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Habitat Management: the Strategy to Preserve the Biodiversity of the Singapore Botanic Gardens Rain Forest

Ruth Kiew & Lena Chan
Singapore Botanic Gardens, Singapore

Home | Contents | Abstract | Introduction | Table 1 | Table 2 | The Need for a Rehabilitation Programme | Table 3 | Strategy to Rehabilitate the SBG Rain Forest | Conclusion | Acknowledgements | References

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Abstract

The Singapore Botanic Gardens Rain Forest is a unique feature of the Gardens being a 4-ha remnant of the original primary forest that once covered Singapore. In the heart of the Gardens, today it still harbours 314 plant species including some forest giants in the Dipterocarpaceae. It is also home to ten species that were thought to have become extinct in Singapore.

However, its structure and floristic composition have not persisted without change. Habitat degradation threatens as canopy structure is fragmented following the death of emergent trees resulting in subsequent rampant growth of climbers and lack of regeneration of canopy species.

Earlier planting of aggressive exotics in the forest are now a problem, not to mention high squirrel populations that may be a factor in low regeneration rates. The first step in habitat rehabilitation is a long-term monitoring programme to enumerate trees and follow their flowering, fruiting and regeneration patterns, as well as active rehabilitation programmes, which include the removal of exotics, control of climbers, collection and germination of seed for enrichment planting and to restore the canopy in gaps.

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Introduction

The Singapore Botanic Gardens (SBG) Rain Forest is a national treasure, not only because of its rich biodiversity but also because its historic records stretch back to the last century when Ridley collected herbarium specimens from it. The rain forest is part of the origin site of the Botanic Gardens founded in 1859 and it was recorded then as primaeval forest and covered 11 acres (4.45 ha).

Burkill's statement in 1918 that "The eye of Sir Stamford Raffles...saw forest where we see it still" remains true today. Apart from reduction in size as the Gardens' developed (such as the siting of the collection of tropical shade palms beneath its canopy), it has persisted with minor disturbance until present when it now measures 4-ha in extent. The SBG Rain Forest boasts forest giants of a size rarely seen now in lowland mixed dipterocarp forest of Peninsular Malaysia, which have been subject to timber extraction, or in Bukit Timah Nature Reserve, Singapore, where many of the large trees were cut down during the Japanese occupation (E.J.H. Corner, pers. comm.).

The tallest trees in the SBG Rain Forest (Dyera costulata and Shorea gratissima both exceed 55 m in height and Koompassia malaccensis, Scaphium macropodum, Shorea leprosula, S. macroptera, S. pauciflora, S. ovalis and Terminalia subspathulata all exceed 45 m (Saiful Anuar, pers. comm.). These are ancient trees.

Biodiversity is extremely rich, the four hectares today harbouring 314 indigenous species (Turner et al., 1996). Many of these species (based on their status in the Singapore Red Data Book, Turner et al., 1994) are of conservation importance (Table 1). Some are extremely rare. For example, Memecylon cantleyi is known in Singapore from just two plants, both of which grow within the SBG Rain Forest (Tan, 1995).

Table 1. The conservation status of species currently growing in the Singapore Botanic Gardens Rain Forest.

Status No. Species Percentage
Extinct 10 3.2
Endangered 19 6.1
Rare 155 49.3
Vulnerable 74 23.6
Common 56 17.8

Table 1 requires some comment. Since the Singapore Red Data Book was prepared, several 'extinct' species have been refound, not only the ten (Table 2) growing in the SBG Rain Forest, but also others from elsewhere in Singapore too. However, it does mean that these ten species are extremely rare in Singapore and at present are known only from the SBG Rain Forest.

Table 2. The ten 'extinct' species which grow in the Singapore Botanic Gardens Rain Forest.

Species Family
Agelaena macrophylla Connaraceae
Alseodaphne nigrescens Lauraceae
Atuna racemosa Chrysobalanaceae
Diopsyros conferta Ebenaceae
Epipremnum pinnatum Araceae
Gnetum latifolium Gnetaceae
Horsfieldia irya Myristicaceae
Lasianthus tomentosus Rubiaceae
Mesua elegans Guttiferae
Pandanus tetradon Pandanaceae

If historic records are taken into account, a total of 448 indigenous species have been recorded from the SBG Rain Forest (Turner et al., 1996). This biodiversity compares well with the 900-odd species recorded from the Bukit Timah Nature Reserve, which covers 70 ha. In addition to its value as a site with rich biodiversity, the forest also contains many native species of economic value. Besides Koompassia malaccensis, the tallest tree species in Singapore, there are another 12 important timber tree species in the Dipterocarpaceae; the majestic Dyera costulata, which in past times was a valuable source of latex and is now in great demand for its wood for making pencils; the eaglewood, Aquilaria malaccensis is the source incense wood; rattans belonging to Calamus, Daemonorops and Korthalsia; many local fruit trees (Baccaurea motleyana, B. pyriformis, Flacourtia rukam, Lansium domesticum, Nephelium lappaceum, Sandoricum koetjape); nuts like Elateriospermum tapos; and medicinal plants such as Eurycoma longifolia, Labisia pumila and Scaphium linearicarpum and S. macropodum.

The latter two are both called in Malay kembang semangkok (fill-a-cup) as a fruit left in a little water for a short while will spectacularly fill the cup with its mucilage, which is used as medicine for coughs, asthma, dysentery and fever (Corner, 1988) and lastly Calophyllum lanigerum var. austrocoriaceus, which was only recently discovered as a possible cure of the HIV virus. (However, the population of this latter species in the SBG Rain Forest does not contain commercial quantities of the bioactive compound).

For the urban population of Singapore and tourists alike (and in 1994, 2.4 million people visited the Gardens of whom 44% were locals, Wong 1998), the SBG Rain Forest represents the most accessible rain forest as the Botanic Gardens are sited at the top of the main shopping street, Orchard Road. Here they can instantly experience the size and complexity of the rain forest and also to see in their natural settings these economically valuable species.

It is therefore a great resource to the Gardens, not only as an attraction in its own right, but also for its conservation and educational values (Phang et al., 1983).

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The Need for a Rehabilitation Programme

However, all is not well with the Rain Forest and it is timely that we embark on an active rehabilitation programme (Kiew & Chan, 1998). There are several reasons that demand this. The first is that there has been a drastic decline in biodiversity since Ridley made collections at the turn of the century. Turner et al. (1996) estimated that overall there has been a loss of 50.9 % of species. Analysis by habit, show that plants that grow in the undergrowth, (herbs and shrubs) are particularly vulnerable, as are epiphytes (Table 3).

Table 3. Species loss by habit. (after Turner et al., 1996)

Habit No. Species Lost Species loss (%)
Herbs 2 85.7
Shrubs 10 73.7
Epiphytes 2 66.7
Climbers 41 60.6
Trees 165 42.3

Possible causes for the high levels of species loss for herbs, shrubs and epiphytes may be due to a change in microclimate as a forest fragment is more subject to through wind, which lowers the relative humidy and raises temperatures in the undergrowth.

The microclimate is further affected by the canopy becoming fragmented by gap formation. (To test this theory, we are now monitoring the undergrowth microclimate).

Another cause for the decrease in understorey species is competition with exotics that were planted to 'brighten up' the rain forest. There are now 80 introduced species established in the rain forest, the most aggressive of which are herbs, such as Costus lucanusianum, Heliconia psitacorrum and Thaumatococcus damiella; climbers, such as Dioscorea sansibarensis, Thunbergia grandiflora and Tanaecium jaroba; and trees, such as Castillea elastica.

The lower level of species loss for trees is nevertheless cause for concern as the difference is probably due to their longer life span, i.e. they persist for much longer periods. Of great concern is the poor regeneration of these forest giants and the fact that more than half the tree species are represented by only one or two individuals.

Poor regeneration could be due to a number of reasons.

One is that populations comprise too few individuals to ensure outcrossing and so seed set is low, or the forest is too small to support pollinator populations or that seed predation by the large resident squirrel population consumes the seed crop. Tree regeneration is one of the problems that we have had to address. Poor regeneration of tree species is a particular cause for concern, as when they do fall they create a gap, which is not being filled by primary forest species. This means that the multi-layered canopy is becoming fragmented and this too has a knock-on effect in making the microclimate of the undergrowth more hot and dry.

In addition, we see that the trees on the edge of the gap are being covered by a curtain of climbers that becomes so heavy that the tree crown is brought down and the twiggy debris, often up to 30 cm deep, smothers any regenerating plants. For all these reasons, a rehabilitation programme is needed to halt and to reverse these detrimental changes to the Rain Forest.

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Strategy to Rehabilitate the SBG Rain Forest

The greatest priority is to restore a complete canopy layer. Although there is some evidence that secondary forest species do invade the gaps, they are not creating a complete canopy mainly because of the problem with climbers. It is therefore necessary to have a programme whereby the growth of these climbers is monitored and they are cut back before they become rampant or, in the case where they have already got a hold, they need to be cut back and the thick debris layer removed to enable more trees to become established.

To compensate for the poor seedling establishment, two strategies are adopted. The first and more urgent strategy is to plant up the gaps to speed the formation of a complete canopy layer by using saplings 3-4 m tall of tree species now or previously recorded from the Rain Forest.

These are saplings available in the SBG nursery that are of known provenance to be sure they are of Singapore origin so as to alter the gene pool as little as possible.

The second is enrichment planting, which involves collecting ripe seed from the Rain Forest to grow in the SBG nursery. This ensures that the saplings grow to good size and will have a better chance of survival before planting back into the Rain Forest.

Enrichment planting will increase the population size of poorly represented tree species. For particularly rare species, such as Memecylon cantleyi known only in Singapore from the two plants in the Rain Forest, we can tap the expertise of the specialists in the Gardens to propagate it by both conventional and micro-propagation.

Another on-going project to rehabilitate the Rain Forest is the removal of exotic species. A most successful programme was that to control Dioscorea sanibariensis, that not only grows rampantly but also produces abundant bulbils that ready detach from the mother plant and sprout (Teh, 1997).

Another project to remove aggressive herbs was carried out by students under the National Youth Achievement Award scheme and involved students in learning about the biology of the rain forest and its unique and interesting plants, the importance of conserving it, and most important how to distinguish exotic from native species, and to learn how to eradicate the exotics with minimum damage to the Rain Forest (Anon., 1992).

This is probably the easiest project to carry out, but it has to be long-term to ensure that the exotics do not become re-established. Borassodendron machadonis is a special case of an exotic, because it is an extremely rare palm from central and north Peninsular Malaysia, where it is know from only about five populations (Kiew, 1991). It was introduced into the Singapore Botanic Gardens early this century and fruits regularly. It appears that the fallen fruits are conveniently deposed of by being thrown into the Rain Forest edge, where they become established. Because of their worldwide conservation status, these palms should be relocated to a more appropriate place rather than exterminated.

The positive side of a project to remove exotics is that it can be tied to nature volunteer programmes for school or college students or for the public and which can create conservation awareness, particularly as it is a satisfying activity that produces an instant positive result and also encourages long-term involvement as the positive effects can be monitored over time.

We have already implemented a system to reverse previous over-development of the Rain Forest by removing excessive concrete paths and also by turning off the lighting at nights. This latter is to return the forest to its natural condition in the hope that the animal life will return to normal. (Another programme is monitoring wildlife).

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Conclusion

Singapore Botanic Gardens is indeed fortunate to possess such a unique natural heritage feature in the Gardens that serves as a focus for scientific, conservation and education programmes. By taking positive action now, we shall be able to maintain the present biodiversity and prevent further degradation so that it remains a resource for future generation.

Acknowledgements

The Rain Forest Rehabilitation Project is a collaborative project involving most units in SBG.

We thank the following for their help and support: S.C. Chin (Director, SBG), Cheryl Chia, P.T. Chew and Saifuddin Suran (Nature Conservation), Ali Ibrahim, Mohd Shah Mohd Noor, Samuri Ahmad (Herbarium), Ohn Set and Saiful Anuar (Horticulture), W.H. Wong and Alan Tan (SBG Management) and G.T. Choo and C.L. Lim (Research).

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References



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