Transition of Adonis Ramose Community in 12 Years by the Effect of the First Filling of a Dam Reservoir

27 Mar Transition of Adonis Ramose Community in 12 Years by the Effect of the First Filling of a Dam Reservoir

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Abstract

三春ダムの試験湛水前に，サーチャージ水位～常時満水位の範囲で，フクジュソウの群生地を確認した．フクジュソウは，確認当時，「我が国における保護上重要な植物種の現状」((財)日本自然保護協会 (1989) の危急種に該当し，ダムの本格運用前の試験湛水によって，生育個体のほとんどが湛水の影響を受けることが明らかであった．試験的な意味合いから，常時満水位～サーチャージ水位付近にかけてベルト上にコドラートを設置し，コドラート内の個体を残存させ，湛水前後の変遷を12年間，追跡した．試験湛水前に全体で106個体あった開花個体のうち，21日以上冠水した範囲では翌年個体を確認できなかった．それ未満の冠水日数では生き残った個体もあるが，冠水の翌春は開花しなかった．試験湛水直後，開花個体は湛水区域外の3個体のみであったが，その後は回復し，試験湛水から10年後の2007年には28個体，11年後には46個体と増加傾向にある．生育適地は，ダム運用後一部は貯水池に，一部はアズマネザサなどの繁茂により減少したが，当初の約70%は残存しており，今後も個体数は増加すると考えられる．

English Abstract

Before the first infilling of Miharu Dam reservoir, a community of vulnerable species, Adonis ramose, was found in 1996 on a slope that was supposed to be submerged by the first filling of the dam reservoir. The main area of A. ramose community was in an area between the normal water level of the reservoir and surcharge water level. Elevation difference of two levels was 7 m. It was obvious that A. ramose community will be submerged in the reservoir during the first filling. For the conservation purpose, we transplanted the most of A. ramose individuals to another place before the first filling. However, for the exami tion purpose, we left a part of the community and tracked it. To examine the influence of the submergence, quadrates of the belt transect were set along the slope including both submerged area and non-submerged. We have specially focused on A. ramose individuals in the quadrates and tracked their growth for 12 years since 1996 of which 2 years are the pre-first filling and 10 years are the post-first filling respectively. 106 blooming individuals were observed in the pre-first filling period. In the first spring of post-first filling, no blooming individual was observed in the area once submerged and 3 individuals were observed in the area not submerged. However, the number of A. ramose recovered to 28 individuals in the 9th year of the post-first filling and to 46 individuals in the 10th year. Within the submerged area, no A. ramose individuals has been observed in the area which was submerged for more than 21 days, and we inferred that all individuals were dead or not germi ted. Some individuals, which were submerged for less than 21 days, survived but did not bloom in the next year.