Aggressive Invasion of Eragrostis curvla in Gravelly Floodplains of Japanese Rivers: Current Status, Ecological Effects and Countermeasures

抄録

　The floodplains on the middle reaches of rapid Japanese rivers are characterized by extensive grav­elly areas with sparse vegetation cover. There are a number of plant species endemic to the gravelly floodplain habitats, including two threatened species, Aster kantoensis and Ixeris tamagawaensiss. The Kinu River is a typical rapid river in central Japan, and the river endemics have remained among the vege­tation of the middle reaches until recently. These unique habitats, however, are rapidly being altered by an aggressive invasion of the alien grass (Eragrostis curvula), which is supposed to have become established through dispersal from upstream stands formed due to active introduction of the species for soil erosion control. The substrate condition of invaded areas tends to be changed from gravelly to sandy/silty, because tussock of E. curvula trap sand and silt during flooding. Thus, the original condition with sparse vegetation cover principally consisting of riparian endemics adapted to gravelly conditions is being lost. E. curvula exerts a strong negative influence on the performance of riparian endemics. A resto­ration project aimed at recovering the original condition of the floodplain by adaptive management techniques was started in 2002. As a ‘first-aid’ minor restoration effort, E. curvula was mechanically removed from a part of the floodplain (1,800 m²), fine sands were washed out by spray the ground with a jet of water to restore the original gravelly condition, and stocked seeds of A. kantoensis were sown. A local population of A. kantoensis was successfully restored, but more extensive exclusion measures are needed in the next stage of management to prevent further negative impact of the invasive species.