The survivorship curve showed a reverse sigmoid pattern (Fig. 4a). That is, the decreasing rate of age-specific survival probability (lx) slowed down in smaller age classes (1–4; i.e., shown as Type III concave curve), but speeded up in larger age classes (4–6; i.e., show as Type I convex curve). This pattern of survivorship trend is the result of a U-shape mortality curve (Fig. 4b). That is, the age-specific death probability (qx) first decreased then increased with age classes. The transition of the temporal direction of death probability occurred in small-medium trees at age class 4.