09 March 2009

Autumn colours on our shores

Shades of scarlet and gold are splashed among our green coasts everywhere I looked this weekend.These are the glorious colours of the Sea almond trees (Terminalia catappa) during their seasonal leaf fall. I could see them blushing all along the coastal forests of Labrador and Sentosa.

Why do leaves change colours?

Autumn colours is more well known for temperate forests, where trees shed their leaves as winter approaches in a spectacular mass display of vivid colours. The following is some information on the phenomenon in temperate forests. I'm not sure if the same thing happens in our Sea almond trees.

The green in leaves comes from chlorophyll, a key pigment involved in photosynthesis. Chlorophyll masks the colours of other pigments that may be present in the leaves such as pigments for yellow (xanthophylls) and orange (carotenoids). These pigments also help in photosynthesis but are not as delicate as cholorophyll.

Chlorophyll must be constantly replaced as it breaks down when exposed to light, in the same way that colored paper fades in sunlight. If cholorophyll replacement slows or stops, the other pigments in the leaves become more visible. These pigments, however, eventually fade as well, leaving tannin pigments which are brown.

Red and purple colours in leaves are due to anthocyanins, a water soluble pigment that becomes redder with greater exposure to sunlight. It is not known why this happens in autumn leaves. One suggestion is that the redness is correlated to the tree's health and resistance to insects, thus discouraging insects from laying their eggs on them.

In tropical forests, trees that drop their leaves rarely change colours the way the Sea almond does. According to Burkill, the Sea almond's habit is "peculiar among Malayan trees" and make the trees very conspicuous. He says such 'autumn leaves' are very rare in the tropics.
According to Corners, the tree sheds its leaves twice a year, in January or February and in July or August. Before falling off, the leaves turn vivid red, in a few cases, yellow. After the crown is bare, all the twigs develop new leaves and the tree is freshly green. The tree then flowers after the new leaves have developed. This habit occurs even in saplings 3-4 years old.
The Sea almond tree grows tall and has a typical pagoda-shaped growth form: it sends out a single stem from the top centre. When the single stem reaches a good height, it sends out several horizontal branches. Leaves large spatula-shaped (10-20cm long) arranged in a spiral at the tip of the twig.
Flowers tiny, white, many arranged on long spikes (10-12cm long). The flowers lack petals and only have a star-shaped calyx. Male flowers are found at the tips of the spike.
The fruit is almond shaped (4-5cm long), developing in clusters. The fruit has a thick, leathery, corky outer layer enclosing air cavities, with a hard thick stone in the centre. Inside the stone is a sliver of edible kernel composed of tightly coiled seed-leaves of the embryo. But this is difficult to extract. The fruit floats and is able to survive for many days in water, during which time the fibrous outer coating rots away.

According to Corners, fruit bats eat the husk of the fruit. I've also seen squirrels nibbling on the fruits. Besides bats, the fruits are also dispersed by monkeys and by water.

This large and beautiful tree has always been common along our coasts and is planted as roadside trees and in villages as shade trees throughout Malaya.

References
  • The Chemistry of Autumn Colours on the Science is Fun Site of the University of Wisconsin-Madison website.
  • The Science of Color in Autumn Leaves on the United States National Arboretum website.
  • 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.
  • Tomlinson, P. B., 1986. The Botany of Mangroves Cambridge University Press. USA. 419 pp.
  • 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.

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