Broad-leaf species composition in Cryptomeria japonica plantations with respect to distance from natural forest

To address the pressing need to evaluate how conifer plantations can serve biodiversity functions in addition to other economic and social roles they play, we assessed the diversity of broad-leaf seedlings and saplings in Cryptomeria japonica plantations in Ogawa, Ibaraki Prefecture, Japan at increasing distances (0–1000 m) from old growth natural forest edge. For saplings, there was no overall significant trend in the frequency, species richness, and Shannon index with respect to distance. Seedlings on the other hand showed a decrease in frequency and species richness with increasing distance from the old growth forest, implying that should recruitment limitation occur in the plantations, it will be stronger at the seed-to-seedling transition than at the seedling-to-sapling transition. Assigning species into groups based on functional traits that are associated with recruitment and regeneration was more revealing. Relative frequency of species that are moderately shade-tolerant, are shrubs, have small seeds, and are frugivore-dispersed increased in the plantations. In comparison, species that are tall trees, have large seeds and are gravity-dispersed decreased in the plantations. Multi-trait analysis showed that propagule size was the trait that could best explain the difference in the distribution of broad-leaf species in the plantations. Based on our results, we suggest to policy-makers that plantation sizes be kept to within a few hundred meters wide, and should ideally be within dispersal distance of species from natural forests that could potentially be seed sources of broad-leaf species. Otherwise, steps must be taken to drive succession such that potentially recruitment-limited species may be able to overcome barriers to regeneration.