Orthothecium sibiricum Ignatov & Ignatova, 2020

4. Orthothecium sibiricum Ignatov & Ignatova , sp. nova. Figs. 10 View Fig , 11A–M, 12F–M, 13A–H.

Type: [ Russia] USSR, Altai Mts., 51°44’N – 87°27’E, 470 m alt., Teletskoye Lake, Bolshoye Istyube Creek. Calcareous cliffs in deep canyon. 11 July 1991, Ignatov 18/119 (Holotype MHA 9029735, isotypes MW!, LE! CBFS!, and in other herbaria, exsiccata “Mosses of USSR” #166). Figs. 10 View Fig , 12G View K L , 13A–C View Fig . [DNA sequences: nuclear ITS MT681162 View Materials , plastid trnF–trnS MT683739 View Materials ].

Diagnosis: Orthothecium sibiricum differs from O. chryseon in having slightly vs. strongly plicate leaves, more abruptly tapered to long and often piliferous acumen, more strongly rounded to base; from O. lapponicum by its smaller size, slightly plicate vs. smooth leaves, more gradually vs. abruptly acuminate leaves with longer acumina.

Description: Plants medium-sized, yellowish-green or brownish-green, glossy, in loose tufts. Stems ascending to erect, 2–4 cm long, terete-foliate, irregularly branched, branches 5–10 mm long. Leaves erect-spreading to loosely appressed, 1.0–1.8(–2.1)× 0.4–0.7 mm, ovate-lanceolate, gradually acuminate, with long, narrow acumina, moderately concave, slightly plicate; margins narrowly recurved from near leaf base to above mid-leaf; cells 75–100×6–8 µm, in mid-leaf with moderately thickened, weakly porose walls, at basalmost part of leaf more thick-walled and strongly porose; alar cells not differentiated. Specialized vegetative reproduction by 4–5- celled clavate axillary gemmae. Apparently dioicous. Gametangia and sporophytes unknown.

Distribution and ecology. Orthothecium sibiricum is currently known from southern Siberia (Altai and Khamar-Daban mountains) and from several distant localities in Yakutia (Sette-Daban, Suntar-Khayata and Orulgan Ranges in Verkhoyansky System, Olenyokskij District, Ulakhan-Chistai Range in Chersky System, Neryungri and Ust-Maya Districts in southern Yakutia) and the lower course of Lena River. It grows in mountain areas at altitudes of 470–1000 m, from the forest zone to the mountain tundra, on wet or damp calcareous cliffs, in niches of cliffs, between rocks and on rock outcrops on slopes.

Additional specimens examined: RUSSIA: Yakutia: Oymyakon District, Mus-Khaya Mt., Knoriy Creek, 14.VII.2011 Ivanov s.n. (MHA 9019891); Eveno-Bytantaysky District, Orulgan Range, upper course of Tumara River, Ignatov 11-3874 (MHA 9019892); Neryngri District, Ivak River, 1.VIII.1987 Volotovsky s.n. (MHA ex SASY); Olenyokskiy District, Sibikte River basin (tributary of Daldyn River, 20.VII.1957 Lukicheva & Samarina s.n. (LE). Krasnoyarsk Territory: Taimyr Peninsula, Anabar Plateau, Fomich River, Fedosov 08-581 (MW 9045911). Buryatia: [Oka Distr.], East Sayan Mts, Bel’sky Range, Urik River at the mouth of Zegen-Gol Creek, 10.VIII.1959 Bardunov & Vysokovsky s.n. (LE).

Differentiation and variation. Before molecular phylogenetic studies, the plants of Orthothecium sibiricum were identified partly as O. chryseon or O. rufescens (for larger plants) or O. strictum (for smaller plants). The larger plants, however, have only slightly plicate leaves ( Fig. 10 View Fig ), and thus cannot be comfortably placed into the two former species. The leaves in larger plants of O. sibiricum are erect-spreading to spreading, whereas in O. chryseon they are usually appressed. Smaller plants (Fig. 11 A–M) have ovate to elliptic leaves and their size approaches the largest expressions of O. strictum , but differ in more strongly concave leaves, piliferous acumina, and leaf bases rather distinctly rounded towards insertion. The latter feature differentiates O. sibiricum from O. chryseon and O. rufescens as well (Fig. 11N–O): in the two latter species the leaves are not strongly rounded to the base.

Small plants of O. sibiricum approach O. lapponicum , but typical expressions of O. lapponicum are larger plants with leaves 1.0–2.3× 0.5–0.9 mm vs. 1.0–2.1× 0.4–0.7 mm in O. sibiricum , more abruptly and shorter acuminate. Populations of O. sibiricum from southern Siberia comprise plants with larger leaves, 1.8–2.1× 0.6–0.7 mm, while most of the Yakutian collections have leaves 1.0– 1.8× 0.4–0.6 mm. Molecularly, the plants named here O. sibiricum have two genotypes (see molecular results and discussion), the characteristic plastid haplotype is present only in a minority of mostly larger expressions, while most analysed plants share the plastid DNA with O. lapponicum . The morphologically distinct plants of the latter species, however, possess a distinct ITS ribotype.