One of the thermokarst relief forms is baidzharakh massif — the group of mounds separated by trenches formed as a result of the underground ice-wedge polygonal networks melting (Fig. 1). Study of baidzharakh vegetation took place on the northeast coast of the Taimyr Peninsula (the Pronchishcheva Bay area) and on the New Siberian Islands (the Kotelny Island) in 1973–1974 (Sumina, 1975, 1976, 1977a, b, 1979 et al.). The aim of this paper is to produce the classification of baidzharakh mound and trenches communities according to the Brown-Blanquet approach (Westhoff, Maarel, 1978) and to compare these data with the community types earlier established on domination principle (Sumina, 1975 et al.). The information obtained in the 1970s could be helpful in a comparative assessment of the thermokarst process dynamics over the past 4 decades, as well as for comparing these processes in other regions of the Arctic. Both studied areas are located in the northern part of the arctic tundra subzone. On the Taimyr Peninsula (and in particular in the Pronchishcheva Bay area) the plakor (zonal) communities belong to the ass. Salici polaris–Hylocomietum alaskani Matveyeva 1998. Our relevés of plakor tundra on the Kotelny Island demonstrate similarity with the zonal communities of the northeast coast of the Taimyr Peninsula (Table 2). Relevés of communities of thermokarst mounds were made within their boundaries, the size of ~ 30 m². In trenches sample plots of the same area had rectangular shape according to trench width. Relevés of plakor tundra were made on 5x6 m plots. There were marked: location in relief, moistening, stand physiognomy, nanorelief, the percent of open ground patches and degree of their overgrowing, total plant cover, that of vascular plants, mosses, and lichens (especially — crustose ons), and cover estimates for each species. The shape of thermokarst mounds depends on the stage of thermodenudation processes. Flat polygons about 0.5 m height with vegetation similar to the plakor tundra are formed at the beginning of ice melting (Fig. 3, a), after which the deformation of the mounds (from eroded flat polygon (Fig. 3, b) to eroded conical mound (Fig. 3, c). Such mounds of maximal height up to 5 m are located on the middle part of steep slopes, where thermodenudation is very active. The last stage of mound destruction is slightly convex mound with a lumpy surface and vegetation, typical to snowbed sites at slope foots (Fig. 3, d, and 5). Both on watersheds and on gentle slopes mounds are not completely destroyed; and on such elongated smooth-conical mounds dense meadow-like vegetation is developed (Fig. 6). On the Kotelny Island thermokarst mounds of all described shapes occur, while in the Pronchishcheva Bay area only flat polygons, eroded flat polygons, and elongated smooth-conical mounds are presented. Under the influence of thermodenudation the plakor (zonal) vegetation is being transformed that allows to consider the most of mound and trench communities as the variants of zonal association. On the base of 63 relevés, made in 14 baidzharakh massifs, 2 variants with 7 subvariants of the ass. Salici polaris–Hylocomietum alaskani Matveyeva 1998 were established, as well as 1 variant of the azonal ass. Poo arcticae– Dupontietum fisheri Matveyeva 1994, which combines the vegetation of wet trenches with dense herbmoss cover. A detailed description of each subvariant is done. All these syntaxa are compared with the types of mound and trenh communities established previously by the domination principle (Sumina, 1975, 1976, 1979 et al.) and with Brown-Blanquet’ syntaxa published by other authors. The Brown-Blanquet approach in compare with domination principle, clearly demonstrates the similarity between zonal and baidzharakh massifs vegetation. Diagnostic species of syntaxa of baidzharakh vegetation by other authors (Matveyeva, 1994; Zanokha, 1995; Kholod, 2007, 2014; Telyatnikov et al., 2017) differ from ours. On the one hand, this is due to the fact that all mentioned researchers worked in another areas, and on the other, with different hierarchial levels of syntaxa, which are subassociations (or vicariants) in cited works or variants and subvariants in the our. Communities of mounds as well as of trenches in different regions have unlike species composition, but similar apearance, which depends on the similarity of the life form composition and community pattern, stage of their transformation and environmental factors. This fact is a base to group communities by physiognomy in order to have an opportunity of comparative analysis of baidzharakh vegetation diversity in different regions of the Arctic. In total, 6 such groups for thermokarst mounds and trenches are proposed: “tundra-like” ― vegetation of flat polygonal mounds (or trenches) is similar to the plakor (zonal) communities; “eroded tundra-like” ― tundra-like vegetation is presented as fragments, open ground occupies the main part of flat polygonal mounds; “eroded mounds with nonassociated vegetation” ― eroded mounds of various shapes up to sharp conical with absent vegetation at the top and slopes, sparse pioneer vascular plants on a bare substrate and crustose lichens and chionophilous grasses at foots; “meadow-like” ― herb stands with a participation of tundra dwarf-shrubs, mosses, and lichens on elongated smooth-conical mounds and in moderately moist trenches; “communities in snowbeds” ― thin plant cover formed by small mosses, liverworts, crustose lichens, and sparse vascular plants in snowbed habitats on destroyed slightly convex mounds with a lumpy surface and in trenches; “communities of cotton grass” or others, depending on the dominant species ― in wet trenches where vegetation is similar to the arctic hypnum bogs with dominant hygrophyte graminoids as Eriophorum scheuchzeri, E. polystachion, Dupontia fischeri et al. This sheme according to physiognomic features of thermokarst mound and trench communities, as a simplier way to assess the current dynamic stage of the baidzharakh massifs, may be useful for monitoring the thermodenudation activity in different areas of the Arctic, particularly in connection with observed climate changes (ACIA, 2004) and a possible dramatic “cascade of their environmental consequences” (Fraser et al., 2018).
