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INTRODUCTION
The floodplain along the middle reach of a river in the Tohoku District, northeast Japan, is covered by willow forests dominated by one or several salicaceous species (Ishikawa, 1982). Every salicaceous species has its distinct distribution pattern along the environmental grathents such as river bed grathent (Naohara, 1945 ; Ishikawa, 1983). Recently, ecological characteristics of salicaceous species have been reported at various phases of their life-cycles : seed dispersal by Walker et al. (1986), seed dormancy by Zasada & Viereck (1983) and Densmore & Zasada (1983), seedling establishment by McLeod & McPherson (1973), flood tolerance by Hosner (1960) and Hosner & Boyce (1962), and sex ratio by Falinski (1980) and Crawford & Balfour (1983). Salicaceous species are regarded as "pioneers" in plant succession on bare lands induced by flood, mud flow, etc. Sakai et al. (1983) examined fourteen characteristics of 27 tree species in a natural forest in Hokkaido and indicated that the time from seedling establishment to the onset of flowering is one of the most important characteristics corresponding to pioneer capacity. But, there have been few studies on this subject.
The purpose of this paper is to examine the flowering behavior of several salicaceous species in relation to their size and age.
I am grateful to Emeritus Professor Shigeru Iizumi of Tohoku University for his guidance during the course of the present study. Thanks are also due to Drs. Takao Kikuchi and Seiki Takatsuki of Tohoku University for their fruitful suggestions and comments on the draft and helpful advice. I am also indebted to Dr. Hideki Sakai for his help in the field survey.
STUDY AREA AND COMMUNITY STRUCTURES OF THE SITES
The Hirose River, a tributary of the Natori River, is about 3108 km2 in catchment area and 45.5 km in length. It originates in the Ouu Mountains and flows eastward to join the Natori River at a point 5.5 km from its mouth. The Ookura Dam was constructed at an upper tributary in 1961. Thereafter, a number of bars expanded in and along the channel, and riverine vegetation including willow forest developed (Iizumi et al., 1975).
Filed surveys were carried out during the flowering period of willows in May 1982, at two sites, Sites A and B, at points...