14 March 2009

Blooming Rhu

The Rhu or Casuarina trees (Casuarina equisetifolia) were blooming!
While the other photographers were doing shoots of the Southernmost point of Singapore (accessible to the public) on Pulau Semakau, I took the opportunity to take a closer look at some of these trees there.

This tree with delicate needle-like twigs is commonly seen on our shores. Many of us are familiar with the durian-like cones that cover the ground on these shores. Especially when we step on them with bare feet...ouch!
The Casuarina is NOT a conifer. While the pine-needles of a conifer are true leaves, those of the Casuarina are merely twigs, with the leaves reduced to tiny teeth, arranged in whorls of 6-10 at the joints of twigs. The 'needle-twigs' are greenish and photosynthesis takes places in these twigs instead of the tiny leaves.

The trees grow very fast. Mr Yu who was kindly driving us around today, shared that he had planted the Rhu trees on the Southernmost point, transplanting them from where they grew wild in the cells of the Semakau Landfill (it's so nice to know the NEA officers rescue tree growing in the cells). He said they grew more than a metre in one year! This ability to thrive in nutrient poor sandy soil is attributed to the fact that the Casuarina harbours nitrogen fixing bacteria in nodules in its roots.

While conifers are non-flowering plants, the Casuarina tree is a flowering plant. Male and female flowers grow on separate trees. The tiny male flowers are held on short spikes (1.5-3cm long). Female flowers form pink tufts on stalks.
The flowers are wind pollinated. These form into cones that are at first green, turning brown. When ripe, the bracts on the cone open up, releasing small winged nuts. The cones are also dispersed by water.

In the wild, the trees cannot settle under shade and thus form only on sandy shores that are advancing into the sea. The seeds sprout in hot, open sand above the high-water mark and the young plants often form a thicket that eventually forms a Casuarina forest. In a suitable spot, the tree grows rapidly. In Singapore, according to Hsuan Keng, these trees were probably wild originally between Tanjung Rhu and Changi. Tanjung Rhu has since been reclaimed and the East Coast Highway now lies on the reclaimed land.

There are many species of Casuarina. The most common species being Casuarina equisetifolia (which some say should be called C. littorea) which has narrow, unbranched needle twigs, while the other species have branched or thicker twigs. Originally from India, the Pacific Islands, northeastern Australia and throughout Southeast Asia. They have also be introduced in many other countries.

The tree is also often planted inland, not so much for its shade but more as a wind break. According to Corners, "while the wind may blow hats off on the shore", behind a depth of three Casuarina trees, the air is "still and heavy". This is attributed to the fine twigs that break the wind.

According to Burkill, the tree was planted where it was desired to allow the soil to dry, as well as to check erosion and to fix drifting sand. It was also planted in India as firewood. Some consider it the best firewood as it will burn even when green. The timber is stronger than teak but splits much. It is sometimes used for beams and rafters, as well as for masts and other heavy duty uses. The bark is used to treat dysentery and diarrhoea, the twigs to treat swellings. According to Giersen, it is also used to make charcoal. The bark yields a resin that is useful for tanning.

  • Hsuan Keng, S.C. Chin and H. T. W. Tan. 1990, The Concise Flora of Singapore: Gymnosperms and Dicotyledons. Singapore University Press. 222 pp.
  • Corners, E. J. H., 1997. Wayside Trees of Malaya: in two volumes. Fourth edition, Malayan Nature Society, Kuala Lumpur. Volume 1: 1-476 pp, plates 1-38; volume 2: 477-861 pp., plates 139-236.
  • Burkill, I. H., 1993. A Dictionary of the Economic Products of the Malay Peninsula. 3rd printing. Publication Unit, Ministry of Agriculture, Malaysia, Kuala Lumpur. Volume 1: 1-1240; volume 2: 1241-2444.
  • Giesen, Wim and Stephan Wulffraat, Max Zieren and Liesbeth Scholten. 2006. Mangrove Guidebook for Southeast Asia (PDF online downloadable). RAP publication 2006/07 Food and Agriculture Organization of the United Nations Regional Office for Asia and the Pacific Bangkok.

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