The downstream effects of dams on the regeneration of riparian tree species in northern Japan

Dams for water regulation substantially alter the regeneration habitats of riparian species, thereby affecting their recruitment, composition and growth rate. This paper examines the effects of darns on the dynamics of riparian forests. First, the distribution of dominant riparian tree species was investigated in the Rekifune River, where no dams have been built and their regeneration habitats were classified into five geomorphic surfaces based upon flooding frequency. Chosenia arbutifolia and Toisusu urubaniana appear on the most flood prone areas such as gravel bars and lower floodplains. Poplus maximowiczii also prefers coarse sediment, but establishes on upper floodplains. The late successional species, such as Ulmus davidiana var. japonica and Froxinus mandshurica var. japonica, occur on stable surfaces referred to as terraces where sand and fine gravel make up the soil. Salix spp. dominates secondary channels. Second, the effects of dams on the regeneration process were examined in the Satsunai River where a flood control dam was completed in 1997. Spring floods in snowy regions mobilize riverbed sediment and create the moist, alluvial substrate ideal for seed germination. These habitats are essential for the recruitment of salicaceous species. Dams, in general, attenuate spring flooding and/or stabilize summer flows. This flow regime limits the development of gravel bars and reduces flood fequency. A decrease in flood frequency and magnitude may reduce the densities of pioneer species and shift species distribution from pioneer to late sucessional species.