霞ケ浦におけるアサザ個体群の衰退と種子による繁殖の現状

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  • Conservation ecological study of Nymphoides peltata in Lake Kasumigaura
  • カスミガウラ ニ オケル アサザ コタイグン ノ スイタイ ト シュシ ニ ヨル ハンショク ノ ゲンジョウ

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Nymphoides peltata is an endangered floating-leaved plant species. Lake Kasumigaura once contained one of the largest populations of the plant in Japan. However, recent monitoring on the metapopulation status revealed a rapid dwindling of this species in the lake. During 1996-2000, the number of the local populations decreased from 34 to 14 and total area occupied by the plant decreased from 99, 497 to 10, 081m2. In 1996, five local populations were constituted from both long- and short-styled morphs of the heterostylous floral polymorphism, however, four of them have been extinct by 2000. Although more or less seeds were produced in all nine local populations examined in 1994 and 1995, only negligible seed set was recorded in 2000 except a local population, in which a considerable amount of seeds were produced. Although some seedlings were observed to emerge on the beach along the shoreline near some remaining local populations, all 1, 768 seedlings marked in 1999 spring failed to establish. Furthermore, we could find no juvenile plants established in the study area covering approximately 2 ha. These results suggest that during recent years recruitment has been unsuccessful in the lake metapopulation of N. peltata. The smaller number of seedlings emerged in 2000 than in previous years may be caused by decrease in seed supply due to the extinction of the local populations. It is feared that futile germination will exhaust the soil seed bank of the species in near future.

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