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While hiking through one of the areas burned in the 1988 Yellowstone fires, I noticed that some of the trees appeared to have grown in spiral patterns.

Most downed trees have crack patterns that imply straight growth, but some look like the one in the picture. The direction shown in the picture seems to be more common, but it's not universal, and it doesn't appear to be related to location -- another picture I took shows two adjacent logs that spiral in opposite directions.

A log showing a distinct spiral crack pattern

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The two commonly put-forward explanations for spiral growth of tree trunks related to stress-loading and damage-tolerance.

The stress-loading explanation states that spiral-grained trees flex more than straight-grained trees before they break. Flexibility under stress-loading is useful in areas with heavy snowfall (as the tree can flex until the snow falls off) and in areas with heavy wind (as the tree can flex in response to wind-buffeting instead of breaking).

The damage-tolerance explanation states that in trees with straight grain, the leaves (or needles, etc) have water delivered to them, via the xylem, from the roots on the same side of the trunk. In spiral-grained trees with several years of growth, water and nutrients from a given root can be transported to branches on all sides of the trunk. Therefore in spiral-grained trees, if one section of the tree's root system is damaged, it does not necessarily mean that a corresponding part of the canopy will have to die off.

Kubler (1991) discusses both the stress-loading and damage-tolerance explanations in much more detail, if you're interested in further reading.

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  • $\begingroup$ Is this essentially to imply that trees spontaneously express this growth pattern in response to specific environmental conditions during their development? $\endgroup$
    – Darren Ringer
    Mar 6, 2017 at 21:00
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    $\begingroup$ From Kubler (1991; linked in answer): "One cannot, however, expect a strict causal relationship between environmental conditions and fiber directions, because spiral grain is "under a considerable degree of genetic control in many species;" only in some species the genetic control of spirality appears to be negligible (Harris 1989, p 159)". So, mostly no, but some species may develop spirally in response to environmental conditions. $\endgroup$
    – bshane
    Mar 7, 2017 at 2:35
  • $\begingroup$ Thanks; sorry, I had just assumed it would be paywalled and didn't even make it to the abstract in this case. $\endgroup$
    – Darren Ringer
    Mar 7, 2017 at 3:26

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