[M-IS23] 山の科学

[MIS23-17] 淀川源流域における山地景観の形成要因：「トチノキ巨木林」を事例として

★招待講演

*手代木 功基¹ (1.摂南大学)

キーワード：トチノキ、山地渓畔林、里山

There are many large old-growth Japanese horse-chestnut (Aesculus turbinata) growing in the Kutsuki region, Shiga Prefecture, central Japan whose diameter at breast height (DBH) is ≧1 m. Large old-growth trees are ecologically important as keystone structures in forests as they provide valuable habitat 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 examine the characteristics of the growth environments of large old-growth Japanese horse-chestnut, and analyse 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 individuals (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 middle-class individuals of <1 m DBH. Additionally, we found that most of them were distributed along belts of 15–20 m 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 probably established under more stable geomorphological conditions with fewer topographical disturbances. Additionally, intensive use of deciduous broad-leaved forest in the Kutsuki region has taken place throughout history, such as periodic gathering of wood for firewood and charcoal production, and branches for fertiliser for rice cultivation. However, the Japanese horse-chestnut trees were unsuitable for these purposes. It was also strictly prohibited to cut them under the customary law of the local government during the Edo Period. Also, local inhabitants collected the nuts for consumption to mix into rice cake. Considering the social aspects, the large old-growth Japanese horse-chestnut trees have been maintained in the region under satoyama 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 established under a combination of relatively stable geomorphological conditions in the catchment area, selective conservation, and periodical disturbances of other tree species by local inhabitants.