Orthothecium retroflexum Ignatov & Ignatova, 2020

2. Orthothecium retroflexum Ignatov & Ignatova , sp. nova. Figs. 7A–B, F–J View Fig , 8K–N View Fig , 9A–C View Fig .

Type: Asian Russia, Yakutia, Eveno-Bytantaysky Distr., Orulgan Range, upper course of Aenigan-Toloono Creek, 68°14'N, 128°06'E, alt. 900 m, wet cliffs, 7 Aug 2011 Ignatov 11-3974 (Holotype: MHA 9029736; isotypes MW, S, CBFS). Figs. 7A–B, F–G View Fig , 8N View Fig . [DNA sequences: nuclear ITS MN794387 View Materials , plastid trnL–rps4 MT683730 View Materials ].

Diagnosis: Orthothecium retroflexum is similar to O. chryseon , but differs in more broadly ovate leaves, abruptly contracted into a short, revolute or hooked apiculus.

Description: Plants large, golden-green, reddish-green or bronze, glossy, in loose or dense tufts. Stems ascending to erect, 2–5(–10) cm long, not or weakly branched, terete-foliate. Leaves erect, appressed, imbricate, 2.1–2.7× 0.8–1.2 mm, ovate, abruptly contracted into short, revolute or hooked apiculus, cochleariform, strongly plicate, slightly narrowed to the insertion, non-decurrent; margins narrowly recurved at most of leaf length; cells in the middle part of leaf 50–80×5–6 µm, thick-walled, porose; basal cells shorter and wider, strongly porose; alar cells not differentiated. Specialized vegetative reproduction unknown. Dioicous. Sporophytes unknown.

Distribution and ecology. Orthothecium retroflexum is widespread in Arctic regions of European and Asian Russia and in permafrost regions throughout Asian Russia. In southern Siberia it is known only from a single record in Eastern Sayan Mts, and the southernmost locality of this species is in Sakhalin Island. It was also revealed in collections from Alaska (see specimens examined), and likely is more widespread in the Arctic, although its distribution needs clarification. It is likely that O. retroflexum was previously reported from Russia under the name O. chryseon , while the records of O. rufescens from northern Russia mostly belong to O. chryseon (e.g., in Abramova et al., 1961). The revision of herbarium collections (MHA, MW, and partly LE) revealed that the distributions of O. chryseon and O. retroflexum largely overlap. Both are widespread in the Arctic and permafrost areas and grow in similar habitats; however, O. chryseon has a wider distribution in the southern regions of Russia. In the northernmost Arctic areas, O. retroflexum often dominates in wet tundra. In permafrost areas of Siberia, it grows in mountain tundra, nival communities, near melting snow beds, along brooks and in creek valleys, on wet cliffs, most often on calcareous rocks. In Sakhalin Island, it was collected at an altitude of ca. 700 m, in a mossy tundra community and so-far this is the southernmost locality of the species. While occurring in the Eastern Sayans in Baikal area, the species is seemingly absent from West Sayans and Altai, as well as from Caucasus, Urals and mountains of Central Europe.

Selected additional specimens examined: EUROPEAN RUSSIA: Arkhangelsk Province, Franz Josef Land: Hooker Island, 2.VIII.1930, V.P. Savicz s.n., Hepaticae et Musci URSS Exsiccati Dec. I (1957) №10 (MHA, MW); Rudolf Island, 2.VIII.2012, Kholod s.n. (LE); Ziegler Island, Konoreva 911 (LE). ASIAN RUSSIA: Krasnoyarsk Territory: Taimyr Peninsula: near Dikson Town, Fedosov 17-2-12-18 (MW 9111718); Syrutaturku Lake, VIII.1994 Pospelova 94/85 (MW 9049063); lower course of Bikada River, 74°50’N. 106°10’E; 15.VIII.1989 Pospelova 37 (MW 9049066); upper course of Ereechka River, Fedosov 13-3-0019 (MW 9050854); middle course of Fomich River, 19.VIII.2003 Pospelov s.n. (MW 9050862). Yakutia: Novosibirskie Islands: Stolbovoy Island, Troeva s.n. (LE); Tomponsky District: Sette-Daban Mts, Sakkyryr River, Ivanov 17-624 (MW 9090216); Segenyakh Creek, Ignatov & Ignatova 17-879 (MW 9090286); Oymyakon District: Mus-Khaya Mt., Knoriy Creek, Ignatov & Ignatova 11-3050 (MHA); Ust-Maya District, Allakh-Yun settlement, 30.VIII.2000 Ivanova s.n., Bryophyta Sibiriae Exsiccata, Fasc. I (2007) №38, MW 9049083); Novosibirskie Islands, Bolshoy Lyakhovsky Island, 5.VIII.1956 Aleksandrova s.n. (MHA); Momsky District, Ulakhan-Chistai Range, Mramornaya Mt., Ignatov & Ignatova 18-1433 (MHA 9027927). Chukotka: Shmidt Cape, 29.VII.1965 Afonina s.n. (LE) Magadan Province: Ol’skoe Basalt Plateau in the upper course of Ola River, 11.VIII.2011, Malashkina MG-30-01-11 (MHA). Buryatia: Oka District, East Sayan Mts., Kitoy Range, 15.VIII.1960 Bardunov s.n., Bryophyta Rossica et civitatum collimitanearum Fasc. IX (2012), №380 (MW 9049072). Sakhalinskaya Province: Sakhalin Island, Smirnykhovsky District, protected area “Vaida Mountain”, Ignatov & Teleganova 06-146 (MHA 9018693).

NORTH AMERICA: U.S.A., Alaska: Utukok River, 7.VII.1969 Steere, Holmen & Mårtensson s.n., Bryophyta Arctica Exsiccata #46 (MW 049092); Point Barrow and vicinity, Steere 19054 (LE); East Oumalik and vicinity, Steere 16610 (LE).

Differentiation and variation. Orthothecium retroflexum differs from O. chryseon in having ovate vs. ovate-lanceolate or ovate-triangular leaves, cochleariform vs. moderately concave, and, most importantly, a hooked, revolute apiculus in all leaves in well-developed shoots (only in young shoots the leaves may have ± straight apices), whereas in O. chryseon the apical portions of leaves are straight or flexuose, only occasionally recurved at 90° in some leaves. This apiculus pattern in O. retroflexum is much alike that in Plagiothecium berggrenianum Frisvoll , where the hooked leaf apices are sufficient for species identification in the field. The leaf cells of O. retroflexum are generally thicker-walled and more strongly porose than in O. chryseon .

Orthothecium retroflexum is usually a tall, robust plant (stems with leaves ca. 2 mm wide when dry); however, slenderer morphotypes occasionally occur (stems with leaves 1–1.5 mm wide when dry). Leaf length and especially leaf width are also variable (2.1–2.7× 0.8–1.2 mm), but the leaves keep their ovate shape with a short, strongly revolute apiculus; the latter character is present in most leaves, though the youngest leaves at some shoot tips may occasionally only be curved at right angle, as in O. chryseon .

The strongly concave, cochleariform leaves raised suspicions that O. chryseon var. cochleariifolium (Lindb.) Limpr. (Laubm. Deutschl. 3(27): 23. 1895. — Leskea chrysea var. cochleariifolia Lindberg, Öfvers. Kongl. Vetensk.-Akad. Förh. 23: 543. 1867) is the appropriate name for this taxon. Its range in North America is confined to the high Arctic (Redfearn Jr., 2014), which indirectly implies that our high Arctic plants may belong to this taxon. However, the type specimen in H-SOL [“ Orthothecium chryseum var. cochlearifolium Lindb. 9 Febr. 67, Spitsbergen, Treurenbern bay, 1861, leg. Malmgren, H-SOL 1834001”!] belongs to O. chryseon , not to the species described here as O. retroflexum , and lacks the characteristic hooked apiculus of the latter (cf. partial image of the holotype in Fig. (9F).