Distribution, Formation, and Classification of Wet Grassland on Mineral Soils in Japan

Abstract

Wet grasslands that occur on mineral soils are widely distributed in Japan. Because these wet grasslands serve as habitats for many indigenous and/or endangered species, their preservation is important for maintaining biodiversity of Japan. However, these wet grasslands have been studied very little. This study surveys the distribution of wet grasslands on mineral soils in Japan by referring to previous reports and discusses the classification of these grasslands on the basis of the physical characteristics of their substrata. Most wet grasslands on mineral soils are located in the Pacific side of the Chubu district or on the Seto Inland Sea sides of the Kinki and Chugoku districts. However, some are located in other, wider areas, including in regions with cold climates. This suggests that the formation of wet grasslands on mineral soils is attributable not only to a temperate climate that promotes the decomposition of plant residues, but also to hydrologic and geomorphologic systems that minimize the accumulation of organic matter. In addition, even in peat-based wetlands, there may be short periods during which, for some reason, no peat covers the ground surface. In consideration of these formation mechanisms, wet grasslands on mineral soils can be roughly classified into three types. The first is the hillside-slope type that is formed on hillside-slopes by seepage water spreading out onto, and down, the ground surface. The second is the valley-floor type that is formed on valley floors, or on the swales between beach ridges, by convergence of groundwater. This type may exist only in warm areas, because they tend to accumulate plant residue. The third type is a pseudo type: that is, a peat-based wet grassland in which the peat is temporarily covered by mineral soil.