Tectona grandis is a valuable timber species with heartwood that is used worldwide. Most of the previous studies on its heartwood and sapwood have focused on dominant or mean trees, while trees with different social status might show different vertical and horizontal distributions of heartwood and sapwood. Studies on their heartwood and sapwood properties could be conducive to increasing heartwood yield at stand level. In 31-year-old plantations of T. grandis in southwest Guangxi, China, the trees were divided into three groups including dominant, mean and suppressed trees. Stem analysis was conducted for sampled trees in each of these groups to explore the differences in the horizontal and vertical distribution of their heartwood and sapwood. The results indicated that the heartwood radius, heartwood and sapwood areas of T. grandis showed significant differences in horizontal and vertical directions among trees of different social status. Heartwood began to form when xylem radius was 2-3 cm, and the heartwood radius ratio tended to be stable when the xylem radius reached about 8 cm. Heartwood radius and area, sapwood area and section heartwood volume all decreased with increasing tree height. The ratios of heartwood radius and area were relatively stable for sections under 50% of tree height. The sapwood width did not vary largely in horizontal and vertical directions among the three social status tree groups, which mainly fluctuated in the range of 1-4 cm. The heartwood volume proportions for dominant, mean and suppressed trees were 60%, 55% and 51%, respectively. There was a significant exponential relationship between heartwood volume and diameter at breast height (DBH) regardless of social status. The model HV = 0.000011 × DBH 2.9787 (R 2 = 0.8601) could accurately estimate heartwood volume for all T. grandis with different social statuses at this age. These findings could provide evidence for stand management and high-quality and large-sized timber production of T. grandis.Forests 2020, 11, 225 2 of 12 T. grandis has been in China for nearly 200 years, and has now been extensively planted in more than 60 counties or cities in 10 provinces of southern China, with a planting area of 35,000 hectares [8].Heartwood normally determines the value of wood [9], while sapwood is closely correlated with the physiological functions of trees [10]. There are a number of references on the heartwood of dominant or mean trees for T. grandis. For example, Pérez and Kanninen [11] studied the effects of thinning intensities on the heartwood volume of dominant T. grandis trees; Tewari and Mariswamy's [1] study showed that the proportion of heartwood increased while sapwood proportion was almost constant with increasing diameter at breast height (DBH) for T. grandis. In addition, Fernández-Sólis et al. [12] predicted the heartwood formation process of T. grandis based on randomly selected trees in each age class. However, mean and suppressed trees also account for a large proportion in stand [13] and their role in wood productio...