Abstract
This paper addresses the management of riparian forests (RF) in the natural levee zones of Hokkaido, based on the relationship between river channel configuration and RF dynamics before and after river improvement. Flow disturbances affect RF dynamics. The distribution of RF reflects river channel form longitudinally and micro-topography transversely. In an RF before improvement, intermediate, post-pioneer species predominate, mainly because the intervals between RF destruction are long for a meandering channel, whose flow changes more slowly than for a braided channel. Pioneer species grow on recently deposited sediment, and such species affect the microtopography by increasing the channel elevation through sediment capture, which promotes the establishment of replacement species and protects the RF. The seedlings of post-pioneer species that can adapt to environmental changes become canopy trees. After river improvement, most RF species are willows, which are not post-pioneer species. The extent to which willows dominate depends on the extent to which the channel floods, which in turn depends on the transverse elevation of the channel. It is thought that after river improvement, an RF cannot reach transition because of frequent natural and artificial disturbances (blowdown removal, river channel improvement, etc.) and land development near the channel. Because of such disturbances, fewer seeds of post-pioneer species are supplied from surrounding forest. This paper points out that natural levee zones do not have a system for restoring RF through flow disturbance, and it suggests that RF at different growth stages in natural levee zones is restored very little, even when the floodplain and meander channel are restored. Toward restoring RF dynamics that correspond to flow disturbances, we propose a river management method that allows the seedlings of post-pioneer species to establish among pioneer species in RFs where transition of vegetation is possible, and that allows the channel to migrate transversely.
