芦生研究林におけるシカ排除柵によるススキ群落の回復過程

Abstract

近年, 日本各地でシカの採食による草地の植生改変が生じている。芦生研究林長治谷作業小屋前の開地ではススキを主とする草本群落が見られたが, 2007年以降シカの採食により衰退した。防鹿柵によるススキ群落の種多様性および現存量の回復過程を把握するため, 柵設置1-3年後に柵内外で植生調査および刈り取り調査を行った。柵設置1年後から柵内は柵外に比べ種多様性が高く, 機能形質を元に種を分類した機能群の多様性も高くなり, 種組成にも明瞭な違いが見られた。調査地近辺で2003年に見られなくなったと報告されていた種を含む77種が柵内で見られた。群落高, 植被率および現存量も2年後には一般的なススキ草地と同程度まで増加した。このようにススキ群落の多様性と現存量が早期に回復したのは, ススキ群落の衰退直後に柵を設置したためと考えられた。柵内では背丈の高いススキ, オカトラノオや小高木・低木種が優占し, 一部の背丈の低い分枝型広葉草本は競争排除され, 高木種の定着も見られなかったため, 今後もしばらくはススキ群落が続き, 多様性が低下すると予想された。柵外では不嗜好性のイグサと分枝型一年生広葉草本のトキンソウの被度が増加し, 植生の単純化が進行した。

Heavy grazing by Japanese sika deer (Cervus nippon) has caused alteration of grasslands in various places over Japan in recent years. Grassland community dominated by Miscanthus sinensis near Choji Valley Station, Ashiu Research Forest has disappeared since 2007 due to grazing by sika deer. To examine recovery process of grassland community by exclosure, we studied changes in diversity and biomass one to three years after fencing by vegetation census and harvest. Since one year after fencing, diversities of species and functional groups classified by functional traits were higher inside the exclosure than the outside, and species compositions were different clearly between inside and outside. Seventy seven species, including species that were reported to have disappeared since 2003 around the research site, were found inside the exclosure. Two years after fencing, vegetation height, cover, and biomass increased to the level of M. sinensis grasslands in Japan. The quick recovery of the grassland in terms of diversity and biomass may be due to early fencing just after the disappearance of M. sinensis grassland. Inside the exclosure, tall plants such as M. sinensis, Lysimachia clethroides, shrub and small trees species dominated resulting in the exclusion of some short branching forbs, and inhibition of tree recruitment. These results suggest that M. sinensis grassland will continue to dominate and that plant diversity will decrease. Plant community outside the exclosure shifted toward a simpler community dominated by an unpalatable species, Juncus effusus var. decipiens and a branching annual forb, Centipeda minima.