Arvernella sibirica Ignatov & Ignatova, 2022

Arvernella sibirica Ignatov & Ignatova View in CoL , species nova.

Figs. 2–3 View Fig View Fig .

Type: Kemerovo Province, Tashtagol District. Gornaya Shoriya; watershed of Taenza and Mrassu Rivers ; small stand of old-growth tall-herbaceous fir forest among young secondary aspen and birch communities (Gs10- 18), 53.03750°N, 88.35981°E; 740 m alt., 4 Aug 2010 Coll. Pisarenko O. Yu. NSK2004768 About NSK (Holotype MHA, Isotypes NSK, DNA isolate OK3221) GoogleMaps .

Diagnosis: Arvernella sibirica is similar to A. pisarenkoi in a very small plant size, shape of leaves, shape and size of leaf cells, but differs in having smaller leaves, 0.17– 0.20× 0.06–0.07 mm vs. 0.27–0.40× 0.10–0.16 mm in A. pisarenkoi ; not differentiated alar groups vs. clearly differentiated, consisting of subquadrate cells; shorter costae, to 0.2 the leaf length vs. to 0.2–0.4 the leaf length; and smaller spores, 8–10 µm vs. 10–13 µm. Arvernella sibirica differs from A. microclada in smooth vs. prorate leaf cells and smaller spores, 8–10 µm vs. 10–15 µm.

Description: Plants minute, in lax, tiny, delicate mats, green to dark green. Stems creeping, to 4 mm long, in transverse sections composed of homogeneous, firm-walled cells, without central strand and hyalodermis, terete foliate, irregularly branched, branches diverging at about right angle, ca. 1 mm long, terete foliate; rhizoids inserted below leaf insertion; axillary hairs 4–5-celled, upper cell ca. 16×10 µm; paraphyllia absent; proximal branch leaves linear to lanceolate. Stem leaves appressed when dry, erect when moist, 0.17–0.20× 0.06–0.07 mm, lanceolate, gradually narrowed to apex, slightly narrowed to base, not decurrent; costa forked, to ca. 0.2 the leaf length, distinctly delimited; margins plane, finely serrulate throughout; upper and median laminal cells rhomboidal or elongate-rhomboidal, 10–15(–20)×4–5 µm, with length to width ratio 1.5–2.0(–2.5):1, firm-walled, smooth; cells at leaf margins in lower 1/2 the leaf length in 2–3 rows subquadrate to short-rectangular, 5–11×5–6 µm, alar cells not differentiated. Branch leaves similar to stem leaves. Autoicous. Perigonia bud-like, perigonial leaves ovate, strongly concave. Perichaetial leaves straight, 0.7–0.8× 0.3 mm, triangular-lanceolate, acuminate, eplicate, with thick, gradually tapered, indistinctly delimited costa to 0.6 the leaf length. Setae to 7 mm, erect to somewhat flexuose, smooth, brownish to reddish-brown. Capsules inclined to somewhat pendent, urn 0.8–1.0× 0.2–0.3 mm, elongate-ovoid, strongly contracted below mouth when dry and empty. Annuli deciduous. Opercula low conic and shortly and broadly rostrate. Exostome teeth 250–260 µm long, cross-striolate below, papillose above; endostome with basal membrane ca. 1/2 of its length, segments narrow, scarcely perforated, slightly shorter than exostome teeth, cilia 1–2, slightly shorter than segments, nodose. Spores 8–10 µm.

Differentiation: Arvernella sibirica shares some morphological characters with both other species of Arvernella . In leaf size it is closer to A. microclada : 0.17– 0.20×0.06–0.07 vs. 0.14–0.35× 0.055 –0.080 mm. Plants of both A. sibirica and A. microclada appear extremely tiny due to closely appressed leaves (contrary to A. pisarenkoi with erect leaves). Mid-leaf cells of these species are also similar: 10–15(–20)×4–5 µm in A. sibirica and 17– 22 ×3–10 µm in A. microclada ; however, in the latter species cells are strongly prorate dorsally but in A. sibirica they are smooth. In addition, spores of A. sibirica are smaller: 8–10 µm vs. 10–15 µm in A. microclada . Arvernella pisarenkoi differs from two other species in slightly larger size of plants, larger leaves (0.27–0.40 × 0.10–0.16 mm) and longer costae. Both A. pisarenkoi and A. sibirica have leaves with distinct, forked costae, whereas leaves of A. microclada are often ecostate or with indistinct costae.

Arvernella sibirica is also similar to Serpoleskea confervoides – another tiny plant with curved capsules and leaves with short double costae; the latter species differs from both species of Arvernella in having stems with differentiated sclerodermis (cells are uniform in stem cross sections of Arvernella ) and ecostate inner perichaetial leaves (with strong costae extending to mid-leaf in Arvernella ).

The differences between three species of the genus can be also described in the key to identification:

1. Costae absent or very short and indistinct; median leaf cells abaxially strongly prorate. A. microclada View in CoL

— Costae distinct, to 0.2–0.4 the leaf length; median leaf cells smooth or abaxially weakly prorate ....... 2

2. Leaves 0.27–0.40× 0.10–0.16 mm; alar groups clearly differentiated; spores 10–13 ìm ......... A. pisarenkoi View in CoL

— Leaves 0.17–0.20× 0.06–0.07 mm; alar groups not differentiated; spores 8–10 ìm ............... A. sibirica View in CoL

Distribution and ecology: The samples of Arvernella sibirica were collected at four points of the northwestern periphery of the Altai-Sayan mountain region ( Fig. 4 View Fig ).

The localities where the species was collected are largely similar. All of them are situated in hyper humid regions surrounded by areas with sharply continental climate. An average annual temperature is about 0°C; the sum of temperatures above 10°C is not more than 1500°C. Annual precipitation everywhere is more than 800 mm, that is at least twice higher than in neighboring plains, and more than half of the amount falls in the warm period. In winter snow depth exceeds 80 cm and due to wind-caused redistribution can reach to 170–190 cm (Pilnikova, 1993). So thick snow layer keeps soil non-freezing during the winter period, with temperatures on a soil surface being about 0°C ( Lashchinsky & Sedelnikov, 1991).

The dominant tree species in all areas is Abies sibirica . Tall-herbaceous communities are the characteristic feature of the vegetation cover of these regions; they cover vast areas and form a mosaic with fir parcels. In most cases tall-herbaceous communities are polydominant; the herb layer is closed, tall, and multi-tiered. The average height of herbage is 1–1.5 m, but generative sprouts of some species often exceed 3 m. Anthriscus sylvestris, Heracleum dissectum, Crepis sibirica, Bupleurum aureum, Aconitum septentrionale, Cirsium helenioides, Saussurea latifolia , and Euphorbia lutescens are the most constant and abundant.

Coarse herbage acts as a significant environmentforming factor: it forms its own phytoclimate, which affects not only the illumination but also the regime of humidity and temperature. So, under the herbage canopy, diurnal temperature fluctuations are smoothed out, and the air humidity during the daytime is 15–30% higher than above it ( Lashchinsky & Sedelnikov, 1991). In winter, under a thick snow cover, the processes of decomposition of plant residues continue to go on. As a result, despite the enormous productivity of the communities, there is almost no litter on soil and nothing besides of low light prevents the growth of mosses. Small stones of several centimeter, laying on soil under this canopy are thus in specific condition. Small species of Amblystegiaceae occur on such rocks and three collections from such environment appeared to be Arvernella sibirica . The fourth collection of this species was gathered from the base of Padus avium trunk in fir forest with tall herbs.

Other mosses abundant under tall-herb canopy are: Oxyrrhynchium hians , Sciuro-hypnum reflexum, Mnium spinosum, Rhodobryum roseum, Fissidens bryoides, F. taxifolius, occasionally Eurhynchium angustirete and Thamnobryum neckeroides , the two latter species being mixed with Arvernella sibirica in the holotype collection.

The specificity of the habitat conditions of this type of vegetation is emphasized by one more interesting species: Rhynchostegium rotundifolium . It is known in all Siberia only from two localities, 2 and 4 in Fig. 4 View Fig ( Pisarenko, 2014), and in collection in loc. 2 it was mixed with Arvernella sibirica .