Tundra vegetation productivity and composition are responding rapidly to climatic changes in the Arctic. These changes can, in turn, mitigate or amplify permafrost thaw. In this Review, we synthesize remotely-sensed and field-observed vegetation change across the tundra biome, and outline how these shifts could influence permafrost thaw. Permafrost ice content appears to be an important control on local vegetation changes; woody vegetation generally increases in ice-poor uplands, whereas replacement of woody vegetation by (aquatic) graminoids following abrupt permafrost thaw is more frequent in ice-rich Arctic lowlands. These locally observed vegetation changes contribute to regional satellite-observed greening trends, although the interpretation of greening and browning is complicated. Increases in vegetation cover and height generally mitigate permafrost thaw in summer, yet increase annual soil temperatures through snow-related winter soil warming effects. Strong vegetation-soil feedbacks currently alleviate the consequences of thaw-related disturbances. However, if the increasing scale and frequency of disturbances in a warming Arctic exceeds the capacity for vegetation and permafrost recovery, changes to Arctic ecosystems could be irreversible. To better disentangle vegetation-soil-permafrost interactions, ecological field studies remain crucial, but require better integration with geophysical assessments. [H1] IntroductionArctic tundra is changing rapidly, with a pervasive trend toward more abundant and taller vegetation as shrubs and trees expand northward 1 . Field and satellite observations suggest that tundra vegetation has become more productive, a phenomenon known as tundra greening. Such increases in the biomass and stature of Arctic tundra vegetation can alter the thermal properties of the ground surface. Canopies can mediate the effect of increasing summer air temperatures on soil temperatures 2-4 and contribute to insulation of soils in winter through trapping of snow [5][6][7][8] .Vegetation and soil characteristics also influence surface energy partitioning and the thermal diffusivity of the soil 9,10 . Permafrost (permanently frozen ground) underlies soil and vegetation, and is the foundation of Arctic tundra ecosystems. In turn, vegetation and near-surface soils insulate permafrost 11 , regulating the effects of atmospheric conditions. However, the Arctic is warming more than twice as fast as the global average, amplified by loss of sea ice cover 1 . Even if Arctic temperatures were to stabilize at 2°C of warming, as aimed for with the Paris Agreement, approximately 40% of near-surface permafrost is still projected to thaw 12 . Permafrost-dominated ecosystems are thus at risk 13 , even under modest CO2 emission scenarios 1 , with consequences for Arctic inhabitants 14 .

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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.

Permafrost thaw can accelerate climate warming by releasing carbon from previously frozen soil in the form of greenhouse gases. Rainfall extremes have been proposed to increase permafrost thaw, but the magnitude and duration of this effect are poorly understood. Here we present empirical evidence showing that one extremely wet summer (+100 mm; 120% increase relative to average June–August rainfall) enhanced thaw depth by up to 35% in a controlled irrigation experiment in an ice-rich Siberian tundra site. The effect persisted over two subsequent summers, demonstrating a carry-over effect of extremely wet summers. Using soil thermal hydrological modelling, we show that rainfall extremes delayed autumn freeze-up and rainfall-induced increases in thaw were most pronounced for warm summers with mid-summer precipitation rainfall extremes. Our results suggest that, with rainfall and temperature both increasing in the Arctic, permafrost will likely degrade and disappear faster than is currently anticipated based on rising air temperatures alone.