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Studies in recent decades have shown strong evidence of physical and biological changes in the Arctic tundra, largely in response to rapid rates of warming. Given the important implications of these changes for ecosystem services, hydrology, surface energy balance, carbon budgets, and climate feedbacks, research on the trends and patterns of these changes is becoming increasingly important and can help better constrain estimates of local, regional, and global impacts as well as inform mitigation and adaptation strategies. Despite this great need, scientific understanding of tundra ecology and change remains limited, largely due to the inaccessibility of this region and less intensive studies compared to other terrestrial biomes. A synthesis of existing datasets from past field studies can make field data more accessible and open up possibilities for collaborative research as well as for investigating and informing future studies. Here, we synthesize field datasets of vegetation and active-layer properties from the Alaskan tundra, one of the most well-studied tundra regions. Given the potentially increasing intensive fire regimes in the tundra, fire history and severity attributes have been added to data points where available. The resulting database is a resource that future investigators can employ to analyze spatial and temporal patterns in soil, vegetation, and fire disturbance-related environmental variables across the Alaskan tundra. This database, titled the Synthesized Alaskan Tundra Field Database (SATFiD), can be accessed at the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for Biogeochemical Dynamics (Chen et al., 2023: 10.3334/ORNLDAAC/2177). [ABSTRACT FROM AUTHOR]

Egg prcdation is the primary cause of nest failure for tundra nesting birds. Taverner's Cackling Geese (Branta hutchinsii taverneri) breeding on the North Slope of Alaska have increased approximately 5-fold over the last 25 years and arc now one of the most encountered waterbird species breeding at our study site on the coastal plain of Arctic National Wildlife Refuge. This rapid population growth may offer a new food resource for egg predators, impacting predator populations, which in turn could have cascading effects for other tundra nesting bird species, many of which are declining. To determine how the population growth of geese may affect the nest-predator community, and to establish a baseline of predators as the area is impacted by climate change and development, we used digital time-lapse cameras to record prcdation events at Cackling Goose nests. We determined fate for 52 nests in 2017, 2019, and 2021, of which 20 were depredated. The most frequent predator differed each year. Red foxes (Vulpes vulpes) were responsible for 89% of depredations in 2017 and wolverines (Gulo gulo) for 67% in 2019, and those species only depredated nests in those single years. In contrast, Arctic foxes (Vulpes lagopus) were responsible for nest failure in all years, but did not exceed 40% of total depredations in any year. Golden Eagles (Aquila chrysaetos) were an unexpected egg predator, depredating 2 nests. Our results were notably different than work on other Arctic goose species, where Arctic foxes were the primary predator. The inter-annual variability of egg predators at this Arctic Refuge study site provides insights into complex predator-prey interactions on the Arctic Coastal Plain. Received 24 October 2022. Accepted 24 September 2023. Key words: Arctic Coastal Plain, Arctic nesting geese, egg predator, Taverner's Cackling Geese (Branta hutchinsii taverneri), wildlife cameras. La depredacion de huevos es la causa primaria del fracaso de nido para aves que anidan en la tundra. El ganso Branta hutchinsii taverneri, que se reproduce en la pendiente Norte de Alaska ha quintuplicado su presencia en los ultimos 25 anos y ahora es una de las especies acuaticas en reproduccion mas observadas en nuestro sitio de estudio en la planicie costera del Arctic National Wildlife Refuge. Este crecimiento poblacional acelerado puede ofrecer una nueva fuente de alimentacion para depredadores de huevos, impactando las poblaciones de depredadores, que podria tener efectos en cascada en otras especies de aves que anidan en la tundra, muchas en declive. Para determinar como el crecimiento poblacional de gansos puede afectar la comunidad de depredadores de nidos y establecer una linea base de depredadores en un area que es impactada por el cambio climatico y el desarrollo, usamos camaras digitales con temporizador automatico para registrar eventos de depredacion en nidos de ganso Branta hutchinsii. Determinamos el destino de 52 nidos en 2017, 2019 y 2021, de los cuales 20 fueron depredados. El depredador mas frecuente vario cada ano. Los zorros rojos (Vulpes vulpes) fueron responsables del 89% de las depredaciones en el 2017, mientras que los glotones (Gulo gulo) lo fueron del 67% en el 2019, y esas especies solo depredaron nidos en esos anos. En contraste, el zorro artico (Vulpes lagopus) fue responsable de fracaso de anidacion durante todos los anos, pero su cuota de depredacion no excedio el 40% en ninguno de los anos. El aguila real (Aquila chtysaetos) fue un depredador inesperado, depredando 2 nidos. Nuestros resultados fueron notablemente diferentes que otros estudios con otras especies de gansos silvestres del Artico, donde los zorros articos fueron los depredadores primarios. La variacion interanual de depredadores de huevos en este sitio de estudio en el Refugio Artico proporciona informacion en interacciones complejas depredador-presa en la Planicie Costera Artica. Palabras clave: camaras para vida silvestre, depredador de huevos, ganso Branta hutchinsii taverneri, gansos de anidacion artica, Planicie Costera Artica., The ecology of Arctic species is complex, and climate-mediated effects have been demonstrated as both direct and rapid, as well as disguised by species interactions (Gauthier et al. 2004, Post [...]

Reproduction of long-distance migrants can be affected by local conditions on the breeding grounds as well as those encountered during the nonbreeding season through carry-over effects. We show that this is true in Long-tailed Jaegers (Stercorarius longicaudus) because individuals that spent less time flying at sea during winter had a high breeding propensity and a reduced pre-laying interval, but breeding propensity and nesting success were also positively associated with food abundance at the breeding site. This seabird switches from a marine to a terrestrial lifestyle to breed in summer in the Arctic, where it primarily feeds on lemmings. We monitored jaeger reproduction and lemming densities on Bylot Island in the Canadian Arctic for 16 years, and we used geolocator to study annual movements. We assessed whether movement parameters (travel distance, migration duration, phenology, and number of flying bouts inferred by saltwater immersions) during the nonbreeding season affected the breeding propensity, phenology, and success of individuals. We also examined whether cyclic lemming fluctuations influenced Long-tailed Jaeger reproduction and whether nesting success affected the phenology of their outbound migration. We found that increased time spent flying during winter and early arrival at the breeding site reduced breeding propensity. Moreover, spending less time flying during winter shortened the pre-laying period, and advancing laying date increased nesting success. Birds may thus face a trade-off to minimize the relative costs associated with arriving too early and breeding too late. Local food availability had a strong effect on reproduction because breeding propensity and nesting success increased sharply with lemming abundance. Failed breeders advanced their outbound migration by 10 days on average compared to successful ones, but migration duration was similar. Therefore, the unpredictability of the highly seasonal Arctic environment, especially fluctuating food abundance, appears to be a strong driver of reproduction that can modulate the strength of carry-over effects. [ABSTRACT FROM AUTHOR]

Arctic tundra largely in response to exceptionally rapid rates of warming. Given the important implications of these changes on ecosystem services, hydrology, surface energy balance, carbon budgets, and climate feedbacks, research on the trends and patterns of these changes is becoming increasingly important and can help better constrain estimates of local, regional, and global impacts as well as inform mitigation and adaptation strategies. Despite this high need, scientific understanding of tundra ecology and change remains limited largely due to the inaccessibility of this region and less intensive study compared to other terrestrial biomes. A synthesis of existing datasets from past field studies can make field data more accessible and open up possibilities for collaborative research as well as for investigating and informing future studies. Here, we synthesize field datasets of vegetation, and active layer properties from the Alaskan tundra, one of the most well-studied tundra regions. Given the potential increasingly intensive fire regimes in the tundra, fire history and severity attributes have been added to data points where available. The resulting database is a resource that future investigators can employ to analyze spatial and temporal patterns in soil, vegetation, and fire disturbance-related environmental variables across the Alaskan tundra. This database, titled Synthesized Alaskan Tundra Field Database (SATFiD), can be accessed at the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for Biogeochemical Dynamics (Chen et al., 2023: https://doi.org/10.3334/ORNLDAAC/2177). [ABSTRACT FROM AUTHOR]

Traditional knowledge about snow and ice conditions on and in the ground is essential in the life of the Yamal Nenets. This holistic knowledge helps the Nenets to travel in the tundra, find good pastures for their domesticated reindeer herds, select proper places for making their camps, find firewood, and locate clean snow or ice for drinking water. It is particularly important for reindeer herders, because looking at different characteristics of snow (layers, hardness, and granularity) enables them to find good pastures for their animals. If there are dangers posed by a crust of ice on the snow, herders have to move their herds to other pastures. Moreover, even reindeer know which kind of snow is easier for them to break with their hooves and where good forage is found. Significantly, the Nenets language has developed a sophisticated terminology describing different types of snow and ice, and similarly permafrost has a special name. Like many other Indigenous peoples of Siberia, the Nenets have noticed that climate change in the Arctic is dramatically affecting their life: it is changing the tundra landscape, the seasons, and the conditions under which they live and herd reindeer. In consequence, the reindeer-herding culture itself helps the people to preserve this knowledge of how to live in the tundra, but to remain relevant, the Nenets' knowledge of tundra ecology and their words for snow, ice, and permafrost must continuously adapt to new realities of the tundra. However, if that culture disappears in the Yamal, this resource will also be difficult to save. [ABSTRACT FROM AUTHOR]

Rain-on-Snow (ROS) events occur across many regions of the terrestrial Arctic in mid-winter. Snow pack properties are changing and in extreme cases ice layers form which affect wildlife, vegetation and soils beyond the duration of the event. Specifically, satellite microwave observations have been shown to provide insight into known events. Only Ku-band radar (scatterometer) has been applied so far across the entire Arctic. Data availability at this frequency is limited, however. The utility of other frequencies from passive and active systems need to be explored to develop a concept for long-term monitoring. Active (radar) records have been shown to capture the associated snow structure change based on time series analyses. This approach is also applicable when data gaps exist and bears capabilities to evaluate the impact severity of events. Active as well as passive microwave sensors can also detect wet snow at the timing of a ROS, if an acquisition is available. Wet snow retrieval methodology is, however, rather mature, compared to identification of snow structure change which needs consideration of ambiguous scattering behaviour. C-band radar is of special interest due to good data availability including a range of nominal spatial resolution (10 m–12.5 km) A combined approach is therefore considered and tested for C-band (active, snow structure change) and L-band (passive, wet snow). Results were compared to in situ observations (snow pit records, caribou migration data) and Ku-band products. Ice crusts were found in the snow pack after detected events. The more crusts (events) the higher the winter season backscatter increase at C-band. ROS events captured on the Yamal and Seward peninsulas have had severe impacts on reindeer and caribou, respectively, due to crust formation. Temperature dependence of C-band backscatter observable down to -40 °C is identified as a major issue for ROS retrieval, but can be addressed by combination with passive microwave wet snow indicators (demonstrated for Metop ASCAT and SMOS). Synthetic Aperture Radar (SAR) from specifically Sentinel-1 (C-band) is promising regarding ice layer identification at better spatial details for all available polarizations. The fusion of multiple types of microwave satellite observations is suggested for the creation of a climate data record, but the consideration of performance differences due to spatial and temporal cover as well as microwave frequency is crucial. Retrieval is most robust north of 65° N, in the tundra biome, where records can be used to identify extremes and to apply the results for impact studies at regional scale. [ABSTRACT FROM AUTHOR]

Rapid Arctic warming is causing permafrost to thaw and exposing large quantities of soil organic carbon (C) to potential decomposition. In dry upland tundra systems, subsidence from thawing permafrost can increase surface soil moisture resulting in higher methane (CH4) emissions from newly waterlogged soils. The proportion of C released as carbon dioxide (CO2) and CH4 remains uncertain as previously dry landscapes transition to a thawed state, resulting in both wetter and drier microsites. To address how thaw and moisture interact to affect total C emissions, we measured CH4 and CO2 emissions from paired chambers across thaw and moisture gradients created by nine years of experimental soil warming in interior Alaska. Cumulative growing season (May–September) CH4 emissions were elevated at both wetter (216.1–1,099.4 mg CH4-C m−2) and drier (129.7–392.3 mg CH2-C m−2) deeply thawed microsites relative to shallow thaw (55.6–215.7 mg CH4-C m−2) and increased with higher deep soil temperatures and permafrost thaw depth. Interannual variability in CH4 emissions was driven by wet conditions in graminoid-dominated plots that generated >70% of emissions in a wet year. Shoulder season emissions were equivalent to growing season CH4 emissions rates in the deeply thawed, warmed soils, highlighting the importance of non-growing season CH4 emissions. Net C sink potential was reduced in deeply thawed wet plots by 4%–42%, and by 3.5%–8% in deeply thawed drier plots due to anaerobic respiration, suggesting that some dry upland tundra landscapes may transition into stronger CH2 sources in a warming Arctic. [ABSTRACT FROM AUTHOR]

Arctic terrestrial ecosystems may be contributing to increasing atmospheric carbon dioxide (CO2) concentrations under amplified climate change at high‐latitudes. This research investigates how summer net ecosystem CO2 exchange (NEE) and its component fluxes, gross primary production (GPP), and ecosystem respiration (Reco) varied over five years (2008, 2009, 2010, 2012, and 2014) at the Cape Bounty Arctic Watershed Observatory (74.92°N, 109.58°W). The eddy covariance technique was used to measure NEE and a combined light and temperature response model was used to partition NEE into GPP and Reco. Total summer NEE varied from −19.8 to 7.9 g C m−2. Despite two summers with net CO2 uptake, this tundra was likely a source of CO2 on an annual basis. In most years, growing degree days with a base 0°C (GDD0) had more predictive power than other environmental variables in random forest analyses of daily NEE. Interannual variability in total summer NEE over the five study years was also attributed to greater variability in GPP than Reco (coefficient of variation: 62% and 27%, respectively). However, total summer GDD0 significantly correlated with total summer Reco but not NEE nor GPP. Instead, summer total NEE and GPP significantly correlated with satellite‐derived normalized difference vegetation index (NDVI), which may have exhibited carry‐over effects from one year to the next to limit the impact of current year GDD0 on total summer NEE at this tundra site. Plain Language Summary: Arctic ecosystems currently store a vast amount of carbon within frozen soils. Over the last two decades, Arctic surface air temperatures have increased at a rate twice the global average, which has the potential to shift terrestrial carbon cycling, ultimately impacting atmospheric carbon dioxide (CO2), a potent greenhouse gas. Whether Arctic terrestrial ecosystems are releasing more CO2 than they are absorbing is important, as this can impact the global climate system. This research measured CO2 exchange between the atmosphere and the tundra over five summers at the Cape Bounty Arctic Watershed Observatory (74.92°N, 109.58°W) on Melville Island, Nunavut, Canada. The ecosystem acted as a CO2 source for three out of five summers and is likely an annual net CO2 source to the atmosphere. Interannual differences in weather appeared to impact plant productivity and CO2 uptake more than decomposition and respiration and CO2 emissions to the atmosphere at this site. The amount of CO2 absorbed or lost from this tundra each summer was correlated to satellite observations, which could help us monitor the carbon balance of these remote but important ecosystems. This research highlights the importance of multiyear observations to constrain understanding of High Arctic tundra CO2 dynamics. Key Points: Over five summers, a High Arctic tundra site with low productivity varied from being a net source to a net sink for atmospheric CO2Summer net CO2 uptake depended more on photosynthesis than respiration and correlated well with satellite‐derived NDVIDaily net ecosystem CO2 exchange could be predicted using growing degree days and other environmental variables using random forests [ABSTRACT FROM AUTHOR]

Warming of the Arctic can stimulate microbial decomposition and release of permafrost soil carbon (C) as greenhouse gases, and thus has the potential to influence climate change. At the same time, plant growth can be stimulated and offset C release. This study presents a 15‐year time series comprising chamber and eddy covariance measurements of net ecosystem C exchange in a tundra ecosystem in Alaska where permafrost has been degrading due to regional warming. The site was a carbon dioxide source to the atmosphere with a cumulative total loss of 781.6 g C m−2 over the study period. Both gross primary productivity (GPP) and ecosystem respiration (Reco) were already likely higher than historical levels such that increases in Reco losses overwhelmed GPP gains in most years. This shift to a net C source to the atmosphere likely started in the early 1990s when permafrost was observed to warm and thaw at the site. Shifts in the plant community occur more slowly and are likely to constrain future GPP increases as compared to more rapid shifts in the microbial community that contribute to increased Reco. Observed rates suggest that cumulative net soil C loss of 4.18–10.00 kg C m−2—8%–20% of the current active layer soil C pool—could occur from 2020 to the end of the century. This amount of permafrost C loss to the atmosphere represents a significant accelerating feedback to climate change if it were to occur at a similar magnitude across the permafrost region. Plain Language Summary: The Arctic is warming at twice the global average. Shifting environmental conditions including the degradation of permafrost affect the storage of carbon in plants and soils of tundra ecosystems. Carbon uptake by plant growth and carbon release by microbial respiration of soil organic matter both appear to have increased prior to and over 15 years of measurements in a tundra ecosystem in Alaska where permafrost has been degrading. But, carbon release to the atmosphere overwhelmed uptake on average, which leads to net release of carbon to the atmosphere and accelerates climate change. Key Points: Fifteen years of measurements reveal tundra to be a persistent annual net source of carbon to the atmosphere where permafrost is degradingPlant and microbial activity increased from historical levels such that respiration losses in most years overwhelmed productivity gainsThe longer successional dynamics of plants suggests that respiration may outpace productivity for decades as an accelerating feedback to climate change [ABSTRACT FROM AUTHOR]

The rates, processes, and controls on Arctic cold period soil carbon loss are still poorly understood. To understand one component of winter CO2 loss in the atmosphere, continuous measurements of soil (CO2) were made and compared to ecosystem scale CO2 fluxes. Measurements of soil (CO2) were made near Utqiaġvik, Alaska from the beginning of soil thaw in summer 2005 to spring 2007. In the summer, soil (CO2) rose with increased soil temperature, reaching value orders of magnitude higher than the atmospheric (CO2). Soil (CO2) initially decreased at the end of summer and beginning of fall but then increased subsequent to soil freezing. Due to complex changes in biological activity, storage, and transport processes, soil (CO2) was then approximately doubled than that was observed in the summer. After reaching peak concentrations in November, soil (CO2) steeply decreased over a couple of weeks, suggesting a substantial release of CO2 into the atmosphere and movement within the soil column. Eddy covariance (EC) measurements showed variable but continued emissions of CO2 to the atmosphere during freezeup. The disconnect between soil (CO2) and landscape level fluxes may be attributed to the spatiotemporal heterogeneity in release of high concentrations of soil (CO2) to the atmosphere during the fall; and when integrated over the area of the EC tower footprint, do not frequently result in detectable emission events. Continued monitoring of fall and winter soil (CO2) and ecosystem fluxes will be vital for further understanding the variability of interannual Arctic CO2 emissions. Key Points: Soil (CO2) doubled in the fall, likely attributable to complex changes in biological activity, storage, and transport processes amidst soil freezingSoil (CO2) was significantly controlled by soil temperature and maintained substantial values well below zero (at −20 cm soil (CO2) concentrations were in the order of 6.40 ± 0.19% through freezing)Peak increases in soil (CO2) were not captured by ecosystem scale CO2 fluxes, suggesting a disconnect between complex and spatially heterogenous driving processes [ABSTRACT FROM AUTHOR]

In this study, we investigated the utility of spectral remote sensing data gathered by a multispectral camera for estimating of vegetation cover in Antarctic vegetation oasis and Arcto-Alpine tundra. The surveys exploiting unmanned aerial vehicles (UAV) and multispectral camera were done in an Antarctic vegetation oasis located at the Northern shore of James Ross Island (Antarctica), and arcto-alpine tundra located in the Jeseníky Mts. (NE Czech Republic, 1 420 m a.s.l.). For the two locations, false colour images of spectral indices (VARI, NGRDI, GLI, RGBVI, ExG, NDVI, PRI) were taken and analysis of vegetation types and components of vegetation cover was done. Additionally, field research was performed by handheld instruments measuring NDVI, PRI and of selected vegetation components: Bryum pseudotriquetrum, Nostoc commune colonies (Antarctica), lichens grown on flat stones and boulders (the Jeseníky Mts.). The results show UAV photo surveys and imaging of spectral reflectance indices can be used to monitor vegetation types forming Antarctic vegetation oases and arcto-alpine tundra. [ABSTRACT FROM AUTHOR]

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 vegetationColonization by aquatic plant species, in particular Sphagnum mosses, is associated with recovery of permafrost in these pondsRecruitment of dwarf shrubs on Sphagnum carpets in thaw ponds indicates potential recovery of the original dwarf shrub‐dominated vegetation [ABSTRACT FROM AUTHOR]

The response of vegetation to recent climate change is a central theme in contemporary biogeography. A tenet of this research is that forests will advance upslope and northward as climate warms, replacing tundra communities. There has been considerable variation, however, in the pace and extent of recent change, particularly in alpine regions. The objectives of this study were to determine how and why tree spatial patterns vary across different topographic features, and to use spatial patterns to infer the mechanisms governing treeline dynamics in an alpine region of subarctic Canada. We mapped trees across different elevations, slope aspects, and slope angles in two QuickBird satellite images, and in field plots established within the bounds of each image. We then quantified the degree of clustering among trees using Ripley's K(t) statistic. We also classified each image into vegetation classes and used class-level landscape metrics to quantify the degree of treeline abruptness in different topographic settings. We found that clustering of stems was more common on south than north aspects, likely due to the high occurrence of overwinter damage on the former. Treelines were also more abrupt on south aspects, likely because high tall shrub abundance on these slopes inhibits tree seedling establishment. We conclude that the spatial patterns of subarctic alpine treelines are strongly influenced by both physical and biological factors that vary strongly with slope aspect. The response of treelines to future climate change will likely be highly variable at the landscape scale, despite experiencing similar climatic conditions. Key Words: forest–tundra ecotone, FragStats, landscape metrics, QuickBird, spatial pattern analysis, Yukon. [ABSTRACT FROM AUTHOR]

The atmospheric methane (CH4) concentration, a potent greenhouse gas, is on the rise once again, making it critical to understand the controls on CH4 emissions. In Arctic tundra ecosystems, a substantial part of the CH4 budget originates from the cold season, particularly during the "zero curtain" (ZC), when soil remains unfrozen around 0 °C. Due to the sparse data available at this time, the controls on cold season CH4 emissions are poorly understood. This study investigates the relationship between the fall ZC and CH4 emissions using long‐term soil temperature measurements and CH4 fluxes from four eddy covariance (EC) towers in northern Alaska. To identify the large‐scale implication of the EC results, we investigated the temporal change of terrestrial CH4 enhancements from the National Oceanic and Atmospheric Administration monitoring station in Utqiaġvik, AK, from 2001 to 2017 and their association with the ZC. We found that the ZC is extending later into winter (2.6 ± 0.5 days/year from 2001 to 2017) and that terrestrial fall CH4 enhancements are correlated with later soil freezing (0.79 ± 0.18‐ppb CH4 day−1 unfrozen soil). ZC conditions were associated with consistently higher CH4 fluxes than after soil freezing across all EC towers during the measuring period (2013–2017). Unfrozen soil persisted after air temperature was well below 0 °C suggesting that air temperature has poor predictive power on CH4 fluxes relative to soil temperature. These results imply that later soil freezing can increase CH4 loss and that soil temperature should be used to model CH4 emissions during the fall. Plain Language Summary: Methane (CH4) is a powerful greenhouse gas, capturing more heat per molecule than carbon dioxide (CO2). Although CH4 is less concentrated in the atmosphere, it is the second most important greenhouse gas with respect to climate change after CO2. Arctic tundra ecosystems are potentially major sources of CH4, given large soil carbon storage and generally wet conditions, favorable to CH4 production. This study investigates if the persistence of unfrozen soils is associated with higher CH4 emissions from the Arctic. We combined long‐term soil temperature measurements, terrestrial CH4 enhancements from the National Oceanic and Atmospheric Administration monitoring station in Utqiaġvik, AK, and CH4 emissions from Arctic tundra ecosystems across four stations in the North Slope of Alaska. Our results show that from 2001 to 2017 the soil is freezing later and that later soil freezing is associated with higher fall CH4 enhancements. Given that unfrozen soils are related to higher CH4 emissions, a later soil freezing could contribute to the observed increase in the regional atmospheric CH4 enhancement. Unfrozen soil layers persisted after the air temperature was well below 0 °C, suggesting that air temperature does not properly predict the sensitivity of CH4 emissions to climate warming. Key Points: During the last two decades soil has been freezing later in the Arctic (from 2001‐2017)The presence of unfrozen soils are associated with sustained emissions of methane until January in tundra ecosystemsAir temperature does not capture soil conditions in the fall, and shallow soil temperature underestimates the duration of unfrozen soils [ABSTRACT FROM AUTHOR]

In permafrost landscapes, ponds and lakes are ubiquitous and important features that support long-term carbon storage and diverse diatom communities. With climate warming, thermokarst lake expansion may modify the ability of these aquatic systems to maintain these functions, as a consequence of changes to nutrient inputs and cycling. A recent (2006–2008) thermokarst shoreline expansion event adjacent to small Frisbee Pond, in the Hudson Bay Lowlands, Canada, presented an opportunity to evaluate the effects of thaw-driven disturbance on pond nutrient dynamics. A sediment core was collected in 2015 and analysed for diatom assemblage composition, total nutrient (carbon, nitrogen and phosphorus) accumulation rates, and stable isotopes of organic carbon (δ13C) and nitrogen (δ15N). Results indicate that recent shoreline expansion in this subarctic peatland pond caused an increase in total diatom accumulation, although shifts in diatom community composition were initiated prior to the shoreline slump, particularly an increase in the relative abundance of Denticula kuetzingii. Nutrient accumulation rates in the sediment increased dramatically both prior to and following shoreline slumping, with isotope values indicating an increased supply of allochthonous carbon and atmospheric nitrogen. Investigations of hydroclimate conditions prior to the shoreline slump indicate warming of summer air temperatures and an increase in precipitation over the long-term mean, consistent with changing climate conditions that caused thermokarst disturbance in other permafrost environments. This study has important implications for understanding hydro-ecological responses of small surface water bodies to general climate forcing, in combination with rapid thermokarst shoreline expansion, under a changing climate. [ABSTRACT FROM AUTHOR]

Summary: The response of vegetation to climate change has implications for the carbon cycle and global climate. It is frequently assumed that a species responds uniformly across its range to climate change. However, ecotypes − locally adapted populations within a species − display differences in traits that may affect their gross primary productivity (GPP) and response to climate change.To determine if ecotypes are important for understanding the response of ecosystem productivity to climate we measured and modeled growing season GPP in reciprocally transplanted and experimentally warmed ecotypes of the abundant Arctic sedge Eriophorum vaginatum.Transplanted northern ecotypes displayed home‐site advantage in GPP that was associated with differences in leaf area index. Southern ecotypes exhibited a greater response in GPP when transplanted.The results demonstrate that ecotypic differentiation can impact the morphology and function of vegetation with implications for carbon cycling. Moreover they suggest that ecotypic control of GPP may limit the response of ecosystem productivity to climate change. This investigation shows that ecotypes play a substantial role in determining GPP and its response to climate. These results have implications for understanding annual to decadal carbon cycling where ecotypes could influence ecosystem function and vegetation feedbacks to climate change. [ABSTRACT FROM AUTHOR]

Mammalian herbivores shape the structure and function of many nutrient-limited or low-productive terrestrial ecosystems through modification of plant communities and plant–soil feedbacks. In the tundra biome, mammalian herbivores may both accelerate and decelerate plant biomass growth, microbial activity and nutrient cycling, that is, ecosystem process rates. Selective foraging and associated declines of palatable species are known to be major drivers of plant–soil feedbacks. However, declines in dominant plants of low palatability often linked with high herbivore densities may also modify ecosystem process rates, yet have received little attention. We present data from an island experiment with a 10-year vole density manipulation, to test the hypothesis that herbivores accelerate process rates by decreasing the relative abundance of poorly palatable plants to palatable ones. We measured plant species abundances and community composition, nitrogen contents of green plant tissues and multiple soil and litter variables under high and low vole density. Corroborating our hypothesis, periodic high vole density increased ecosystem process rates in low-productive tundra. High vole density was associated with both increasing relative abundance of palatable forbs over unpalatable evergreen dwarf shrubs and higher plant N content both at species and at community level. Changes in plant community composition, in turn, explained variation in microbial activity in litter and soil inorganic nutrient availability. We propose a new conceptual model with two distinct vole–plant–soil feedback pathways. Voles may drive local plant–soil feedbacks that either increase or decrease ecosystem process rates, in turn promoting heterogeneity in vegetation and soils across tundra landscapes. [ABSTRACT FROM AUTHOR]

Water is crucial for plant productivity and survival as a fundamental resource, but water conditions can also cause physiological stress and mechanical disturbance to vegetation. However, these different influences of water on vegetation patterns have not been evaluated simultaneously. Here, we demonstrate the importance of three water aspects (spatial and temporal variation of soil moisture and fluvial disturbance) for three ecologically and evolutionary distinct taxonomical groups (vascular plants, mosses and lichens) in Fennoscandian mountain tundra. Fine‐scale plant occurrence data for 271 species were collected from 378 × 1 m2 plots sampled over broad environmental gradients (water, temperature, radiation, soil pH, cryogenic processes and the dominant allelopathic plant species). While controlling all other key environmental variables, water in its different aspects proved to be a crucial environmental driver, acting on individual species and on community characteristics. The inclusion of the water variables significantly improved our models. In this high‐latitude system, the importance of spatial variability of water exceeds the importance of temperature for the fine‐scale distribution of species from the three taxonomical groups. We found differing responses to the three water variables between and within the taxonomical groups. Water as a resource was the most important water‐related variable in species distribution models across all taxonomical groups. Both water resource and disturbance were strongly related to vascular plant species richness, whereas for moss species richness, water resources had the highest influence. For lichen species richness, water disturbance was the most influential water‐related variable. These findings demonstrate that water variables are not only independent properties of tundra hydrology, but also that water is truly a multifaceted driver of vegetation patterns at high‐latitudes. [ABSTRACT FROM AUTHOR]

The water balance of the Arctic tundra is shifting as permafrost stability, seasonality, and the ratio of precipitation to evaporation respond to amplified Arctic warming. While in some northern tundra locations there has been a notable increase in the number of water bodies, generally, the tundra landscape has experienced a decline in the number and area of lakes. We analyzed changes in small lake count (<10,000 m2), large lake count (>10,000 m2), and lake surface area across the periglacial tundra of western Greenland, using historical satellite and aerial imagery and weather data from the late 1960s to present. Overall, we found a decrease in lake count (21%) and surface area (2%) across our study region. Specifically, smaller ponds were particularly prone to change, with decreases of 28% in count and 15% in surface area. Shrinking lakes often became revegetated by both emergent aquatic and terrestrial vegetation, which captures potential successional trajectories following Arctic lake drying. Additionally, while annual precipitation may be increasing, it occurred primarily during the winter months in the form of snow, which may or may not contribute to the overall growing season water budget. Conversely, the peak growing season months of June, July, and August all have experienced significant increases in potential evaporation rates, thus likely creating a water deficit for much of the growing season. These results suggest that a large section of deglaciated Greenland appears likely to become drier in the summer months, which may result in widespread ecological consequences. Plain Language Summary: Arctic tundra environments may become drier as the climate continues to warm, due to changes in precipitation and evaporation patterns, and losses of permanent ground ice. In many Arctic systems, lakes are widely distributed and thus can serve as an early indicator of drying conditions. We predicted that our study region of west Greenland would be particularly susceptible to drying due to low levels of precipitation and observed warming trends at a rate of approximately 0.5 °C per decade. Using historical and modern satellite and aerial imagery, we observed a decrease in the number of small lakes by 28% and a 15% decrease in the total area of smaller water bodies between 1969 and 2017. Additionally, we found that many of the disappeared lakes from 1969 appeared to have become vegetated. An analysis of historical weather data suggested that water losses to the atmosphere via evaporation have likely increased, especially in the summer months, which may be contributing to observed lake decline. Declining small lake area across our study region and other parts of the Arctic can have landscape‐wide effects such as changes to available animal habitat, increases in the risk of fire, and to the prevalence of drought conditions. Key Points: Over the last 50 years, the Kangerlussuaq region of west Greenland has seen a 21% decrease in the total number of lakesSmall lakes (<10,000 m2) are the most sensitive to lake dryingEvaporation appears to be a key driver to lake drying across the landscape [ABSTRACT FROM AUTHOR]

Arctic shrub expansion is occurring across large parts of the tundra biome and its potential ecological repercussions have been widely discussed. But while the term "shrub expansion" often implicitly refers to an increase in tall, deciduous species such as birch and willow, several studies have also found a strong increase in evergreen dwarf shrubs in response to warming, a fact which has received far less attention.The effects of an evergreen dwarf shrub expansion are markedly different from the effects of an increase in taller, deciduous species. While deciduous shrubs may increase carbon (C) cycling through changes in albedo, litter input, and snow depth, the low stature of evergreen dwarf shrubs means that they are unlikely to influence snow cover. They also produce more recalcitrant litter, which reduces microbial activity. Furthermore, recent research suggests that ericoid mycorrhiza associated with evergreen shrubs may help to decelerate litter and soil organic matter turnover rates through the production of melanized hyphae that resist decomposition. Through selective browsing, herbivores may promote evergreen shrubs and facilitate C storage.Synthesis. In this mini review, we argue that basing predictions of how shrub expansion will affect tundra ecosystems on characteristics only applicable to tall deciduous shrubs hampers our understanding of the complex feedbacks related to Arctic vegetation shifts. We postulate that climate‐driven evergreen dwarf shrub expansion has counteracting biogeophysical processes compared to an increase in taller, deciduous species. This has implications for ecosystem functioning and global carbon dynamics. We outline several ecological differences between deciduous and evergreen shrubs and how they may affect ecosystem processes in opposing ways and highlight the key mediating role played by herbivores. [ABSTRACT FROM AUTHOR]

Abstract Higher temperatures in northern latitudes will increase permafrost thaw and stimulate above- and belowground plant biomass growth in tundra ecosystems. Higher plant productivity increases the input of easily decomposable carbon (C) to soil, which can stimulate microbial activity and increase soil organic matter decomposition rates. This phenomenon, known as the priming effect, is particularly interesting in permafrost because an increase in C supply to deep, previously frozen soil may accelerate decomposition of C stored for hundreds to thousands of years. The sensitivity of old permafrost C to priming is not well known; most incubation studies last less than one year, and so focus on fast-cycling C pools. Furthermore, the age of respired soil C is rarely measured, even though old C may be vulnerable to labile C inputs. We incubated soil from a moist acidic tundra site in Eight Mile Lake, Alaska for 409 days at 15 °C. Soil from surface (0–25 cm), transition (45–55 cm), and permafrost (65–85 cm) layers were amended with three pulses of uniformly 13C-labeled glucose or cellulose every 152 days. Glucose addition resulted in positive priming in the permafrost layer 7 days after each substrate addition, eliciting a two-fold increase in cumulative soil C loss relative to unamended soils with consistent effects across all three pulses. In the transition and permafrost layers, glucose addition significantly decreased the age of soil-respired CO 2 -C with Δ14C values that were 115‰ higher. Previous field studies that measured the age of respired C in permafrost regions have attributed younger Δ14C ecosystem respiration values to higher plant contributions. However, the results from this study suggest that positive priming, due to an increase in fresh C supply to deeply thawed soil layers, can also explain the respiration of younger C observed at the ecosystem scale. We must consider priming effects to fully understand permafrost C dynamics, or we risk underestimating the contribution of soil C to ecosystem respiration. Highlights • Glucose addition increased permafrost soil C loss two-fold. • Glucose addition to surface soils did not elicit a priming response. • Cellulose addition to surface soils elicited a negative priming response. • Glucose addition decreased the age of soil-respired carbon in deep layers. [ABSTRACT FROM AUTHOR]

The current assessments of the carbon turnover in the Arctic tundra are subject to large uncertainties. This problem can (inter alia) be ascribed to both the general shortage of flux data from the vast and sparsely inhabited Arctic region, as well as the typically high spatiotemporal variability of carbon fluxes in tundra ecosystems. Addressing these challenges, carbon dioxide fluxes on an active flood plain situated in the Siberian Lena River Delta were studied during two growing seasons with the eddy covariance method. The footprint exhibited a heterogeneous surface, and the mixed flux signal associated therewith could extensively be decomposed: respiratory loss and photosynthetic gain were not only modelled for the overall footprint, but also for each of two vegetation classes. This downscaling of the observed fluxes unveiled a differing seasonality in the net uptakes of bushes (0.89 μmol m-2 s-1) and sedges (0.38 μmol m-2 s-1) in 2014. That discrepancy, which was concealed in the net signal, resulted from a comparatively warm spring in conjunction with an early snow melt and a varying canopy structure. Thus, the representativeness of footprints may adversely be affected in response to prolonged unusual weather conditions. In 2015, when air temperatures on average corresponded to climatological means, both vegetation class-specific flux rates were of similar magnitude (0.69 μmol m-2 s-1). A comprehensive set of measures (e.g. phenocam) approved the reliability of the partitioned fluxes, and hence confirmed the utility of the flux decomposition for enhanced flux data analysis. This scrutiny encompassed insights into both the phenological dynamic of individual vegetation classes, plus their respective functional flux to flux driver relationships with the aid of ecophysiologically interpretable parameters. For the purpose of comparison with other sites, the decomposed fluxes were employed in a vegetation class area-weighted upscaling that was based on a classified high-resolution orthomosaic of the flood plain. In this way, robust budgets that take the heterogeneous surface characteristics into account were estimated. In relation to the average sink strength of various Arctic flux sites, the flood plain constitutes a distinctly stronger carbon dioxide sink. Roughly 42 % of this net uptake, however, was on average offset by methane emissions lowering the sink strength for greenhouse gases. With growing concern about rising greenhouse gas emissions in high-latitude regions, providing robust carbon budgets from tundra ecosystems is critical in view of the thawing permafrost, whose released carbon can impact the global climate for centuries. [ABSTRACT FROM AUTHOR]

Byline: S. B. Rasheed, M. Ali, F. Zaidi And S. Noreen Keywords: Diversity; Formicidae; New records; Pakistan; Swabi. INTRODUCTION Ants are among the most abundant organisms, accounting for 90% of [...]