Arctic Terrestrial Ecosystems Research Articles

High-resolution 4D electrical resistivity tomography and below-ground point sensor monitoring of High Arctic deglaciated sediments capture zero-curtain effects, freeze–thaw transitions, and mid-winter thawing

Abstract. Arctic regions are under immense pressure from a continuously warming climate. During the winter and shoulder seasons, recently deglaciated sediments are particularly sensitive to human-induced warming. Understanding the physical mechanisms and processes that determine soil liquid moisture availability contributes to the way we conceptualize and understand the development and functioning of terrestrial Arctic ecosystems. However, harsh weather and logistical constraints limit opportunities to directly observe subsurface processes year-round; hence automated and uninterrupted strategies of monitoring the coupled heat and water movement in soils are essential. Geoelectrical monitoring using electrical resistivity tomography (ERT) has proven to be an effective method to capture soil moisture distribution in time and space. ERT instrumentation has been adapted for year-round operation in high-latitude weather conditions. We installed two geoelectrical monitoring stations on the forefield of a retreating glacier in Svalbard, consisting of semi-permanent surface ERT arrays and co-located soil sensors, which track seasonal changes in soil electrical resistivity, moisture, and temperature in 3D. One of the stations observes recently exposed sediments (5–10 years since deglaciation), whilst the other covers more established sediments (50–60 years since deglaciation). We obtained a 1-year continuous measurement record (October 2021–September 2022), which produced 4D images of soil freeze–thaw transitions with unprecedented detail, allowing us to calculate the velocity of the thawing front in 3D. At its peak, this was found to be 1 m d−1 for the older sediments and 0.4 m d−1 for the younger sediments. Records of soil moisture and thermal regime obtained by sensors help define the conditions under which snowmelt takes place. Our data reveal that the freeze–thaw shoulder period, during which the surface soils experienced the zero-curtain effect, lasted 23 d at the site closer to the glacier but only 6 d for the older sediments. Furthermore, we used unsupervised clustering to classify areas of the soil volume according to their electrical resistivity coefficient of variance, which enables us to understand spatial variations in susceptibility to water-phase transition. Novel insights into soil moisture dynamics throughout the spring melt will help parameterize models of biological activity to build a more predictive understanding of newly emerging terrestrial landscapes and their impact on carbon and nutrient cycling.

Browning events in Arctic ecosystems: Diverse causes with common consequences

Arctic ecosystems are experiencing extreme climatic, biotic and physical disturbance events that can cause substantial loss of plant biomass and productivity, sometimes at scales of >1000 km2. Collectively known as browning events, these are key contributors to the spatial and temporal complexity of Arctic greening and vegetation dynamics. If we are to properly understand the future of Arctic terrestrial ecosystems, their productivity, and their feedbacks to climate, understanding browning events is essential. Here we bring together understanding of browning events in Arctic ecosystems to compare their impacts and rates of recovery, and likely future changes in frequency and distribution. We also seek commonalities in impacts across these contrasting event types. We find that while browning events can cause high levels of plant damage (up to 100% mortality), ecosystems have substantial capacity for recovery, with biomass largely re-established within five years for many events. We also find that despite the substantial loss of leaf area of dominant species, compensatory mechanisms such as increased productivity of undamaged subordinate species lessen the impacts on carbon sequestration. These commonalities hold true for most climatic and biotic events, but less so for physical events such as fire and abrupt permafrost thaw, due to the greater removal of vegetation. Counterintuitively, some events also provide conditions for greater productivity (greening) in the longer-term, particularly where the disturbance exposes ground for plant colonisation. Finally, we find that projected changes in the causes of browning events currently suggest many types of events will become more frequent, with events of tundra fire and abrupt permafrost thaw expected to be the greatest contributors to future browning due to their severe impacts and occurrence in many Arctic regions. Overall, browning events will have increasingly important consequences for ecosystem structure and function, and for feedback to climate.

Oceanic evasion fuels Arctic summertime rebound of atmospheric mercury and drives transport to Arctic terrestrial ecosystems

Mercury (Hg) contamination poses a persistent threat to the remote Arctic ecosystem, yet the mechanisms driving the pronounced summer rebound of atmospheric gaseous elemental Hg (Hg0) and its subsequent fate remain unclear due to limitations in large-scale seasonal studies. Here, we use an integrated atmosphere–land–sea-ice–ocean model to simulate Hg cycling in the Arctic comprehensively. Our results indicate that oceanic evasion is the dominant source (~80%) of the summer Hg0 rebound, particularly driven by seawater Hg0 release facilitated by seasonal ice melt (~42%), with further contributions from anthropogenic deposition and terrestrial re-emissions. Enhanced Hg0 dry deposition across the Arctic coastal regions, especially in the Arctic tundra, during the summer rebound highlights the potential transport of Hg from the pristine Arctic Ocean to Arctic terrestrial ecosystems. Arctic warming, with a transition from multi-year to first-year ice and tundra greening, is expected to amplify oceanic Hg evasion and intensify Hg0 uptake by the Arctic tundra due to increased vegetation growth, underlining the urgent need for continued research to evaluate Hg mitigation strategies effectively in the context of a changing Arctic.

Key determinants of soil labile nitrogen changes under climate change in the Arctic: A meta-analysis of the responses of soil labile nitrogen pools to experimental warming and snow addition

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 (NH4+), and nitrate (NO3-) 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 NH4+; and soil layer for NO3-. 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 NH4+; and soil pH for NO3-. 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.

Arctic Greening Trends: Change Points in Satellite-Derived Normalized Difference Vegetation Indexes and Their Correlation with Climate Variables over the Last Two Decades

In this study, we utilized NDVI data from the moderate resolution imaging spectroradiometer (MODIS) alongside climatic variables obtained from a reanalyzed dataset to analyze Arctic greening during the summer months (June–September) of the last two decades. This investigation entailed a detailed analysis of these changes across various temporal scales. The data indicated a continuous trend of Arctic greening, evidenced by a 1.8% per decade increment in the NDVI. Notably, significant change points were identified in June 2012 and September 2013. A comparative assessment of NDVI pre- and post-these inflection points revealed an elongation of the Arctic greening trend. Furthermore, an anomalous increase in NDVI of 2% per decade was observed, suggesting an acceleration in greening. A comprehensive analysis was conducted to decipher the correlation between NDVI, temperature, and energy budget parameters to elucidate the underlying causes of these change points. Although the correlation between these variables was relatively low throughout the summer months, a distinct pattern emerged when these periods were dissected and examined in the context of the identified change points. Preceding the change point, a strong correlation (approximately 0.6) was observed between all variables; however, this correlation significantly diminished after the change point, dropping to less than half. This shift implies an introduction of additional external factors influencing the Arctic greening trend after the change point. Our findings provide foundational data for estimating the tipping point in Arctic terrestrial ecosystems. This is achieved by integrating the observed NDVI change points with their relationship with climatic variables, which are essential in comprehensively understanding the dynamics of Arctic climate change, particularly with alterations in tundra vegetation.

Bioclimatic atlas of the terrestrial Arctic

The Arctic is the region on Earth that is warming at the fastest rate. In addition to rising means of temperature-related variables, Arctic ecosystems are affected by increasingly frequent extreme weather events causing disturbance to Arctic ecosystems. Here, we introduce a new dataset of bioclimatic indices relevant for investigating the changes of Arctic terrestrial ecosystems. The dataset, called ARCLIM, consists of several climate and event-type indices for the northern high-latitude land areas > 45°N. The indices are calculated from the hourly ERA5-Land reanalysis data for 1950–2021 in a spatial grid of 0.1 degree (~9 km) resolution. The indices are provided in three subsets: (1) the annual values during 1950–2021; (2) the average conditions for the 1991–2020 climatology; and (3) temporal trends over 1951–2021. The 72-year time series of various climate and event-type indices draws a comprehensive picture of the occurrence and recurrence of extreme weather events and climate variability of the changing Arctic bioclimate.

Sentinel responses of Arctic freshwater systems to climate: linkages, evidence, and a roadmap for future research

While the sentinel nature of freshwater systems is now well recognized, widespread integration of freshwater processes and patterns into our understanding of broader climate-driven Arctic terrestrial ecosystem change has been slow. We review the current understanding across Arctic freshwater systems of key sentinel responses to climate, which are attributes of these systems with demonstrated and sensitive responses to climate forcing. These include ice regimes, temperature and thermal structure, river baseflow, lake area and water level, permafrost-derived dissolved ions and nutrients, carbon mobilization (dissolved organic carbon, greenhouse gases, and radiocarbon), dissolved oxygen concentrations, lake trophic state, various aquatic organisms and their traits, and invasive species. For each sentinel, our objectives are to clarify linkages to climate, describe key insights already gained, and provide suggestions for future research based on current knowledge gaps. We suggest that tracking key responses in Arctic freshwater systems will expand understanding of the breadth and depth of climate-driven Arctic ecosystem changes, provide early indicators of looming, broader changes across the landscape, and improve protection of freshwater biodiversity and resources.

Climate change and mercury in the Arctic: Abiotic interactions

Dramatic environmental shifts are occuring throughout the Arctic from climate change, with consequences for the cycling of mercury (Hg). This review summarizes the latest science on how climate change is influencing Hg transport and biogeochemical cycling in Arctic terrestrial, freshwater and marine ecosystems. As environmental changes in the Arctic continue to accelerate, a clearer picture is emerging of the profound shifts in the climate and cryosphere, and their connections to Hg cycling. Modeling results suggest climate influences seasonal and interannual variability of atmospheric Hg deposition. The clearest evidence of current climate change effects is for Hg transport from terrestrial catchments, where widespread permafrost thaw, glacier melt and coastal erosion are increasing the export of Hg to downstream environments. Recent estimates suggest Arctic permafrost is a large global reservoir of Hg, which is vulnerable to degradation with climate warming, although the fate of permafrost soil Hg is unclear. The increasing development of thermokarst features, the formation and expansion of thaw lakes, and increased soil erosion in terrestrial landscapes are increasing river transport of particulate-bound Hg and altering conditions for aquatic Hg transformations. Greater organic matter transport may also be influencing the downstream transport and fate of Hg. More severe and frequent wildfires within the Arctic and across boreal regions may be contributing to the atmospheric pool of Hg. Climate change influences on Hg biogeochemical cycling remain poorly understood. Seasonal evasion and retention of inorganic Hg may be altered by reduced sea-ice cover and higher chloride content in snow. Experimental evidence indicates warmer temperatures enhance methylmercury production in ocean and lake sediments as well as in tundra soils. Improved geographic coverage of measurements and modeling approaches are needed to better evaluate net effects of climate change and long-term implications for Hg contamination in the Arctic.

Toward UAV-based methane emission mapping of Arctic terrestrial ecosystems

Methane is an important greenhouse gas, and emissions are expected to rise in Arctic wetland ecosystems when temperatures increase due to climate change. However, current emission estimates are associated with large uncertainties because methane shows high spatial variability. A central problem is that existing methods are often spatially restricted due to limitations in access, cost, power availability, and in need of high maintenance levels. Our study explores how a setup consisting of an unmanned aerial vehicle and a high-precision trace gas analyzer can complement well-established methods, like mobile flux chambers and eddy covariance towers, by providing independent maps of spatial variability in emissions at the landscape scale.In Zackenberg Valley, Northeast Greenland, we mapped concentration measurements from a high-precision trace gas analyzer with a reported precision of 0.6 parts per billion in a high-Arctic tundra fen ecosystem. We connected the analyzer via a long tube to a consumer-grade quadcopter, finding that the combined setup could differentiate near-surface methane concentrations of less than 5 parts per billion within a few meters under favorable weather conditions. Five of ten campaigns showed that relative methane concentration hot spots and cold spots significantly correlated with areas showing relatively high and low emissions (ranging from 1.40 to 7.4 mg m−2 h−1) during study campaigns in previous years. Concurrent measurements in a stationary automated chamber setup showed comparatively low methane emissions (~0.1 to 3.9 mg m−2 h−1) compared to previous years, indicating that a further improved UAV-analyzer setup could demonstrate clear differences in an ecosystem where methane emissions are generally higher. Calm conditions with some degree of air mixing near the surface were best suited for the mapping. Windy and wet conditions should be avoided, both for the reliability of the mapping and for safely navigating the unmanned aerial vehicle.

Disproportionate microbial responses to decadal drainage on a Siberian floodplain.

Permafrost thaw induces soil hydrological changes which in turn affects carbon cycle processes in the Arctic terrestrial ecosystems. However, hydrological impacts of thawing permafrost on microbial processes and greenhouse gas (GHG) dynamics are poorly understood. This study examined changes in microbial communities using gene and genome-centric metagenomics on an Arctic floodplain subject to decadal drainage, and linked them to CO2 and CH4 flux and soil chemistry. Decadal drainage led to significant changes in the abundance, taxonomy, and functional potential of microbial communities, and these modifications well explained the changes in CO2 and CH4 fluxes between ecosystem and atmosphere-increased fungal abundances potentially increased net CO2 emission rates and highly reduced CH4 emissions in drained sites corroborated the marked decrease in the abundance of methanogens and methanotrophs. Interestingly, various microbial taxa disproportionately responded to drainage: Methanoregula, one of the key players in methanogenesis under saturated conditions, almost disappeared, and also Methylococcales methanotrophs were markedly reduced in response to drainage. Seven novel methanogen population genomes were recovered, and the metabolic reconstruction of highly correlated population genomes revealed novel syntrophic relationships between methanogenic archaea and syntrophic partners. These results provide a mechanistic view of microbial processes regulating GHG dynamics in the terrestrial carbon cycle, and disproportionate microbial responses to long-term drainage provide key information for understanding the effects of warming-induced soil drying on microbial processes in Arctic wetland ecosystems.

Changes in net ecosystem exchange of CO2 in Arctic and their relationships with climate change during 2002–2017

Arctic warming leads to permafrost degradation, which can increase ecosystem respiration and release more greenhouse gas into the atmosphere. Meanwhile, climate warming also promotes the plant growth and increases carbon assimilation. Presently, it is largely unknown about the carbon budget and their responses to climate change in the Arctic regions. In this study, to investigate the seasonal and annual net ecosystem carbon exchange (NEE), we collected 71 observation stations for net ecosystem exchange (NEE) of CO2 in the high latitude permafrost regions during 2002–2017. The results showed that the annual NEE was −8.2 ± 4.1 g CO2 m−2 d−1 for forest, −3.3 ± 2.6 g CO2 m−2 d−1 for shrub, −4.8 ± 4.1 g CO2 m−2 d−1 for grassland, −3.6 ± 3.0 g CO2 m−2 d−1 for wetland and 0.02 ± 0.62 g CO2 m−2 d−1 for tundra, respectively. From 2002 to 2017, the CO2 emissions of grassland (carbon source) showed a decreasing trend, and the CO2 assimilation of shrub and forest (carbon sink) has been increased. The wetland and tundra are shifting from carbon sources to sinks. There were great variations in temperature sensitivities (Q10) of NEE in different seasons, with larger values in winter and lower values in summer. These findings indicate that the Arctic terrestrial ecosystem presently acts as a carbon sink, while there is a possibility that future warming, especially the warming in winter, may decrease the carbon sink of the Arctic terrestrial ecosystem.

Export of nutrients and suspended solids from major Arctic rivers and their response to permafrost degradation

The rapid warming of the Arctic has led to permafrost degradation, accelerating the transport of terrestrial materials by rivers. The quantitative assessment of riverine nutrients and total suspended solids (TSS) flux is important to clarify the land–ocean connections in the Arctic. However, much is unknown about the estimates of these components from direct measurements in the Arctic rivers and the response of the components to permafrost degradation. Here, we report the results from the Arctic Great Rivers Observatory (Arctic-GRO) for the six major Arctic rivers (Yenisey, Lena, Ob’, Mackenzie, Yukon, and Kolyma) to investigate the riverine exports of TSS, total dissolved nitrogen (TDN), nitrate (NO3−), bicarbonate (HCO3−), total dissolved phosphorus (TDP), and phosphate (PO43−). The results showed that from 2004 to 2017, the annual TSS, TDN, and NO3− exports to the Arctic Ocean were approximately 106,026 Gg, 692 Gg, and 130 Gg, respectively, and the HCO3−, TDP, and PO43− exports were approximately 79,092 Gg, 32 Gg, and 18 Gg, respectively. There were remarkable variations in component concentrations and fluxes between seasons. More than 80% of the TDN, TDP, PO43−, and TSS exports mainly occurred in spring and summer, and a high HCO3− flux was recorded in summer, while a high NO3− flux in some rivers occurred in winter. The active layer thickness was significantly positively correlated with the annual TDN, NO3−, and HCO3− exports. In addition, the HCO3− flux of the six Arctic rivers increased by 247 Gg per year during 2004–2017. The positive relationship between the active layer thickness and river discharge indicates that permafrost degradation accelerated riverine carbonate, nitrogen, and phosphorus exports. This study demonstrates that riverine exports play an important role both in the Arctic terrestrial and marine ecosystems, and permafrost degradation will likely increase the riverine material exports to the ocean.

Growing season leaf carbon:nitrogen dynamics in Arctic tundra vegetation from ground and Sentinel-2 observations reveal reallocation timing and upscaling potential

Plant nitrogen (N) use is an essential component of the N cycle in Arctic terrestrial ecosystems, and important processes include plant N uptake and reallocation during the growing season. While the availability of N to deciduous tundra plants in part relies on their internal reallocation of N from leaves to stems and roots during autumn senescence, the species-specific importance of reallocation timing and its community-wide implications on landscape- and regional-scales remains not well known. Here, we quantified leaf N contents and C:N ratios of four widespread shrub species in West Greenland from June through October and compared plot observations to landscape scale based on a new Sentinel-2-derived index. Our Sentinel-2 index captures overall N reallocation trends well across time and space at the plot level (R2 = 0.81, p < 0.001). Using this novel approach is therefore relevant for upscaling of current and future changes in plant C:N dynamics. Using satellite data, we estimate the leaf N mass reallocated during senescence equaled about 0.8 g N m−2 from leaves to stems and roots. We conclude that (1) in-situ data from the entire growing season is critical to quantify timing and contrasting strategies of N allocation at species level, (2) key species such as Salix glauca are capable of halving their leaf N content during senescence, (3) Sentinel-2 (S-2) satellite data is a strong candidate for quantifying these plant functional dynamics in space and time. Our study has implications for research on competition among Arctic plants, litter decomposition and consequently carbon accumulation in tundra soils from a climate change perspective. Further, it captures plant functional dynamics during critical parts of the growing season, including autumn senescence which is generally more complex than capturing spring greenup. Lastly, our new index has important implications for remotely mapping temporal and spatial variation in substrate quality for both wild and domesticated herbivores, as it is a first step towards a tool to assess the tundra vegetation and fodder quality for cold region animal husbandry.

Distinct Microbial Communities in Adjacent Rock and Soil Substrates on a High Arctic Polar Desert.

Understanding microbial niche variability in polar regions can provide insights into the adaptive diversification of microbial lineages in extreme environments. Compositions of microbial communities in Arctic soils are well documented but a comprehensive multidomain diversity assessment of rocks remains insufficiently studied. In this study, we obtained two types of rocks (sandstone and limestone) and soils around the rocks in a high Arctic polar desert (Svalbard), and examined the compositions of archaeal, bacterial, fungal, and protistan communities in the rocks and soils. The microbial community structure differed significantly between rocks and soils across all microbial groups at higher taxonomic levels, indicating that Acidobacteria, Gemmatimonadetes, Latescibacteria, Rokubacteria, Leotiomycetes, Pezizomycetes, Mortierellomycetes, Sarcomonadea, and Spirotrichea were more abundant in soils, whereas Cyanobacteria, Deinococcus-Thermus, FBP, Lecanoromycetes, Eurotiomycetes, Trebouxiophyceae, and Ulvophyceae were more abundant in rocks. Interestingly, fungal communities differed markedly between two different rock types, which is likely to be ascribed to the predominance of distinct lichen-forming fungal taxa (Verrucariales in limestone, and Lecanorales in sandstone). This suggests that the physical or chemical properties of rocks could be a major determinant in the successful establishment of lichens in lithic environments. Furthermore, the biotic interactions among microorganisms based on co-occurrence network analysis revealed that Polyblastia and Verrucaria in limestone, and Atla, Porpidia, and Candelariella in sandstone play an important role as keystone taxa in the lithic communities. Our study shows that even in niches with the same climate regime and proximity to each other, heterogeneity of edaphic and lithic niches can affect microbial community assembly, which could be helpful in comprehensively understanding the effects of niche on microbial assembly in Arctic terrestrial ecosystems.

Response of Arctic biodiversity and ecosystem to environmental changes: Findings from the ArCS project

Arctic ecosystems are altered profoundly by climate changes. However, the responses of Arctic marine and terrestrial ecosystems as well as their biodiversity to global warming remain largely unknown. This article provides comprehensive insights into the results and major findings from the Arctic Challenge for Sustainability (ArCS) Project – an Arctic region research program initiated in Japan, which aims to address and advance our understanding of these uncertainties. Marine ecosystem studies have identified several biogeochemical processes that are associated with sea ice decline and northward transport and shift of marine species across multiple trophic levels over the Bering and Chukchi Sea shelves. Studies of the terrestrial ecosystem have identified factors that are important for the understanding of terrestrial biodiversity and ecosystems, including Arctic lakes, under the presence of global warming. Novel fungal species from the Arctic terrestrial ecosystem have also been isolated and described. Overall, these results could contribute to the conservation and sustainable management of the Arctic ecosystem services.

High Resolution Mapping of Tundra Ecosystems on Victoria Island, Nunavut – Application of a Standardized Terrestrial Ecosystem Classification

Rapid warming of Arctic climate is driving complex ecological changes in Arctic terrestrial ecosystems that are not well understood. These ecological changes have important implications for northern communities and Arctic ecosystem resilience. Researchers require baseline information on tundra ecosystem compo sition, structure and function at a range of scales to begin to understand how and why tundra ecosystems are changing to anticipate future changes and their impacts. Here we develop and assess a process for obtaining a high-resolution ecosystem map of terrestrial ecological communities for the Intensive Monitoring Area (IMA) of the Canadian High Arctic Research Station (CHARS) in Cambridge Bay, Nunavut. The methodology is based on a standardized classification of tundra ecosystems, and will provide an ecological template for designing and implementing ecosystem research, long-term monitoring experiments, and for linking local scale research to regional scales using remote sensing tools and models. Finally, we discuss opportunities and challenges regarding the mapping approach tested for the ecosystem classification, and the usefulness of the mapped result for a range of potential applications.

The needle mycobiome of Picea glauca – A dynamic system reflecting surrounding environment and tree phenological traits

Fungi play a crucial role in terrestrial Arctic ecosystems as symbionts of vascular plants and nutrient recyclers in soil, with many species persistently or temporarily inhabiting the phyllosphere of the vegetation. In this study we apply high-throughput sequencing to investigate the mycobiome of 172 samples of fresh (current year) and aged (3 year old) needles of Picea glauca from three sites over a distance of 500 km in Alaska (USA). We analysed Illumina-generated ITS2 sequences to relate mycobiome data with phenotypic tree traits, measures of genetic variation and climate variables obtained from long-term monitoring of the sites. Alpha-diversity declined with increasing environmental stress/climate harshness. Fungal communities differed in richness and taxonomic composition between sites, with a pronounced difference in the relative abundance of OTUs assigned to species of the rust genus Chrysomyxa, plant pathogens which seem to have been in an outbreak at two sites at the time of sampling. Beside climate parameters, needle age was the second strongest explanatory variable of the mycobiome composition, whereas we found no effect of tree genetic variation, indicating that environmental and tree trait specific variables mainly determined individual white spruce mycobiomes at Alaska's treelines.

Model-data fusion to assess year-round CO2 fluxes for an arctic heath ecosystem in West Greenland (69°N)

Quantifying net CO2 exchange (NEE) of arctic terrestrial ecosystems in response to changes in climatic and environmental conditions is central to understanding ecosystem functioning and assessing potential feedbacks of the carbon cycle to future climate changes. However, annual CO2 budgets for arctic tundra are rare due to the difficulties of performing measurements during non-growing seasons. It is still unclear to what extent arctic tundra ecosystems currently act as a CO2 source, sink or are in balance. This study presents year-round eddy-covariance (EC) measurements of CO2 fluxes for an arctic heath ecosystem on Disko Island, West Greenland (69 °N) over five years. Based on a fusion of year-round EC-derived CO2 fluxes, soil temperature and moisture, the process-oriented model (CoupModel) has been constrained to quantify an annual budget and characterize seasonal patterns of CO2 fluxes. The results show that total photosynthesis corresponds to -202 ± 20 g C m−2 yr-1 with ecosystem respiration of 167 ± 28 g C m-2 yr-1, resulting in NEE of -35 ± 15 g C m-2 yr-1. The respiration loss is mainly described as decomposition of near-surface litter. A year with an anomalously deep snowpack shows a threefold increase in the rate of ecosystem respiration compared to other years. Due to the high CO2 emissions during that winter, the annual budget results in a marked reduction in the CO2 sink. The seasonal patterns of photosynthesis and soil respiration were described using response functions of the forcing atmosphere and soil conditions. Snow depth, topography-related soil moisture, and growing season warmth are identified as important environmental characteristics which most influence seasonal rates of gas exchange.

Evaluation of the use of reindeer droppings for monitoring essential and non-essential elements in the polar terrestrial environment

Excess or toxic metals, non-metals and metalloids can be eliminated from the organism by deposition in inert tissue (e.g. fur) or excretion with body secretions, urine and faeces. Droppings are one of the main routes for the elimination of multiple elements and they can be collected without direct contact with the animal. Contaminant concentration has been examined in non-lethally collected tissues of several species (especially reptilian, avian and mammalian). However, studies on species residing in polar areas are still limited, especially of mammals from the European Arctic. Reindeers are the only large herbivores living in Svalbard, being an essential part of the Arctic terrestrial ecosystem. Although reindeer presence has a high impact on their surroundings, those huge mammals are rarely part of ecotoxicological studies regarding metal pollution. In this paper, the droppings of Svalbard reindeer were used as a non-invasively collected tissue to examine the excretion pathway of 30 elements. Samples were collected in mesic and moss tundra, representing summer, winter and winter-transitional excretion. For more than a half of the studied elements, significant differences occurred between the samples collected in the two tundra types. The feasibility of older and fresh samples was assessed based on summer droppings, and significant differences were found for K, As, Mn, Na, Ni, and Sb concentrations. No relevant differences in element levels were observed for samples collected from adult females, adult males and calves, except for zinc and potassium. Results show that reindeer droppings are an important vector for the transfer of many metals, non-metals and metalloids including calcium, phosphorus, zinc, aluminium and lead. As a sedentary species, feeding on local food sources, Svalbard reindeer is a valuable indicator of trace element presence in the polar terrestrial ecosystem.

Density‐dependent and phenological mismatch effects on growth and survival in lesser snow and Ross's goslings

Strong seasonality of high‐latitude environments imposes temporal constraints on forage availability and quality for keystone herbivores in terrestrial arctic ecosystems, including hyper‐abundant colonial geese. Changes in food quality due to intraspecific competition, or food availability relative to the breeding phenology of birds, may have consequences for growth and survival of young. We used long‐term data (1993–2014) from the Karrak Lake nesting colony in the Canadian central arctic to study relative roles of density and phenological mismatch (i.e. days between seasonal peaks in vegetation quality and hatching) as drivers of annual variations in gosling survival among lesser snow Anser caerulescens caerulescens and Ross's geese A. rossii. Survival of Ross's goslings was consistently higher compared to snow geese. For both species, annual gosling survival was greatest when phenological mismatch was minimal and when nesting population size was low. We also examined gosling structural size (1999–2014) in relation to density and mismatch hypotheses to understand whether changes in survival were preceded by a parallel response in growth stemming from a density‐dependent effect on annual forage conditions. After controlling for sex, age and random effects of capture group and year × species, structural size of both snow and Ross's goslings was reduced in years when phenological mismatch was greater. However, there was no significant evidence that body size of goslings was negatively related to breeding population size at the colony. Our results lend support to the notion that both broad‐scale changes in seasonality from observed and predicted warming in the arctic and, to a lesser extent, density‐dependence on brood‐rearing areas may result in changes to offspring quality or survival, with implications for population recruitment.