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Widespread changes in arctic and boreal Normalized Difference Vegetation Index (NDVI) values captured by satellite platforms indicate that northern ecosystems are experiencing rapid ecological change in response to climate warming. Increasing temperatures and altered hydrology are driving shifts in ecosystem biophysical properties that, observed by satellites, manifest as long-term changes in regional NDVI. In an effort to examine the underlying ecological drivers of these changes, we used field-scale remote sensing of NDVI to track peatland vegetation in experiments that manipulated hydrology, temperature, and carbon dioxide (CO 2 ) levels. In addition to NDVI, we measured percent cover by species and leaf area index (LAI). We monitored two peatland types broadly representative of the boreal region. One site was a rich fen located near Fairbanks, Alaska, at the Alaska Peatland Experiment (APEX), and the second site was a nutrient-poor bog located in Northern Minnesota within the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment. We found that NDVI decreased with long-term reductions in soil moisture at the APEX site, coincident with a decrease in photosynthetic leaf area and the relative abundance of sedges. We observed increasing NDVI with elevated temperature at the SPRUCE site, associated with an increase in the relative abundance of shrubs and a decrease in forb cover. Warming treatments at the SPRUCE site also led to increases in the LAI of the shrub layer. We found no strong effects of elevated CO 2 on community composition. Our findings support recent studies suggesting that changes in NDVI observed from satellite platforms may be the result of changes in community composition and ecosystem structure in response to climate warming., (© 2018 John Wiley & Sons Ltd.)

Long‐term atmospheric CO2 concentration records have suggested a reduction in the positive effect of warming on high‐latitude carbon uptake since the 1990s. A variety of mechanisms have been proposed to explain the reduced net carbon sink of northern ecosystems with increased air temperature, including water stress on vegetation and increased respiration over recent decades. However, the lack of consistent long‐term carbon flux and in situ soil moisture data has severely limited our ability to identify the mechanisms responsible for the recent reduced carbon sink strength. In this study, we used a record of nearly 100 site‐years of eddy covariance data from 11 continuous permafrost tundra sites distributed across the circumpolar Arctic to test the temperature (expressed as growing degree days, GDD) responses of gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (ER) at different periods of the summer (early, peak, and late summer) including dominant tundra vegetation classes (graminoids and mosses, and shrubs). We further tested GPP, NEE, and ER relationships with soil moisture and vapor pressure deficit to identify potential moisture limitations on plant productivity and net carbon exchange. Our results show a decrease in GPP with rising GDD during the peak summer (July) for both vegetation classes, and a significant relationship between the peak summer GPP and soil moisture after statistically controlling for GDD in a partial correlation analysis. These results suggest that tundra ecosystems might not benefit from increased temperature as much as suggested by several terrestrial biosphere models, if decreased soil moisture limits the peak summer plant productivity, reducing the ability of these ecosystems to sequester carbon during the summer. [ABSTRACT FROM AUTHOR]

[Display omitted] • Meta-analysis reveals Arctic soil labile nitrogen (N) responses to climate change. • Warming shows no overall effect, while snow addition alters soil labile N pools. • Decision tree analysis identifies key determinants of soil labile N changes. • Climate, soil, vegetation, and experimental factors shape soil labile N responses. • Provide insights into the responses of Arctic N dynamics to climate change. The Arctic terrestrial ecosystems are undergoing rapid climate change, causing shifts in the dynamics of soil nitrogen (N), a pivotal but relatively underexplored component. To understand the impacts of climate change on soil labile N pools, we performed meta- and decision-tree analyses of 391 observations from 38 peer-reviewed publications across the Arctic, focusing on experimental warming and snow addition. Soil dissolved organic nitrogen (DON), ammonium (N H 4 +), and nitrate (N O 3 -) pools under experimental warming exhibited overall standard mean differences (SMDs) ranging from −0.08 to 0.02, with no significance (P > 0.05); however, specific conditions led to significant changes. The key determinants of soil labile N responses to warming were experimental duration and mean annual summer temperature for DON; annual precipitation, soil moisture, and sampling timing for N H 4 + ; and soil layer for N O 3 -. Snow addition significantly increased all labile N pools (overall SMD = 0.23–0.36; P < 0.05), influenced by factors such as sampling timing and vegetation type for DON; experimental duration and soil moisture for N H 4 + ; and soil pH for N O 3 -. By consolidating and reprocessing datasets, we not only showed the overall responses of soil labile N pools to climate manipulation experiments in Arctic tundra ecosystems but also identified key determinants for changes in soil N pools among environmental and experimental variables. Our findings demonstrate that warming and snow-cover changes significantly affect soil labile N pools, highlighting how the unique environmental characteristics of different sites influence terrestrial N cycling and underscoring the complexity of Arctic N dynamics under climate change. [ABSTRACT FROM AUTHOR]

Wildfire frequency and expanse in the Arctic have increased in recent years and are projected to increase further with changes in climatic conditions due to warmer and drier summers. Yet, there is a lack of knowledge about the impacts such events may have on the net greenhouse gas (GHG) balances in Arctic ecosystems. We investigated in situ effects of an experimental fire in 2017 on carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) surface fluxes in the most abundant tundra ecosystem in West Greenland in ambient and warmer conditions. Measurements from the growing seasons 2017 to 2019 showed that burnt areas became significant net CO2 sources for the entire study period, driven by increased ecosystem respiration (ER) immediately after the fire and decreased gross ecosystem production (GEP). Warming by open‐top chambers significantly increased both ER and GEP in control, but not in burnt plots. In contrast to CO2, measurements suggest that the overall sink capacity of atmospheric CH4, as well as net N2O emissions, were not affected by fire in the short term, but only immediately after the fire. The minor effects on CH4 and N2O, which was surprising given the significantly higher nitrate availability observed in burnt plots. However, the minor effects are aligned with the lack of significant effects of fire on soil moisture and soil temperature. Net uptake and emissions of all three GHG from burnt soils were less temperature‐sensitive than in the undisturbed control plots. Overall, this study highlights that wildfires in a typical tundra ecosystem in Greenland may not lead to markedly increased net GHG emissions other than CO2. Additional investigations are needed to assess the consequences of more severe fires. [ABSTRACT FROM AUTHOR]

Climate-driven permafrost thaw alters the strongly coupled carbon and nitrogen cycles within the Arctic tundra, influencing the availability of limiting nutrients including nitrate (NO3−). Researchers have identified two primary mechanisms that increase nitrogen and NO3− availability within permafrost soils: (1) the 'frozen feast', where previously frozen organic material becomes available as it thaws, and (2) 'shrubification', where expansion of nitrogen-fixing shrubs promotes increased soil nitrogen. Through the synthesis of original and previously published observational data, and the application of multiple geospatial approaches, this study investigates and highlights a third mechanism that increases NO3− availability: the hydrogeomorphic evolution of polygonal permafrost landscapes. Permafrost thaw drives changes in microtopography, increasing the drainage of topographic highs, thus increasing oxic conditions that promote NO3− production and accumulation. We extrapolate relationships between NO3− and soil moisture in elevated topographic features within our study area and the broader Alaskan Coastal Plain and investigate potential changes in NO3− availability in response to possible hydrogeomorphic evolution scenarios of permafrost landscapes. These approximations indicate that such changes could increase Arctic tundra NO3− availability by ~250–1000%. Thus, hydrogeomorphic changes that accompany continued permafrost degradation in polygonal permafrost landscapes will substantially increase soil pore water NO3− availability and boost future fertilization and productivity in the Arctic. [ABSTRACT FROM AUTHOR]

Climate warming is causing significant changes in the Arctic, leading to increased temperatures and permafrost instability. The active layer has been shown to be affected by climate change, where warmer ground surface temperatures result in progressive permafrost thaw and a deepening active layer. This study assessed the effects of thermal modeling parameters on permafrost ground response to climate warming using the fifth phase of the Coupled Model Intercomparison Project (CMIP5) and TEMP/W software. We analyzed how variations in depth, water content, and soil type affect predictions of future active layer depths and settlement under various climate scenarios using the soil characteristics along Hudson Bay Railway corridor. The results indicate that, for fine-grained soils, the depth of the model is a more significant parameter than for coarse-grained soils. The water content of all soil types is a critical factor in determining the time at which permafrost thaws and the depth at which the active layer is located, as higher water content leads to larger active layer changes and more settlement in most cases. Our findings have important implications for infrastructure and land use management in the Arctic region. • Arctic regions are experiencing degradation due to climate warming trends. • RCP8.5 predicts the highest future settlement, but RCP4.5 may also exhibit significant settlement due to various factors. • Water content plays a vital role in thermal modeling of permafrost, affecting embankment settlement predictions. • Model depth in thermal modeling's impact varies with water content; depths exceeding 30 m are suitable for similar studies. [ABSTRACT FROM AUTHOR]

High latitude organic soils form a significant carbon storage and deposition of these soils is largely driven by climate. Svalbard, Norway, has experienced millennial-scale climate variations and in general organic soil processes have benefitted from warm and humid climate phases while cool late Holocene has been unfavourable. In addition to direct effect of cool climate, the advancing glaciers have restricted the vegetation growth, thus soil accumulation. Since the early 1900's climate has been warming at unprecedented rate, assumingly promoting organic soil establishment. Here we present results of multiple organic soil profiles collected from Svalbard. The profiles have robust chronologies accompanied by soil property analyses, carbon stock estimations and testate amoeba data as a proxy for soil moisture. Our results reveal relatively recent initiation of organic soils across the Isfjorden area. The initiation processes could be linked to glacier retreat, and improvement of growing conditions and soil stabilization. Carbon stock analyses suggested that our sites are hot spots for organic matter accumulation. Testate amoebae data suggested drying of soil surfaces, but the reason remained unresolved. If continued, such a process may lead to carbon release. Our data suggest that detailed palaeoecological data from the Arctic is needed to depict the on-going processes and to estimate future trajectories. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) are released from biogenic sources in a temperature-dependent manner. Consequently, Arctic ecosystems are expected to greatly increase their VOC emissions with ongoing climate warming, which is proceeding at twice the rate of global temperature rise. Here, we show that ongoing warming has strong, increasing effects on Arctic VOC emissions. Using a combination of statistical modeling on data from several warming experiments in the Arctic tundra and dynamic ecosystem modeling, we separate the impacts of temperature and soil moisture into direct effects and indirect effects through vegetation composition and biomass alterations. The indirect effects of warming on VOC emissions were significant but smaller than the direct effects, during the 14-y model simulation period. Furthermore, vegetation changes also cause shifts in the chemical speciation of emissions. Both direct and indirect effects result in large geographic differences in VOC emission responses in the warming Arctic, depending on the local vegetation cover and the climate dynamics. Our results outline complex links between local climate, vegetation, and ecosystem-atmosphere interactions, with likely local-to-regional impacts on the atmospheric composition. [ABSTRACT FROM AUTHOR]

The recent widespread expansion of deciduous shrubs across much of the Arctic has been largely attributed to climate warming. This study investigated decadal growth rates of dwarf birch (Betula glandulosa) across a low Arctic landscape in the continental interior of Canada. Detailed birch cover (100 m2 replicate plots) and individual shrub stature measurement datasets for five representative habitat-types were compared between 2006 and 2016, and evaluated in relation to environmental characteristics. Furthermore, dendrochronologically-based annual growth rates were assessed in relation to the 20-year climate record. Birch height, lateral dimensions, and patch groundcover all increased 20–25% relative to 2006 values, but these increases were similar among the habitat-types. Together, the limited evidence of recent warming at this site, the absence of significant habitat-type growth rate differences, and the lack of correlation between annual climate and stem secondary growth strongly suggest that climate change was not the principal cause. Instead, we propose that release from caribou impacts following the recent severe herd decline may explain the net shrub growth. Individual shrub growth rates were correlated with soil nutrient availability, but the latter was highly variable, suggesting that growth rates are primarily determined by fine-scale rather than habitat-scale spatial heterogeneity in nutrient supply. Together, our results demonstrate that birch growth has been enhanced across a variety of habitat-types in the Daring Lake landscape over the decade since 2006, and suggest that the recent severe caribou herd declines may be at least as significant as climate warming in driving birch shrub expansion in the Canadian central low Arctic. [ABSTRACT FROM AUTHOR]

Impacts of increased winter snowfall and warmer summer air temperatures on nitrous oxide (N 2 O) dynamics in arctic tundra are uncertain. Here we evaluate surface N 2 O dynamics in both wet and dry tundra in West Greenland, subjected to field manipulations with deepened winter snow and summer warming. The potential denitrification activity (PDA) and potential net N 2 O production (N 2 O net) were measured to assess denitrification and N 2 O consumption potential. The surface N 2 O fluxes averaged 0.49 ± 0.42 and 2.6 ± 0.84 μg N 2 O–N m−2 h−1, and total emissions were 212 ± 151 and 114 ± 63 g N 2 O–N scaled to the entire study area of 0.15 km2, at the dry and wet tundra, respectively. The experimental summer warming, and in combination with deepened snow, significantly increased N 2 O emissions at the dry tundra, but not at the wet tundra. The deepened snow increased winter soil temperatures and growing season soil N availability (DON, NH 4 +-N or NO 3 −-N), but no main effect of deepened snow on N 2 O fluxes was found at either tundra ecosystem. The mean PDA was 5- and 121-fold higher than the N 2 O net at the dry and wet tundra, respectively, suggesting that N 2 O might be reduced and emitted as dinitrogen (N 2). Overall, this study reveals modest but evident surface N 2 O fluxes from tundra ecosystems in Western Greenland, and suggests that projected increases in winter precipitation and summer air temperatures may increase N 2 O emissions, particularly at the dry tundra dominating in this region. • We studied climate change effects on N 2 O fluxes in dry and wet tundra West Greenland. • Generally, N 2 O fluxes were relatively modest in both types of vegetation. • Summer warming combined with deepened snow enhanced N 2 O emissions in dry tundra. • Climate change did not affect N 2 O fluxes in wet tundra. [ABSTRACT FROM AUTHOR]

Refugia the sites preserving conditions reminiscent of suitable climates are projected to be crucial for species in a changing climate, particularly at high latitudes. However the knowledge of current locations of highlatitude refugia and particularly their ability to retain suitable conditions under future climatic changes is limited. Occurrences of refugia have previously been mainly assessed and modelled based solely on climatic features, with insufficient attention being paid to potentially important landscape-scale factors. Here, climate-only models and 'full' models incorporating topo-edaphic landscape-scale variables (radiation, soil moisture and calcareousness) were developed and compared for 111 arctic-alpine plant species in Northern Fennoscandia. This was done for both current and future climates to determine cells with resilient climatic suitability harbouring refugia. Our results show that topographic and edaphic landscape-scale predictors both significantly improve models of arctic-alpine species distributions and alter projections of refugia occurrence. The predictions of species-climate models ignore landscape-scale ecological processes and may thus provide inaccurate estimates of extinction risk and forecasts of refugia where species can persist under a changing climate. [ABSTRACT FROM AUTHOR]

Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH4 emissions may represent a significant climate forcing feedback, long-term impacts of changes in snow accumulation on CH4 fluxes remain uncertain. We measured ecosystem CH4 fluxes and soil CH4 and CO2 concentrations and 13C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH4 flux over the growing season (Jun-Aug) after 18 years of experimental snow depth increases and decreases. Deeper snow increased soil wetness and warming, reducing soil %O2 levels and increasing thaw depth. Soil moisture, through changes in soil %O2 saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH4 sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH4 (Fox) by 75-120% compared to Ambient, switching the system from a small source to a net CH4 sink (21 ± 2 and -31 ± 1 mg CH4 m-2 season-1 at Ambient and RS). Deeper snow reduced Fox by 35-40% and 90-100% in medium-(MS) and high-(HS) snow additions relative to Ambient, contributing to increasing the CH4 source strength of moist acidic tundra (464 ± 15 and 3561 ± 97 mg CH4 m-2 season-1 at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant-mediated CH4 transport associated with the expansion of tall graminoids. Deeper snow enhanced CH4 production within newly thawed soils, responding mainly to soil warming rather than to increases in acetate fermentation expected from thaw-induced increases in SOC availability. Our results suggest that increased winter precipitation will increase the CH4 source strength of Arctic tundra, but the resulting positive feedback on climate change will depend on the balance between areas with more or less snow accumulation than they are currently facing. [ABSTRACT FROM AUTHOR]

Nitrogen (N) mineralization, nutrient availability, and plant growth in the Arctic are often restricted by low temperatures. Predicted increases of cold-season temperatures may be important for plant nutrient availability and growth, given that N mineralization is also taking place during the cold season. Changing nutrient availability may be reflected in plant N and chlorophyll content and lead to increased photosynthetic capacity, plant growth, and ultimately carbon (C) assimilation by plants. In this study, we increased snow depth and thereby cold-season soil temperatures in high Arctic Svalbard in two vegetation types spanning three moisture regimes. We measured growing-season availability of ammonium (NH), nitrate (NO), total dissolved organic carbon (DOC) and nitrogen (TON) in soil; C, N, δN and chlorophyll content in Salix polaris leaves; and leaf sizes of Salix, Bistorta vivipara, and Luzula arcuata at peak season. Nutrient availability was significantly higher with increased snow depth in the two mesic meadow vegetation types, but not in the drier heath vegetation. Nitrogen concentrations and δN values of Salix leaves were significantly higher in all vegetation types, but the leaf sizes were unchanged. Leaves of Bistorta and Luzula were significantly larger but only significantly so in one moist vegetation type. Increased N and chlorophyll concentrations in leaves indicate a potential for increased growth (C uptake), supported by large leaf sizes for some species. Responses to cold-season soil warming are vegetation type- and species-specific, with potentially stronger responses in moister vegetation types. This study therefore highlights the contrasting effect of snow in a tundra landscape and has important implications for projections of whole tundra responses to climate change. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to discuss the range of temperature and moisture differences and variability in tundra vegetation and Arctic soils in the context of weather changes during the growing season. Research was carried out in Wedel Jarlsberg Land (SW Spitsbergen) in a small non-glaciated catchment in the vicinity of the Polish Polar Station in Hornsund. Measurements of air and soil temperature and changes in volumetric soil water content were carried out between June 24 and September 9 of 2008. The studied sites represented: (1) Turbic Cryosol, poorly covered by vegetation and exhibiting evidences of strong cryogenic processes; (2) Hyperskeletic Cryosol covered by a community of wet moss tundra vegetation with variable moisture which evolves under the influence of water flowing from the adjacent mountain slopes occupied by Little Auk colonies; and (3) Haplic Cryosol covered by lichen–herb–heath tundra vegetation, located on a raised marine terrace. Hyperskeletic Cryosol is the wettest of all the soil profiles studied. This profile shows the largest fluctuations in water content, related to the rate of snow melting on mountain slopes and the outflow of ablation water. The volumetric soil water content in Hyperskeletic Cryosol varies from 29% to 71%, with an average value of 49%. The mean volumetric soil water content in the driest Haplic Cryosol is 6% and ranges from 3% to 10% throughout the growing season. This is related to its sandy texture and a large number of stones, which leads to fast infiltration of snowmelt water and rainfall. Turbic Cryosol shows an intermediate volumetric water content ranging from 21% to 38%, with a mean value of 30%. The thermal regimes of the soils studied are also variable. The warmest soil was the driest Haplic Cryosol, as no influence of cold thawing water was observed. During the measurement period, the mean soil temperature at a depth of 10cm in Haplic Cryosol was 6.3°C. Turbic Cryosol was ca. 1.2°C–1.5°C colder than Haplic Cryosol. Hyperskeletic Cryosol was the coldest in comparison with the other soils studied because of the moss cover having insulating capacity. The temperatures recorded in this profile at a depth of 10cm reached a mean value of 3.2°C. The results indicate that cold water inflow from melting snow cover greatly affects soil temperature in the first part of summer (ablation season). This is related to an increase in solar radiation and air temperature leading to more intensive snow melting. This relationship is particularly evident in the first ten days of July. The highest soil surface temperatures (>20°C) were recorded in the beginning of July under intense solar radiation (25–27MJ/m2 d−1). In the second part of August, thermal gradients were weaker and soil temperatures in all the pedons studied were almost the same, ranging between +3°C and +6°C. This was due to limited solar energy inflow and heat migration into the soil transported with rainfall. [ABSTRACT FROM AUTHOR]

Rapid climate warming has widely been considered as the main driver of recent increases in Arctic tundra productivity. Field observations and remote sensing both show that tundra "greening" has been widespread, but heterogeneity in regional and landscape-scale trends suggest that additional controls are mediating the response of tundra vegetation to warming. In this study, we examined the relationship between changes in vegetation productivity in the western Canadian Arctic and biophysical variables by analyzing trends in the Enhanced Vegetation Index (EVI) obtained from nonparametric regression of annual Landsat surface reflectance composites. We used Random Forests classification and regression tree modelling to predict the trajectory and magnitude of greening from 1984 to 2016 and identify biophysical controls. More than two-thirds of our study area showed statistically significant increases in vegetation productivity, but observed changes were heterogeneous, occurring most rapidly within areas of the Southern Arctic that were: (1) dominated by dwarf and upright shrub cover types, (2) moderately sloping, and (3) located at lower elevation. These findings suggest that the response of tundra vegetation to warming is mediated by regional- and landscape-scale variation in microclimate, topography and soil moisture, and physiological differences among plant functional groups. Our work highlights the potential of the joint analysis of annual remotely sensed vegetation indices and broad-scale biophysical data to understand spatial variation in tundra vegetation change. • A 33-year time-series of Landsat EVI was analyzed using machine learning. • Over 68% of the western Canadian Arctic greened significantly from 1984 to 2016. • Greening was most rapid in the Southern Arctic region in shrub dominated terrain. • Tundra responses to warming are mediated by microclimate, topography, and moisture. [ABSTRACT FROM AUTHOR]