The effects of human impact on spatial structure of the riparian vegetation along the Ashida river, Japan

A vegetation map was prepared to clarify both the differences of patch size in each vegetation type and the changes of patch size in each river reach in the Ashida river running through the east part of Hiroshima Prefecture, Japan. With use of this map, the area and perimeter of each patch were measured. The microscale landform and distance from the river mouth of each patch were also recorded to analyze the habitat of vegetation and land-use. The vegetation and land-use types in the river were reflected in different patch size distributions. These were roughly divided into following four groups by cluster analysis: group A characteristic of large size patches occupying the major part; group B comprising a variety of patches from large size to small size; group C consisting of small patches covering the major part and group D with small number of patches. The vegetation types in group A tended to be observed mainly on river bank slope and major bed, while group B had a trend to occur mainly on minor bed and waterside lowland. For further scrutinization, the distributions of patch size in vertical course of the river were addressed after dividing the major part of the stream into four sections namely, 0–10, 11–20, 21–30 and 31–43 km from the river mouth, respectively. The smaller patches become dominant as the river reaches go upstream. The major vegetation types in group A were observed on artificial lands including bank slope and major bed, suggesting that human impacts such as flood protection works and regular mowing affected the spatial structure of riparian vegetation.