24 January 2010

Remarkable Rhu

The Rhu tree (Casuarina equisetifolia) is abundant on our shores.
Rhu seeds sprout in hot, open sand above the high-water mark and the young plants grow quickly, often form a thicket that eventually forms a Rhu forest.

The plants cannot settle under shade and thus form only on sandy shores that are advancing into the sea. This seems to be the case in the shoreline above, with a row of young Rhu trees growing in front of older ones. But on other parts of the shore, erosion is toppling over mature Rhu trees.
Further inland, I explored the Rhu forest. It is nice and shady, away from the open sunny grasslands that grow all around the forest. The Rhu tree harbours nitrogen fixing bacteria in nodules in its roots, thus allowing it to grow in apparently infertile areas.
Under the shade of the great tall trees, there is little undergrowth.
At first sight mistaken for a conifer (a non-flowering plant), the Rhu tree is actually a flowering plant. While the pine-needles of a conifer are true leaves, those of the Rhu are merely twigs, with the leaves reduced to tiny teeth. Photosynthesis takes place in these green twigs. The ground in the Rhu forest is thick with fallen twigs, forming a soft springy carpet that seems to suppress the growth of other trees and plants.
But the carpet of twigs does provide shelter for small animals like well camouflaged little moths. I've seen reference to Casuarina peat swamps, so perhaps the twigs eventually build up to form some kind of peaty ground?The countless tiny twigs of the Rhu tree is known for breaking the winds blowing in from the shore. The tree is in fact often purposely planted inland, not so much for its shade but more as a wind break. "While the wind may blow hats off on the shore", behind a depth of three Rhu trees, the air is "still and heavy", according to Corners.

Indeed, in the still air of the Rhu forest, there were lots of little animals clinging onto the fine twigs. You can see the reduced, scale-like leaves in the photo of the fat fly.
I saw this magnificent dragonfly perched on a Rhu twig. Tang later shared that it was a female dragonfly. He also added that the dragonflies seen on the water tend to be males who are protecting potentially good egg-laying sites. Females only come to the water to mate and lay eggs and otherwise rest elsewhere. A Rhu forest full of twigs seems a good place for a girl to hang out!
The twigs also seem handy for little creatures to create hiding places. I saw many different kinds of mysterious bundles of different constructions. While there were also several silken cocoons.
And this fuzzy caterpillar. I wonder what kind of moth or butterfly it will turn into?
But the most interesting creature among the twigs was this spider that looks exactly like a Rhu twig! Can you spot it?
It's a little hard to see when it has most of its skinny legs lined up along its long stick-like body. This one has one skinny leg sticking out.
Here's another one with a few more legs sticking out. The Whip spider (Argyrodes flagellum) is commonly seen in mangroves. But I've never seen as many as I have today in the Rhu forest.
It does actually have eight legs like other spiders. And a very long tapered abdomen. Here's one that's a little more lively. It's probably a male as it seems to have palps (the little appendages on the front) which are used to insert sperm into the female.
I only started looking for the spiders when I saw their egg cases. A tear-drop shaped structure. The spider doesn't build a web but uses a single line trap in a Y-shape. When it is disturbed, it falls to the forest floor but clings onto a silken line. It climbs back up when all is clear.
There sure is a lot to see in our coastal habitats!

  • Hsuan Keng, S.C. Chin and H. T. W. Tan. 1990, The Concise Flora of Singapore: Gymnosperms and Dicotyledons. Singapore University Press. 222 pp.
  • Tee Swee Ping and Wee Mei Lynn (eds). 2001. Trees of our Garden City. National Parks Board. 202 pp.
  • Wee Yeow Chin. 1992. A Guide to Medicinal Plants. The Singapore Science Centre. 160pp
  • 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.
  • Joseph K H Koh, A Guide to Common Singapore Spiders, Singapore Science Centre.

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