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Kamchatka, due to its position in hemiarctic northeast Asia on the migration pathways of taxa from Asia to America and vice versa, which has an insular geographical position and provides numerous pieces of evidence regarding current active volcanism, has a peculiar flora. The study of the liverwort flora of the southeastern part of the Kamchatka Peninsula (South Kamchatka Nature Park), which, until now, has been very poorly explored, showed high taxonomic richness and some specificity due to volcanic evidence. In total, 132 species have been recorded in this sector of the “Pacific Ring of Fire”. This number is quite high compared to other floras of hemiarctic Asia. The influence of volcanism on flora formation is prominent due to the regular renewal of substrates, the appearance of specific habitats (for example, thermal water outlets), the displacement of vegetation belts, and ashfalls leading to the presence of extended spaces free of vegetation cover and is an area that is open for invaders. The altitudinal zonality, despite the difference of 2000 m in elevation, is not clearly expressed in the flora of the liverworts: arctomontane species descend far down the altitudinal profile, reaching the Betula ermanii forests. The “plantless” uppermost belt in the mountains, commonly described in floristic studies of vascular plants, is not actually free of plants if liverwort occurrences are considered. A number of species normally occupy unusual habitats in the studied flora due to contemporary volcanism evidence, e.g., sulfur-rich substrates.

Marsupella sect. Stolonicaulon is not speciose and is a commonly neglected section within the genus, which currently includes three species with somewhat similar morphologies (wiry shoots with distanced leaves) and distributions in the mountains of tropical and subtropical regions (SE (Southeast) Asia, the Venezuelan Andes, and the high mountains of SE Brazil). After studying materials that were found to be dissimilar to the “traditional” Marsupella that were collected in the last decade by the authors of this article, it was found that these plants belong to three new-for-science species, and all of these species should be included in Marsupella sect. Stolonicaulon. The newly described species have expanded the boundaries of morphological variability, not only for the section itself, but also for the genus based on two findings: (1) the leaves of Marsupella sect. Stolonicaulon can be either appressed and entire or spaced and deeply divided (thus, the plants could occasionally be similar to Cephaloziella or Anastrophyllum); (2) some species of the section possess regular underleaf production. The first discovery of regular underleaves in Marsupella, as noted in two of the three newly described taxa, is the main morphological novelty described in this paper. The development of regular underleaves is a presumable relict character that brings Marsupella closer to Nardia, which was recently transferred to the Gymnomitriaceae and occupies an isolated position within its own subfamily, Nardioideae.