Impact of extreme run‐off events from the River Sakasagawa on the Senjogahara ecosystem, Nikko National Park III. Pattern of alluvial deposition and effects on the growth of Malus Toringo and Betula platyphylla var. japonica

In the study area at Senjogahara, there are 13 vegetational groups forming a space‐time continuum in response to alluvial deposition from the River Sakasagawa. The pattern of deposition changes markedly from time to time, and the vegetation is thus a mosaic of recovery phases in different stages of development. There is marked zonation of layers of gravel, silt and organic matter in the soil profile, running sequentially from the mouth of the river back to the moor. Furthermore, there is a decrease in the depth of the water table with distance from the river mouth. The growth increment of Malus Toringo is poor under the canopy of Ulmus Davidiana var. japonica forest, with maximum growth occurring in the areas of active alluvial deposition. The changing patterns of Malus growth have been used to approximate the dates of major changes in the erosion channels and areas of alluvial deposition. There were five main channels in the study area. The oldest was on the west side and active ca. 35–40 yr prior to 1984, the year of sampling. Subsequently, adjacent areas further east became major drainage and erosion channels about 25–30 yr before 1984. However, after 1985 the erosion channels again showed a marked change in pattern, and currently flow into an entirely different sector, being responsible for the continuous dynamic patterns of vegetational change in this area.