Environmental changes, such as climate warming and higher herbivory pressure, are altering the carbon balance of Arctic ecosystems; yet, how these drivers modify the carbon balance among different habitats remains uncertain. This hampers our ability to predict changes in the carbon sink strength of tundra ecosystems. We investigated how spring goose grubbing and summer warming-two key environmental-change drivers in the Arctic-alter CO2 fluxes in three tundra habitats varying in soil moisture and plant-community composition. In a full-factorial experiment in high-Arctic Svalbard, we simulated grubbing and warming over two years and determined summer net ecosystem exchange (NEE) alongside its components: gross ecosystem productivity (GEP) and ecosystem respiration (ER). After two years, we found net CO2 uptake to be suppressed by both drivers depending on habitat. CO2 uptake was reduced by warming in mesic habitats, by warming and grubbing in moist habitats, and by grubbing in wet habitats. In mesic habitats, warming stimulated ER (+75%) more than GEP (+30%), leading to a 7.5-fold increase in their CO2 source strength. In moist habitats, grubbing decreased GEP and ER by similar to 55%, while warming increased them by similar to 35%, with no changes in summer-long NEE. Nevertheless, grubbing offset peak summer CO2 uptake and warming led to a twofold increase in late summer CO2 source strength. In wet habitats, grubbing reduced GEP (-40%) more than ER (-30%), weakening their CO2 sink strength by 70%. One-year CO2-flux responses were similar to two-year responses, and the effect of simulated grubbing was consistent with that of natural grubbing. CO2-flux rates were positively related to aboveground net primary productivity and temperature. Net ecosystem CO2 uptake started occurring above similar to 70% soil moisture content, primarily due to a decline in ER. Herein, we reveal that key environmental-change drivers-goose grubbing by decreasing GEP more than ER and warming by enhancing ER more than GEP-consistently suppress net tundra CO2 uptake, although their relative strength differs among habitats. By identifying how and where grubbing and higher temperatures alter CO2 fluxes across the heterogeneous Arctic landscape, our results have implications for predicting the tundra carbon balance under increasing numbers of geese in a warmer Arctic.

Litter decomposition represents a major path for atmospheric carbon influx into Arctic soils, thereby controlling below-ground carbon accumulation. Yet, little is known about how tundra litter decomposition varies with microenvironmental conditions, hindering accurate projections of tundra soil carbon dynamics with future climate change. Over 14 months, we measured landscape-scale decomposition of two contrasting standard litter types (Green tea and Rooibos tea) in 90 plots covering gradients of micro-climate and -topography, vegetation cover and traits, and soil characteristics in Western Greenland. We used the tea bag index (TBI) protocol to estimate relative variation in litter mass loss, decomposition rate (k) and stabilisation factor (S) across space, and structural equation modelling (SEM) to identify relationships among environmental factors and decomposition. Contrasting our expectations, microenvironmental factors explained little of the observed variation in both litter mass loss, as well as k and S, suggesting that the variables included in our study were not the major controls of decomposer activity in the soil across the studied tundra landscape. We use these unexpected findings of our study combined with findings from the current literature to discuss future avenues for improving our understanding of the drivers of tundra decomposition and, ultimately, carbon cycling across the warming Arctic.

Our understanding of tundra fire effects in Northern Alaska is limited because fires have been relatively rare. We sampled a 70+ year -old burn visible in a 1948 aerial photograph for vegetation composition and structure, soil attributes, terrain rugosity, and thermokarst pit density. Between 1948 and 2017 the burn initially became wetter as ice wedges melted but then drained and dried as the troughs became hydrologically connected. The reference tundra has become wetter over the last few decades and appears to be lagging through a similar sequence. The burn averaged 2.5 degrees C warmer than the reference tundra at 30 cm depth. Thinning of organic soil following fire appears to dramatically accelerate the background degradation of ground-ice features in response to climate change and promotes a plant community that is distinct in terms of taxa and structure, dominated by tall willows and other competitive, rather than cold-tolerant, species. The cover of sedges and mosses is low while that of willows and grass is high relative to the reference tundra. The changes in plant community composition and structure, increasing ground temperature, and thermokarst lead us to expect the observed biophysical changes to the tundra will persist centuries into the future.

Significant progress in permafrost carbon science made over the past decades include the identification of vast permafrost carbon stocks, the development of new pan-Arctic permafrost maps, an increase in terrestrial measurement sites for CO2 and methane fluxes, and important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Process-based modeling studies now include key elements of permafrost carbon cycling and advances in statistical modeling and inverse modeling enhance understanding of permafrost region C budgets. By combining existing data syntheses and model outputs, the permafrost region is likely a wetland methane source and small terrestrial ecosystem CO2 sink with lower net CO2 uptake toward higher latitudes, excluding wildfire emissions. For 2002-2014, the strongest CO2 sink was located in western Canada (median: -52 g C m-2 y-1) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: -5 to -9 g C m-2 y-1). Eurasian regions had the largest median wetland methane fluxes (16-18 g CH4 m-2 y-1). Quantifying the regional scale carbon balance remains challenging because of high spatial and temporal variability and relatively low density of observations. More accurate permafrost region carbon fluxes require: (a) the development of better maps characterizing wetlands and dynamics of vegetation and disturbances, including abrupt permafrost thaw; (b) the establishment of new year-round CO2 and methane flux sites in underrepresented areas; and (c) improved models that better represent important permafrost carbon cycle dynamics, including non-growing season emissions and disturbance effects. Climate change and the consequent thawing of permafrost threatens to transform the permafrost region from a carbon sink into a carbon source, posing a challenge to global climate goals. Numerous studies over the past decades have identified important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Overall, studies show high wetland methane emissions and a small net carbon dioxide sink strength over the terrestrial permafrost region but results differ among modeling and upscaling approaches. Continued and coordinated efforts among field, modeling, and remote sensing communities are needed to integrate new knowledge from observations to modeling and predictions and finally to policy. Rapid warming of northern permafrost region threatens ecosystems, soil carbon stocks, and global climate targets Long-term observations show importance of disturbance and cold season periods but are unable to detect spatiotemporal trends in C flux Combined modeling and syntheses show the permafrost region is a small terrestrial CO2 sink with large spatial variability and net CH4 source

Climate change poses a serious threat to permafrost integrity, with expected warmer winters and increased precipitation, both raising permafrost temperatures and active layer thickness. Under ice-rich conditions, this can lead to increased thermokarst activity and a consequential transfer of soil organic matter to tundra ponds. Although these ponds are known as hotspots for CO2 and CH4 emissions, the dominant carbon sources for the production of greenhouse gases (GHGs) are still poorly studied, leading to uncertainty about their positive feedback to climate warming. This study investigates the potential for lateral thermo-erosion to cause increased GHG emissions from small and shallow tundra ponds found in Arctic ice-wedge polygonal landscapes. Detailed mapping of fine-scale erosive features revealed their strong impact on pond limnological characteristics. In addition to increasing organic matter inputs, providing carbon to heterotrophic microorganisms responsible for GHG production, thermokarst soil erosion also increases shore instability and water turbidity, limiting the establishment of aquatic vegetation-conditions that greatly increase GHG emissions from these aquatic systems. Ponds with more than 40% of the shoreline affected by lateral erosion experienced significantly higher rates of GHG emissions (similar to 1200 mmol CO2 m-2 yr-1 and similar to 250 mmol CH4 m-2 yr-1) compared to ponds with no active shore erosion (similar to 30 mmol m-2 yr-1 for both GHG). Although most GHGs emitted as CO2 and CH4 had a modern radiocarbon signature, source apportionment models implied an increased importance of terrestrial carbon being emitted from ponds with erosive shorelines. If primary producers are unable to overcome the limitations associated with permafrost disturbances, this contribution of older carbon stocks may become more significant with rising permafrost temperatures.

Tundra is one of the most sensitive environments of the world in relation to climate changes, since its ecosystems exist close to the limits of plant community tolerance. Besides, tundra vegetation in most of Arctic regions resides on permafrost, which is thermally unstable media. Thus, vegetation and frozen soils are extremely vulnerable to external impacts and are balancing in fragile thermodynamic equilibrium. Thermal and moisture regime shifting lead to changing of thermophysical properties of vegetation cover and thus, the thermal balance of underlying permafrost. In this study we present the results of 2001-2024 in-situ monitoring of vegetation cover and permafrost conditions in remote region of the Chukchi Peninsula, Russian Northeast. The study combines the yearly data on active layer thickness and vegetation cover from two sites of Circumpolar Active Layer Monitoring (CALM) program located within the key site of Eastern Chukotka Coastal Plains (ECCP). The study reveals long-term trajectories of climate, permafrost, and vegetation cover characteristics. Although common biological productivity was growing and active layer was thickening, the particular plant species respond to these changes differently. On sloping plots, the increasing of active layer thickness (ALT) led to correspondent lowering of the permafrost table, drainage of thawing ice and thus, soil drying, which caused the decrease in moss and sedge covers. Meanwhile, within flat poorly-drained surfaces the permafrost thawing contributes to soil moisture with correspondent sedge expansion. Thermokarst-affected terrain triggers the growth of tundra vegetation bioproductivity and serves as a shelter for plants from Arctic winds and facilitates higher snow accumulation.

Climate change has had a significant impact on the seasonal transition dates of Arctic tundra ecosystems, causing diverse variations between distinct land surface classes. However, the combined effect of multiple controls as well as their individual effects on these dates remains unclear at various scales and across diverse land surface classes. Here we quantified spatiotemporal variations of three seasonal transition dates (start of spring, maximum normalized difference vegetation index (NDVImax) day, end of fall) for five dominating land surface classes in the ice-free Greenland. Using a distributed snow model, structural equation modeling, and a random forest model, based on ground observations and remote sensing data, we assessed the indirect and direct effects of climate, snow, and terrain on seasonal transition dates. We then presented new projections of likely changes in seasonal transition dates under six future climate scenarios. The coupled climate, snow cover, and terrain conditions explained up to 61% of seasonal transition dates across different land surface classes. Snow ending day played a crucial role in the start of spring and timing of NDVImax. A warmer June and a decline in wind could advance the NDVImax day. Increased precipitation and temperature during July-August are the most important for delaying the end of fall. We projected that a 1-4.5 degrees C increase in temperature and a 5%-20% increase in precipitation would lengthen the spring-to-fall period for all five land surface classes by 2050, thus the current order of spring-to-fall lengths for the five land surface classes could undergo notable changes. Tall shrubs and fens would have a longer spring-to-fall period under the warmest and wettest scenario, suggesting a competitive advantage for these vegetation communities. This study's results illustrate controls on seasonal transition dates and portend potential changes in vegetation composition in the Arctic under climate change. Employing the distributed snow modeling, structural equation modeling, and random forest model, we assessed the effect of climatic, snow, and terrain conditions on the seasonal transition dates of Arctic tundra ecosystems and projected their changes by 2050 in ice-free Greenland, between diverse land surface classes. We found that the start of spring and the end of fall respond differently to various environmental conditions. Future temperature and precipitation changes could significantly affect the spring-to-fall period for different land surface classes, with potential implications for Arctic vegetation composition, providing valuable insights into the controls on seasonal transition dates under climate change.image

Arctic warming and changing precipitation patterns are altering soil nutrient availability and other processes that control the greenhouse gas balance of high-latitude ecosystems. Changes to these biogeochemical processes will ultimately determine whether the Arctic will enhance or dampen future climate change. At the Cape Bounty Arctic Watershed Observatory, a full-factorial International Tundra Experiment site was established in 2008, allowing for the investigation of ten years of experimental warming and increased snow depth on nutrient availability and trace gas exchange in a mesic heath tundra across two growing seasons (2017 and 2018). Plots with open-top chambers (OTCs) had drier soils (p < .1) that released 1.5 times more carbon dioxide (p < .05), and this effect was enhanced in the drier growing season. Increased snow depth delayed the onset of thaw and active layer development (p < .1) and corresponded with greater nitrous oxide release (p < .05). Our results suggest that decreases to soil moisture will lead to an increase in nitrate availability, soil respiration, and nitrous oxide fluxes. Ultimately, these effects may be moderated by the magnitude of future shifts and interactions between climate variability and ecological responses to permafrost thaw.

Tundra soils are one of the world's largest organic carbon stores, yet this carbon is vulnerable to accelerated decomposition as climate warming progresses. The landscape-scale controls of litter decomposition are poorly understood in tundra ecosystems, which hinders our understanding of the global carbon cycle. We examined the extent to which the thermal sum of surface air temperature, soil moisture and permafrost thaw depth influenced litter mass loss and decomposition rates (k), and at which spatial thresholds an environmental variable becomes a reliable predictor of decomposition, using the Tea Bag Index protocol across a heterogeneous tundra landscape on Qikiqtaruk-Herschel Island, Yukon, Canada. We found greater green tea litter mass loss and faster decomposition rates (k) in wetter areas within the landscape, and to a lesser extent in areas with deeper permafrost active layer thickness and higher surface thermal sums. We also found higher decomposition rates (k) on north-facing relative to south-facing aspects at microsites that were wetter rather than warmer. Spatially heterogeneous belowground conditions (soil moisture and active layer depth) explained variation in decomposition metrics at local scales (< 50 m(2)) better than thermal sum. Surprisingly, there was no strong control of elevation or slope on litter decomposition. Our results reveal that there is considerable scale dependency in the environmental controls of tundra litter decomposition, with moisture playing a greater role than the thermal sum at < 50 m(2) scales. Our findings highlight the importance and complexity of microenvironmental controls on litter decomposition in estimates of carbon cycling in a rapidly warming tundra biome.

Tundra is primarily a habitat for shrub growth, not trees, but growth of prostrate forms of trees has been reported occasionally from the subarctic tundra region. In the light of on-going climate change, climate sensitivity studies of these unique trees are essential to predict vegetation dynamics and potential northward expansion of boreal forest tree species into tundra. Here we studied one of the northernmost Larix Mill. trees and Betula nana L. shrubs (72 degrees N) from the Siberian tundra for the common period 1980-2017. We took advantage of the discovery of a single cohort of prostrate Larix trees within a tundra ecosystem, i.e., ca. 60 km northwards from the northern treeline, and compared climate-growth relationships of the two species. Both woody plants were sensitive to the July temperature, however this relationship was stable across the entire study period (1980-2017) only for Betula nana chronology. Additionally, radial growth of Larix trees became negatively correlated to temperatures during the previous summer. In recent period moisture sensitivity between Larix trees and Betula nana shrubs was contrasting, with generally wetter soil conditions favoring Larix trees growth and dryer conditions promoting Betula nana growth. Our study indicates that Larix trees radial growth in recent years is more sensitive to moisture than to summer air temperatures, whereas temperature sensitivity of Betula nana shrub is stable over time. We provide first detailed insight into the annual resolution on Larix tree growth sensitivity to climate in the heart of the tundra. The potentially higher Betula nana shrub resistance to warmer and drier climate versus Larix trees on a tundra revealed in our study needs to be further examined across habitats of various soil, moisture and permafrost status.