Invasive alien plants have encroached and outcolonized many native species in almost all Japanese river floodplains in recent years. Sicyos angulatus is one of the most notorious invasive lianas. To monitor the impact of S. angulatus and its association with other species, S. angulatus colonies were explored by volunteer bio-monitors in the Tama River floodplain from 2006 to 2009. The data were analyzed to determine the relationship among the composition of S. angulatus colonies and the flooding regime of the Tama River using detrended correspondence analysis (DCA). The results revealed that Miscanthus sacchariflorus was the most frequent (60.4%) understory herbaceous species found in association with S. angulatus colonies. The DCA coordination triplot gave that the association of S. angulatus changed after a major flood and its understory layer was replaced byLolium X hybirdum, Phalaris arundinacea and Bromus catharticus. M. sacchariflorus was almost absent in the understory group, and the soil depth of quadrats became deeper than it was in the pre-flood condition. We concluded that S. angulatus were strongly invading M. sacchariflorus, and that influence in combination with both the invasion of S. angulatus and the disturbance from flooding caused M. sacchariflorus colonies to change to pasture.
Key words: Flood disturbance, Sicyos angulatus, species association, Tama River, riparian, volunteer bio-monitoring.
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