Induction of compression wood inhibits development of spiral grain in radiata pine

Spiral grain refers to the helical patterns formed by the wood grain in the trunks of many tree species. In most gymnosperms, grain near the pith is vertical but wood formed after several years of growth has a slight to pronounced left-handed twist. Grain changes presumably involve the slow rotation of cells within the vascular cambium, but the mechanisms that allow this reorientation to occur remain unclear. Understanding this process is, however, important as the presence of strong spiral grain within the corewood of gymnosperms is a major wood quality issue devaluing cut timber. In this study, we measured wood grain in stems of Pinus radiata (radiata pine) saplings through reconstructions of resin canals that follow the grain, visualised by serial sectioning and scanning with circularly polarised light, and through X-ray computed microtomography (μCT) and image analysis in ImageJ. Vertical trees retained a symmetrical grain pattern that was weakly right-handed near the pith, but which became progressively more left-handed during the first eight months of growth. In tilted trees, however, the development of left-handed grain was inhibited by the formation of compression wood on the lower side of the tree whereas the wood on the upper side of the tree developed increasingly more left-handed grain as in the vertical controls. These results demonstrate that a previously unidentified link exists between compression wood formation and the inhibition of grain development.