Abstract
The spatial pattern of species composition in weed communities and the effects of water flow connection were studied in paddy fields on the bottom of the small valley, in Tochigi Prefecture, central Japan. In fall, the species composition and water content were investigated a series of 4-m2 quadrat in 47 paddy fields. Eighty-six quadrats divided into two clusters and 13 subclusters from species composition using the cluster analysis. By using the results of cluster analysis of the species composition, 86 quadrats were divided into 2 clusters and 13 subclusters. The 2 clusters corresponded to the difference in water content. Paddy fields integrated into the same subcluster were observed to hold together spatially. The combination of all paddy fields showed a negative correlation between the similarity index and the horizontal distance. The correlation coefficient values, which were calculated for the similarity index and 3 types of distances (horizontal distance, 3-dimensional distance, and 3-dimensional distance along the path of water flow) in the quadrat at the lowest elevation, were the highest for the distance along the path of water flow. The similarity index between two adjacent paddy fields with a water inlet was significantly higher than that of adjacent paddy fields without one. Six species had a significantly higher presence in a quadrat that was connected by a water inlet to a field at a higher elevation. It was suggested that factors related to irrigation such as the distance of water flow or a water inlet between paddy fields affected the spatial patterns of species composition in weed communities due to seed dispersal by water.
