Plant indication of thermocarst forms in palsa mires in the south of Nature Park Numto (West Siberia)

Abstract

Currently, palsa mires in the southern limit of circumpolar permafrost zone are characterized by an increased intensity of thermokarst processes. In the south of Nature Park Numto the cyclic character of frozen mound formation was replaced by their unidirectional degradation as a result of thaw. We analyze plant indicators of various thermokarst forms occurring in mounds: denudation spots, slumps and funnels. Depending on the stages of thermokarst and type of palsas (flat mound or large mound ones) a number of plant indicators of thawing is revealed: “the drunken forest”, the loss of dwarf shrubs and lichens and their secondary colonization, eutrophication of communities, the emergence of the “ridges” of vegetatively mobile species and others. Brown mosses and secondary colonizing Cladonia lichens with cup-shaped and club-like podetia are indicators of slow thermokarst on patches of denudation in large mounds. Meanwhile, the role of Cladonia lichens with richly branched podetia is markedly reduced. Mesotrophic communities, dominated by mesotrophic sphagnum mosses, are formed in deep funnels on large mounds under the conditions of frozen bed existence. The development of eutrophic communities indicates the release of nutrients during the melting of peat. The stages of thermokarst on the flat palsa mounds correspond to the following succession row: lichens + dwarf shrubs → hygrophyte sphagnum mosses and / or brown mosses + liverworts → hygro-hydrophyte sphagnum mosses + Warnstorfia fluitans. Reliable and the most long-term indicator of thermokarst on large mounds is the downy birch (Betula pubescens).