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

J Thomas1, S Dijkstra1 and D Collings1,2

  1. University of Canterbury, Christchurch, New Zealand
  2. University of Newcastle, Callaghan, NSW Australia

Spiral grain is the systematic deviation of wood grain away from the stem axis, and is problematic for the timber industry as cut timber with grain exceeding 5° can develop a twist. Grain in radiata pine, like most gymnosperms, is initially vertical but develops a left-handed helix during the first years of growth, often reaching 10° from vertical. In subsequent years, the direction of grain change reverses, and grain angle decreased and often becomes right-handed. The mechanisms of spiral grain formation, which presumably require the rotation of cambial initials, remain poorly understood. We have used ImageJ to measure spiral grain development in young, vertically-grown radiata pine trees through multiple, independent approaches. Resin canals (running parallel to grain) were imaged in serial sections with circularly-polarised light, appearing as black spots against birefringent tracheid walls, and were reconstructed and measured following image alignment. Grain was also measured from Fast Fourier Transforms of tangential longitudinal sections derived from X-ray microtomography. These experiments confirmed that left-handed spiral grain was initiated in the first year of growth and only several millimetres outside the central pith, and that grain angles steadily increase after that. In tilted trees, however, the induction of compression wood on the lower-side of the tree interfered with spiral grain development, with grain in these regions failing to become more left-handed, although spiralling developed normally on the upper side. This asymmetry in grain development between the upper and lower sides of the same stem confirms the link between spiral grain and compression wood formation, and suggests an entry point through which the mechanisms that generate spiral grain might be studied.