Factors affecting the occurrence of woody plants in understory of sugi (Cryptomeria japonica D. Don) plantations in a warm-temperate region in Japan

We compared the occurrence of woody plants present in sugi (Cryptomeria japonica D. Don) plantations and evergreen broad-leaved natural forests in a warm-temperate region in order to characterize the understory vegetation of sugi plantations. The influence of stand attributes on species occurrence in the plantations was evaluated using logistic regression analysis. Out of 163 species, 65 plantation-preferring species and 31 natural forest-preferring species were detected. The plantation-preferring species group contained deciduous species (38 of 65 species) that are characteristic of coppice woodland, but all the natural forest-preferring species were evergreen plants. This indicates that the composition of the woody understory in sugi plantations of the region is characterized by the species of open, disturbed habitats. Stand age and the distance from the nearest natural forests were found to significantly affect the occurrence of evergreen shrubs and plants with gravity-dispersed seeds, respectively, but the light environment of the stand had less effect. According to the regression model analyses, long-term strategies such as lengthening the rotation (logging age) or creating patch mosaics of plantations in the matrix of natural forests appeared to be effective in conserving woodland species that are characteristic of evergreen, broad-leaved forests. On the other hand, increasing light penetration through thinning practices is a short-term strategy that is less likely to be effective in promoting these species.