ミヤコザサの維持と樹木実生の更新にエゾシカの採食が与える影響

Abstract

エゾシカによる森林衰退が報告されている北海道, 白糠において, エゾシカがミヤコザサの形態, 現存量に与える影響を調査した.またシードバンク, 実生の調査を行うことにより, 白糠の森林におけるエゾシカ, ミヤコザサ, 樹木実生の関係を解明しようとした.エゾシカの採食を受けることによりミヤコザサの地上部は矮小化し, 現存量も減少していた.一方, 地下部現存量は軽度に採食を受けている区で多くなっており, 地下茎は長くなっていた.しかし過度の採食を受けた場合には地下部現存量も極度に減少し, ミヤコザサの消滅が危倶された.ミヤコザサは採食耐性が高いと言われるものの, その耐性には限界があることが示唆された.過度の採食を受けていたミヤコザサでもエゾシカを排除することにより回復がみられ, その過程ではまず地上部に投資していた.また他植物種の実生調査では, ミヤコザサが軽度に採食を受けている区で実生数が多く, 実生の生存のためには適度な現存量のササは好影響を与えると考えられた.調査地では種子供給とシードバンクの発達が期待できるため, エゾシカの密度管理によってミヤコザサを適量に維持し, 実生更新を促進できる可能性が示唆された.

Browsing effects of deer on the growth of dwarf bamboo were studied at Shiranuka, Hokkaido where forest degradation caused by deer is reported. We also tried to reveal the relations among deer, dwarf bamboo and seedlings of tree species by investigation of seedbanks and seedlings. The aboveground part of dwarf bamboo was dwarfed and the biomass was reduced under browsing pressure. On the other hand, the underground biomass of dwarf bamboo increased and the rhizomes became longer under the light browsing pressure. However, the underground biomass was reduced seriously and seemed to vanish under the severe browsing pressure. Although dwarf bamboo is regarded to be tolerant to browsing pressure, the limit of its tolerance is shown. When the overbrowsed dwarf bamboo was protected from browsing pressure, dwarf bamboo started to recover its aboveground part. The appearance of more seedlings at the site where dwarf bamboo is under the light browsing pressure suggests that the moderate biomass and density of dwarf bamboo gives a good influence on the survival of seedlings. Enough seed supply and developed seedbanks show that the seedling regeneration can be promoted by controlling the biomass of dwarf bamboo under the controlled deer density.