Tree Trunk Twist

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Tree Trunk Twist
Say that 5 times fast!

If you have ever walked down a forest path you may have noticed that some tree trunks have a twist in them - either right-handed or left-handed (as shown above). It is as if some Paul Bunyon had torqued them with a giant twist. Most types of trees rarely have such twists but others commonly do. Some of the most twisted are in the juniper family,* and, in particular, the "Utah" or desert junipers of the southwestern parts of United States or northern Mexico. Thus this is a project that might be best done by students in rather arid places, but with a lot of scouting students in other climates can do it also.

First you will want to make a "twist-survey" of several thousand of these scrubby junipers. Make sure you include various habitats such as flat ground, north-facing and south-facing slopes, valley or canyon bottoms and mesa tops. Keep track of how many are twisted to the right, how many to the left, and how many have no appreciable twist at all. It might even be good for you to make a map. You might get a topographic map of your area. In the United States, these are produced by the U.S. Geological Survey (almost every small town in the desert country has a map store).

Once you have your data, you will want to make some speculations (hypotheses) as to NOT ONLY why the twists go in the directions they do, BUT ALSO why there are twists in the first place. As a biologist, you know that living things are the way they are because of both genetics and the nurturing environment. Thus your first question will be to ascertain the degree to which these two fundamental factors have in the twist phenomenon.

Let's suppose you made the speculation that the twists were genetic. Now go to your data to see if the data support that hypothesis. Let's pretend your map indicates that the trees growing on north-facing slopes are twisted left-handed, and those on the south-facing slopes are twisted right-handed, and those in protected box canyons have little or no twist. Well-, looks like genetics isn't the reason for the twisting. This doesn't mean that genetics plays no role at all. Indeed, the junipers' genetics may lend it much moreeasily to some environmental factor than what other plant species have.

Continuing with our same pretend map (this author doesn't know if it is true or not!), we speculate that the twisting must be due to some environmental factor. You decide to chose wind - and more specifically, the prevaling wind. In the latitude of the North American Deserts, the prevailing wind comes from the west and blows eastward. Thus, you notice something interesting with regard how the wind blows through your valley or canyon and the direction of twist on the two sides of the valley. It seems as if the wind must strike the branches of the trees that are furthest from the slope - perhaps because the wind is slower because it encounters more surface resistance on the upslope sides of the trees than on the outward sides of the trees. To determine if this is true or not, you need to make a device that will demonstrate just that! Better yet, it would be good if you installed such devices at or in each tree so that you can definitively check on the individual micro-climates. This might be especially helpful in explaining why a slope might have 90% right-handed twists and 10% left-handed twists because the prevailing wind's direction would dictate the right-handed torque, but a few of the trees live behind rocks or other trees and experience the opposite torque.

If you are planning to survey the torques imposed on many trees, you will need lots of inexpensive devices such as shown here. To the left is a wire bow in which a two-sided tag is affixed in the middle of a thread that is only loosely fitted to the bow. In this case, the wind was slightly stronger when hitting side "A" and so the tag was rotated around and around causing the thread to twist as shown. To the right, is an even easier proposed model that is merely tied to a branch of the tree and dangles in the wind. The small blue bead helps prevent the tag,s fluttering too violently. Again, this picture shows that side "A" was struck by a stronger breeze than side "B". One advantage of this type of device is that it will tell you the prevailing rotational force. With the vicissitudes of the wind, momentary reverse winds will unwind the thread a little, but then upon resumption of the prevailing wind, the thread will continue to twist in its "prevailing" direction.

While you are contemplating wind forces, you should not forget another environmental variable - solar exposure. Trees growing on south-facing slopes have much more intense light (and heat!) than do those on north-facing slopes. Remember that light might be stimulatory to growth on the light-side of the tree, but its heat could be detrimental (promotes loss of precious water).

IN SUMMARY, this is a project that contains a number of variables of possibly different significance. It will teach surveying techniques, as well as test your ingenuity for making devices to measure the different variables.


* Many people in North America mistakenly call these "cedars", but junipers have small waxy blue berries while true cedars have cones. In the "old days," the berries were collected and dropped into boiling water. The wax would float to the top and be skimmed off for making candles. Juniper berries can also be crushed, fermented and distilled to yield gin (as in giniper berries) - an old custom of the Hollanders, who dropped anchor on this side of the Atlantic Ocean, and wanted to take a little euphoric time off from their daily labors.


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