Groundwater flow in spring-fed wetland and the formation of vegetation patterns in Hiroshima Prefecture, southwestern Japan

Abstract

The relationship between vegetation and groundwater flow in a small spring-fed wetland without a peat was surveyed in Hiroshima Prefecture, southwestern Japan. Detailed groundwater flow was measured with piezometer nests consisting of piezometers and groundwater wells. These piezometers were laid out into two lines extending from hillslope to the valley bottom. Plant communities of field layer and profiles of vegetation were surveyed along settled piezometer nests. All surveyed communities were classified into nine vegetation types. Wetland vegetation was characterized by Moliniopsis japonica and it was classified into two types, i.e. graminoids dominant open type and wetland forest type. In spite of the differences in vegetation physiognomy, both open and forested habitats featured the following groundwater table : over 20 cm in moist period and over 40 cm in common period. The difference of the two period was below 15 cm. On the other hand, the hydraulic gradient explained the difference of these communities. Thus, gramineous vegetation developed at the center of the wetland where upward seepage occurred. Trees were not found right above the upward seepage but were distributed in marginal zone of the wetland where downstream movement of ground water emerged. These results suggest that groundwater seepage offers a habitat of graminoids dominant vegetation thus preventing forest development. In contrast, groundwater-sinking flow provides a habitat for a wetland forest development.