Introduction -- Succession

These plants are not usually found in primary undisturbed forest. In South East Asia, the forests are characterized by the presence of dipterocarps, members of the family Dipterocarpaceae, which is known for its winged seeds and good timber. When these are cleared from the land, or when gaps appear in the forest, the plants which emerge to quickly take over the land are known as pioneers. As mentioned above, they tend to grow quickly as well as flower and fruit fast. They are also better dispersed than dipterocarps, hence pioneers can get to the open land faster than the dipterocarps. The exposed nature of gaps and clearings also is unsuitable for many primary forest plants, which have adapted to the humid and shaded climate of the understorey, that is the part of the forest covered by the foliage canopies of trees.

Eventually, the vegetation will recover from the clearing or disturbance, and revert to its original state. This process is known as 'succession', so called because different types of plants succeed each other as conditions change. Corlett described four stages of succession on degraded land in Singapore. In stage 1, the recently cleared and abandoned land is invaded by herbaceous and smaller woody pioneers and there is no distinct foliage canopy, in stage 2, the woody pioneers form a canopy over the ground, and eventually shade out the herbaceous pioneers, which need light for full growth. In stage 3, a transition occurs and the pioneers are slowly replaced by a different set of species, and this culminates in stage 4, the tall secondary forest. Previously, a distinction had been made between the so-called low and tall secondary forests, the former being shorter than 10 metres, and the latter being taller. (Hill, 1977) However, this separation is rather arbitrary. Stages 1 and 2 also appear to correspond with the old concepts of belukar muda ('muda' is Malay for 'young') and belukar tua ('tua' is Malay for 'old'). At present, Kent Ridge appears to be at stage 2 of Corlett's model of succession, being still dominated by the original species of pioneers.

The 'original state', or 'climax' that the Ridge belukar should revert to is the lowland tropical dipterocarp rainforest (Whitemore, 1975). However, this will take an extremely long time, as dipterocarps (as mentioned above) are very poorly dispersed. The seeds, although winged, can only fly up to a maximum of 100 metres (100 yards) in a very strong wind, and according to Ridley (1930), dipterocarps take about 30 years to reach maturity and start fruiting, hence in the most ideal case, they would take 58666 years to cover 100 miles of land, and this is discounting other threats to dipterocarp survival, e.g. the need to be tall to catch drafts of fast wind, and the possibility of the seeds being consumed by rodents. In order for the belukar to proceed beyond stage 2, new species of plants from the older forest need to recolonize the land. However, Kent Ridge is in the middle of the urban jungle, and the only sources of seed for these plants are several kilometers away in Bukit Timah and the Central Catchment area, the former being the only primary forest in Singapore, and the latter the most mature secondary forest. (Corlett, 1992) Hence, in this state of isolation, the future of Ridge vegetation is uncertain.

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