Effect of Time Lag of Establishment of Deer-proof Fences on the Recovery of Perennial Herbs in a Cool Temperate Deciduous Forest Diminished by Sika Deer Browsing in the Tanzawa Mountains, Central Japan

27 Mar Effect of Time Lag of Establishment of Deer-proof Fences on the Recovery of Perennial Herbs in a Cool Temperate Deciduous Forest Diminished by Sika Deer Browsing in the Tanzawa Mountains, Central Japan

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Abstract

ニホンジカの採食圧を受けてきた時間の長さによる植生保護柵(以下,柵)設置後の多年生草本の回復しやすさを検討するために,同一斜面上の設置年の異なる柵で,12種の出現頻度と個体数,成熟個体数を比較した。柵はニホンジカの強い採食圧を10年程度受けた後に設置された柵3基(1997年柵)と16年程度受けた後に設置された柵4基(2003年柵)である。両方の柵で出現頻度が同程度であった種が6種,1997年柵で高い傾向にある種が6種であった。出現頻度が1997年柵で高い傾向のある6種のうちのあさ種は,シカの採食圧の低かった時代において調査地にまんべんなく分布していた可能性があった。そのため12種のうち9種は両方の柵で潜在的な分布は同じだったと考えられた。これら9種において個体数を比較したところ,4種は1997年柵で個体数が有意に多かった。これらの種は,シカの採食圧を長く受けた後に柵を設置しても回復しにくいことを示している。一方で,9種のうちの5種は,両方の柵で個体数に有意差はなかった。このことは,これら5種が林床植生退行後の柵設置までに要した10年ないし16年程度のシカの採食圧では出現に影響しないことを示唆している。ただし,このうちの1種は成熟個体数の比率が2003年柵で低かった。以上のことから,柵の設置が遅れると回復しにくい種があることが明らかになった。したがって,それらの生育地では退行後,遅くとも10年以内に柵を設置することが望ましいと結論した。

English Abstract

Deer-proof fences have been used to protect and restore endangered perennial herbs in some areas of Japan where the forest floor vegetation has been diminished by browsing sika deer (Cervus nippon Temminck). However, little work has been done to evaluate the optimal timing for installing such deer-proof fences. Thus, I surveyed the occurrence, number of individuals, and number of mature individuals of 12 perennial species inside seven deer-proof fences over 5 years post-establishment, three of which were installed in 1997 (1997 fences), and the other four in 2003 (2003 fences). The occurrence of six species was equal between the 1997 fences and the 2003 fences, while that of the remaining six species was higher within the 1997 fences than the 2003 fences. Past records indicated that three of these latter six species were distributed equally between the areas of the 1997 and 2003 fences. These results suggest that the potential distribution of nine species found within the 1997 fences was equal to that of the 2003 fences. The number of individuals of four of these nine species was greater in the 1997 fences than in the 2003 fences, or showed differing relative rankings. These findings suggest that these four species have difficulty recovering, given the assumption that the fences were constructed in regions where the forest floor was subjected to the continual sika deer browsing. In contrast, the remaining five species did not show significantly different numbers of individuals between the 1997 and 2003 fences. Thus, these five species appear to be u ffected by 10 or 16 years, respectively, of sika deer browsing. These findings demonstrate that some species have difficulty recovering once the forest floor has been diminished by browsing sika deer. Thus, the early establishment of deer-proof fences is suggested to e ble the best recovery of perennial herbs.