Abstract: In this study, we examined hydro- and hygrophytic vegetation in small, seasonal wetlands along the shoreline of the Taguchi Oohora Pond, an irrigation reservoir constructed ~130 years ago in Inuyama, central Japan. Traditional rice cultivation practices in the region intermittently but significantly reduce the water level of the reservoir. During these irrigation periods, submerged shoals are transformed into sand and gravel wetlands. These shoals formed through the abundant supply of friable deposits from the geological features of the watershed, and because the reservoir is no longer dredged to maintain the water capacity. Few reservoirs in the area exhibit this type of wetland formation, as water levels are increasingly maintained by water supplied from rivers or other reservoirs via irrigation channels that cross catchment boundaries. We identified over 100 hydro- and hygrophytic taxa, including eight endangered species, in the target reservoir and nearby wet environments including riversides, rice fields, and low-lying swamps, both up- and downstream from the target reservoir. Given the transitional nature of these habitats, it is uncertain whether the plant species observed in this study will persist. In local communities, discussions have just started regarding stopping vegetational succession at a desirable stage to preserve endangered species or to maintain species diversity. The associated problems are common to many substitutional conservation sites.
