Abstract
AbstractThermokarst 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.
Highlights
The Arctic is warming twice as fast as the global average
Our results suggest that thaw ponds display potentially cyclic vegetation succession associated with permafrost degradation and recovery
We investigate whether shallow arctic thaw ponds in north‐eastern Siberia, a large and understudied lowland tundra region, follow the quasi‐successional succession model described above
Summary
It is expected that by the end of the 21st century, the global area of near‐surface permafrost will decrease by 24 ± 16% (IPCC scenario RCP2.6) to 69 ± 20% (IPCC scenario RCP8.5) (Meredith et al, 2019) This thawing of permafrost exposes previously frozen soil organic carbon to microbial decomposition, resulting in the release of greenhouse gases (GHG). Abrupt thaw can lead to formation and expansion of surface water in continuous permafrost (Jorgenson et al, 2006; Raynolds et al, 2014) and is expected to increase the area of small lakes by 50% by 2,100 under IPCC scenario RCP8.5 (Meredith et al, 2019) It thereby enhances GHG emission through increased wetting and disturbance of the existing vegetation (Abbott et al, 2016). Most studies on permafrost carbon dynamics have focused on gradual thaw, and abrupt thaw processes are currently not considered in climate models (Meredith et al, 2019; Schuur et al, 2015; Van Huissteden & Dolman, 2012)
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