Composition, size structure and local variation of naturally regenerated broadleaved tree species in hinoki cypress plantations: a case study in Shikoku, south-western Japan

摘要

We aimed to assess the potential of the natural regeneration of broadleaved tree species in relatively dense and approximately 30-year-old hinoki cypress (Chamaecyparis obtusa) plantations on the Pacific side of Shikoku in south-western Japan. The composition and size structure of naturally established canopy and sub-canopy species were investigated in 18 plots at two hinoki cypress plantation sites, namely Okuono and Karakawa (OKU and KRK). Local variation in the density and species richness of naturally established stems were analysed in relation to overstorey stand structure, local topography, soil properties and understorey vegetation. OKU was noteworthy for its relatively high proportion of sub-canopy species and L-shaped height distribution of saplings (>200 cm), while KRK was characterized by a wider range in the height distribution of saplings and a low density of seedlings (<200 cm) despite the relatively higher dominance of canopy species. This was attributed to the differences in the characteristics of component species and the thinning history of the sites. The density and species richness of the saplings at both sites and the seedlings at OKU were higher in plots with lower mean heights of hinoki cypress and higher soil C/N ratios. These results suggest that variation in the growth of hinoki cypress, which is related to the soil nitrogen status, brings about local variation in the regeneration potential at these sites.