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Vol. 14 (1997) No. 2 p. 91-104




We surveyed the distribution of vegetation and substratum of the ground surface on the depositional landform developed by some debris flows in a subalpine area of the Oku-Nikko region, Central Japan, and discussed the effect of subsequent movement of debris flow deposits on succession of vegetation. The substratum of the ground surface was divided into three types according to the difference in deposited materials : 1) BS-type, consisting of boulder with sandy soil matrix, and retaining the features of debris flow deposits ;2) B-type, consisting of boulder without matrix, and suffering erosion ; 3) S-type, consisting of sandy materials transported by water flow. This depositional landform contained some depositional surfaces formed by past debris flows. However the three types of deposits were located in order of B, BS, S (B in the upper part of the depositional landform, down to S), and not corresponding to the age of depositional surfaces. This reflects the subsequent movement of debris flow deposits through the different depositional surfaces. There were young forests dominated by Alnus matsumurae, Pterocarya rhoifolia, and Salix sachalinensis which are regarded as the primary phase, or old forests of A. matsumurae and P. rhoifolia with juveniles of Tsuga diversifolia in the B-type deposit areas. Abies veitzcii forest, more than 55 years old, was dominant in the BS-type deposit area. Furthermore, S. sachalinensis was dominant in the area buried by S-type deposits. This distributional pattern is considered to be developed by the subsequent movement of debris flow deposits which affect on the speed and direction of succession on the depositional landform. In the B-type areas, the pioneer stage of succession occurs repeatedly due to the destruction of the site by erosion, however, these areas have potentiality to proceed T. diversifolia forests. In the BS-type area, which had been stable since a major debris flow more than 55 years ago, succession proceeded to A. veitzcii forest. In the S-type areas, only S. sachalinensis survived deposition, while conifer trees failed to invade.

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