Vegetation of segde-hypnum and wooded rich fens and swamps on the north border of their distribution in Western Siberia

The vegetation of sedge-hypnum and forest swamps rich in soil nutrition was studied in “Chistoe Boloto” (Open Mire), located in the low left-bank terrace of the Irtysh River valley in the environs of Tobolsk city (58.25°–58.09°N, 68.15°–68.34°E) near the northern limit of this mire type distribution in Western Siberia. A total of 7 associations, 2 subassociations, 1 variant, and 1 community type are described, of which one association (Hamatocauli vernicosae–Caricetum lasiocarpae) and 2 subassociations (Telypterido palustris–Betuletum pubescentis inops and Thelypteridetum palustris brachythecietosum mildeanae) are introduced as new ones. Communities are assigned to 4 alliances of 3 orders and 2 classes. The class Alnetea glutinosae Br.-Bl. et Tx. ex Weshoff et al. 1946 of coniferous and small-leaved forests of Eurasia includes the order Calamagrostio purpureae–Piceetalia obovatae Lapshina 2010 combining forest-like swamp communities with dark coniferous species in the tree layer, called “sogra” in Siberia, with two associations within 2 alliances in the study area. Ass. Mnio stellari–Pinetum sibiricae Lapshina 2010 (Fig. 2) of the alliance Carici cespitosae–Piceion obovatae Lapshina 2010 includes communities of the dark coniferous sogra. Their considerable geographical remoteness from the main distribution area in the south of the forest zone of Western Siberia and noticeable differences in the species composition allowed to establish within it var. Viola epipsila. Ass. Carici appropinquatae–Pinetum sylvestris Lapshina 2010 (Fig. 3) of the alliance Carici appropinquatae–Laricion sibiricae Lapshina 2010 combines birch–pine herb-tussock sedge communities, rich in soil nutrition, dominated by Carex apropinquata, Thelypteris palustris. The class Scheuchzeria palustris–Caricetea nigrae Tx. 1937 is represented by the alliance Saxifrago-Tomentypnion Lapshina 2010 in the study area. Diverse communities of sedge-hypnum and Menyanthes-low sedge-hypnum fens, which are visually well distinguished in the vegetation cover by the dominance of different sedge species (Сarex diandra, C. dioica, C. limosa, C. rostrata) and Menyanthes, correspond well in their floristic composition to the diagnosis of the ass. Brachythecio mildeanae–Caricetum limosae Lapshina 2010 (Fig. 6) described on the hypnum fens in the southeast of Western Siberia. With the distance from the Tobolsk slope and gradual reduction of the groundwater influence, the domination in Menyanthes-low sedge-hypnum communities gradually flows from the ass. Brachythecio mildeanae–Caricetum limosae to Carex lasiocarpa — ass. Hamatocauli vernicosae–Caricetum lasiocarpae ass. Nov (Fig. 7). A characteristic feature of the studied mire is the wide distribution of sedge-fern and fern communities among low sedge-hypnum fens dominated by Carex lasiocarpa and Thelypteris palustris, which are assigned to associations Thelypterido–Caricetum lasiocarpae Lapshina 2010 (Fig. 8) and Thelypteridetum palustris Lapshina et al. 2018 (Fig. 9). The latter is established as subass. brachythecietosum mildeanae subass. nov. Comparison of the of mire habitats is made based on L. G. Ramenskiy ecological scales (Ramensky et al., 1956). Ecological indices (grades) of water supply and nutrient condition for each relevé are calculated. The ecological ordination of mire habitats (Fig. 10) shows values for mineral nutrition (Y-axis), and for water supply (wetness) (X-axis). According to the scale of mineral nutrition, the t communities exist in a narrow ecological range corresponding to mesotrophic conditions (6.5–9.0 grades). This is due to the fact that at high values of electrical conductivity and a medium close to neutral (EC=180–430; pH=6.5–7.5), peat soils rich in ground nutrition are poor in nitrogen and phosphorus (Wassen et al., 2005). According to the humidity scale, the studied vegetation may be divided into 3 groups: communities of hydromesophilic (77–88 degrees), subhydrophilic (89–94 degrees), and aerohydrophilic (95–103 degrees) habitats corresponding to the main classes of mire vegetation. The first group includes syntaxa of two alliances of wooded swamps of the class Alnetea glutinosae, which occur in moderately humid habitats. The low birch swamps communities of the class Alnetea glutinosae are common in higher water saturated habitats. Syntaxa of low sedge-hypnum, fern, and sedge-fern fens of the class Scheuchzerio–Caricetea nigrae are common to the most wet fen habitats, often associated with high level of water horizons buried in peat deposits. The classification results and ordination of syntaxa are confirmed by the t-SNE method, which allows displaying multidimensional hyperspaces on the plane. The right side of the ordination diagram (Fig. 11) shows well-differentiated low sedge-hypnum fens communities of the class Scheuchzerio–Caricetea. The rare sedge-hypnum communities with the Paludella squarrosa dominance are located somewhat apart, bonded to the exits of springs. The opposite part of the diagram shows well-differentiated communities of wooded swamps within the class Alnetea glutinosae. They are adjoined by communities of the ass. Thelypterido palustris–Betuletum pubescentis, within of the class Alnetea glutinosae. The most difficult challenging was to determine the position of Carex lasiocarpa-dominated fern, sedge-fern and sedge-hypnum communities within the higher units of classification. They occur in ecologically similar habitats as low sedge-hypnum communities of the alliance Saxifrago-Tomentypnion with well-represented diagnostic species in these. Due to that and the presence of hypnum moss layer, we assigned these communities to the alliance Saxifrago-Tomentypnion of the class Scheuchzeria-Caricetea.