SEASONAL VARIATION OF THE ALIEN PLANTS INVATION INTO RIVER ECOSYSTEM

Abstract

Many alien species have invaded into fluvial environment. The invasion of alien species affect
seriously damages to the biological diversity. Authors pointed out that invasion of alien plants is mainly
occurred in spring, and recommended that the improvement of river environment should be done from
summer to autumn in order for prevent invasion of alien species. However, there is little report about the
phenomenon when the improvement of river was occurred from summer to autumn. So we investigated how
the vegetation recovered at the improvement river area with no burial seeds in summer. During the
investigation we removed all emerging seedlings in the half of the investigation area and always kept as the
bare ground. As a result four of the following became clear, 1) seedlings of alien species could not easily
invade bared area in summer, 2) only some seedlings of a few native species can be established in summer,
3) the vegetation by native species that recovered in summer prevented the invasion of alien species, 4)
seedling removal hasten the invasion of alien species. The character that the seeds are able to emerge anytime
and anywhere like the alien plants species are advantageous at the fluvial environment where rising of the
river sometimes occurs. But the condition of river bed in summer is too severe to invade open area. Only a
few native species can established their seedlings.