Stem density and basal area
During the observation period, stem density in both PC (control) and PT (treated) areas decreased by ca. 30% from 6848 (/ha) and 6310 in 1992 to 4,608 and 4,460 in 2015, respectively (Fig. 2a; Table 1). The decrease in stem density of the forest interior (FI) and the gap edge (GE) in PT was similar to that observed in the total PT. However, in the gap (G), stem density decreased by 64% (from 10,133 to 3,598), which is attributed to a large decrease of stems in several multi-stem individuals of Q. salicina that had had large numbers of stems within clumps in 1992.
In contrast to stem density, the basal area (BA) increased during the observation period in all parts except for G (Fig. 2b). The BA as the total of PT increased by 36% from 35.6 (m2/ha) in 1992 to 48.3 in 2015, but the increase was smaller than that in PC (57%, from 40.7 in 1992 to 63.8 in 2015). BA in G decreased by 38% from 56.6 in 1992 to 35.2 in 2015 due to the loss of large multi-stem individuals of Q. salicina.
DBH- and height-class distribution
The DBH-class distribution of stems in 1992 showed similar inverse-J shaped patterns for PC and PT (Fig. 3). In 2015, the range of DBH expanded to more than 30 cm both in PC and PT. The stems in G consisted of stems of DBH less than 10 cm in 2015, while the maximum DBH reached 23 cm.
The height-class distribution of individuals was a mono-modal pattern with a peak at 8–10 m in 1992 for both of PC and PT (Fig. 4). The distribution in 2016 for both PC and PT showed patterns that could be regarded as a mono-modal distribution, though the peaks became unclear with the expanded height ranges. The individuals in G also showed a wide height range and included the highest individual exceeding 14 m in height.
Number of species
The total number of species in PC and PT was 20 in 1992 (Fig. 5), including almost equivalent numbers of evergreen broadleaved tree species (15 and 14 species in PC and PT, respectively). Of the total species in 1992, 15 species, including 11 evergreen broadleaved tree species, were common to PC and PT (Fig. 6a). Although there was a slight increase or decrease in the number of species in 2015, the number of species common and specific to PC and PT showed almost the same tendency as in 1992 (Fig. 6a).
Among FI, GE, and G in PT, FI had the largest species number in 1992 (20 species including 14 evergreen broadleaved tree species) (Figs. 5, 6b). Of these 20 species, five evergreen broadleaved tree species were common to GE and G (Fig. 6b). Seven species including three evergreen broadleaved tree species were common to FI and GE. No species specific to GE or G was observed. No species common to FI and G and GE and G were found. A similar tendency as in 1992 was observed in 2015; 7 species out of all 17 species that occurred in PT were specific to FI, and six and four species were common to FI–G and GE–G, respectively. No species specific to GE or G was found, the same as in 1992.
Recovery of felled individuals
The stem number per clump (multi-stem individual) in G in 2015 was positively correlated with that in 1992 (r = 0.69, p < 0.01) and decreased in most individuals (Fig. 7a). Basal area also showed a positive correlation between 1992 and 2015; however, in contrast to the stem number, they increased in 2015 up to 1.5 or 2 times that of 1992 (Fig. 7b).