Hardwood recruitment into conifer plantations in Japan : Effects of thinning and distance from neighboring hardwood forests

Abstract Conservation of plant species diversity in managed and natural forests is a concern worldwide. To clarify the effects of thinning and distance from neighboring hardwood forests on hardwood establishment in conifer plantations, we measured the abundance and diversity of hardwoods at different life stages (seed-fall, soil seed bank, seedling, sapling) and environmental conditions (light, soil temperature, litter accumulation, and composition) from −2 to 40 m from the hardwood-coniferous forest boundary into the coniferous forest interior in five unthinned and four thinned stands in Cryptomeria japonica plantations in Japan. In unthinned stands, hardwood abundance and diversity decreased with increasing distance into the coniferous forest interior for all life stages, perhaps because of a distance-dependent decrease in seed-fall and the subsequent soil seed bank for wind- and bird-dispersed seeds. Higher light and soil temperatures and shallower litter accumulation were observed at the hardwood-coniferous forest boundary compared to the coniferous forest interior, suggesting favorable environmental conditions for germination, emergence, and seedling growth for hardwood species, and resulting in higher species diversity near the boundary. In thinned stands, hardwood abundance and diversity also decreased with increasing distance into the coniferous forest interior for seed-fall and the soil seed bank, but did not decrease for seedlings and saplings, probably because of distance-independent improvement of environmental conditions due to thinning. Higher light availability and soil temperature and lower litter accumulation in thinned stands enhance seedling establishment for not only shade-tolerant but also shade-intolerant and light-demanding species. Our results suggest that distance-dependent diversity of juvenile hardwoods in dense coniferous plantations is closely related to distance-dependent gradients in both seed-fall and environmental conditions, but thinning enhances diversity, even in the forest interior.