Aulacomniuaceae. Genus Aulacomnium

1. Plants large, tumid, leaves strongly concave; leaf apices broadly rounded; margins entire; northern and high mountain species 4. A. turgidum

Aulacomnium turgidum is a widespread species in the Arctic, and one of the most common species in permafrost areas of Asian Russia growing in forest/lichen communities above tree line. It has a restricted distribution in European Russia where it extends only to the northernmost part of the boreal zone, is very rare in most parts of the Urals and absent in the Caucasus. Helpful features in identifying A. turgidum include its delicate texture and tumid plants. It can be confused with A. palustre var. imbricatum; for distinguishing features that separate the two taxa see discussion under A. palustre.

— Plants large, medium sized or small, laxly foliate, leaves not or slightly concave; leaf apices acute (rarely rounded in Arctic/alpine A. palustre var. imbricatum); leaf margins serrulate, serrate or subentire; hygrophylic species found throughout Russia or rare terrestrial species in western European Russia 2

2. Plants 5–10)–15) cm high; leaves straight or weakly flexuose when dry; leaf apices gradually acuminate or slenderly acute; leaf margins subentire; all stems without leafless distal ends, caducous leaves absent 3. A. acuminatum

A widespread species in East Siberia, extending westwards to Yenisei River (Central Siberia). Aulacomnium acuminatum is strongly associated with calcareous areas where it may completely replace A. palustre, the otherwise common Aulacomnium species in the region. Aulacomnium acuminatum is distinguished from A. palustre by its larger plant size; stronger, rich-yellowish color; often gradually acuminate leaf apices (enabling the species to be recognized in the field); and complete absence of caducous leaves at the shoot ends.

— Plants 1–7(–12) cm high; leaves lexuose when dry; leaf apices acute; leaf margins serrulate or serrate; sterile stems often with leafless, distal ends, caducous leaves present 3

3. Plants medium-sized, rarely small; leaf margins serrulate above, teeth perpendicular to leaf margins; caducous leaves in terminal groups and also sparingly scattered below, oblong, 4–5(–7) cells wide, 3–5-celled at base; widespread in Russia 2. A. palustre

This is a very widespread Russian species throughout the Arctic to the steppe zone. It is mainly a peat-bog species in the forest zone of European Russia, and a common moss in all kinds of East Siberian forests. It can be recognized by its yellowish-green color that strongly contrasts with its bright-whitish costa, and leaves that are usually flexuose above when dry. Plants from the Arctic and high mountains may have leaves with rounded apices somewhat similar to those of A. turgidum: these forms are often segregated as A. palustre var. imbricatum. This variety is best separated from A. turgidum by examining all the stem leaves. Inevitably, collections of var. imbricatum have some acuminate leaves, and some stems with terminal bundles of caducous leaves. Confusion with A. androgynum is also possible, especially for very small plants of A. palustre found in steppe areas with small depressions of swampy Betula forests. In fact, in this environment plants of A. palustre are even smaller than average plants of A. androgynum. In these cases, the shape, size and arrangement of cauducous leaves are the most important features distinguishing the two species (see key).

— Plants small; leaf margins coarsely serrate to serrulate above, teeth directed upwards; caducous leaves in terminal groups only, elliptic to spindle-shaped, 2(–3) cells wide, 1-celled at base; rare species in westernmost regions of European Russia 1. A. androgynum

Aulacomnium androgynum is known from scattered localities in the western parts of European Russia (North West, the Caucasus, and Rostov Province), and is not rare only in Kaliningrad Province. The species grows on wet peaty and sandy soil banks and sandstones.