Clarifying factors affecting above-ground floristic composition and soil seed banks for the conservation and restoration of plant species in rice paddy fields

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Cultivated and abandoned paddy field are floristically highly diverse, but the mechanisms affecting this diversity are not clear. The following studies were carried out. First, field surveys in cultivated rice paddy fields showed that influence of land improvement intensity on floristic composition was different among different land form types. This study also implied the importance of the period of rice harvest as a factor controlling floristic composition. A germination test was, hence, carried out using surface soils of a cultivated rice paddy. The results suggest that floristic composition just after the crop harvest in autumn was strongly different according to the timing of rice harvest, while in the subsequent spring floristic composition was less affected by the timing of rice harvest in the previous year. As the third study topic, floristic composition in the soil seed banks was investigated along a chronosequence in abandoned paddy fields, where soil seed banks of rare wetland floristic species are often maintained in the surface soils. This study illustrates that duration of abandonment clearly decreased the number of wetland species, whereas no less than half species observed in the start of abandonment persisted in the seed banks under germinable condition.