| Summary: | Arctic tundra ecosystems are characterized by low vegetation and permanently frozen ground (permafrost). Permafrost thaw and the resulting release of greenhouse gases from decomposing soil organic carbon has the potential to accelerate climate warming. In recent decades, tundra shrub expansion has been observed in many places, in response to changing climate conditions. However, in our long-term field experiment at a Northeast-Siberian tundra site we have observed that the lowland tundra ecosystem is extremely vulnerable to increased thawing, e.g. due to disturbance, leading to thaw pond development and shrub decline . Removing the shrub part of the vegetation in the experiment initiated thawing of ice- rich permafrost, resulting in collapse of the originally elevated shrub patches into waterlogged depressions.If permafrost thawing can more frequently trigger such local permafrost collapse, methane-emitting wet depressions could become more abundant in the lowland tundra landscape, at the cost of permafrost-stabilizing low shrub vegetation, particularly in poorly-drained ice-rich permafrost regions. Shrub decline associated w ith increasing wetland extent seems an underestimated trajectory in response to climate warming, but would be in line with reports of lake expansion, and drownin,g larch trees close to the treeline in Siberian lowland tundra.
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