The vertical distribution of the cross-sectional area increment of the lower trunk in aged Sugi (Cryptomeria japonica) trees was analyzed using annual ring data from disks. We collected data from three approximately 90-year-old mature plantations under different density management and three old-growth stands of 160 years or more. Of the nine sample trees in the thinning experiment plots, only three had significantly higher cross-sectional area increments at a relative height of 0.2 × tree height (0.2h) than the mean cross-sectional area increment. This result did not strongly support the hypothesis that several trees in mature stands that are thinned regularly may show a heightening trend in the cross-sectional area increment of the lower trunk. In contrast, the cross-sectional area increment at 0.2h was significantly higher than the mean cross-sectional area increment for all three sample trees in the old-growth stands. A generalized linear mixed model with the presence or absence of a heightening trend in the cross-sectional area increment of the lower trunk as a response variable was used for 20 sample trees. The results showed that the model with the ratio of height to diameter at breast height (H/D ratio) and relative spacing index (Sr) as fixed effects was optimal. The probability of trees showing a heightening trend in the cross-sectional area increment of the lower trunk increased with a lower H/D ratio and a higher Sr at the most recent values. We estimated that if stand density is maintained at an Sr≥19%, where the forest canopy is sparse, a heightening trend of cross-sectional area increment at the lower part of the trunk occurs in trees with a H/D ratio of ≤71.
