Alaska Science Forum
More on Why Tree Trunks Spiral
Article #783
by Larry Gedney
This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Larry Gedney is a seismologist at the Institute.
In an earlier column, I asked if any readers could explain why the grain in trees seemed to spiral up the trunk-in a clockwise direction. That is, spiral marks in old trees crack open from the upper right to lower left around the trunk. Professor (now Emeritus) Neil Davis, the originator of this column, posed the same question in this column over ten years ago, and it's time for an update.
I received numerous replies to the query, and thank all those who answered. I'd like to repeat two of the better responses here. First, David J. Friis of Anchorage included not only some solid reasoning, but a bit of whimsy. He said:
"When I worked for NOM 's Environmental Research Labs in Boulder, we were once asked why lightning sometimes spirals down the trunk of a tree. While the answer was not proven, we observed that the path of least resistance might follow the spiral grain of the wood. I eventually found a tree with a spiral lightning mark and it followed the spiral grain exactly. One tree, of course, proves nothing.
"But why should the tree spiral? More speculation here: Foliage tends to be thicker on the south side of the tree because of better sunlight. Prevailing winds, in most of the tree-growing northern hemisphere, are from the west. Combine these factors, and the westerly wind pushing on the thicker south side of the tree, year after year, causes an asymmetrical wind loading which slowly twists the tree around in the observed direction.
"This reasoning is so obvious that there must be something wrong with it or else it would be well known to plant biologists. I can think of several ways to test this hypothesis. My favorite is to examine trees in the southern hemisphere for an opposite rotation. I have several countries in mind. Do you know where I can get a grant?"
(I should add here that I checked with retired University of Alaska Professor Tunis Wentink, an expert in wind power, and John Lingaas of the National Weather Service in Fairbanks to find the prevailing direction of wind in the Alaskan Interior. Both agreed that it was extremely variable, but that it was most often from the north-northeast during the winter, with stronger winds from the southwest during the growing season of June and July.) .
I applaud Mr. Friis's ingenuity, but it was matched by that of Hans Nielsen of the Geophysical Institute. Professor Nielsen was the first to come up with the clockwise-spiraling question in this column back in 1976. He apparently has had time to think the matter over further.
Hans told me recently that he thinks the matter can be related to the Coriolis effect of the earth's rotation. In the northern hemisphere, all moving objects are diverted ever so slightly to the right. (This is why hurricanes rotate counterclockwise--air moving toward the storm is diverted to the right and thus imparts a counterclockwise spiral to the storm at the center.) Nielsen thinks that possibly when a tree is rocked by winds, the tip might tend to rotate in a counterclockwise circle when viewed from above. This would lead to a clockwise spiral twist. (That sounds like a contradiction until you think about it for a while.)
Granted, not all trees exhibit the same twist, but the majority of them do. The phenomenon can be likened to the claim that water will always spiral out of a drain in a counter-clockwise direction in the northern hemisphere. It is well known that you can make it spiral out in either direction, if you give it a little shove first. Local effects such as topography of the landscape (or irregularities in the bowl) play a much greater role than does the very minuscule Coriolis effect.
I found it intriguing that even though they had entirely different suggestions about the cause, both Friis and Nielsen suggested that a good test would be a trip to the southern hemisphere to check the direction of twists in trees there. In the southern hemisphere, the prevailing winds at similar latitudes would be in the same direction, but the Coriolis force would be to the left and the sun would lie to the north, rather than to the south. The observed sense of twisting in trees should therefore be clockwise around the trunk, resulting in a counterclockwise spiral of cracks (upper left to lower right). If all this makes you slightly dizzy, as it does me, try twisting a roll of putty or bread dough, and you'll see what I mean.
A final note: Barry Donnellan, a Fairbanks attorney, observes that the term "spiral" is not correct in the sense that we've been using it here. The preferred use of spiral, he points out, is the description of a plane curve like a neatly coiled garden hose lying flat on the driveway. If you raise one end of the coiled hose, you would have the shape that we're talking about, which is a helix. But, as he says, who ever speaks of a "helical" staircase?
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