2006 Volume 23 Issue 1 Pages 37-54
Factors affecting plant community types in relation to soil and groundwater environments were studied in abandoned terraced paddy fields in the southeastern part of Sado Island, Niigata Prefecture, Japan. Plant communities were classified into forest, shrub, and reed communities based on growth form and height of dominant species. The forest and shrub communities were further divided into mesic and wet forest communities, and mesic and wet shrub communities, respectively, based on the species composition of the herb layer. Both the ploughed layer and the subsoil that were formed when the site was used as a paddy field were distinguished in all investigated soil profiles. A humus-rich black layer was formed in the top portion of the ploughed layer. A gley layer was recognized in most investigated soil profiles. Both the carbon and nitrogen contents of the soil in the reed community were higher than those in the other community types owing to the peat layer that accumulated in the top portion of the ploughed layer. The Ca content and pH of the soil were higher in the reed and shrub communities than in the forest communities. The average groundwater table level was significantly lower in the mesic forest and mesic shrub communities than in the wet forest, wet shrub, and reed communities. The depth of the gley layer showed a strong correlation with the average height of the groundwater table level. The fluctuation range of the groundwater table level was small, and the cation contents and pH of the groundwater were higher in the reed and wet shrub communities, which were located on the lower portions of the slope, than in the mesic and wet forest communities. Detrended correspondence analysis (DCA) using understory vegetation data showed significant correlations between the first axis of the DCA and environmental factors, i.e., the average groundwater table level and chemical properties of both the soil and the groundwater. The surrounding vegetation and levees around the terraced paddy field may have played important roles in plant succession after the terraced paddy fields were abandoned. Topographical conditions, such as slope position and inclination, may have affected the establishment of the different types of plant community because they affected both the height and fluctuation pattern of the groundwater table after the terraced paddy fields were abandoned.
