I am a primary teacher in Oxford, and at the end of last term my class Year 3 (7-8 year olds) discussed what they would like to find out for their science topic 'helping plants grow well'. One little girl in my class gave this much thought and asked 'why don't plam trees fall down during a tsunami? We could have simply googled this, but I know it would mean a lot to her and my class to have an expert answer it for us. Any help you could provide would be greatly appreciated.

Best Wishes,

Miss Burt.

I have never seen the impacts of a tsunami myself (thankfully!) but have been through many cyclones, including the massive tropical cyclone Yasi and the very fast moving tropical cyclone Larry in north-eastern Australia. I have subsequently done quite a lot of work on how palms are affected by strong winds, and tsunami impacts will be very similar, as both concern very high pressure (from water or wind) passing the palm from a single direction.
Essentially, palm trees have three significant features that help them survive. 1. Palms tend to have a very large number of relatively short roots which spread out around them in the upper layers of the soil - something like the head of a toilet brush. This holds a huge amount of soil around the root ball, making a very heavy and therefore stable anchor, as long as the soil is reasonably dry. When a tsunami or cyclone hits, this anchor therefore helps to hold the base of the palm in place, rather than relying on one or two very strong roots like cables to anchor it to the ground. However, if the soil is very wet beforehand (such as when a slow moving cyclone causes very heavy rain for several days before the winds hit) the soil gets quite soupy, and the root ball can move in it and the palm may tip up. 2. The stem of a palm is not woody in a radial growth pattern like a pine or oak tree, where the annual rings effectively make a series of hollow cylinders inside each other, but composed of lots of smaller bundles of woody material, more link the bunches of wires inside a telephone cable. The cylinder approach provides great strength to support weight (compressive strength) which means that an oak tree's trunk can support a huge weight of branches, but limited flexibility compared to the bundle approach, which allows the palm stem to bend over through 40 or 50 degrees without snapping. Again, palms will get broken in cyclones and tsunamis, but they are much more resilient than other trees. 3. Most trees have a canopy made up of branches, twigs and leaves. This enables them to spread their leaves over a wide area and catch lots of light, like a huge umbrella, but this also means that they catch a lot of wind or water, which either pulls them over like a sail or causes the branches to break and the canopy to be ripped off. Palms do not have branches, but have huge leaves with a central, flexible mid-rib, like enormous feathers. In good weather these leaves may spread several metres each side of the palm stem providing a canopy, but wind or waves will fold the leaves up like a feather duster, allowing them to largely survive intact. Some leaves might get broken but they are much 'cheaper' for the palm to replace than a whole canopy of branches would be.

The image shows a patch of rain forest near Mission Beach in Far North Queensland soon after severe Tropical Cyclone Yasi crossed the coast slightly to the south. A closed canopy forest of 25-30m tall trees has been reduced to sticks, but the palms are still standing and with their leaves tattered but intact.

Last edited by Dan Metcalfe (8th Apr 2013 04:08:19)

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