Orthothecium brunnescens Ignatova & Ignatov, 2020

1. Orthothecium brunnescens Ignatova & Ignatov , sp. nov. Figs. 4–6 View Fig View Fig .

Type: Asian Russia, Yakutia, Tomponsky District, near the pass between Kuraanakh and Dyby Rivers,

62°48'40"N, 139°00'30"E, alt. 1100 m, on cliffs in mountain tundra. 27 Aug 2017 N. Bysyin 17-482. Holotype MHA 9025568, isotypes MW, LE, CBFS. Figs. 4A–C, F–J View Fig ; 5A; 6C–D. [DNA sequences: nuclear ITS MT681126 View Materials , plastid trnF–trnS MT683698 View Materials ].

Diagnosis: Orthothecium brunnescens differs from the most similar species, O. chryseon and O. sibiricum , in having leaves homomallous to slightly falcate vs. straight, and plant colour olivaceous, brownish to dark brown vs. golden-yellow, bronze or vinaceous; additional distinctions from O. chryseon include erect-spreading to spreading vs. appressed leaves, not or scarcely plicate vs. strongly plicate leaves, and from O. sibiricum acute vs. narrowly acuminate/apiculate leaves.

Description: Plants medium-sized, olivaceous or yellowish-green, often with brownish tint, occasionally dark brown, glossy. Stems 2–6 cm long, 1.5–2.0 mm wide with leaves, terete-foliate. Leaves loosely or moderately densely arranged, erect-spreading to spreading, often slightly homomallous, 1.6–2.1(–2.6)× 0.6–0.8 mm, ovate-lanceolate to oblong ovate, acute or short-acuminate, rounded at base, not decurrent, slightly concave, not plicate or, rarely, scarcely plicate; leaf margins plane or, rarely, weakly recurved at places, entire or slightly serrulate in apical part; laminal cells 50–80×7–9 µm. Specialized asexual reproduction by brood bodies 60–80 µm long. Gametangia and sporophytes unknown.

Distribution and ecology. Orthothecium brunnescens is so far only known from several localities in the eastern Verkhoyansky Range, Yakutia, in the basins of Vostochnaya Khandyga and Maya Rivers. It was collected at altitudes between 370–1100 m, on wet calcareous cliffs near waterfalls, at riverbanks, in niches and on ledges of cliffs in larch forest and in mountain tundra.

Additional specimens examined: RUSSIA: Yakutia: Tomponsky District:Suntar-Khayata Mts, “Suntar Khayata” protected area, 774 m alt., At-Moole brook, 22.VII.2003 Ivanova & Zolotov s.n. (MW 9049081); Sette Daban Mts, left bank of Vostochnaya Khandyga River opposite Segenyakh (Rosomakha) Creek mouth, 447 m alt., Ignatov & Ignatova 17-767 & 17-771 (MHA 9025345 & 9025340); Sette Daban Mts, western slope of Okraina Range, Nadezhda Creek, 500 m alt., Ignatov & Ignatova 16-989 (MHA 9022742).

Differentiation and variation. Superficially, Orthothecium brunnescens is not similar to any other species of the genus and can be confused in the field with Pseudohygrohypnum species, e.g. P. subeugyrium which is not rare on wet cliffs in Asian Russia. It differs, however, by undifferentiated alar cells and only slightly falcate leaves. The combination of eplicate leaves, medium-sized to large plants, homomallous to slightly falcate leaves and dark brownish, instead of reddish colour make O. brunnescens rather easy to identify, despite a rather variable leaf shape: plants from relatively dry and shaded cliffs in the forest have lanceolate leaves (Figs. 5F, 6F), while in wet sunny places leaves are broad (Figs. 5E, 6E). Large plants of O. sibiricum and O. lapponicum have straight, non-falcate leaves, and their reddish colour is conspicuous, especially in plants from exposed habitats, although it might disappear in not properly dried herbarium specimens. The colour of O. brunnescens is, however, variable: it is yellowish-green in shady habitats and usually with a brownish tint in exposed places ( Fig. 6E View Fig ). Large phenotypes of O. strictum might have a similar aspect, but their leaves are not homomallous (except for one case discussed under that species), and are generally considerably smaller, having leaves 0.8–1.7×0.2–0.6 µm vs. 1.6–2.1(–2.6)×0.6– 0.8 µm in O. brunnescens . The leaf margins recurved for most of the leaf length are commonly present in O. strictum , but never in O. brunnescens .