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Official Magazine of the Nature Society (Singapore)
 

Plant diversity at Bukit
Timah Nature Reserve
Photo by Shawn Lum
Is Our Oldest Forest Dying?
Part 1 | Part 2

For many hundreds, perhaps even thousands, of years the primary forest of what we now call Bukit Timah Nature Reserve regenerated itself. Trees died of old age but the many birds and animals of the forest, along with the wind and insects, ensured that seeds were being continuously dispersed and therefore new trees would take the place of the old.

But today the reserve has shrunk to a small, isolated fragment and many of its birds and animals are gone. So the crucial question being asked is: 'What is really happening to the forest composition in Bukit Timah? Ayesha Ercelawn reports on a Tree Census covering more than 13,000 trees in a 2 hectare plot in Bukit Timah.
Bukit Timah Nature Reserve has, with the opening of the Bukit Timah Expressway, become a more isolated fragment of primary forest in Singapore, cut off from the patches of secondary forest that it was once vitally linked to. That many of its birds and animals have long disappeared is no surprise. But what has happened or is happening to the plants that grow there? We can speculate of course: it is probably hotter, perhaps also drier than it used to be in the forest and this is affecting moisture-sensitive plant species. Hornbills and other large birds are gone, so fruit that they dispersed now sit there and may not germinate successfully.

Seed sources from other forested areas are non-existent and the remaining tiny patches of primary forest in the Central Catchment Area are cut off, so while some plant species disappear, no others will re-colonise in their place. But what is really happening to the forest composition in Bukit Timah is a hard question to answer.

Trees are long-lived, so an individual tree may be around long after conditions suitable for its reproduction have changed. It is only by looking at exactly what species are dying and which species are recruiting (i.e., reproducing) successfully, that we can begin to get some answers.

The best way to do this is through an intensive study in an established permanent plot where a census of every tree in an area is repeated over the years. It was primarily for this reason that a two-hectare plot was set up in 1993 by Dr James LaFrankie and Dr Lee Sing Kong and it was a joint effort between the Center for Tropical Forest Science (CTFS) of the prestigious Smithsonian Institute in the USA and the National Institute of Education in Singapore (NIE). (CTFS has other large-scale permanent plots throughout the world engaged in similar studies).

These brightly coloured
forest nutmegs attract birds
like the hornbills which
help to disperse the seeds
Photo by Ayesha Ercelawn


Field researcher Kai Li
measuring a tree sapling
at the plot
Photo by Shawn Lum


Kai Li and Mislia
collecting data
Photo by Shawn Lum
The Bukit Timah plot is located just off the Catchment and Tiup Tiup Paths and it was at this location that our team of five converged for the second census in November of 1995. I had the help of four students from NIE, namely Mislia Supar, Ooi Kai Li, Tan Chuan Lim and Ng Lee Fui. Dr Shawn Lum (co-author of the book View from the Summit: Bukit Timah), set us forth learning the botany of Bukit Timah and also learning what field research was all about.

For me (coming from the temperate USA), it was a brand new encounter with the tropics and the incredible diversity present. I was suddenly faced with several hundred new trees species diversity I had never experienced before. Our plot alone, 200m by 100m equivalent to two hectares out of the 75 hectares that is Bukit Timah 1 Nature Reserve, has 321 species. Over the weeks all of us learned tremendous amounts. Some days Shawn or Jim LaFrankie were able to help us and they taught us a lot of tricks for telling species apart.

But we also had maps from the first census to help us locate the trees within the plot which is divided into squares of 5m by 5m. There is a permanent metal post embedded in the ground in each corner. Our job was to locate which square (or quadrant) we were in, measure the diameter of all trees within it, and map out any new trees that had appeared since 1993, and identify the new recruits.

We broke into teams with one person recording data and the others finding the trees, scrabbling through prickly rattan, locating aluminium tree tags, and measuring the tree diameters. Every tree that is larger or equal to 1 cm in diameter is tagged with a unique identification number, has been given a paint mark at a height of 1.3m from the ground, has had its diameter measured at that height, is mapped, and then identified as accurately as possible. We also tagged all new recruits (any new tree larger than 1 cm) and collected leaves for identification as well as later comparison with herbarium specimens at the Botanic Gardens.

Measuring with a Tree Hug!
We found that large trees, such as the serayas, were more than a meter in diameter and needed a tree hug, with two to three people helping wrap the measuring tape around. We measured more than 13,000 trees at the end of this second census. (A third census is now underway). By repeating this kind of census every few years a better picture can finally emerge about what species are present, what species are dying, what species are showing new regeneration, and whether or not there are any patterns to the species composition.

Working with my field assistants was an experience in itself. Some days I really wondered what they were getting out of it all. It was their November/December school holiday and I can think of better ways to relax! Chuan Lim simply walked around most days muttering "stress, stress". Yet she managed to look remarkably calm when it rained relentlessly and she worked at keeping the data sheets dry under her umbrella. Li Fui was another calm one, despite muddy slopes and missing trees. Kai Li, aside from getting furious at all the rattan, had the horrible habit of excitedly pointing out every single spider that she saw—an element of nature I can always do without. I finally learned not to turn when she called out. Without Mislia, well, we would probably have been lost. She had an uncanny ability to recognise landmarks within the forest and could take us back without fail to the place we had left off the day before. All of them contributed to my vocabulary as well so that by the time the census was over I not only knew what a tempines tree was, but also that a tree doing poorly was either "botak" or "died already" or just "dieded".

Ayesha Ercelawn is from the USA. Her field is environmental science and management.

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