<論文>長治谷草地のシカ排除柵内外の植生変遷と柵の開放試験

Abstract

シカ排除柵は植生復元に有効な方法であるが, 従来とは異なる植生を生ずることがあり, 十分ではない. 柵を用いた植生復元方法の改善には一時的な柵の開放が有効かもしれない. そこで本研究では, 柵の開放が植生に及ぼす影響を調べるため, 5年前に柵を設置した長治谷草地において柵内外の植生の現状を把握すると共に一時的に柵を開放し, 全種および種ごとの地上部バイオマス量を開放前後で比較した. 柵内ではススキが優占していたが, 先の調査で報告されていた陽生植物は少なかった. 柵外ではイグサに加え, イワヒメワラビが多かった. これらは, 柵内では陽生植物が減少し, 柵外ではシカ不嗜好性植物が増加したことを示唆しており, クラスター解析の結果も柵内外の植生遷移が異なる方向に進んだことを示した. 柵開放の前後に調べた全種の地上部バイオマス量に有意な差はなかったが, オカトラノオ, バライチゴ, ドクダミの地上部バイオマス量は開放後に減少したことが示唆された. したがって, 柵開放に伴うシカによる採食は植生量を減少させるが, 採食は優占種のススキではなくシカ嗜好性の高い種に偏ることを示唆しており, 開放時には嗜好性種の保護にも配慮が必要である.

Deer exclosure is not useful enough to restore previous vegetation, because the deer exclosure sometimes result in unexpected vegetation. Temporal opening of the exclosure may be effective to improve the way to restore previous vegetation. We investigated (1)species composition inside and outside the deer exclosure, which was constructed five years ago, and (2)aboveground biomass of total and each species before and after temporal opening of the exclosure at grassland in Japan, to examine the effect of opening of the exclosure on vegetation. Miscanthus sinensis was dominant inside the deer exclosure. Lysimachia clethroides, Potentilla freyniana, and Viola hondoensis were also frequently found but Carex ischnostachya, Hedyotis lindleyana, Agrostis clavata, Viola verecunda, which were frequently found in previous survey, were not. Outside the deer exclosure, Juncus effusus were dominant and Hypolepis punctanta was frequent. These suggest that heliophyte decreased inside the exclosure, whereas non-preferred plants increased outside the exclosure. Results of cluster analysis also support that the succession inside and outside the exclosure have proceeded differently. In temporal opening of the exclosure, aboveground biomass after the opening were significantly smaller than those before opening in three forb species, despite that the total aboveground biomass was not different between before and after opening the exclosure. These results suggested that the preferred species would decrease whereas M. sinensis would be still dominant, if the exclosure was removed for short period. Therefore, protection for the preferred species may be required to open the deer exclosure, to improve the way to restore previous vegetation.