In the study of plant ecology, the adaptive features of plants in a competition-rich environment are often studied and considered. With the ever presence of strong competition for life-sustaining materials such as sunlight, water, nutrients and space, it is indeed a marvel for a great diversity of plant species, highly characteristic of a tropical forest, to exist. Due to the great differences in both the physical (biotic) and biological (biotic) environments, special adaptations to the environment must have developed to help these plant species through the struggles of the different phases of their life cycle. Each species of plant, thus, is evolutionarily primed and suited for a specific niche in the forest. This constitutes the niche theory. Specifically, in ascending order, the forest is broken into four layers: the forest floor, the understory, the canopy and the emergent layer1; humidity, light and wind intensities, nutrient and water availability are different across these four layers.
The study of plant ecology has brought us to Bukit Timah Nature Reserve on the 28th of October. At 164 hectares and standing 164 metres, Bukit Timah Nature Reserve is an important tropical rainforest in Singapore. Being the oldest and best-known protected area, a great diversity of plants in a mixture of primary and secondary forests can be found here. Unlike a temperate forest, there is more plant diversity at the nature reserve. In this field trip, we looked at a select few plants that correspond to the primary and secondary forest.
Bukit Timah Nature Reserve comprises primary and secondary forests. According to our tour guide Chua Siew Chin, both forests differ in the speed of growth of the trees present where the primary forest trees grow much slower compared to the secondary trees. A good example of a tree found in a primary forest at the nature reserve is Seraya, Shorea curtisii, an important dipterocarp of Singapore belonging to the Dipterocarpaceae family. As the name of the family suggests, Seraya produces fruits that contain two typical wings which allow the seeds to be dispersed by wind. Along with the other emergent dipterocarps in the primary forest, Seraya has the height advantage to allow efficient wind dispersal which is facilitated by stronger wind. However, an interesting fact mentioned is that these trees depend on animal dispersal of the seeds too. Because Seraya trees, together with the other dipterocarps in the primary forest, flower and fruit in synchrony during a periodic event called mast, a large amount of fruits is produced. A saturation effect is elicited with the excess of fruits and rodents, such as squirrels, bury the fruits that they cannot eat and eventually forget about them. With the help of these rodents, the seeds are now well-situated in the soil and will generate the next batch of trees in the primary forest away from the parent when the desired set of conditions is available for germination to take place.
Of course, there are pros and cons associated with each mode of dispersal for these dipterocarps. A disadvantage of both methods is wastage. As fruits are produced which are eventually lost by wind or eaten, it seems wasteful for the trees to produce these many fruits. After all, for the fruits to develop, energy is required. Nevertheless, with respect to the saturation effect mentioned above, there are more than enough fruits to ensure the successful dispersal of the seeds in both modes of dispersal. Furthermore, wind dispersal can also prevent progenies from growing too close to the parent tree. Hence, on the whole, the combination of both methods of dispersal plays an important role for the perpetuation of these dipterocarps.
Dipterocarps are entomophilous, or insect pollinated. Contrary to wind pollination, insect pollination is more advantageous when the insects, as pollen carrier, are specific in pollination. Not only are these insects specific for the dipterocarps they pollinate, they are also capable of travelling long distances to transfer the pollen they carry to a non-sibling tree. These are not possible with wind pollination, especially when massive amount of pollen has to be produced and wasted in the process. The trade-off, of course, is the production of nectar, scent and brilliantly coloured petals.
There are also other adaptive features for the dipterocarps. As informed by the guide, the morphology of the leaves is different at different growth stages. As the tree gains height, the size of the leaves reduces. This decrease in leave size serves an adaptive role for the adult plant which will eventually rise above the canopy and encounter greater wind intensity and sunlight than the lower layers of the forest. By having smaller leaves, the tree is able to withstand the shearing winds and at the same time, reduce the rate of transpiration. The latter is also prevented by mid-day closure of stomata in response to the intense heat which is a driving force behind transpiration. Cooling to reduce transpiration can also be achieved by the leaves that are whiter on the underside.
The secondary forest is different from the primary forest. Apart from the fact that the secondary forest trees grow much faster, it was also observed that it is less densely populated. For the secondary forest, we looked at Terentang (Campnosperma auriculata). With respect to the fast growth of Terentang, it is possible that Terentang grows taller and bigger faster leading to the eventual “shading out” of light demanders which depend heavily on sunlight for growth. This effectively inhibits the growth of these light demanders and kills them at the end.
Symbiosis with animals is crucial for the survival and proliferation of plants in the forests. The dipterocarps are one example of this. Another example is the figs species. As a keystone species in the forest, figs provide for the animals more since dipterocarps flower and fruit irregularly. However, in order for the figs to fruit, pollination by wasps is necessary. This method of pollination is extremely specific. Firstly, the figs provide a part in its flowering body (a gall) for female wasps to lay their eggs. This ensures the safe development of the eggs. In return, pollination is performed by the young wasps.
Macaranga bancana, too, employs a symbiotic strategy to protect itself from its many primary consumers. By providing nesting sites in its stem and nectaries, the plant enlists ant in its defence as the latter are encouraged to take up residence. In reciprocation, the ants are conscripted to protect the plant from any feeding animal such as caterpillars. Having mentioned that, the macaranga plant that we studied was still attacked by what seemed like a caterpillar. It is believed that some form of evolutionary trickery was performed when these ants that are enlisted for protection could be tricked not to attack the animal.
The symbiotic strategies outlined above involve mutual benefits. However, symbiosis also occurs when only one party benefits. The rattan plant is an example of such an interaction. Armed with spines on its leaves, the rattan is able to cling and climb up tall trees to obtain more sunlight. This does not benefit the host tree. The same strategy to rely on a host tree to gain height, and consequently sunlight, is also seen with epiphytic Bird’s nest fern and Staghorn fern, as well as the liana. Like the rattan plant, all three plants are commensalistic: they do not help the host tree, neither do they harm it. For the ferns to grow at the top of the tree and isolated from the ground, a novel mechanism, however, is adopted to derive nutrients and water to support growth. Each fern has a basket to collect rainwater and dead leaves. The liana, though, is a climber and will only produce the first set of leaves when it reaches the top of the tree.
Regeneration of the forest in a gap depends on the adaptations of the plants to the nutrient availability and their interaction with other plants on the land. It was seen at Bukit Timah that the distribution of the plants on the reserve is highly heterogeneous. Varied nutrient level is one factor to cause this heterogeneity. Normally, the nutrient cycle is maintained by dead fallen leaves and dead trees. Notwithstanding, due to the run-offs, the lower levels of the land, including the valley areas, are rendered more fertile. Hence, a greater density of plants is found at those areas. Besides nutrient availability, plant interactions can also prevent plant growth. As mentioned before, shade created by taller trees can in fact inhibit the growth of saplings. The germination and subsequent establishment of other plants can also be hindered by a thick cover of understory plants.
In conclusion, the survival of the fittest has produced the great diversity of plants that we see today. Through interactions with plants, animals and the physical environment, plants have adopted important adaptive features to establish themselves in unique niches. These interactions are sometimes highly intimate, as seen in figs. The removal of any one party could start an adverse chain reaction in forest ecology. Thus, it is of high importance for forest diversity to be preserved.
