Table 1 shows the individual densities, species richness, and tree height for each tree species group at the UI-plot and UC-plot. At both plots, the dominant species group in terms of individual density in the first year after clearcutting was pioneer species, followed by lucidophyllous shrubs and lucidophyllous trees (Table 1). In 2017, the individual density of pioneer species decreased markedly at both plots, whereas individual densities of lucidophyllous shrubs and lucidophyllous trees decreased slightly (UI-plot) or increased (UC-plot) (Table 1). The decline of the pioneer species densities was mainly due to the decrease in seedlings (SE) (Table S1).
The total species richness at each plot was almost unchanged between the first year and 2017 (Table 1). Pioneer species decreased from 18 species in 2006 to 10 species in 2017 at UI-plot and 17 species in 2004 to 14 species in 2017 at UC-plot. The species richness of lucidophyllous trees increased at both plots.
The mean tree height in 2006 at UI-plot and in 2004 at UC-plot were less than 1 m for all species groups (Table 1). However, the tree height of lucidophyllous trees at UI-plot in 2006 was higher than that at UC-plot in 2004 (0.8 m and 0.4 m, respectively). In 2017, the height of lucidophyllous trees were almost same between the two plots.
Individual density for the different regeneration origins and seed dispersal types
Table 2 shows the individual densities for the different regeneration origins for all tree species and lucidophyllous tree species at the UI-plot and the UC-plot. At the UI-plot, most of the S-AR and R-AR in all species in 2006 survived until 2017 (Table 2a). Similarly, at the UC-plot, more than 80% of R-AR found in 2004 have also survived until 2017 (Table 2a). Values of the ARG density slightly decreased both in UI-plot (2810/ha in 2006 to 2411/ha in 2017) and in UC-plot (3830/ha in 2004 to 3164/ha in 2017) (Table 2a). SE remarkably decreased their number (down to 22% of 2006 in UI-plot and 39% of 2004 in UC-plot) in the both plots by 2017, but were still higher than ARG at both plots (Table 2a). The ratios of ARG to total densities increased until 2017 compared to the first year after clearcutting at both plots (from 5.7 to 18.7% in UI-plot, and from 8.5 to 16.3% in UC-plot, respectively; p < 0.001) by the decrease of SE densities.
For lucidophyllous tree species, the densities of ARG and SE in 2017 did not differ markedly or were almost same compared to those in 2006 or 2004 (Table 2a). SE of lucidophyllous tree species decreased from 4832 to 2926/ha in UI-plot, but were maintained as 60% of the initial value. In UC-plot, SE of lucidophyllous tree species increased from 1988 to 2303/ha (Table 2a).
For the difference in the individual densities by seed dispersal types, in both all species and lucidophyllous species, the density of gravity-dispersal-type in ARG slightly decreased but maintained 90% of 2006 in UC-plot and 77% of 2004 in UI-plot (Table 2b). For SE in all species and in lucidophyllous species, most of them were the frugivore-dispersal-type in the both plots in the both censuses (the first year after clearcutting and 2017). In all species, the density of gravity-dispersal-type SE increased in the both plots (242/ha to 716/ha in UI-plot and 909/ha to 1188/ha in UC-plot), but were still lower compared to that of ARG (Table 2b). Similar trends were found in lucidophyllous tree species in the both plots. The ratios of gravity-dispersal-type SE to total densities (0.5–2.9%) were far lower than those of ARG (22.3–26.9%) at both plots during each census (p < 0.001).
Species richness for the different regeneration origins and seed dispersal types
Table 3 shows the difference in the species richness for the different regeneration origins and seed dispersal types for all tree species and lucidophyllous tree species at the UI-plot and the UC-plot. At the UI-plot, the species richness of ARG in 2006 was 36 species and many of these were counted in S-AR (35 species) (Table 3a). In 2017, out of the same 36 species in ARG, 34 were counted for S-RA. Species richness of SE in UI-plot were larger than that of ARG in the both censuses (57 and 63 species in 2006 and 2017, respectively). The ratios of species richness derived from ARG to the total species richness were 51.4% in 2006 and 52.9% in 2017, which was less than that of SE (81.4% in 2006 and 92.6% in 2017) (p < 0.001, Table 3a).
At the UC-plot, 31 R-AR species were observed in 2004, and decreased to 28 species in 2017 (Table 3a). The ratios of species number of SE to total species were higher than those counted for ARG in both 2004 and 2017 (p < 0.001).
For lucidophyllous tree species, ARG at the UI-plot accounted for 83.3% of total species in 2006, which was not significantly different from that of SE (54.2%) (p = 0.060); no significant difference was also observed in 2017 (p = 0.177), though SE raised their contribution up to 89.3% (Table 3a). At the UC-plot, the ratios of species richness in ARG and in the SE to the total species were 71–88% during study period (Table 3a).
The species richness of the gravity-dispersed species in ARG at the UI-plot was 6 species in 2006, which was two species more than that of SE (4 species) (Table 3b). For the frugivore-dispersed species, a total of 63 species were observed at the UI-plot in 2006 and 52 species were in SE, which was 1.7 times that of the ARG (30 species). Similar seed-dispersal-type compositions were found at the UI-plot in 2017 and at the UC-plot in 2004 and 2017 (Table 3b).
For lucidophyllous tree species, the species richness of frugivore-dispersed species was higher for ARG (15) than SE (9) at the UI-plot in 2006 (Table 3b). However, no difference was observed in 2017 due to the increase in SE species (19). At the UC-plot, the species richness of frugivore-dispersed species in ARG was similar to that of SE throughout the study period (Table 3b). Species richness of gravity-dispersed SE increased by 1–2 species in each plot (Tables 3b, S1).
In all species, the UI-plot had a wider range of tree height distribution than that in the UC-plot in the first year after clearcutting (Fig. 2). Most S-AR in UI-plot were the 1–5 m height class and some reached the tree height class of 19 m in 2006 (Fig. 2a), while most R-AR in UC-plot only reached up to the 1–2 m height class in 2004 (Fig. 2b). In 2017, the height distribution of R-AR in the UC-plot was almost same as that of A-SR in the UI-plot (Fig. 2b).
For SE, almost all of the individuals occupied the 1 m height class in both plots at the first year after clearcutting (Fig. 3a, b). However, in 2017, these individuals showed a right-skewed distribution in the both plots. Majority of SE were less than 4 m in their height, but some of them reached 13–14 m in the both plot (Fig. 3a, b).
In the lucidophyllous trees, the tree height distributions of individuals in the ARG and SE were similar to the distribution in all tree species at both plots during the study periods (Figs. 4 and 5).