Radial growth rate does not affect radial variation of latewood tracheid length in aged trees of Thujopsis dolabrata var. hondae

Tracheid length is lowest at the pith and then it increases rapidly as cambial age increases up until it reaches an asymptote value in what is known as ‘Sanio’s first law’. However, it is still unclear the effect of radial growth rate on latewood tracheid length and the effect of radial variation pattern of latewood tracheid length on the boundary position between juvenile and mature wood in aged trees. Radial variation modeling was applied to annual ring width and latewood tracheid length in aged trees (79–238 years old) of Thujopsis dolabrata var. hondae grown in natural forests and a plantation in Shimokita Peninsula, Aomori, Japan. In addition, the effect of differences in radial variation patterns of latewood tracheid length on the boundary age between juvenile and mature wood was evaluated. Cambial age showing the maximum values of current annual increment and mean annual increment was 42–121 and 65–189 years for natural forests and 30–33 and 46–52 years for the plantation, respectively. A mixed-effects model based on a logarithmic function with an explanatory variable of only cambial age, and random intercepts of stand and individual tree, was selected as the one best explaining the radial variation of latewood tracheid length. The estimated boundary cambial age was approximately 20 years in all trees regardless of radial growth rate. We concluded that (1) the regularity of radial variation in tracheid known as ‘Sanio’s first law’ can be adapted to at least about 250 annual rings from the pith in T. dolabrata trees, (2) the boundary cambial age is similar to that in younger trees, and (3) the effect of radial growth rate on latewood tracheid length is minimal in T. dolabrata.