Emerged shore vegetation of irrigation ponds in western Japan
Artificial ponds were constructed for rice field irrigation all over Japan. Particularly, numerous ponds were constructed in the Seto Inland Sea District of western Japan because of the area's limited rainfall and the difficulty of taking water from rivers. Pond water level is low during the rice-growing season, and reaches the lowest level in the late summer and early autumn. In ponds with large water level fluctuations the vegetation is composed mainly of small annuals that develop on the emerged shores and bottoms at low water levels, or only in drought years. In ponds surrounded by cultivated lands, bare shores and bottoms are colonized by weedy annuals, which are also common in artificial sites. Emerged shore communities of ponds surrounded by woods were described as the Deinostemato-Eriocauletum hondoensis and the Eriocaulion hondoensis. The Eriocaulion communities are characterized by annuals such as Eriocaulon spp. and Deinostema violaceum, which are non-aquatic plants, but very tolerant to submersion. Constituent species also contain amphibious perennials such as Myriophyllum ussuriense and Scirpus lineolatus. Some terrestrial wetland plants including Utricularia are also members of these communities. The Eriocaulion communities occupy an intermediate position between aquatic and terrestrial ones in space and in species composition. The optimal habitats of the alliance are nutrient-poor ponds with gently-sloping shores and water level fluctuations. Some of the Eriocaulion constituent species are threatened in Japan. Local communities have carried out the maintenance and management of ponds for hundreds of years. Recently traditional communal activities are getting difficult because of rapid social changes such as farm depopulation, advanced age of farmers, and urbanization. The rural changes have produced several negative effects on the pond environment and vegetation.