Riparian forest invasion by Robinia pseudoacacia and its effects on riverside vegetation

Abstract

We investigated the invasion of Robinia pseudoacacia and subsequently affected riparian vegetation of the Saigawa and Azusagawa Rivers, which are upper tributaries of the Shinano River, the longest river in Japan.　Habitats that had been affected by river water disturbance in recent years were located at a height relatively near the river surface and were composed of coarse gravel with numerous annual plants.　We found subtrees and shrubs of R. pseudoacacia under willow canopies, but small willow individuals did not occur under the R. pseudoacacia canopies.　Furthermore, forest floors in willow communities hosted more wetland and riverside species, including herb species, than those beneath R. pseudoacacia canopies.　Mature R. pseudoacacia stands reduced the distribution of wetland and riverside species.　Furthermore, the species composition at these sites was not riparian, instead consisting of various forest species.　The resulting forest landscape is unlike the natural riparian zone in this area.