Heartwood formation progresses basipetally over years or decades in each conically shaped annual growth layer of Cryptomeria japonica stems

Key message Cryptomeria japonica stems included successive annual growth layers with sapwood, intermediate wood, and heartwood acropetally. Parenchyma cell death and heartwood formation in each layer progress basipetally over years or decades. In the secondary xylem of trees, heartwood formation is a physiological process accompanied by parenchyma cell death in the inner sapwood; however, the trigger of this cell death remains unclear. Direct visualization of living or dead parenchyma cells within stems is difficult; alternatively, it is possible to analyze the distribution of sapwood, which contains living parenchyma cells within stems. By employing X-ray and surface images of 85 cross-sectional disks obtained from the entire height of five mature Cryptomeria japonica stems, the number of annual rings in, and the radial width of, each sapwood, intermediate wood, and heartwood were measured and axially reconstructed. The effect of stem height on those numbers and widths was analyzed by fitting linear mixed-effects models. The axial distribution of sapwood, intermediate wood, or heartwood in each conically shaped annual growth layer of the whole stems was also reconstructed. The numbers of annual growth layers from the cambium to the intermediate wood and heartwood decreased with stem height. In contrast, radial distances from the cambium to the intermediate wood and heartwood were constant regardless of stem height. All stems included several successive tri-state annual growth layers, with sapwood, intermediate wood, and heartwood acropetally; heartwood formation in each layer progressed basipetally over years or decades. These results confirm that parenchyma cell aging after their formation in the cambium is not the determinant of their death or heartwood formation. Instead, the radial distance affects the death of parenchyma cells and heartwood formation.