Spiral of Wood Grain in Tree Trunks at San Jacinto Mountain
Pinus lambertiana, sugar pine | Pinus ponderosa, ponderosa pine | |
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| Fig. 1. Left: The left-hand spiral grain of the wood of sugar pine, Pinus lambertiana. Note that the grain has no spiral at the very base of the tree. Right: The right-hand spiral grain of the wood of ponderosa pine, P. ponderosa. Note that the outermost wood layer has a larger spiral angle than the older layer immediately underneath it. Click on the pictures for larger versions. | ||
Table of Contents
Introduction, Observations and Photographs
Possible Causes for Spiral Grain
Introduction, Observations and Photographs
Deb Nelson asked us an interesting question when we met her at our turnaround point at 9000 feet elevation on the PCT north of Saddle Junction: why do some trees have spiral wood grain? Furthermore, she piqued our interest by saying she had observed trees with spiral grain right next to trees with no spiral grain, that were presumably of the same species
Hence on the way back down to Humber Park, we paid attention to the dead tree trunks we saw, to try to understand this phenomenon. (The spiral can only be observed in dead trunks that have lost their bark.)
In the following, a right-hand spiral is one in which the grain spirals to the right as you follow the grain up the tree trunk. A left-hand spiral is one in which the grain spirals to the left as you follow the grain up the tree trunk.
We observed the following:
- Every tree we observed had spiral grain, although the spiral angle for small-diameter trees of white fir was very small and almost zero.
- The spiral angle is greater near the bottom of the tree than near the top (except at the very base of the tree, where the grain is straight).
- The spiral angle becomes larger with increasing size of the tree.
- The spiral angle varies with species:
- The grain of white fir exhibits only a very slight spiral, especially for small-diameter trees, usually a left-hand spiral
- The grain of Jeffrey pine and ponderosa pine exhibits a very strong right-hand spiral, in which the spiral twists up and to the right along the trunk. However, the grain of small ponderosa pine trees has a low spiral angle, and can spiral in either direction.
- The grain of sugar pine exhibits a very strong left-hand spiral, in which the spiral twists up and to the left along the trunk.
- The grain in incense cedar may have a left-hand spiral, if the grain has the same spiral as the bark.
Pictures showing some of the dead trees we observed are given in Fig. 1 above and Fig. 2 below.
Abies concolor, white fir | ||
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Calocedrus decurrens, incense-cedar(bark still present, so the underlying spiral of the wood is not known for sure) | Pinus lambertiana, sugar pine | |
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Pinus jeffreyi, Jeffrey pine | ||
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Pinus ponderosa, ponderosa pine | ||
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| Fig. 2. Photographs of dead trees for various species showing variation in spiral grain between and within species. Click on the pictures for larger versions. | ||
Possible Causes for Spiral Grain
Any theory to explain spiral grain must also explain why the maximum spiral angle varies with species, and that even within species there is variation in the spiral grain angle. Those facts appear to imply that there is not a single dominant cause of spiral grain, and that instead it depends on the characteristics of each individual species and the conditions under which each species evolved.
One clear benefit of spiral grain is that it allows water from each individual root to reach every branch on the tree (Kubler 1991). A species with spiral grain is therefore more resistant to having a portion of its root system die on one side of the plant. Species that grow on steep mountain slopes might well have very different conditions for their roots on the uphill side of the tree than on the downhill or lateral sides, such as a boulder preventing development of roots on one side.
Less clear is the benefit for the structural integrity of the tree. Leelavanichkul and Cherkaev 2006 find that spiral grain does not make the trunk stronger against forces that would break it (wind load), and in fact makes the trunk weaker for angles greater than some maximum value. However, Kubler states that spiral grain allows a tree to bend and flex more under wind load, resulting in less wind resistance than if the grain were not spiraled. He also states that the bending and twisting allows snow to slide down the branches more easily.
It may be that only tree species that develop massive trunks, and are relatively long-lived, benefit the most from both the improved water distribution, and the flexibility of the trunk, given by spiral grain, and thus evolved the capacity of having large spiral angles.