Changes in natural forest fragmentation and their driving factors across Japan from 1985 to 2022

Forest fragmentation reduces biodiversity and ecological function. However, some regions now show signs of defragmentation, a recovery process in which forest cover becomes more continuous. Defragmentation owing to the recovery of natural forests is especially important because natural forests generally support higher biodiversity than planted forests. Despite this, most previous studies into forest defragmentation have not distinguished natural forests from planted forests, leaving natural forest defragmentation insufficiently understood. This study provides the first nationwide assessment of changes in natural forest fragmentation in Japan from 1985 to 2022 and identified the factors promoting fragmentation dynamics. Using annual land cover maps, we quantified long-term changes in landscape metrics and evaluated the effect of site factors and land use factors using Bayesian spatial regression models implemented through INLA-SPDE framework. Across Japan, natural forests became less fragmented, while the proportion of natural forests showed a slight increase. Although the number of small, isolated patches increased after 2000, simultaneous increases in area and spatial aggregation metrics suggest that this trend was consistent with ongoing defragmentation. Bayesian spatial regression analysis showed that defragmentation was promoted by clear-cutting and occurred most readily in remote areas with high elevation, steep slopes, and low population density. In these areas, conversion from planted forests to natural forests following clear-cutting was strongly associated with natural forest defragmentation. This study provides the first nationwide evidence that natural forests in Japan are undergoing defragmentation and highlights the potential role of conversion from planted forests to natural forests following clear-cutting in natural forest defragmentation.