Using microgeomorphological approach, colonization of pioneer forest on the bottom plain of a mountain valley was investigated. In the synthetic study of vegetation-landform relationship, the arrangement with temporal and spatial scales for each objective phenomenon is of importance. First, the basic scale of a topographic segment related to habitat of plant individuals was set up, and micro-landrom units consisting of several topographic segements were determined.Yoko-o Valley is characterized by considerable change of runoff and sediment supply from alpine zone in the Northern Japanese Alps. This characteristics lead to the development of micro-landforms in response to flood magnitude, which provide different habitats to pioneer plant species. Thus Larix leptolepis and Alnus hirsuta was established on relatively dry and high surface (“flood terrace”) formed by flood, in contrast, Populus maximowiczii and Chosenia arbutifolia was located on wetter and lower one formed by small flood.Based on the DBH structure of forest stand, the relationship between annual rings and DBH of Larix leptolepis, and the formation and habitat condition of micro-landforms on the riverbeds, the forest dynamics on the Yoko-o valley bottom for the last ca. 150 years was reconstructed as follows. The forest on the Yoko-o valley bottom is divided into L. leptolepis forest I, II, L. leptolepis scrub, Chosenia arbutifolia forest, C. arbutifolia scrub and so on. L. leptolepis forest I appeared on high, immense and dry surface (“flood terrace”) which was formed by a catastrophic flood about 150 years ago. Subsequently, the river channel shifted northward, and then flood magunitude reduced gradually with small oscilation of the river channel. In concordance with change of river landforms, L. leptolepis forest II also appeared on almost dry but partly wet surface (“flood terrace”) which was formed by a middle-magnitude flood about 80 years ago. Furthermore small flood about 30 years ago formed almost dry but partly wet and higher surface in upper and middle reaches and wet and lower surface in lower reach near the junction of the Yoko-o Valley and the Azusa River. The almost dry and higher surface was occupied by L. leptolepis scrub, and the wet and lower surface by C, arbutifolia scrub.Pioneer stage of the prisere in Yoko-o valley bottom recognized commonly is that Larix leptolepis forest or Chosenia arbutifolia forest and Populus maximowiczii forest are colonized on bare surfaces with different habitat conditions. In next stage Abies veitchii forest succeed with negligible disturbance by flood. However, each forest is considered to have a cycle of destruction and recolonization under the control of the geomorphological processes with different magnitude and frequency from the point of view that forest age is regarded to indicate the return period of disturbance caused by a flood. Thus destruction and recolonization is considered to arise cyclicaly in the period of 150 years in L. leptolepis forest I, 80 years in L. leptolepis forest II, and 30 years in L. leptolepis scrub and C, arbutifolia scrub, and in the period shoter than 10 years in sparse vegetetion on riverbeds.