Physiographic factors controlling the degradation of the moor at the foot of an alluvial fan in the Senjogahara, Nikko National Park, Japan

Abstract

The extent of the moor is decreasing in the Senjogahara, an upland moor in a volcanic area. Topographic maps and field observations suggest the invasion of Larix kaempferi and Betula platyphylla into the moor. This paper discusses the physiographic factors controlling the degradation of the moor in the transitional zone between an alluvial fan and the moor based on field investigations of vegetation, groundwater level, snow depth, micro-landforms, and deposits. The L. kaempferi forest is accompanied by lobate forms and cobbles that are characteristics of alluvial fans, whereas the B. platyphylla forest lacks these features. The boundary between the two types of vegetation well defines the distribution of the lobate forms and alluvial fan deposits. Geomorphic factors are considered to influence the vegetation both directly and indirectly. The direct factor is fluvial sedimentation, which favors the L. kaempferi forest as a pioneering plant community. The indirect factor is the microrelief, which affects the depth of the groundwater table that controls vegetation through moisture conditions. The invasion of tree species into the moor influences hydrologic and micrometeorologic processes. The large roughness and small albedo of the L. kaempferi forest result in deep and variable groundwater levels because of the intensive interception of rainfall and evapotranspiration. The B. platyphylla forest, which is surrounded by the moor lacking trees, favors deep snow since the wind velocity is reduced and the snow is trapped. The B. platyphylla forest is newer than the L. kaempferi forest. This apparently indicates that the B. platyphylla forest precedes the L. kaempferi forest in the succession. However, the B. platyphylla forest is unlikely to be replaced by the L. kaempferi forest in the present environment, because the shallow groundwater level and deep snow depth in the B. platyphylla forest prevent the succession. The environmental conditions play a more important role in the landscape than the succession stages. The results of the field investigations show that the L. kaempferi forest is different from the B. platyphylla forest in the age and process of formation, and that the major control on the landscape is geomorphic processes. This indicates that the degradation of the Senjogahara moor depends primarily on geomorphic factors.