Visualizing the extremely narrow geographic and habitat range of the endemic species Cirsium charkeviczii in northeastern Japan, with conservation implications

Cirsium charkeviczii Barkalov is a Japanese thistle endemic to wetlands in the Nemuro region and the southern Kuril Islands (known as the Northern Territories). Although its restricted distribution and specialized niche suggest high conservation importance, this species remains poorly studied in Japan, and its conservation value is not currently recognized. In the Nemuro region, it is known only from high moors; however, the available distributional information is insufficient. To elucidate the distribution and habitat status of this species in mainland Japan, field surveys were conducted between 2023 and 2025 at 13 wetland sites across various wetland environments on the Nemuro Peninsula. In addition, we re-identified 54 specimens of Cirsium species from the three herbaria and compiled a distribution map based on these data. The results showed that C. charkeviczii in mainland Japan occurs exclusively in Sphagnum bogs on marine terraces in the eastern part of the Nemuro Peninsula, highlighting the need for conservation. The habitat in the Nemuro region was different from that in the southern Kuril Islands, possibly reflecting regional environmental differences of the species. In addition, reconciling different taxonomic recognition between Japanese and Russian researchers remains a challenge for effective conservation.