Permafrost thaw can cause an intensification of climate change through the release of carbon as greenhouse gases. While the effect of air temperature on permafrost thaw is well quantified, the effect ...of rainfall is highly variable and not well understood. Here, we provide a literature review of studies reporting on effects of rainfall on ground temperatures in permafrost environments and use a numerical model to explore the underlying physical mechanisms under different climatic conditions. Both the evaluated body of literature and the model simulations indicate that continental climates are likely to show a warming of the subsoil and hence increased end of season active layer thickness, while maritime climates tend to respond with a slight cooling effect. This suggests that dry regions with warm summers are prone to more rapid permafrost degradation under increased occurrences of heavy rainfall events in the future, which can potentially accelerate the permafrost carbon feedback.
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.
Vegetation change, permafrost degradation and their interactions affect greenhouse gas fluxes, hydrology and surface energy balance in Arctic ecosystems. The Arctic shows an overall “greening” trend ...(i.e. increased plant biomass and productivity) attributed to expansion of shrub vegetation. However, Arctic shrub dynamics show strong spatial variability and locally “browning” may be observed. Mechanistic understanding of greening and browning trends is necessary to accurately assess the response of Arctic vegetation to a changing climate. In this context, the Siberian Arctic is an understudied region. Between 2010 and 2019, increased browning (as derived from the MODIS Enhanced Vegetation Index) was observed in the Eastern Siberian Indigirka Lowlands. To support interpretation of local greening and browning dynamics, we quantified changes in land cover and transition probabilities in a representative tundra site in the Indigirka Lowlands using a timeseries of three very high resolution (VHR) (0.5 m) satellite images acquired between 2010 and 2019. Using spatiotemporal Potts model regularization, we substantially reduced classification errors related to optical and phenological inconsistencies in the image material. VHR images show that recent browning was associated with declines in shrub, lichen and tussock vegetation and increases in open water, sedge and especially Sphagnum vegetation. Observed formation and expansion of small open water bodies in shrub dominated vegetation suggests abrupt thaw of ice-rich permafrost. Transitions from open water to sedge and Sphagnum, indicate aquatic succession upon disturbance. The overall shift towards open water and wetland vegetation suggests a wetting trend, likely associated with permafrost degradation. Landsat data confirmed widespread expansion of surface water throughout the Indigirka Lowlands. However, the increase in the area of small water bodies observed in VHR data was not visible in Landsat-derived surface water data, which suggests that VHR data is essential for early detection of small-scale disturbances and associated vegetation change in permafrost ecosystems.
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•The Indigirka Lowlands show a wetting trend and divergent greening/browning.•VHR data revealed shrub decline and surface water and wetland vegetation expansion.•Thermokarst and terrain wetting explain local shrub decline and tundra browning.•Accurate VHR change detection is essential for monitoring of Arctic ecosystems.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Contrary to the general “greening of the Arctic”, the Siberian Indigirka Lowlands show strong “browning” (a decrease in the Normalized Difference Vegetation Index or “NDVI”) in various recent ...satellite records. Since greening and browning are generally indicative of increases and losses in photosynthetically active biomass, this browning trend may have implications for the carbon balance and vegetation of this Arctic tundra region. To explore potential mechanisms responsible for this trend break from general Arctic greening, we studied timeseries of Landsat summer maximum NDVI, weather data, and high‐resolution maps of vegetation compositional change, topography, geomorphology and hydrology. We find that a significant proportion of browning (lower summer NDVI) is explained by moisture dynamics, with high snow depths and resulting floods as well as summer drought coinciding with low NDVI. Relations between seasonal weather variables and NDVI are spatially heterogeneous, with floodplains, drained thaw lake basins and Yedoma ridges showing different patterns of association with weather variables. Low summer NDVI after high snowfall was particularly evident in floodplains, likely explained by early summer floods. Local small‐scale vegetation changes explained only small amounts of variance in browning rates in Landsat NDVI. Local expansion of Sphagnum vegetation in particular may have contributed to recent browning of our study site, but higher resolution NDVI timeseries are necessary to accurately constrain the role of small‐scale vegetation shifts. Overall, associations identified in this study suggest that future increases in Arctic precipitation variability and extremes may limit tundra greening, but to different extents even across comparatively small topographical contrasts.
Plain Language Summary
Across Arctic regions, satellite images show that tundra ecosystems have been “greening”. This suggests that plant growth, and thereby uptake of carbon from the atmosphere, is increasing in the Arctic. The Indigirka Lowlands in Siberia provide a stark contrast; in this lowland tundra region, satellite images show “browning”, which suggests a decline in plant growth. We explored why this might be the case, in order to better assess whether browning should be expected across the wider Arctic in a changing climate. We compare local browning rates, calculated as changes in spectral indices from Landsat satellite images, to annual weather data, detailed maps of local vegetation changes and maps of terrain properties such as elevation and hydrology. We find that browning is associated with high snow depths and resulting floods, but also with dry summer conditions. Browning was not strongly related to local, small‐scale changes in vegetation (the most common of which was expansion of peat moss). We also find that the degree of association with potential drivers of browning (e.g., flooding, dry summers) differs across landforms. Floodplains show particularly strong browning following high snowfall, likely explained by early summer floods. This suggests that expected increases in year‐to‐year variability and extremes in Arctic precipitation may limit future greening.
Key Points
Our study site in the Indigirka Lowlands shows strong (−0.0039 NDVI units yr−1) and ubiquitous (76% of area) recent browning in Landsat NDVI
High snowfall and low summer rainfall were associated with lower summer NDVI, with differential impacts across topographical gradients
Local small‐scale shifts in vegetation types and thermokarst activity show weak association with Landsat NDVI trends
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 vegetation
Colonization by aquatic plant species, in particular Sphagnum mosses, is associated with recovery of permafrost in these ponds
Recruitment of dwarf shrubs on Sphagnum carpets in thaw ponds indicates potential recovery of the original dwarf shrub‐dominated vegetation
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
•Whether CWD has a positive effect on C sequestration in forest soils remains to be debated.•We need a comparison of the role of C from CWD and from leafy litter in soil C stabilization.•To elucidate ...the contribution of CWD to stable soil C we need to trace individual compounds.•Management of CWD should also focus on increasing sequestration in stable C pools.
Worldwide, forests have absorbed around 30% of global anthropogenic emissions of carbon dioxide (CO2) annually, thereby acting as important carbon (C) sinks. It is proposed that leaving large fragments of dead wood, coarse woody debris (CWD), in forest ecosystems may contribute to the forest C sink strength. CWD may take years to centuries to degrade completely, and non-respired C from CWD may enter the forest soil directly or in the form of dissolved organic C. Although aboveground decomposition of CWD has been studied frequently, little is known about the relative size, composition and fate of different C fluxes from CWD to soils under various substrate-specific and environmental conditions. Thus, the exact contribution of C from CWD to C sequestration within forest soils is poorly understood and quantified, although understanding CWD degradation and stabilization processes is essential for effective forest C sink management. This review aims at providing insight into these processes on the interface of forest ecology and soil science, and identifies knowledge gaps that are critical to our understanding of the effects of CWD on the forest soil C sink. It may be seen as a “call-to-action” crossing disciplinary boundaries, which proposes the use of compound-specific analytical studies and manipulation studies to elucidate C fluxes from CWD. Carbon fluxes from decaying CWD can vary considerably due to interspecific and intraspecific differences in composition and different environmental conditions. These variations in C fluxes need to be studied in detail and related to recent advances in soil C sequestration research. Outcomes of this review show that the presence of CWD may enhance the abundance and diversity of the microbial community and constitute additional fluxes of C into the mineral soil by augmented leaching of dissolved organic carbon (DOC). Leached DOC and residues from organic matter (OM) from later decay stages have been shown to be relatively enriched in complex and microbial-derived compounds, which may also be true for CWD-derived OM. Emerging knowledge on soil C stabilization indicates that such complex compounds may be sorbed preferentially to the mineral soil. Moreover, increased abundance and diversity of decomposer organisms may increase the amount of substrate C being diverted into microbial biomass, which may contribute to stable C pools in the forest soil.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Abstract
Global warming has pronounced effects on tundra vegetation, and rising mean temperatures increase plant growth potential across the Arctic biome. Herbivores may counteract the warming ...impacts by reducing plant growth, but the strength of this effect may depend on prevailing regional climatic conditions. To study how ungulates interact with temperature to influence growth of tundra shrubs across the Arctic tundra biome, we assembled dendroecological data from 20 sites, comprising 1,153 individual shrubs and 22,363 annual growth rings. Evidence for ungulates suppressing shrub radial growth was only observed at intermediate summer temperatures (6.5-9°C), and even at these temperatures the effect was not strong. Multiple factors, including forage preferences and landscape use by the ungulates, and favourable climatic conditions enabling effective compensatory growth of shrubs, may weaken the effects of ungulates on shrubs, possibly explaining the weakness of observed ungulate effects. Earlier local studies have shown that ungulates may counteract the impacts of warming on tundra shrub growth, but we demonstrate that ungulates’ potential to suppress shrub radial growth is not always evident, and may be limited to certain climatic conditions.
The fate of the Arctic affects us allArctic regions are considered the canaries in the coalmine of global change. They are now warming three times faster than the planet on average and show evidence ...of rapid and drastic changes in structure and functioning. Changes include loss of sea ice and snow cover, increasing disturbances of ecosystems due to wildfire and permafrost thaw and changes to hydrology (e.g. precipitation, river discharge). Beside global impacts on sea levels and local impacts on livelihoods, these disturbances can cause additional emissions of greenhouse gasses to the atmosphere, reinforcing global warming. Hence, the development of Arctic ecosystems may also have a critical role in determining our future climate. Widespread degradation of permafrost (frozen ground of which only the top layer thaws in summer) strongly contributes to this reinforcement of warming. As permafrost soils thaw, previously frozen carbon in the soil can decompose and be emitted as greenhouse gas. This warming effect may, however, be partly offset by atmospheric cooling through enhanced uptake of atmospheric carbon in plant biomass. Field observations and satellite images of the Earth’s surface show that warmer conditions in the Arctic promote plant growth and especially that of shrubs. This thesis focuses on the balance between these two opposing drivers of global change.Teasing out the web of interactions between permafrost, vegetation and climateIt is challenging to predict exactly to what extent the response of permafrost and vegetation growth in Arctic regions will affect our future climate. Although permafrost degradation and vegetation change are evident throughout the Arctic, they are also influenced by local conditions and interactions. Permafrost provides the foundation for Arctic tundra vegetation and determines essential properties of the ground surface. Especially ice-rich permafrost tends to lose structure once it thaws and melting ice leaves depressions in the terrain, which affects wetness and other growth conditions for vegetation. At the same time, vegetation regulates the amount of heat and water from the atmosphere that can penetrate into frozen Arctic soils. With this thesis, I addressed three specific knowledge gaps on the interface of climate, vegetation and permafrost to help understand the influence of rapid changes in Arctic ecosystems on our global climate. I focused particularly on the lowland tundra ecosystem in Northeastern Siberia near the remote settlement of Chokurdakh. The Siberian lowland tundra region occupies a large proportion of the world’s tundra area. Due to its continental climate with warm summers and soils rich in ice and carbon, this region has high potential for permafrost degradation and greenhouse gas emissions. Yet, this region has been studied very little in Western scientific literature. I assessed (1) to what extent this Siberian tundra ecosystem shows expansion of shrub vegetation and “greening” of the land surface or “browning” due to shrub drowning in thaw ponds, (2) how this permafrost ecosystem reacts to predicted increases in extreme rainfall events in summer and, lastly, (3) how areas affected by local degradation of ice-rich permafrost evolve over time and whether the permafrost and shrub vegetation can recover again. I answered these three questions using a combination of field experiments, field monitoring, modeling, remote sensing and tree ring analyses of shrubs.
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