suspected seepage from upstream reservoirs (Upper Peirce Reservoir) results in significantly changed circumstances for many swamp forest species. So for example, the Myristicaceae family (Nutmegs) which produce fairly large fruits that were known to be primarily distributed by hornbills will now rely more
Sterculia cordata in flower
heavily on other animals such as rodents (squirrels and rats) to distribute their seeds. The acidity and amount of water is also likely to have changed favouring some species over others likely resulting in a change in makeup of the habitat. 
It is likely that the eight vegetation zones identified by Corner for the Sedili river system of Johore would have been substantially attenuated for the Singapore systems due to the shorter length of the river. Corner did in fact make a study of the swamp forest of Sg Seletar in the 1930's  and his activities here are fully documented in the Gardens Bulletin supplement "The Freshwater Swamp-forest of South Johore and Singapore". Interestingly the forest was at the time of his study being cleared for the small dam that was a precursor to the Upper Seletar Reservoir that we know today.
Stilt roots of Knema conferta
Corners  Mandai plots were located at a bend in the old Mandai road immediately north of the original Seletar Reservoir and just west of the old Nee Soon village. Two plots (A & B) were submerged by 1948 and Plot C is likely to have survived up until early 1960's at which time the reservoir was expanded to become the Upper Seletar Reservoir that we see today.
Corner has enumerated the tree species in that plot and this could be used as a baseline for comparing existing distribution of swamp species in the Nee Soon Swamp Forest since they both were part of the original Sg Seletar swamp forest system. The only weakness in this idea is that the area of this plot was small compared to the total area of swamp forest and Corner did not identify all trees to species level, Nutmegs for example were not fully identified, possibly because it is difficult to do so without fertile specimens.