Rapid Vegetation Succession and Coupled Permafrost Dynamics in Arctic Thaw Ponds in the Siberian Lowland Tundra.

Thermokarst features, such as thaw ponds, are hotspots for methane emissions in warming lowland tundra. Presently we lack quantitative knowledge on the formation rates of thaw ponds and subsequent vegetation succession, necessary to determine their net contribution to greenhouse gas emissions. This study sets out to identify development trajectories and formation rates of small‐scale (<100 m2), shallow arctic thaw ponds in north‐eastern Siberia. We selected 40 ponds of different age classes based on a time‐series of satellite images and measured vegetation composition, microtopography, water table, and thaw depth in the field and measured age of colonizing shrubs in thaw ponds using dendrochronology. We found that young ponds are characterized by dead shrubs, while older ponds show rapid terrestrialization through colonization by sedges and Sphagnum moss. While dead shrubs and open water are associated with permafrost degradation (lower surface elevation, larger thaw depth), sites with sedge and in particular Sphagnum display indications of permafrost recovery. Recruitment of Betula nana on Sphagnum carpets in ponds indicates a potential recovery toward shrub‐dominated vegetation, although it remains unclear if and on what timescale this occurs. Our results suggest that thaw ponds display potentially cyclic vegetation succession associated with permafrost degradation and recovery. Pond formation and initial colonization by sedges can occur on subdecadal timescales, suggesting rapid degradation and initial recovery of permafrost. The rates of formation and recovery of small‐scale, shallow thaw ponds have implications for the greening/browning dynamics and carbon balance of this ecosystem. Plain Language Summary: Global warming results in dramatic changes across Arctic landscapes, for instance the thawing of permafrost. Thawing of ice‐rich permafrost creates local depressions that may fill in with water, forming thaw ponds. In thaw ponds, previously frozen carbon becomes available for decomposition into the greenhouse gas methane. It is presently unknown how long thaw ponds remain sources of methane. We studied whether the permafrost and original dwarf shrub vegetation can recover after thaw pond formation. In the Siberian lowland tundra, we selected 40 shallow ponds of various ages, based on a series of aerial photographs. In these ponds, we assessed vegetation composition and the status of the permafrost. We found that once formed, thaw ponds are rapidly colonized by sedges. In older thaw ponds, carpets of peat moss appear, on which shrubs can reestablish. We found that the permafrost thaws less deeply under peat moss carpets than under open water or sedges. This indicates that thaw ponds go through a succession of dead shrub vegetation toward sedges and peat moss and potentially back to shrubs, and that this is related to the degradation and recovery of the permafrost. Key Points: Thaw ponds formed by small‐scale thermokarst in the Siberian lowland tundra display rapid formation and colonization by aquatic vegetationColonization by aquatic plant species, in particular Sphagnum mosses, is associated with recovery of permafrost in these pondsRecruitment of dwarf shrubs on Sphagnum carpets in thaw ponds indicates potential recovery of the original dwarf shrub‐dominated vegetation [ABSTRACT FROM AUTHOR]