Irrigation and drainage canals are elements of the paddy landscape, forming potential habitats for many wetland plants. However, pattern of plant species occurrence across these canals have been rarely quantified. We considered plant species diversity was lower in concrete canals than in earth canals. Moreover, scale of them may also affect the occurrence of plants.

In this study, we aimed to clarify the pattern of plant occurrence in canals of a lowland paddy field area. They are affected by specific disturbances through control of water levels in rice paddies seasonally.

Investigation was conducted in 103 smaller canals and in 55 larger canals, located on the Echigo Plain, Niigata. In vegetation study, we placed 10 and 60 continuous quadrats in tertiary and lateral canals, respectively. As environmental factors, canal form, water level, thickness of sediment deposit, and source river were recorded.

As a result, we found 71 and 70 wetland plants in tertiary and lateral canals, respectively. There was no difference between canal scales regarding the number of species, but species composition slightly varied. Canal structure and depth of sediment deposit are important factors affecting occurrence of wetland plant species.