Abstract
Paddy farmland consolidation in Japan has been widely recognized as having a negative impact on biodiversity in rural areas. In particular, drainage ditches made of concrete that are introduced to lessen farmers’ burden during maintenance activities to expell vegetation raise concern about their potential strong impact on the ecosystem, as some of the vegetation has established itself in the ditches. In this study our goal was to clarify the conditions for vegetation intrusion into concrete drainage ditches so that appropriate drainage canals can be selected that will lessen the environmental burden. A field survey was performed from 2004 to 2006 in a consolidated paddy field in Joso City, Ibaraki Prefecture, Japan (139°58’ E, 36°2’ N). The results were as follows: (1) Three-year vegetation distribution in concrete drainage ditches was almost steady, with the same dominant species. (2) Water depth and sediment deposition measured at all points of vegetation observed in two selected ditches suggested that most vegetation was found in water less than 10 cm deep. (3) An index of average vegetation coverage (AVC) was introduced to obtain the spatial average of vegetation coverage. High AVC occurred in water less than 15 cm deep, regardless of the wide range in sediment deposition. (4) Based on a germination test for soils in different conditions, deep water appeared to suppress germination in the drainage ditches. (5) The found conditions of vegetation suppression in the two selected ditches can explain the distribution of vegetation over the whole study area.
