京都府の多雪地におけるニホンジカ Cervus nippon Temminck によるハイイヌガヤ Cephalotaxus harringtonia var. nana の採食にみられる積雪の影響

Abstract

近畿地方の日本海側に位置する京都大学芦生フィールドステーションにおいて, ニホンジカ(Cervus nippon Temminck)の採食が常緑低木であるハイイヌガヤ(Cephalotaxus harringtonia var. nana)の生存に及ぼす影響と, 採食と積雪の関係を調べた.調査地内の87地点を5㎞に渡って踏査し.ハイイヌガヤの被度および分布を記録した.また, ハイイヌガヤ群落にコドラートを設け, 地上茎の生死と食痕の地上高および被食時期を記録した.調査地におけるハイイヌガヤの分布と被度は1984年以降の20年間で劇的に低下していた.調査対象とした地上茎には多数の食痕が観察され, 2000年から2004年の間にほとんどの地上茎が枯死した.枝葉は無雪期にも積雪期にも採食されていたが, 特に積雪期にニホンジカが通常採食できない高さの枝葉も採食されることが明らかになった.積雪期にハイイヌガヤが集中的に採食された原因は, 常緑低木であること, 地上茎が雪の重みでしなり低く下がっていたこと, 分布地が谷部で, 川沿いに移動するニホンジカの種雪期の行動ルートと重なっていたことだと考えられた.

In Ashiu field station, Sea of Japan region in central Japan, we investigated the effects of deer browsing on the survival of evergreen coniferous shrub species, Cephalotaxus harringtonia var. nana, focusing on the relationship between deer browsing and snow cover. We monitored the distribution of C. harringtonia and recorded the coverage at 87 points along 5㎞ transect. Further, we established five quadrats in the study site and recorded the survival of the ramets and periodically checked bite marks and its vertical height. Coverage of C. harringtonia drastically declined, or otherwise disappeared between 1984 and 2004. Many bite marks were observed on the study ramets and most ramets died between 2000 and 2004. Shoots were browsed both in snowless season and snowy season. Though shoots at high vertical positions were less browsed in snowless season, many of these shoots were browsed in snowy season. Based on our findings, we concluded that severe browsing during snowy season was attributed to the ecological feature of C. harringtonia ; (1) they retained leaves during snowy season, (2) they bent down by snow pressure and shoot vertical height was low and (3) they were distributed along valleys which deers frequently visited by moving along the nearby streams.