Low-sedge vegetation of waterlogged bog hollows and fens in the north of Western Siberia

The classification of West Siberian mire vegetation is more or less well developed in the southern part of the forest zone (Lapshina, 2010) while in the northern part of the West Siberian Plain it has received much less study. There are only a small number of publications containing descriptions of mire types and plant communities (Pyavchenko, 1955; Boch et al., 1971; Kirpotin et al., 1995; Smagin, 2003; Neshatayev et al., 2002). This paper presents the classification results for the low-sedge vegetation of waterlogged hollows and Sphagnum lawns, within flat palsa-bogs, ombrotrophic raised bogs and transitional mire complexes, which is assigned to two alliances — Stygio–Caricion limosae Nordhagen 1943 and Scheuchzerion palustris Nordhagen ex Tx. 1937 of the class Scheuchzerio–Caricetea nigrae Tx. 1937. The classification is based on 422 relevés performed in 2004–2019 at 22 plots located between 63° and 75° N in the northern taiga, forest tundra, and southern tundra subzones of West Siberia (Fig. 1). In the most recent summary “Vegetation of Europe…” (Mucina et. al., 2016), the alliance Stygio–Caricion limosae is assigned to the order Sphagno watnstorfii–Tomentypnetalia Lapshina 2010, however this does not seem conclusive. Communities of this order are closely associated with rich fens, often spring fens fed by ground water, which does not correspond to the real conditions in which communities of this alliance are developed. Ecologically, in the current structure of the class Scheuchzetio–Caricetea nigrae (Peterka et al, 2017), the alliance Stygio–Caricion limosae has taken the true place of the alliance Rhynchosporion albae Koch 1926 (ICPN, Art. 36), which was initially unambiguously associated with the order Caricetalia nigrae Koch 1926 based on the original relevés and diagnostic species (Rhynchspora alba, Agrostis canina, sphagnum mosses of sec. Subsecunda). Therefore, we also consider the alliance Stygio–Caricion limosae belonging to the order Caricetalia nigrae, where it fits better judging by its ecological and floristic features. The differential species combination of the alliance Stygio–Caricion limosae in the northern part of West Siberia includes Carex limosa, Drosera obovata, Juncus stygius, Gymnocolea inflata, Sphagnum perfoliatum, S. platyphyllum, S. subsecundum, Utricularia minor, U. ochroleuca, Warnstorfia exannulata, and W. fluitans. Within this alliance, two new associations with subassociations have been described: Utricularo ochroleucae–Caricetum limosae and Sphagno perfoliati–Caricetum rotundatae, of which the first one occurs in the northern taiga mires, while the second one in the forest tundra and southern tundra subzones. The order Scheuchzerietalia palustris Nordhagen ex Tx. 1937 comprises ombrotrophic vegetation of Sphagnum lawns and bog hollows (Mucina et al., 2016) and currently includes the only alliance Scheuchzerion palustris. Its typical boreal suballiance Scheuchzerienion palustris suball. nov. (nomenclature type — lectotypus hoc. loco: ass. Scheuchzerietum palustris Tüxen, 1937: 61) is represented by two associations: Eriophoro vaginati–Sphagnetum baltici and Carici limosae–Sphagnetum jenseni. Their distribution to the north is limited by the mire complexes of the northern taiga. Further north similar habitats are occupied mainly by communities of the predominantly subarctic suballiance Caricion rariflorae. Within this suballiance, two associations — Carici rotundatae–Sphagnetum baltici and Carici rotundatae–Sphagnetum lindbergii — are widely distributed over the entire gradient from the northern taiga to the southern tundra. The ass. Carici rariflorae–Sphagnetum baltici occurs only occasionally and is bound to the forest tundra and southern tundra. Statistical processing of the entire data set was performed to confirm the classification results and make a number of syntaxonomic decisions. The results of t-SNE ordination (t-distributed stochastic neighbor embedding method) (van der Maaten, Hinton, 2008) confirmed the validity and expediency of separating oligotrophic and mesooligotrophic low-sedge communities of hollows and fens not only at the alliance level, but also at the order level. Despite certain physiognomic and floristic similarities, the location points of the two alliances in multidimensional space are well differentiated and do not overlap with each other (Fig. 10). Calculation of the floristic similarity degree of relevés with regard to species abundance and visualization of the statistical processing results have clearly demonstrated that the entire relevé array of oligotrophic sphagnum lawns in the alliance Scheuchzerion palustris can be divided by the dominant sphagnum moss species into separate clusters, within which dominant grass layer clusters could also be distinguished. Given that, in the future, the formal statistical processing of large sets of geobotanical data will become an increasingly important tool to underpin syntaxonomic decisions, this fact cannot be ignored. In this connection, we propose to review the current practice of identifying associations of mire vegetation by the dominant species of vascular plants and sub-associations by the dominant moss species. The latters are of primary importance in the poor-species plant communities of waterlogged hollows and fens, because they are more sensitive to ecological conditions of habitats, which ultimately determine the entire floristic composition and community structure.