The Mystery of the Twisted Tree

Abstract

Why are trees twisted? To a varying degree, the wood fibers in a tree are arranged in a helical pattern known as spiral grain. Often most clearly visible when the bark has peeled off, or when the wood is cracked by frost or drought, such helices can also be visible in the bark itself, especially in older trees. It is a spectacular phenomenon, and important for the forestry industry, but why? The literature is vast and contradictory. The oldest reference in a review paper by Kubler (1991) is from 1854. In his classic book Curves of Life, Cook (1914) speculates that trees in the northern hemisphere should show right-handed helices, in contrast with left-handed trees south of the Equator. Vague theories about the Earth’s rotation and prevailing winds are offered, only to be seemingly disproved later in the book by the observation of two chestnut trees from the same locality in England, but with opposite chirality. Astonishingly, hundreds of scientific papers later, this discussion has not been put to rest. According to Skatter and Kucera (1998), trees in the northern hemisphere grow faster on the southern side because of phototropism, making the crown asymmetric. This is supported by statistics, at least at high northern latitudes (Eklund and Säll 2000). The prevailing westerly winds at these latitudes would then cause a counterclockwise torsion of the tree as seen from above, and right-handed spiral grain is supposed to make the tree more resistant to these forces. Later papers have questioned several aspects of this idea (e.g., Wing et al. 2014), and data from south of the equator are still sparse (Fig. 53.1).