Saturday, September 6, 2008

Hurricane-proof.

Bermuda Palmetto, Sabal bermudiana.

The official symbol for a hurricane is a red washing machine agitator. The unofficial, more dramatic symbol, is a palm tree, its wiggery almost tearing off.

I was in Bermuda during a heavy summer storm, not a hurricane, but a good blow nonetheless. I stood in a sheltered doorway and watched a group of the native palms—the Bermuda palmetto, Sabal bermudiana, as they thrashed about. I watched the behaviour of the leaves, which seemed to bend somewhat, then rapidly twist to the side. A Douglas fir, a tree prominent in my current locale, and so different from a palm tree I’m not even sure why I’m mentioning this, is nowhere near as animated in a windstorm. The branches sway back and forth as the wind moans through the thick mats of needles. Eventually, if the wind gusts luck onto the harmonic frequency of a bough, it explodes from the tree near its base and falls to earth, shuttlecock orientation, and plants itself in your lawn—instant tree. (Kind of cool, and probably why I mentioned it.)

But this behaviour is nothing like that of palms, which lack boughs, whose leaf petioles are their branches, although it probably isn’t correct to call them petioles (the stems of leaves) either, but it's too late now. The petioles are celery stalk-shaped in cross-section, concave surface up. They are widest at the attachment to the “trunk,” and taper to the leafy tip. The concavity is most pronounced proximally, decreasing distally.


I took the picture for the lizard of course, a Jamaican Anole, but it gives a view of the upper (facing) concave surface of a Sabal petiole.

In that doorway in Bermuda I wondered if anyone had done a functional morphological study of palm leaves and wind. Recently someone has. Referring to palm petioles, Windsor-Collins et al. (2008) state:

“The petiole has to be both stiff in order to maintain the blade in a position to intercept the sun’s rays as well as flexible in order to withstand the external forces of the wind, rain and snow. The vascular system also has to remain intact during torsion if the plant is to survive. A certain amount of bending and torsion ability is required to withstand the environment and also gain sunlight without breaking. Generally, it was found from a shape factor perspective that the mid and distal parts of the petioles showed greater resistance to bending than torsion meaning that their shapes were easier to twist than to bend. It must be remembered that the resistance to force values may only be attributed to shape.”

It bends a bit, but then quickly twists to lessen the stress that would otherwise snap it from the plant. The leaves of these palms are adapted (in the cross-sectional shape of the fronds/petioles/insert correct term) to withstand destructive winds and pounding rain. If you are in a hurricane, (perhaps you are a reporter for CNN), check out what a palm tree does as the eye-wall approaches. You’ll see the cleverness in the design of the leaves, just before that big sheet of corrugated met

By the way, according to this site, Sabal palms (palmettos) are particularly clever at withstanding wind:

“Sabal palms are second only to pygmy date palms in wind tolerance. Even the old dead leaves are almost always attached to the palm after wind speeds of 145 mph or more. Ninety-two percent of the palms were still standing after hurricanes Jeanne and Charley.”

The genus Sabal is native to the southern warm & steamy Atlantic coastal US, Bermuda, the Caribbean, nearby Mexico and the northern fringe of South America . The presence of these trees should be a red-flag: if you live in these places, your house should be of the brick variety.

Reference:

Windsor-Collins, A.G., M.A.Atherton, M.W.Collins & D.F.Cutler. 2008. Section properties of palm petioles, Part 1: The influence of section shape on the flexural and torsional properties of selected palm petioles. International Journal of Design & Nature and Ecodynamics. 2(4):328-347. http://hdl.handle.net/2438/2320

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