I noticed a funny thing out in the woods the other day. I came across a fallen log, its bark long stripped away by the gleaning of industrious birds and mammals. The exposed wood, rather than running in straight furrows from end to end, spiraled around the trunk, helix upon helix from end to end. I was mystified. If the shortest distance between two points is a straight line, and trees in a forest are under intense competitive pressure to reach the canopy as quickly as possible, why would a tree ever spend precious energy and resources growing in circles? What could possibly make a tree behave so strangely?
As with many natural phenomena, a lack of a definitive explanation provides fertile ground for speculation, and theories abound as to why some trees take on this curious growth form. A couple of likely reasons are:
Spiral growth is beloved by photographers for the beautiful curves to be found in the bark and wood of the trees. Sawmills, on the other hand, are not so pleased to see a spiral-grained log in their yard; the wood is often weaker and spiral-grained boards often twist as they dry. Despite the reduction in the raw strength of the wood itself, spiral growth gives a tree greater flexibility than its straight-grained neighbors, making the tree more resistant to high winds or heavy snow loads.
Another possible cause for spiral grain is to better distribute water around the tree. In straight-grained trees, the needles (or leaves, though spiral growth is more common in conifers) share water, photosynthate, and nutrients with the roots directly below them. If the roots or branches were to be damaged on one side of a straight-grained tree, the corresponding roots and branches would wither. A spiral grain, on the other hand, allows resource sharing all around the tree, distributing water from a single root to branches all around the tree, and sugars made in the leaves on a single branch to all the roots. By distributing water and nutrients evenly, a spiral-grained tree can more easily survive periods of drought or windstorms, as well as the chronic stresses of growing in dry, windy areas. Unsurprisingly, spiral grained-trees are often found on dry ridgetops where the twin forces of exposure and desiccation are at their greatest. Bristlecone pine in particular, famous both for its longevity and for its habitat high on the windswept Sierras, often exhibits spiral growth.
A few other theories involve genetics, heliotropism (tracking the sun), the Coriolis effect (which causes the clockwise pattern of winds and ocean currents in the northern hemisphere, and mechanical torque from prevailing winds. What do you think?