Natural and social environments in a large old-growth Japanese horse-chestnut forest in Shiga Prefecture, Central Japan

large old-growth Japanese horse-chestnut trees growing in the Kutsuki region, Shiga prefecture, central Japan, with diameter at breast height (DBH) ≥ 1 m. Large old-growth trees are ecologically important as keystone structures in forests as they provide valuable habitats for many animal and plant species. In recent years, however, large old-growth trees have rapidly declined in many places around the world. In this study, we examined the characteristics of the growth environments of large old-growth Japanese horse-chestnut trees and analyzed the natural and social factors related to the establishment of these trees in a catchment area in the Kutsuki region. In our field survey, 230 Japanese horse-chestnut individuals including young trees were identified in the riparian zone, and 47 (20%) were large old-growth trees. These old-growth trees were mostly located in the upper parts of the catchment area and partly on the knick line of the side slope and upper part of the head hollow zone compared with the location of the small- and medium-sized individuals with DBH <1 m. Additionally, we found that most of them were distributed along belts 15‒20 m in height above the riverbed and that the smaller individuals tended to grow in the lower zone. These results show that the large old-growth trees were probably established under more stable geomorphological conditions with fewer topographical disturbances. Additionally, intensive use of the deciduous broad-leaved forest in the Kutsuki region has occurred throughout history, such as periodic gathering of wood for firewood and charcoal production, and branches for fertilizer for rice cultivation. However, Japanese horse-chestnut trees are unsuitable for these purposes. It was also strictly prohibited to cut them under the customary law of the local government during the Edo period. Local inhabitants also collected the nuts to mix into rice cakes for consumption. Considering the social aspects, the large old-growth Japanese horse-chestnut trees have been maintained in the region under satoyama (traditional border zone between mountainous and arable land) conditions and by the selective conservation policies of the local government and local inhabitants. We conclude that the large old-growth Japanese horse-chestnut forest was established under a combination of relatively stable geomorphological conditions in the catchment area, selective conservation, and periodical disturbances by other tree species transplanted by local inhabitants.