30 巻 (2013) 1 号 p. 1-15
River gravel floodplains are important habitats for plants which grow in dry and periodically disturbed environments. However, as the result of human impacts, most of those floodplains are covered with thick sandy deposits which allow dominance of tall-growing perennial grasses, such as Miscanthus sacchariflorus and M. sinensis. Prescribed field burning is considered to be one of the effective methods to manage the floodplain vegetation dominated by tall grasses. We aim to clarify the effects of the field burning on the flora of the floodplain covered by dense Miscanthus species. The research was conducted in the Houki River, Tochigi Prefecture, where floodplain vegetation has been burnt at the end of February every year by local people. Between 36 burnt stands and 14 unburnt stands, there was a clear floristic difference in both M. sacchariflorus and M. sinensis communities. Jaccard's coefficients between the burnt and unburnt stands were lower than 0.5, showing that field burning changed the floristic composition drastically. In addition, field burning increased the number of native plants per area without increase of alien plants. Twenty eight species including endangered species were recorded with a higher frequency in the burnt stands than in the unburnt stands, for both M. sacchariflorus and M. sinensis communities. Especially, the occurrence of spring ephemerals was unique in M. sacchariflorus community. On the other hand, 7 and 14 species occurred with a higher frequency in the unburnt stands for M. sacchariflorus community and M. sinensis community, respectively. Furthermore, field burning showed a positive effect on plants which possess particular traits; Geophytes and herbs which flower in early spring were more prominently represented in the burnt stands. The major reason for the floristic differences can be litter removal which interrupts sunlight and prohibits shoot emergence. On the other hand, field burning showed a negative effect on winter annuals which suffer from fire directly. Although the floristic composition after burning appeared to be quite different from original flora of river gravel floodplains, it may still include many species of conservation value. In addition, field burning did not induce alien species invasions. From these results we concluded that prescribed burning is the effective tool for the vegetation management in river floodplains under the artificially modified environment.