Current practices of retention forestry in plantations across Japan

To implement retention forestry in conifer plantations in Japan on a broad scale, it is necessary to understand the characteristics of suitable forest stands and regional differences in feasibility. We focused on retention forestry and similar practices that involve retaining a part of the forest structure in young, planted forests after high-ratio harvesting. We gathered information on these existing practices through personal contacts, published reports, academic presentations, and websites. We organized them in terms of (1) their geographical distribution, (2) characteristics of pre-harvest stands, (3) species and amounts of retained trees, and (4) forest managers and their aims/reasons for implementation. We found that the distribution of practices retaining broad-leaved trees in harvested areas was spatially biased across Japan and was concentrated in Hokkaido, Chugoku (western Honshu), and Shikoku (the fourth‑largest island). In most cases, the retained tree density was 10–20 trees/ha, and many practices have been implemented by public institutes. The reasons for implementation, excluding biodiversity conservation, varied among forest managers and included enhanced timber production (e.g. promoting broad-leaved tree growth and natural regeneration) and reducing labor costs. In some cases, although canopy trees were scarcely retained, regenerating and sprouting trees were retained. Our results suggest that the species and number of retained trees vary across regions. The development of criteria and assessment systems is required to evaluate the ecological and social benefits of practices with varying targets and aims.