Fedosov, V.E., A.V. Fedorova & E.A. Ignatova. On the two poorly known Orthotrichum species from north Asia
Arctoa (2017) 26: 144–153
Recent revision of Asiatic specimens of the genus Orthotrichum s. str. revealed two species widely distributed and rather frequent in northern Asia, which however differ from all currently recognized species. One of them includes plants growing on silted bases of willows and fallen trees in flood valleys; its specimens were previously referred to O. holmenii, but in fact it fits O. sibiricum, a species described from the lower course of Yenisei River in 1890, but later reduced to the synonymy of O. pallens. However, O. sibiricum differs from European O. pallens in high, branched papillae and pointed, not blunt, leaf apex, and they are also clearly distinct in ITS sequences. Orthotrichum sibiricum occurs in Nenets Autonomous disctict in NE European Russia, the Polar and Subpolar Urals, the lower course of the Yenisei River, on the Anabar Plateau, Orulgan Range and in Transbaikalia. The second species is also superficially somewhat similar to O. pallens, at least most its collections were so named in herbaria. However, it is characterized by having hyaline cells at leaf apices, and usually 1–2 teeth nearby it, as well as branched papillae on laminal cells, immersed to emergent capsules with 8 ribs at entire urn length, stomata half-covered by subsidiary cells, exostome teeth in pairs, 8 or 16 endostome segments, and smooth, hairless calyptra. This species grows mostly on siliceous rocks in cold areas of Siberia, both in northern regions and in high mountains. It is described as a new species, O. hyperboreum; its range includes Polar Urals, Byrranga Mts. in Taimyr, Anabar Plateau, Orulgan Range in the lower course of Lena River, and Chukotka.