A comparative study of the seed banks of abandoned paddy fields along a chronosequence in Japan

Since paddy weed communities are declining with intensification and abandonment of agricultural activities, protecting these species has become important. Seed banks of paddy weeds may decline with increasing duration of abandonment. To determine the potential role of seed banks in the restoration of paddy weeds, we aimed to clarify whether the germinable seed bank of paddy weeds changes with increasing successional age. We investigated the established vegetation and soil seed banks of 25 abandoned plots between 6 and 37 years old, and of four cultivated plots. The target species were defined as specialist rice-paddy species, which are observed only in cultivated paddies and have not been reported in other habitats. A statistical description of the species response along environmental gradients was obtained by using linear mixed-effects models. Duration of abandonment is a major factor influencing the pattern of specialist rice-paddy species from soil seed banks. Both the species richness and the number of viable seeds in specialist rice-paddy species decreased significantly with time, with the number of species declining by ca. 50% over 40 years. Although seed banks of paddy weeds have relatively high resilience for maintaining populations after abandonment, our results indicated that younger abandoned sites were more beneficial for the restoration of paddy weed communities. Nevertheless, several threatened species persisted in seed banks in longer-abandoned sites, which are thus available to conserve specific species of paddy weeds. The number of viable seeds from non-specialist species remained large throughout the 40-year chronosequence. If paddies with longer periods of abandonment are selected as restoration sites, management practices should be carried out to prevent the dominance of non-specialist species.