Improvement of Habitat Quality for Ranunculus Nipponicus Var. Submersus by the Removal of Phragmites Japonica

27 Mar Improvement of Habitat Quality for Ranunculus Nipponicus Var. Submersus by the Removal of Phragmites Japonica

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Abstract

兵庫県北部を流れる田君川は,標高10数mという西日本としては極めて低標高にバイカモが生育する河川である.かつては大規模なバイカモ群落が見られたが,河川の全域を覆うほどツルヨシの繁茂が著しく,近年ではそうした群落が減少しつつあった.そこで,バイカモ群落を復元する手法を検討する目的でツルヨシの除去実験を行った.ツルヨシの除去は手刈り及び重機(バックホウ)を用いて行った.その結果,手刈りによるツルヨシ除去は1ヶ月に1回の刈り取りを3回以上行うことでツルヨシの群落高の抑制に有効であること,水面上の照度は年間2回以上の刈り取りで増加すること,ツルヨシの衰退とバイカモの植被率の増加には相関があることなどが確認された.重機によるツルヨシ除去はバケット部をシャベル形とし河床材料とツルヨシを全て取り除く方法(河床浚渫法)とバケット部をフォーク形として株状に成長したツルヨシのみをつかみ取る方法(ツルヨシ株除去法)で行った.その結果,河床浚渫法,ツルヨシ株除去法ともに処理後バイカモの植被率は増加したが,ツルヨシ株除去法を用いた方がより早くバイカモの植被率が増加した.また,バイカモの植被率の増加は重機処理区よりも手刈り処理区の方が早いことが確認された.

English Abstract

In 1980’s, there occurred a luxuriant growth of submerged macrophyte, Ranunculus nipponicus (Makino) kai var. submersus Hara, in Tagimi River, Hyogo Prefecture, Japan. However, its distributio l range has been diminishing recently with the progressive growth of emergent Phragmites japonica Steud. To improve and restore the habitat quality for R. nipponicus, we attempted to conduct several methods of P. japonica removal and evaluated the effectiveness. In the field, we carried out hand-cutting using sickles with various frequencies of cuttings from once to four times from May / June to August 1997. In addition, we compared effects of two removal methods of P. japonica using back-hoe on the regrowth of R. nipponicus; one method that excavated the plant wholly with bottom sediment (excavation method) and the other dug out only plant shoots (shoot removal method). Frequent hand-cuttings with more than three times during growth were effective in reducing the growth of P. japonica, and R. nipponicus recovered noticeably with decreasing P. japonica growth. While the plant cover of R. nipponicus increased after the application of either removal method of P. japonica, earlier recovery of R. nipponicus was observed after the application of shoot removal method than after the application of excavation method.