上高地・梓川における河畔林のモザイク構造とその形成過程

抄録

The dynamics of The mosaic pattern of riparian forests was studied in the floodplain along the Azusa River, Kamikochi, where braided channels are developing. Various types of disturbance such as frequent channel shifts, partial destruction of mature forest by floods, and secondary channel formation by summer floods in the forest, were observed on aerial photographs taken over a 50-year period. Since 1947, there has been no catastrophic disturbance, but about 10% of the river bed has been repeated destruction and revegetation every year. These disturbances have created open sites for colonization of riparian trees. First, pioneer trees comprising the salicaceous species Chosenia arbutifolia, Populus maximowiczii, Toisusu urbaniana and Salix rorida and some betulaceous species colonized the bars near active channels characterized by thick buried sediments made up of coarse particles. The dominant species in each patch varied with the habitat conditions. These patches of scrub developed into mature pioneer species forests, unless destroyed by floods. Ulmus davidiana var. japonica and Abies homolepis appeared about 50 years later than the pioneer trees. Their diameter increased more rapidly than those of the pioneer species, and therefore they tended to succeed the pioneer trees, which have a relatively short life span. Another successional trend was also observed on abandoned channels characterized by wet conditions and thin buried sediment, where Alnus hirsuta was replaced by Fraxinus mandshurica var. japonica. It is concluded that the overall mosaic pattern and differences in the successional stages of riparian forests develop through frequent channel shifts, which destroy the riparian forest and create colonization sites.