Ooyamazawa Riparian Forest Research Site and Network Group

Hitoshi Sakio

Sado Station, Field Center for Sustainable Agriculture and Forestry,

Faculty of Agriculture, Niigata University (Japan)

Forest Research Site

     Research theme is understanding long-term dynamics of a riparian forest on Ooyamazawa, Chichibu Mountains from viewpoints of the tree life history, tree physiology and natural riparian disturbances. The study site (35°57'30"N, 138°45'32"E) is located in a riparian zone along a small stream (Ooyamazawa) of the Nakatsugawa branch of the Arakawa River, in Chichibu Mountains, central Japan. This site is located in the protected Chichibu-Tama National Park and ranged from 1210-1530 m above sea level. Annual precipitation averaged about 1100 mm, and the maximum snow depth measured approximately 30 cm between January and March. The estimated mean annual temperature at the study site (1450 m a.s.l.) was 6.2°C. The study site was situated in the cool temperate zone that deciduous broad-leaved forest zone extended from 700-1600 m a.s.l. This riparian forest in this area phytosociologically belongs to the Chrysosplenio-Fraxinetum spaethianae, a typical riparian forest. Dominant canopy species in this area are Fraxinus platypoda, Pterocarya rhoifolia and Cercidiphyllum japonicum over 30 m in tree height. The subcanopy specie are Acer shirasawanum and Acer mono, and the understory is primarily composed of Acer carpinifolium and Acer argutum. The area around the study plot is no human disturbances such as logging or erosion control works. This study site is a part of the Monitoring Sites 1000 Project launched by the Ministry of the Environment, Japan, and also is one of a Japan Long-Term Ecological Research (JaLTER) Sites.

     We have studied on regeneration dynamics and coexistence mechanisms of tree species in this research site from 1987. We have surveyed tree size, litter and seed production. And we have researched the flowering and seeds production of Fraxinus platypoda.

     Three canopy tree species (Fraxinus platypoda, Pterocarya rhoifolia, and Cercidiphyllum japonicum) coexist in riparian forests in the Chichibu Mountains of central Japan. F. platypoda was the dominant canopy species. It produced many saplings and grew in abandoned channels and floodplains, and was able to invade both large and small disturbance sites. P. rhoifolia was a subdominant species that occurred on the deposits of large–scale landslides and grew in patches containing even-aged trees. C. japonicum was the other subdominant species that produced few saplings and invadedlarge disturbance sites together with P. rhoifolia. Establishment sites of C. japonicum were restricted to fine mineral soils and fallen logs. We found tradeoffs in reproductive characteristics (seed size, seed number, irregular seed production, and sprouting) among the three canopy species. F. platypoda and P. rhoifolia had large seeds and fruited irregularly. C. japonicum produced many small seeds every year and sprouted prolifically around the main stem. The causes of the coexistence mechanism of the three riparian canopy tree species may be both niche- and chance-determined to varying degrees. In riparian areas, the three canopy species were well adapted to disturbances throughout their life-history.