Upscaling vascular aboveground biomass and topsoil moisture of subarctic fens from Unoccupied Aerial Vehicles (UAVs) to satellite level

Arctic and subarctic ecosystems are experiencing rapid changes in vegetation composition and productivity due to global warming. Tundra wetlands are especially susceptible to these changes, which may trigger shifts in soil moisture dynamics. It is therefore essential to accurately map plant biomass and topsoil moisture. In this study, we mapped total, wood, and leaf above ground biomass and topsoil moisture in subarctic tundra wetlands located between Norway and Finland by linking models derived from Unoccupied Aerial Vehicles with multiple satellite data sources using the Extreme Gradient Boosting algorithm. The most accurate predictions for topsoil moisture (R2 = 0.73) used a set of red edge-based vegetation indices with a spatial resolution of 20 m per pixel. On the contrary, wood biomass showed the lowest accuracies across all tested models (R2 = 0.38). We found a trade-off between the spatial resolution and the performance of upscaling models, where smaller pixel sizes generally led to lower accuracies. However, we were able to compensate for reduced accuracy at smaller pixel sizes using Copernicus phenology metrics. A modelling uncertainty assessment revealed that the uncertainty of predictions increased with decreasing pixel sizes and increasing number of co-predictors. Our approach could improve efforts to map and monitor changes in vegetation at regional to pan-Arctic scales.

Vegetation biomass and topography are associated with seasonal habitat selection and fall translocation behavior in Arctic hares

Habitat selection theory suggests that environmental features selected at coarse scales reveal fundamental factors affecting animal fitness. When these factors vary across seasons, they may lead to large-scale movements, including long-distance seasonal migrations. We analyzed the seasonal habitat selection of 25 satellite-tracked Arctic hares from a population on Ellesmere Island (Nunavut, Canada) that relocated over 100 km in the fall. Since no other lagomorph is known to perform such extensive movements, this population offered an ideal setting to test animal movement and habitat selection theory. On summer grounds hares selected low elevation areas, while on winter grounds they selected high vegetation biomass, high elevation, and steep slopes. During fall relocation, they alternated between stopover and traveling behavioral states (ratio 2:1). Stopover locations were characterized by higher vegetation heterogeneity and lower rugosity than traveling locations, while vegetation biomass and elevation interacted to explain stopover locations in a more complex way. The selected combination of environmental features thus varied across seasons and behavioral states, in a way broadly consistent with predictions based on the changing food and safety needs of hares. Although causality was not demonstrated, our results improve our understanding of long-distance movements and habitat selection in Arctic hares, as well as herbivore ecology in the polar desert. Results also provide strong support to animal movement and habitat selection theory, by showing how some important hypotheses hold when tested in a species phylogenetically distinct from most animal models used in this research field.

Flowering time responses to warming drive reproductive fitness in a changing Arctic

BACKGROUND AND AIMS

The Arctic is warming at an alarming rate, leading to earlier spring conditions and plant phenology. It is often unclear to what degree changes in reproductive fitness (flower, fruit, seed production) are a direct response to warming versus an indirect response through shifting phenology. This study aims to quantify the relative importance of these direct and indirect pathways and project the net effects of warming on plant phenology and reproductive fitness under current and future climate scenarios.

METHODS

We used two long-term datasets on twelve tundra species in the Canadian Arctic as part of the International Tundra Experiment (ITEX). Phenology and reproductive fitness were recorded annually on tagged individual plants at both Daring Lake, Northwest Territories (64.87, -111.58) and Alexandra Fiord, Nunavut (78.83, -75.80). Plant species encompass a wide taxonomic diversity across a range of plant functional types with circumpolar/boreal distributions. We use Hierarchical Bayesian Structural Equation models to compare the direct and indirect effects of climate warming on phenology and reproductive fitness across species, sites and years.

KEY RESULTS

We find that warming, both experimental and ambient, drives earlier flowering across species, which leads to higher numbers of flowers and fruits produced, reflecting directional phenotypic selection for earlier flowering phenology. Furthermore, this indirect effect of climate warming mediated through phenology was generally ~2-3x stronger than the direct effect of climate on reproductive fitness. Under future climate predictions, individual plants showed a ~2 to 4.5-fold increase in their reproductive fitness (flower counts) with advanced flowering phenology.

CONCLUSIONS

Our results suggest that, on average, the benefits of early flowering, such as increased development time and subsequent enhanced reproductive fitness, may outweigh its risks. Overall, this work provides important insights into population-level consequences of phenological shifts in a warming Arctic over multi-decadal time scales.

Soil microbial responses to simulated climate change across polar ecosystems

The polar regions are among the most biologically constrained in the world, characterized by cold temperatures and reduced liquid water. These limitations make them among the most climate-sensitive regions on Earth. Despite the overwhelming constraints from low temperatures and resource availability, many polar ecosystems, including polar deserts and tundras across the Arctic and Antarctic host uniquely diverse microbial communities. Polar regions have warmed more rapidly than the global average, with continued warming predicted for the future, which will reduce constraints on soil microbial activity. This could alter polar carbon (C) cycles, increasing CO2 emissions into the atmosphere. The objective of this study was to determine how increased temperature and moisture availability impacts microbial respiration in polar regions, by focusing on a diversity of ecosystem types (polar desert vs. tundra) that are geographically distant across Antarctica and the Arctic. We found that polar desert soil microbes were co-limited by temperature and moisture, though C and nitrogen (N) mineralization were only stimulated at the coldest and driest of the two polar deserts. Only bacterial biomass was impacted at the less harsh of the polar deserts, suggesting microbial activity is limited by factors other than temperature and moisture. Of the tundra sites, only the Antarctic tundra was climate-sensitive, where increased temperature decreased C and N mineralization while water availability stimulated it. The greater availability of soil resources and vegetative biomass at the Arctic tundra site might lead to its lack of climate-sensitivity. Notably, while C and N dynamics were climate-sensitive at some of our polar sites, P availability was not impacted at any of them. Our results demonstrate that soil microbial processes in some polar ecosystems are more sensitive to changes in temperature and moisture than others, with implications for soil C and N storage that are not uniformly predictable across polar regions.

Close coupling of plant functional types with soil microbial community composition drives soil carbon and nutrient cycling in tundra heath

Aims

This study aimed at elucidating divergent effects of two dominant plant functional types (PFTs) in tundra heath, dwarf shrubs and mosses, on soil microbial processes and soil carbon (C) and nutrient availability, and thereby to enhance our understanding of the complex interactions between PFTs, soil microbes and soil functioning.

Methods

Samples of organic soil were collected under three dwarf shrub species (of distinct mycorrhizal association and life form) and three moss species in early and late growing season. We analysed soil C and nutrient pools, extracellular enzyme activities and phospholipid fatty acid profiles, together with a range of plant traits, soil and abiotic site characteristics.

Results

Shrub soils were characterised by high microbial biomass C and phosphorus and phosphatase activity, which was linked with a fungal-dominated microbial community, while moss soils were characterised by high soil nitrogen availability, peptidase and peroxidase activity associated with a bacterial-dominated microbial community. The variation in soil microbial community structure was explained by mycorrhizal association, root morphology, litter and soil organic matter quality and soil pH-value. Furthermore, we found that the seasonal variation in microbial biomass and enzyme activities over the growing season, likely driven by plant belowground C allocation, was most pronounced under the tallest shrub Betula nana.

Conclusion

Our study demonstrates a close coupling of PFTs with soil microbial communities, microbial decomposition processes and soil nutrient availability in tundra heath, which suggests potential strong impacts of global change-induced shifts in plant community composition on carbon and nutrient cycling in high-latitude ecosystems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11104-023-05993-w.

Impacts of elevation on plant traits and volatile organic compound emissions in deciduous tundra shrubs

The northernmost regions of our planet experience twice the rate of climate warming compared to the global average. Despite the currently low air temperatures, tundra shrubs are known to exhibit high leaf temperatures and are increasing in height due to warming, but it is unclear how the increase in height will affect the leaf temperature. To study how temperature, soil moisture, and changes in light availability influence the physiology and emissions of climate-relevant volatile organic compounds (VOCs), we conducted a study on two common deciduous tundra shrubs, Salix glauca (separating males and females for potential effects of plant sex) and Betula glandulosa, at two elevations in South Greenland. Low-elevation Salix shrubs were 45% taller, but had 37% lower rates of net CO₂ assimilation and 63% lower rates of isoprene emission compared to high-elevation shrubs. Betula shrubs showed 40% higher stomatal conductance and 24% higher glandular trichome density, in the low-elevation valley, compared to those from the high-elevation mountain slope. Betula green leaf volatile emissions were 235% higher at high elevation compared to low elevation. Male Salix showed a distinct VOC blend and emitted 55% more oxygenated VOCs, compared to females, possibly due to plant defense mechanisms. In our light response curves, isoprene emissions increased linearly with light intensity, potentially indicating adaptation to strong light. Leaf temperature decreased with increasing Salix height, at 4 °C m^-1, which can have implications for plant physiology. However, no similar relationship was observed for B. glandulosa. Our results highlight that tundra shrub traits and VOC emissions are sensitive to temperature and light, but that local variations in soil moisture strongly interact with temperature and light responses. Our results suggest that effects of climate warming, alone, poorly predict the actual plant responses in tundra vegetation.

Impact of ambient temperature, precipitation and seven years of experimental warming and nutrient addition on fruit production in an alpine heath and meadow community

Alpine and polar regions are predicted to be among the most vulnerable to changes in temperature, precipitation, and nutrient availability. We carried out a seven-year factorial experiment with warming and nutrient addition in two alpine vegetation communities. We analyzed the relationship between fruit production and monthly mean, maximum, and min temperatures during the fall of the pre-fruiting year, the fruiting summer, and the whole fruit production period, and measured the effects of precipitation and growing and thawing degree days (GDD & TDD) on fruit production. Nutrient addition (heath: 27.88 ± 3.19 fold change at the end of the experiment; meadow: 18.02 ± 4.07) and combined nutrient addition and warming (heath: 20.63 ± 29.34 fold change at the end of the experiment; meadow: 18.21 ± 16.28) increased total fruit production and fruit production of graminoids. Fruit production of evergreen and deciduous shrubs fluctuated among the treatments and years in both the heath and meadow. Pre-maximum temperatures had a negative effect on fruit production in both communities, while current year maximum temperatures had a positive impact on fruit production in the meadow. Pre-minimum, pre-mean, current mean, total minimum, and total mean temperatures were all positively correlated with fruit production in the meadow. The current year and total precipitation had a negative effect on the fruit production of deciduous shrubs in the heath. GDD had a positive effect on fruit production in both communities, while TDD only impacted fruit production in the meadow. Increased nutrient availability increased fruit production over time in the high alpine plant communities, while experimental warming had either no effect or a negative effect. Deciduous shrubs were the most sensitive to climate parameters in both communities, and the meadow was more sensitive than the heath. The difference in importance of TDD for fruit production may be due to differences in snow cover in the two communities.

Atmospheric CCl4 degradation in Antarctic tundra soils and the evaluation on its partial atmospheric lifetime with respect to soil

Carbon tetrachloride (CCl₄) is an anthropogenic gas with a long atmospheric lifetime and can catalyze the destruction of stratospheric ozone. Natural soils are believed to be important and widespread sinks of atmospheric CCl₄, although poorly characterized due to a limited number of measurements. In this study, for the first time in situ static-chamber measurements and laboratory-based incubations for CCl₄ fluxes were conducted at coastal Antarctic tundra. Results showed that soil in remote Antarctica is also acting as a CCl₄ sink, with an average uptake rate of -2.2 ± 0.6 nmol m^-2 d^-1, which is comparable to the reported soil sinks in other regions of the world. No significant difference (p > 0.05) was found across different types of tundra, such normal upland tundra, coastal marsh tundra, and tundra in the sea animal colonies. Soil CCl₄ fluxes did not show significant correlations (p > 0.05) with soil moisture, pH, TOC, TN, TP and Cl contents. Laboratory-based anoxic incubations showed that the uptake rates of CCl₄ in tundra soil were suppressed; post-thermal sterilization incubations showed that soil CCl₄ sink was enhanced; these results suggested that CCl₄ degradation in tundra soil was likely an abiotic process preferring oxic environments. A rough extrapolation suggested that Antarctic tundra may degrade about 2.4 metric tons of atmospheric CCl₄ each year. Combining soil CCl₄ fluxes from this study and other literature reports, CCl₄ partial lifetime with respect to the soil sink was evaluated to be 354 (235-474) years, which supported the recent viewpoint that the soil sink of CCl₄ is smaller than previously thought.

Temporal Variations Rather than Long-Term Warming Control Extracellular Enzyme Activities and Microbial Community Structures in the High Arctic Soil

In Arctic soils, warming accelerates decomposition of organic matter and increases emission of greenhouse gases (GHGs), contributing to a positive feedback to climate change. Although microorganisms play a key role in the processes between decomposition of organic matter and GHGs emission, the effects of warming on temporal responses of microbial activity are still elusive. In this study, treatments of warming and precipitation were conducted from 2012 to 2018 in Cambridge Bay, Canada. Soils of organic and mineral layers were collected monthly from June to September in 2018 and analyzed for extracellular enzyme activities and bacterial community structures. The activity of hydrolases was the highest in June and decreased thereafter over summer in both organic and mineral layers. Bacterial community structures changed gradually over summer, and the responses were distinct depending on soil layers and environmental factors; water content and soil temperature affected the shift of bacterial community structures in both layers, whereas bacterial abundance, dissolved organic carbon, and inorganic nitrogen did so in the organic layer only. The activity of hydrolases and bacterial community structures did not differ significantly among treatments but among months. Our results demonstrate that temporal variations may control extracellular enzyme activities and microbial community structure rather than the small effect of warming over a long period in high Arctic soil. Although the effects of the treatments on microbial activity were minor, our study provides insight that microbial activity may increase due to an increase in carbon availability, if the growing season is prolonged in the Arctic.

Patterns of free amino acids in tundra soils reflect mycorrhizal type, shrubification, and warming

The soil nitrogen (N) cycle in cold terrestrial ecosystems is slow and organically bound N is an important source of N for plants in these ecosystems. Many plant species can take up free amino acids from these infertile soils, either directly or indirectly via their mycorrhizal fungi. We hypothesized that plant community changes and local plant community differences will alter the soil free amino acid pool and composition; and that long-term warming could enhance this effect. To test this, we studied the composition of extractable free amino acids at five separate heath, meadow, and bog locations in subarctic and alpine Scandinavia, with long-term (13 to 24 years) warming manipulations. The plant communities all included a mixture of ecto-, ericoid-, and arbuscular mycorrhizal plant species. Vegetation dominated by grasses and forbs with arbuscular and non-mycorrhizal associations showed highest soil free amino acid content, distinguishing them from the sites dominated by shrubs with ecto- and ericoid-mycorrhizal associations. Warming increased shrub and decreased moss cover at two sites, and by using redundancy analysis, we found that altered soil free amino acid composition was related to this plant cover change. From this, we conclude that the mycorrhizal type is important in controlling soil N cycling and that expansion of shrubs with ectomycorrhiza (and to some extent ericoid mycorrhiza) can help retain N within the ecosystems by tightening the N cycle.

Herbivory changes soil microbial communities and greenhouse gas fluxes in a high-latitude wetland

Herbivory can have strong impacts on greenhouse gas fluxes in high-latitude ecosystems. For example, in the Yukon-Kuskokwim (Y-K) Delta in western Alaska, migratory goose grazing affects the magnitude of soil carbon dioxide (CO₂) and methane (CH₄) fluxes. However, the underlying drivers of this relationship are unclear, as few studies systematically tease apart the processes by which herbivores influences soil biogeochemistry. To examine these mechanisms in detail, we conducted a laboratory incubation experiment to quantify changes in greenhouse gas fluxes in response to three parameters altered by herbivores in situ: temperature, soil moisture content, and nutrient inputs. These treatments were applied to soils collected in grazing lawns and nearby ungrazed habitat, allowing us to assess how variation in microbial community structure influenced observed responses. We found pronounced differences in both fungal and prokaryotic community composition between grazed and ungrazed areas. In the laboratory incubation experiment, CO₂ and CH₄ fluxes increased with temperature, soil moisture, and goose fecal addition, suggesting that grazing-related changes in the soil abiotic environment may enhance soil C losses. Yet, these abiotic drivers were insufficient to explain variation in fluxes between soils with and without prior grazing. Differences in trace gas fluxes between grazed and ungrazed areas may result both from herbivore-induced shifts in abiotic parameters and grazing-related alterations in microbial community structure. Our findings suggest that relationships among herbivores and soil microbial communities could mediate carbon-climate feedbacks in rapidly changing high-latitude ecosystems.

Contrasting responses of major and minor volatile compounds to warming and gall-infestation in the Arctic willow Salix myrsinites

Climate change is altering high-latitude ecosystems in multiple facets, including increased insect herbivory pressure and enhanced emissions of volatile organic compounds (VOC) from vegetation. Yet, joint impacts of climatic drivers and insect herbivory on VOC emissions from the Arctic remain largely unknown. We examined how one-month warming by open-top plastic tents, yielding a 3-4 °C air temperature increase, and the natural presence of gall-forming eriophyoid mites, Aculus tetanothrix, individually and in combination, affect VOC emissions from whortle leaved willow, Salix myrsinites, at two elevations in an Arctic heath tundra of Abisko, Northern Sweden. We measured VOC emissions three times in the peak growing season (July) from intact and gall-infested branches using an enclosure technique and gas chromatography-mass spectrometry, and leaf chemical composition using near-infrared reflectance spectroscopy (NIRS). Isoprene accounted for 91% of the VOCs emitted by S. myrsinites. Isoprene emission rates tended to be higher at the high than low elevation during the measurement periods (42 μg g^-1 DW h^-1 vs. 23 μg g^-1 DW h^-1) even when temperature differences were accounted for. Experimental warming increased isoprene emissions by approximately 54%, but decreased emissions of some minor compound groups, such as green leaf volatiles (GLV) and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT). In contrast, gall-infestation did not affect isoprene emissions but stimulated emissions of DMNT, sesquiterpenes and GLVs, particularly under ambient conditions at the low elevation. The NIRS-based chemical composition of the leaves varied between the two elevations and was affected by warming and gall-infestation. Our study suggests that under elevated temperatures, S. myrsinites increases emissions of isoprene, a highly effective compound for protection against oxidative stress, while an infestation by A. tetanothrix mites induces emissions of herbivore enemy attractants like DMNT, sesquiterpenes and GLVs. Under both conditions, warming effects on isoprene remain but mite effects on DMNT, sesquiterpenes and GLVs diminish.

Trait-based responses to cessation of nutrient enrichment in a tundra plant community

Plant communities worldwide show varied responses to nutrient enrichment-including shifts in species identity, decreased diversity, and changes in functional trait composition-but the factors determining community recovery after the cessation of nutrient addition remain uncertain. We manipulated nutrient levels in a tundra community for 6 years of nutrient addition followed by 8 years of recovery. We examined how community recovery was mediated by traits related to plant resource-use strategy and plant ability to modify their environment. Overall, we observed persistent effects of fertilization on plant communities. We found that plants with fast-growing traits, including higher specific leaf area, taller stature and lower foliar C:N, were more likely to show a persistent increase in fertilized plots than control plots, maintaining significantly higher cover in fertilized plots 8 years after cessation of fertilization. Additionally, although graminoids responded most strongly to the initial fertilization treatment, forb species were more vulnerable to fertilization effects in the long-term, showing persistent decline and no recovery in 8 years. Finally, these persistent fertilization effects were accompanied by modified environmental conditions, including persistent increases in litter depth and soil phosphorous and lower soil C:N. Our results demonstrate the potential for lasting effects of nutrient enrichment in nutrient-limited systems and identify species traits related to rapid growth and nutrient-use efficiency as the main predictors of the persistence of nutrient enrichment effects. These findings highlight the usefulness of trait-based approach for understanding the persistent feedbacks of nutrient enrichment, plant dynamics, and niche construction via litter and nutrient build-up.

Plant CSR types in the north: comparing the morphological and morpho-physiological approaches

Grime's competition-stress-ruderal (CSR) theory is widely used to study plant species' responses to multiple environmental factors. We compared two models to allocate CSR types the global "StrateFy" model (Pierce et al. Funct Ecol, 31:444-457, 2017) and a locally developed morpho-physiological model (Novakovskiy et al. Int J Ecol, p e1323614, 2016). The "StrateFy" model is based on three morphological leaf traits: leaf area (LA), leaf dry matter content (LDMC) and specific leaf area (SLA). The morpho-physiological model additionally uses plant height (PH), leaf dry weight (LDW), photosynthetic capacity (PN) and respiration rate (RD), leaf nitrogen, and carbon concentration (LNC, LCC). We applied both models to 74 plant species, the traits of which were measured at mountain (Northern Urals) and plane (Komi Republic, Russia) landscapes of European Northeast. The comparison of the calculated C, S, and R scores showed two groups of species with large and unidirectional differences. The first group consists of species with a shift from S (morpho-physiological model) to CR (StrateFy model) strategy. Species of this group are typical for deep shaded habitats and characterized by low LDMC (10-25%) and high SLA (30-60 mm² mg^-1). The second group consists of C species (morpho-physiological model) which were classified as S (StrateFy model) strategy. This group includes mainly tall shrubs, graminoids, and forbs with relatively small leaves (300-2000 mm²). In our opinion, the CSR strategies obtained by the morpho-physiological model showed better agreement with the basic principles underlying Grime's theory. The use of a limited number of morphological traits (LA, LDMC, SLA) in the StrateFy model does not always allow to determine the life strategy correctly. For example, these traits are insufficient for a clear separation of deeply shaded stress-tolerant species and ruderals. On the other hand, the use of the morpho-physiological model requires a large number of field measurements, which makes it difficult to use this model to allocate CSR strategies for a large number of species.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00973-9.

Volatile organic compound emission in tundra shrubs - Dependence on species characteristics and the near-surface environment

Temperature is one of the key abiotic factors during the life of plants, especially in the Arctic region which is currently experiencing rapid climate change. We evaluated plant traits and environmental variables determining leaf temperature in tundra shrubs and volatile organic compound (VOC) emissions with field measurements on deciduous tundra shrubs, Salix myrsinites and Betula nana, and evergreen Cassiope tetragona and Rhododendron lapponicum. Higher leaf-to-air temperature difference was observed in evergreen, compared to deciduous shrubs. Evergreen shrubs also showed continuously increasing photosynthesis with increasing temperature, suggesting high thermal tolerance. For the deciduous species, the optimum temperature for net photosynthesis was between our measurement temperatures of 24 °C and 38 °C. Air temperature and vapor pressure deficit were the most important variables influencing leaf temperature and VOC emissions in all the studied plants, along with stomatal density and specific leaf area in the deciduous shrubs. Using climate data and emission factors from our measurements, we modelled total seasonal tundra shrub VOC emissions of 0.3-2.3 g m-2 over the main growing season. Our results showed higher-than-expected temperature optima for photosynthesis and VOC emission and demonstrated the relative importance of plant traits and local environments in determining leaf temperature and VOC emissions in a subarctic tundra.

Complexation of lead and cadmium ions with humic acids from arctic peat soils

Humic acids (HAs) have many significant environmental and geochemical functions in soils, bottom sediments, and aquatic environments. Their interaction with toxic heavy metal ions affects their transport and bioavailability. This study suggests that binding of heavy metal ions to HAs could potentially help to develop strategies for recovering metal-contaminated soils and groundwater. This study is aimed at investigation of sorption properties of HA preparations from peat soils based on determining the kinetic and thermodynamic parameters of the sorption processes of Cd²⁺ and Pb²⁺ ions. Based on a model experiment, the binding ability of HAs of Hemic Folic Cryic Histosol to Cd²⁺ and Pb²⁺ ions was revealed. It is shown that during the initial stage (first 20 min for Cd²⁺ ions and 30 min for Pb²⁺ ions) the kinetics of the process of sorption of metal ions by suspended HA preparations is better described by pseudo-second order equation. This indicates the chemisorption mechanism and limiting contribution of chemical reaction. The kinetics sorption process within the Boyd - Adamson - Myers model is described with high precision for both ions during the whole experiment. This indicates the intra-diffusion limitation of the sorption process. The thermodynamic characteristics (the limiting sorption capacity, constant of sorption equilibrium, Gibbs free energy change, entropy change and enthalpy change of sorption) of the sorption process of selected heavy metal ions have been calculated. It was found that the limiting specific sorption of Pb²⁺ ions is almost an order of magnitude higher than that of Cd²⁺ ions and amounts to 0.16-0.29 mmol/dm³ and 0.0078-0.034 mmol/dm³, respectively. The sorption enthalpy variance of 48.4 kJ/mol for Cd²⁺ ions and 22.6 kJ/mol for Pb²⁺ ions indicate the endothermic nature of sorption on HA solid particles. It was shown that the limiting stage of sorption for Cd²⁺ and Pb²⁺ ions is ion diffusion to the sorbent, while chemisorption itself proceeds quite quickly. Values ΔS> -10 J/(mol ∙ K) indicate a dissociative sorption mechanism for both metals, i.e. non-sorbed ions in the solution are in a more ordered state than after sorption. The negative values of the Gibbs free energy change for Cd²⁺ and Pb²⁺ ions indicate that the interaction of ions with HA preparations from peat soils is a spontaneous process with a complex mechanism involving complexation and ion exchange processes.

Soil thawing regulates the spring growth onset in tundra and alpine biomes

Soil temperature remains isothermal at 0 °C and water shifts to a liquid phase during soil thawing. Vegetation may receive this process as a signal and a key to restore physiological activity. We aimed to show the relationship between the timing of soil thawing and the spring growth onset. We estimated the delay between the soil thawing and the spring growth onset in 78 sites of the FLUXNET network. We built a soil thawing map derived from modeling for the northern hemisphere and related it to the greenness onset estimated with satellite imagery. Spring onset estimated with GPP time series occurred shortly after soil surface thawing in tundra (1.1 ± 3.5 days) and alpine grasslands (16.6 ± 5.8 days). The association was weaker for deciduous forests (40.3 ± 4.2 days), especially where soils freeze infrequently. Needleleaved forests tended to start the growing season before the end of thawing (-17.4 ± 3.6 days), although observations from remote sensing (MODIS Land Cover Dynamics) indicated that the onset of greenness started after the thawing period (26.8 ± 3.2 days). This study highlights the role of soil temperature at the spring growth onset at high latitudes. Soil thawing becomes less relevant in temperate forests, where soil is occasionally frozen and other climate factors become more important.

Can reindeer husbandry management slow down the shrubification of the Arctic?

Rapid climate change is threatening the stability and functioning of Arctic ecosystems. As the Arctic warms, shrubs have been widely observed to expand, which has potentially serious consequences for global climate regulation and for the ecological processes characterising these ecosystems. However, it is currently unclear why this shrubification has been spatially uneven across the Arctic, with herbivory being suggested as a key regulating factor. By taking advantage of freely available satellite imagery spanning three decades, we mapped changes in shrub cover in the Yamal Peninsula and related these to changes in summer temperature and reindeer population size. We found no evidence that shrubs had expanded in the study site, despite increasing summer temperatures. At the same time, herbivore pressure increased significantly, with the local reindeer population size growing by about 75%. Altogether, our results thus point towards increases in large herbivore pressure having compensated for the warming of the Peninsula, halting the shrubification of the area. This suggests that strategic semi-domesticated reindeer husbandry, which is a common practice across the Eurasian Arctic, could represent an efficient environmental management strategy for maintaining open tundra landscapes in the face of rapid climate change.

A screening of select toxic and essential elements and persistent organic pollutants in the fur of Svalbard reindeer

Reindeers play an important role in the polar ecosystem, being long-lived sole vegetarians feeding on local vegetation. They can be used as a valuable bioindicator, helping us to understand contaminants' impact on the polar terrestrial ecosystem. Still, scarce data exist from research in which polar herbivores (especially those from the European parts of the Arctic) were a major study subject for trace elements and persistent organic pollutant determination. Here, Svalbard reindeer fur has been used to determine metals, non-metals and metalloids using ICP-MS, and several persistent organic pollutants including polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) using gas chromatography coupled to a tandem mass spectrometer (GC-MS/MS). Samples were collected from reindeer populations living in the area near Ny-Ålesund and Longyearbyen. Essential elements like Fe, Mg, Zn, K, Ca, Cu predominated in the trace elements profile. Median values of As, Cd, Co, Li, Ni, Se and V were all below 0.5 μg/g dw. Mercury was below detection limit in all samples, while the Pb median varied from 0.35 to 0.74 μg/g dw. Except acenaphthylene and fluorene, PAHs were detectable only in samples collected in the vicinity of Longyearbyen. Of 15 studied pesticides, only DDT and its metabolites were above the detection limit, and, of PCBs, only PCB28.

Status and trends of tundra birds across the circumpolar Arctic

Tundra-breeding birds face diverse conservation challenges, from accelerated rates of Arctic climate change to threats associated with highly migratory life histories. Here we summarise the status and trends of Arctic terrestrial birds (88 species, 228 subspecies or distinct flyway populations) across guilds/regions, derived from published sources, raw data or, in rare cases, expert opinion. We report long-term trends in vital rates (survival, reproduction) for the handful of species and regions for which these are available. Over half of all circumpolar Arctic wader taxa are declining (51% of 91 taxa with known trends) and almost half of all waterfowl are increasing (49% of 61 taxa); these opposing trends have fostered a shift in community composition in some locations. Declines were least prevalent in the African-Eurasian Flyway (29%), but similarly prevalent in the remaining three global flyways (44-54%). Widespread, and in some cases accelerating, declines underscore the urgent conservation needs faced by many Arctic terrestrial bird species.

Patch use in the arctic ground squirrel: effects of micro-topography and shrub encroachment in the Arctic Circle

We investigated the roles of vegetation structure, micro-topographic relief, and predator activity patterns (time of day) on the perception of predatory risk of arctic ground squirrels (Urocitellus parryii), an abundant pan-Arctic omnivore, in Arctic Circle tundra on the North Slope of Alaska, where tundra vegetation structure has been predicted to change in response to climate. We quantified foraging intensity by measuring the giving-up densities (GUDs) of the arctic ground squirrels in experimental foraging patches along a heath-graminoid-shrub moist tundra gradient. We hypothesized that foraging intensity of arctic ground squirrels would be greatest and GUDs lowest, where low-stature vegetation or raised micro-topography improves sightlines for predator detection. Furthermore, GUDs should vary with time of day and reflect 24-h cycles of varying predation risk. Foraging intensity varied temporally, being highest in the afternoon and lowest overnight. During the morning, foraging intensity was inversely correlated with the normalized difference vegetation index (NDVI), a proxy for vegetation productivity and cover. Foraging was additionally measured within landscapes of fear, confirming that vegetative and topographic obstructions of sightlines reduces foraging intensity and increases GUDs. We conclude that arctic ground squirrels may affect Arctic Circle vegetation of tundra ecosystems, but these effects will vary spatially and temporally.

Mercury in tundra vegetation of Alaska: Spatial and temporal dynamics and stable isotope patterns

Vegetation uptake of atmospheric mercury (Hg) is an important mechanism enhancing atmospheric Hg deposition via litterfall and senescence. We here report Hg concentrations and pool sizes of different plant functional groups and plant species across nine tundra sites in northern Alaska. Significant spatial differences were observed in bulk vegetation Hg concentrations at Toolik Field station (52 ± 9 μg kg^-1), Eight Mile Lake Observatory (40 ± 0.2 μg kg^-1), and seven sites along a transect from Toolik Field station to the Arctic coast (36 ± 9 μg kg^-1). Hg concentrations in non-vascular vegetation including feather and peat moss (58 ± 6 μg kg^-1 and 34 ± 2 μg kg^-1, respectively) and brown and white lichen (41 ± 2 μg kg^-1 and 34 ± 2 μg kg^-1, respectively), were three to six times those of vascular plant tissues (8 ± 1 μg kg^-1 in dwarf birch leaves and 9 ± 1 μg kg^-1 in tussock grass). A high representation of nonvascular vegetation in aboveground biomass resulted in substantial Hg mass contained in tundra aboveground vegetation (29 μg m^-2), which fell within the range of foliar Hg mass estimated for forests in the United States (15 to 45 μg m^-2) in spite of much shorter growing seasons. Hg stable isotope signatures of different plant species showed that atmospheric Hg(0) was the dominant source of Hg to tundra vegetation. Mass-dependent isotope signatures (δ²⁰²Hg) in vegetation relative to atmospheric Hg(0) showed pronounced shifts towards lower values, consistent with previously reported isotopic fractionation during foliar uptake of Hg(0). Mass-independent isotope signatures (Δ¹⁹⁹Hg) of lichen were more positive relative to atmospheric Hg(0), indicating either photochemical reduction of Hg(II) or contributions of inorganic Hg(II) from atmospheric deposition and/or dust. Δ¹⁹⁹Hg and Δ²⁰⁰Hg values in vascular plant species were similar to atmospheric Hg(0) suggesting that overall photochemical reduction and subsequent re-emission was relatively insignificant in these tundra ecosystems, in agreement with previous Hg(0) ecosystem flux measurements.

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.

Soil organic carbon depletion and degradation in surface soil after long-term non-growing season warming in High Arctic Svalbard

Arctic tundra active-layer soils are at risk of soil organic carbon (SOC) depletion and degradation upon global climate warming because they are in a stage of relatively early decomposition. Non-growing season (NGS) warming is particularly pronounced, and observed increases of CO₂ emissions during experimentally warmed NGSs give concern for great SOC losses to the atmosphere. Here, we used snow fences in Arctic Spitsbergen dwarf shrub tundra to simulate 1.86 °C NGS warming for 9 consecutive years, while growing season temperatures remained unchanged. In the snow fence treatment, the 4-11 cm thick A-horizon had a 2% lower SOC concentration and a 0.48 kg C m^-2 smaller pool size than the controls, indicating SOC pool depletion. The snow fence treatment's A-horizon's alkyl/O-alkyl ratio was also significantly increased, indicating an advance of SOC degradation. The underlying 5 cm of B/C-horizon did not show these effects. Our results support the hypothesis that SOC depletion and degradation are connected to the long-term transience of observed ecosystem respiration (ER) increases upon soil warming. We suggest that the bulk of warming induced ER increases may originate from surface and not deep active layer or permafrost horizons. The observed losses of SOC might be significant for the ecosystem in question, but are in magnitude comparatively small relative to anthropogenic greenhouse gas enrichment of the atmosphere. We conclude that a positive feedback of carbon losses from surface soils of Arctic dwarf shrub tundra to anthropogenic forcing will be minor, but not negligible.

Tundra landscape heterogeneity, not interannual variability, controls the decadal regional carbon balance in the Western Russian Arctic

Across the Arctic, the net ecosystem carbon (C) balance of tundra ecosystems is highly uncertain due to substantial temporal variability of C fluxes and to landscape heterogeneity. We modeled both carbon dioxide (CO₂ ) and methane (CH₄ ) fluxes for the dominant land cover types in a ~100-km² sub-Arctic tundra region in northeast European Russia for the period of 2006-2015 using process-based biogeochemical models. Modeled net annual CO₂ fluxes ranged from -300 g C m^-2  year^-1 [net uptake] in a willow fen to 3 g C m^-2  year^-1 [net source] in dry lichen tundra. Modeled annual CH₄ emissions ranged from -0.2 to 22.3 g C m^-2  year^-1 at a peat plateau site and a willow fen site, respectively. Interannual variability over the decade was relatively small (20%-25%) in comparison with variability among the land cover types (150%). Using high-resolution land cover classification, the region was a net sink of atmospheric CO₂ across most land cover types but a net source of CH₄ to the atmosphere due to high emissions from permafrost-free fens. Using a lower resolution for land cover classification resulted in a 20%-65% underestimation of regional CH₄ flux relative to high-resolution classification and smaller (10%) overestimation of regional CO₂ uptake due to the underestimation of wetland area by 60%. The relative fraction of uplands versus wetlands was key to determining the net regional C balance at this and other Arctic tundra sites because wetlands were hot spots for C cycling in Arctic tundra ecosystems.

Experimental herbivore exclusion, shrub introduction, and carbon sequestration in alpine plant communities

Background

Shrub cover in arctic and alpine ecosystems has increased in recent decades, and is predicted to further increase with climate change. Changes in shrub abundance may alter ecosystem carbon (C) sequestration and storage, with potential positive feedback on global C cycling. Small and large herbivores may reduce shrub expansion and thereby counteract the positive feedback on C cycling, but herbivore pressures have also changed in the alpine-arctic tundra; the increased shrub cover together with changes in herbivore pressure is leading to unpredictable changes in carbon sequestration and storage. In this study we investigate the importance of herbivory and shrub introduction for carbon sequestration in the short term. We measured standing biomass and daytime mid-growing season carbon fluxes in plots in a full factorial design where we excluded small and large mammalian herbivores and introduced Salix by planting Salix transplants. We used three study sites: one Empetrum-dominated heath, one herb- and cryptogam-dominated meadow, and one Salix-dominated shrub community in the low-alpine zone of the Dovre Mountains, Central Norway.

Results

After 2 years, significant treatment effects were recorded in the heath community, but not in the meadow and shrub communities. In the heath community cessation of herbivory increased standing biomass due to increased biomass of dwarf shrubs. Cessation of herbivory also reduced biomass of bryophytes and ecosystem respiration (ER). Except for an increase in biomass of deciduous shrubs caused by the Salix introduction, the only effect of Salix introduction was an increase in biomass of graminoids in the heath.

Conclusions

Our short-term study demonstrated that herbivore exclusion had small but still significant effects on heath vegetation, whereas such effects were not apparent in the herb-and cryptogam-dominated meadow and the Salix-dominated shrub community. Following the treatments over more years is needed to estimate the long-term effects on community structure and the consequences for C sequestration in the three plant communities. Such data are important for predicting the impact of shrub expansion on C budgets from alpine regions.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0185-9) contains supplementary material, which is available to authorized users.

Svalbard reindeer as an indicator of ecosystem changes in the Arctic terrestrial ecosystem

Over the years, noticeable effort has been directed towards contaminant determination in multiple biotic samples collected from the inhabitants of the Arctic. Little consideration has been given to polar herbivores, however, especially those from the European parts of the Arctic. To provide a broader perspective, we aimed to decipher trace element concentration in hairs of the key species in the Arctic, namely the Svalbard reindeer (Rangifer tarandus platyrhynchus), and to recognise whether diet variations could correspond with forward exposure. The effect of habitat and diet was investigated using the ratios of stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N), and previous literature studies on vegetation from the areas of interest. Analysis was performed for eighteen elements in total, both toxic and essential. Metals were present in a decreasing order Fe > Zn > Ba > Cu > Pb > Cr > Ni > V > Ga =La > Rb > As > Li > Co > Hg > Cd > Cs > Be. Similarity in trends in the studied subpopulations was observed for many metals. A significant log-linear correlation was observed for most of the elements, excluding nitrogen and carbon isotopes signature. Extremely high iron levels were determined in some of the samples, suggesting past iron overload. Zinc, in contrast to the remaining metals, did not correlate well with any other element. Mercury was determined at very low levels, in accordance with previous literature regarding its concentrations in moss and lichen species in Svalbard. The analysis of stable isotopes showed a high variation in nitrogen isotopes signatures. Further research is required to properly evaluate the potential health risks and ecological implications of elevated exposure.

Understanding the drivers of extensive plant damage in boreal and Arctic ecosystems: Insights from field surveys in the aftermath of damage

The exact cause of population dieback in nature is often challenging to identify retrospectively. Plant research in northern regions has in recent decades been largely focussed on the opposite trend, namely increasing populations and higher productivity. However, a recent unexpected decline in remotely-sensed estimates of terrestrial Arctic primary productivity suggests that warmer northern lands do not necessarily result in higher productivity. As large-scale plant dieback may become more frequent at high northern latitudes with increasing frequency of extreme events, understanding the drivers of plant dieback is especially urgent. Here, we report on recent extensive damage to dominant, short, perennial heath and tundra plant populations in boreal and Arctic Norway, and assess the potential drivers of this damage. In the High-Arctic archipelago of Svalbard, we recorded that 8-50% of Cassiope tetragona and Dryas octopetala shoots were dead, and that the ratios of dead shoots increased from 2014 to 2015. In boreal Norway, 38-63% of Calluna vulgaris shoots were dead, while Vaccinium myrtillus had damage to 91% of shoots in forested sites, but was healthy in non-forested sites. Analyses of numerous sources of environmental information clearly point towards a winter climate-related reason for damage to three of these four species. In Svalbard, the winters of 2011/12 and 2014/15 were documented to be unusually severe, i.e. insulation from ambient temperature fluctuation by snow was largely absent, and ground-ice enforced additional stress. In boreal Norway, the 2013/14 winter had a long period with very little snow combined with extremely low precipitation rates, something which resulted in frost drought of uncovered Calluna plants. However, extensive outbreaks of a leaf-defoliating geometrid moth were identified as the driver of Vaccinium mortality. These results suggest that weather and biotic extreme events potentially have strong impacts on the vegetation state of northern lands.

Variations in Pu isotopic composition in soils from the Spitsbergen (Norway): Three potential pollution sources of the Arctic region

Although the polar regions have not been industrialised, numerous contaminants originating from human activity are detectable in the Arctic environment. This study reports evidence of ²⁴⁰Pu/²³⁹Pu atomic ratios in the tundra and initial soils from different parts of west and central Spitsbergen and recognizes possible environmental inputs of non-global fallout Pu. The average atomic ratio of ²⁴⁰Pu/²³⁹Pu equal to 0.179 (ranging between 0.129 and 0.201) in tundra soils are comparable to the characteristic ratio for global fallout (0.180). However, the ²⁴⁰Pu/²³⁹Pu atomic ratios in the initial soils from proglacial zone of glaciers change within wide range between 0.1281 and 0.234 with the mean value of 0.169. By combining alpha and mass spectrometry, the three-sources model was used to identify the Pu sources in initial soils. Our study indicated that the main source of Pu is nuclear tests and that a second source with lower Pu ratio may come from weapons grade Pu (unexploded weapons grade Pu ie. material from bomb which didn't undergo nuclear explosions for example for security tests). Additionally, we found samples with high ²³⁸Pu/^239+240Pu activity ratios and with typical global fallout ²⁴⁰Pu/²³⁹Pu atomic ratios, which are associated with separate sources of pure ²³⁸Pu from the SNAP-9A satellite burn up in the atmosphere.

Topographic and geomorphologic controls on the distribution of vegetation formations in Elephant Point (Livingston Island, Maritime Antarctica)

This article focuses on the spatial distribution of vegetation formations in Elephant Point, an ice-free area of 1.16km² located in Livingston Island (South Shetland Islands, Antarctica). Fieldwork carried out in January 2014 consisted of floristic surveys and designation of a vegetation map. We have examined these data in a GIS environment together with topographical and geomorphological features existing in the peninsula in order to infer the factors controlling vegetation distribution. This has allowed quantifying the total area covered by the four different vegetation formations distributed across the peninsula, proliferating mainly on bedrock plateaus and Holocene raised beaches. Grass formation is essentially composed of Deschampsia antarctica, distributed almost exclusively on raised beaches, and covering 4.1% of the ice-free surface. The remaining three formations are fundamentally composed of cryptogam species. The first of which is fruticose lichen and moss formation, present on high bedrock plateaus and principally formed by lichens such as Usnea aurantiaco-atra. The next is the crustose lichen formation, spreading on bedrock plateaus near the coast populated by bird colonies. In this case, ornitocoprophilous lichens such as Caloplaca regalis, Xanthoria elegans and Haematomma erythromma are predominant. Together, both formations have colonised 5.1% of the peninsula. The last variety, moss carpet and moss cushion formation, occupies 1.4% of the deglaciated surface, spreading primarily in flooded areas, stabilised talus slopes, and bedrock plateaus as well. Therefore, the total surface colonised by vegetation is 12.2ha, which comprises 10.5% of the peninsula. Due to the retreat of the Rotch Dome glacier, 20.1ha remain ice-free since 1956 (17.3% of the deglaciated area). Ever since, even though the Antarctic Peninsula has registered one of the most significant temperature rises on Earth, vegetation has only colonised 0.04ha of this new space, which merely represents 0.3% of the vegetated area in Elephant Point.

Spatiotemporal variability in surface energy balance across tundra, snow and ice in Greenland

The surface energy balance (SEB) is essential for understanding the coupled cryosphere-atmosphere system in the Arctic. In this study, we investigate the spatiotemporal variability in SEB across tundra, snow and ice. During the snow-free period, the main energy sink for ice sites is surface melt. For tundra, energy is used for sensible and latent heat flux and soil heat flux leading to permafrost thaw. Longer snow-free period increases melting of the Greenland Ice Sheet and glaciers and may promote tundra permafrost thaw. During winter, clouds have a warming effect across surface types whereas during summer clouds have a cooling effect over tundra and a warming effect over ice, reflecting the spatial variation in albedo. The complex interactions between factors affecting SEB across surface types remain a challenge for understanding current and future conditions. Extended monitoring activities coupled with modelling efforts are essential for assessing the impact of warming in the Arctic.

Toward a statistical description of methane emissions from arctic wetlands

Methane (CH₄) emissions from arctic tundra typically follow relations with soil temperature and water table depth, but these process-based descriptions can be difficult to apply to areas where no measurements exist. We formulated a description of the broader temporal flux pattern in the growing season based on two distinct CH₄ source components from slow and fast-turnover carbon. We used automatic closed chamber flux measurements from NE Greenland (74°N), W Greenland (64°N), and Svalbard (78°N) to identify and discuss these components. The temporal separation was well-suited in NE Greenland, where the hypothesized slow-turnover carbon peaked at a time significantly related to the timing of snowmelt. The temporally wider component from fast-turnover carbon dominated the emissions in W Greenland and Svalbard. Altogether, we found no dependence of the total seasonal CH₄ budget to the timing of snowmelt, and warmer sites and years tended to yield higher CH₄ emissions.

A synthesis of the arctic terrestrial and marine carbon cycles under pressure from a dwindling cryosphere

The current downturn of the arctic cryosphere, such as the strong loss of sea ice, melting of ice sheets and glaciers, and permafrost thaw, affects the marine and terrestrial carbon cycles in numerous interconnected ways. Nonetheless, processes in the ocean and on land have been too often considered in isolation while it has become increasingly clear that the two environments are strongly connected: Sea ice decline is one of the main causes of the rapid warming of the Arctic, and the flow of carbon from rivers into the Arctic Ocean affects marine processes and the air-sea exchange of CO₂. This review, therefore, provides an overview of the current state of knowledge of the arctic terrestrial and marine carbon cycle, connections in between, and how this complex system is affected by climate change and a declining cryosphere. Ultimately, better knowledge of biogeochemical processes combined with improved model representations of ocean-land interactions are essential to accurately predict the development of arctic ecosystems and associated climate feedbacks.

Assessing the contribution of combustion-derived contaminants to a remote subarctic environment from traffic on the Tibbitt to Contwoyto winter road (Northwest Territories, Canada)

Remote mining operations in Canada's Northwest Territories and Nunavut are supported by a 600 km winter road, which spans the transition from subarctic boreal forest in Yellowknife to low Arctic tundra. Each year, thousands of truckloads of fuel, large equipment, and other heavy loads are hauled up the winter road. We investigated whether diesel emissions from commercial truck traffic is a major source of metals and polycyclic aromatic compounds (PACs) to aquatic ecosystems along the winter road. In March 2014, at the end of the hauling season, we collected integrated snow samples, water, and sediment from nine lakes located along the winter road, as well as from six lakes located within the city of Yellowknife. Examination of PAC composition and diagnostic ratios in snow samples showed that wildfires are an important source of PACs to lakes along the winter road, while anthropogenic sources are more prevalent in snow from Yellowknife lakes. Concentrations of PACs, including those associated with diesel emissions, were variable in snow, water, and sediment across all sites. The highest concentrations of PACs in snow were reported in winter road lakes located in the subarctic boreal forest, where forest fires are common. No compositional differences were observed for PACs in sediment and water samples between Yellowknife and winter road lakes. We did not observe any evidence of metal contamination in snow collected along the winter road, and metal concentrations in snow from winter road sites were consistently lower than Yellowknife sites. Our results show that a high contribution of PACs from natural sources can obscure potential contributions from diesel traffic emissions along the winter road.

Microbial community composition shapes enzyme patterns in topsoil and subsoil horizons along a latitudinal transect in Western Siberia

Soil horizons below 30 cm depth contain about 60% of the organic carbon stored in soils. Although insight into the physical and chemical stabilization of soil organic matter (SOM) and into microbial community composition in these horizons is being gained, information on microbial functions of subsoil microbial communities and on associated microbially-mediated processes remains sparse. To identify possible controls on enzyme patterns, we correlated enzyme patterns with biotic and abiotic soil parameters, as well as with microbial community composition, estimated using phospholipid fatty acid profiles. Enzyme patterns (i.e. distance-matrixes calculated from these enzyme activities) were calculated from the activities of six extracellular enzymes (cellobiohydrolase, leucine-amino-peptidase, N-acetylglucosaminidase, chitotriosidase, phosphatase and phenoloxidase), which had been measured in soil samples from organic topsoil horizons, mineral topsoil horizons, and mineral subsoil horizons from seven ecosystems along a 1500 km latitudinal transect in Western Siberia. We found that hydrolytic enzyme activities decreased rapidly with depth, whereas oxidative enzyme activities in mineral horizons were as high as, or higher than in organic topsoil horizons. Enzyme patterns varied more strongly between ecosystems in mineral subsoil horizons than in organic topsoils. The enzyme patterns in topsoil horizons were correlated with SOM content (i.e., C and N content) and microbial community composition. In contrast, the enzyme patterns in mineral subsoil horizons were related to water content, soil pH and microbial community composition. The lack of correlation between enzyme patterns and SOM quantity in the mineral subsoils suggests that SOM chemistry, spatial separation or physical stabilization of SOM rather than SOM content might determine substrate availability for enzymatic breakdown. The correlation of microbial community composition and enzyme patterns in all horizons, suggests that microbial community composition shapes enzyme patterns and might act as a modifier for the usual dependency of decomposition rates on SOM content or C/N ratios.

Effects of human trampling on abundance and diversity of vascular plants, bryophytes and lichens in alpine heath vegetation, Northern Sweden

This study investigated the effects of human trampling on cover, diversity and species richness in an alpine heath ecosystem in northern Sweden. We tested the hypothesis that proximity to trails decreases plant cover, diversity and species richness of the canopy and the understory. We found a significant decrease in plant cover with proximity to the trail for the understory, but not for the canopy level, and significant decreases in the abundance of deciduous shrubs in the canopy layer and lichens in the understory. Proximity also had a significant negative impact on species richness of lichens. However, there were no significant changes in species richness, diversity or evenness of distribution in the canopy or understory with proximity to the trail. While not significant, liverworts, acrocarpous and pleurocarpous bryophytes tended to have contrasting abundance patterns with differing proximity to the trail, indicating that trampling may cause shifts in dominance hierarchies of different groups of bryophytes. Due to the decrease in understory cover, the abundance of litter, rock and soil increased with proximity to the trail. These results demonstrate that low-frequency human trampling in alpine heaths over long periods can have major negative impacts on lichen abundance and species richness. To our knowledge, this is the first study to demonstrate that trampling can decrease species richness of lichens. It emphasises the importance of including species-level data on non-vascular plants when conducting studies in alpine or tundra ecosystems, since they often make up the majority of species and play a significant role in ecosystem functioning and response in many of these extreme environments.

Effect of thaw depth on fluxes of CO₂ and CH₄ in manipulated Arctic coastal tundra of Barrow, Alaska

Changes in CO₂ and CH₄ emissions represent one of the most significant consequences of drastic climate change in the Arctic, by way of thawing permafrost, a deepened active layer, and decline of thermokarst lakes in the Arctic. This study conducted flux-measurements of CO₂ and CH₄, as well as environmental factors such as temperature, moisture, and thaw depth, as part of a water table manipulation experiment in the Arctic coastal plain tundra of Barrow, Alaska during autumn. The manipulation treatment consisted of draining, controlling, and flooding treated sections by adjusting standing water. Inundation increased CH₄ emission by a factor of 4.3 compared to non-flooded sections. This may be due to the decomposition of organic matter under a limited oxygen environment by saturated standing water. On the other hand, CO₂ emission in the dry section was 3.9-fold higher than in others. CH₄ emission tends to increase with deeper thaw depth, which strongly depends on the water table; however, CO₂ emission is not related to thaw depth. Quotients of global warming potential (GWPCO₂) (dry/control) and GWPCH₄ (wet/control) increased by 464 and 148%, respectively, and GWPCH₄ (dry/control) declined by 66%. This suggests that CO₂ emission in a drained section is enhanced by soil and ecosystem respiration, and CH₄ emission in a flooded area is likely stimulated under an anoxic environment by inundated standing water. The findings of this manipulation experiment during the autumn period demonstrate the different production processes of CO₂ and CH₄, as well as different global warming potentials, coupled with change in thaw depth. Thus the outcomes imply that the expansion of tundra lakes leads the enhancement of CH₄ release, and the disappearance of the lakes causes the stimulated CO₂ production in response to the Arctic climate change.

Input of easily available organic C and N stimulates microbial decomposition of soil organic matter in arctic permafrost soil

Rising temperatures in the Arctic can affect soil organic matter (SOM) decomposition directly and indirectly, by increasing plant primary production and thus the allocation of plant-derived organic compounds into the soil. Such compounds, for example root exudates or decaying fine roots, are easily available for microorganisms, and can alter the decomposition of older SOM ("priming effect"). We here report on a SOM priming experiment in the active layer of a permafrost soil from the central Siberian Arctic, comparing responses of organic topsoil, mineral subsoil, and cryoturbated subsoil material (i.e., poorly decomposed topsoil material subducted into the subsoil by freeze-thaw processes) to additions of ¹³C-labeled glucose, cellulose, a mixture of amino acids, and protein (added at levels corresponding to approximately 1% of soil organic carbon). SOM decomposition in the topsoil was barely affected by higher availability of organic compounds, whereas SOM decomposition in both subsoil horizons responded strongly. In the mineral subsoil, SOM decomposition increased by a factor of two to three after any substrate addition (glucose, cellulose, amino acids, protein), suggesting that the microbial decomposer community was limited in energy to break down more complex components of SOM. In the cryoturbated horizon, SOM decomposition increased by a factor of two after addition of amino acids or protein, but was not significantly affected by glucose or cellulose, indicating nitrogen rather than energy limitation. Since the stimulation of SOM decomposition in cryoturbated material was not connected to microbial growth or to a change in microbial community composition, the additional nitrogen was likely invested in the production of extracellular enzymes required for SOM decomposition. Our findings provide a first mechanistic understanding of priming in permafrost soils and suggest that an increase in the availability of organic carbon or nitrogen, e.g., by increased plant productivity, can change the decomposition of SOM stored in deeper layers of permafrost soils, with possible repercussions on the global climate.

Simulated global change: contrasting short and medium term growth and reproductive responses of a common alpine/Arctic cushion plant to experimental warming and nutrient enhancement

Cushion plants are important components of alpine and Arctic plant communities around the world. They fulfill important roles as facilitators, nurse plants and foundation species across trophic levels for vascular plants, arthropods and soil microorganisms, the importance of these functions increasing with the relative severity of the environment. Here we report results from one of the few experimental studies simulating global change impacts on cushion plants; a factorial experiment with warming and nutrient enhancement that was applied to an alpine population of the common nurse plant, Silene acaulis, in sub-arctic Sweden. Experimental perturbations had significant short-term impacts on both stem elongation and leaf length. S. acaulis responded quickly by increasing stem elongation and (to a lesser extent) leaf length in the warming, nutrient, and the combined warming and nutrient enhancements. Cover and biomass also initially increased in response to the perturbations. However, after the initial positive short-term responses, S. acaulis cover declined in the manipulations, with the nutrient and combined warming and nutrient treatments having largest negative impact. No clear patterns were found for fruit production. Our results show that S. acaulis living in harsh environments has potential to react quickly when experiencing years with favorable conditions, and is more responsive to nutrient enhancement than to warming in terms of vegetative growth. While these conditions have an initial positive impact, populations experiencing longer-term increased nutrient levels will likely be negatively affected.

Nitrogen dynamics in Turbic Cryosols from Siberia and Greenland

Turbic Cryosols (permafrost soils characterized by cryoturbation, i.e., by mixing of soil layers due to freezing and thawing) are widespread across the Arctic, and contain large amounts of poorly decomposed organic material buried in the subsoil. This cryoturbated organic matter exhibits retarded decomposition compared to organic material in the topsoil. Since soil organic matter (SOM) decomposition is known to be tightly linked to N availability, we investigated N transformation rates in different soil horizons of three tundra sites in north-eastern Siberia and Greenland. We measured gross rates of protein depolymerization, N mineralization (ammonification) and nitrification, as well as microbial uptake of amino acids and NH4+ using an array of 15N pool dilution approaches. We found that all sites and horizons were characterized by low N availability, as indicated by low N mineralization compared to protein depolymerization rates (with gross N mineralization accounting on average for 14% of gross protein depolymerization). The proportion of organic N mineralized was significantly higher at the Greenland than at the Siberian sites, suggesting differences in N limitation. The proportion of organic N mineralized, however, did not differ significantly between soil horizons, pointing to a similar N demand of the microbial community of each horizon. In contrast, absolute N transformation rates were significantly lower in cryoturbated than in organic horizons, with cryoturbated horizons reaching not more than 32% of the transformation rates in organic horizons. Our results thus indicate a deceleration of the entire N cycle in cryoturbated soil horizons, especially strongly reduced rates of protein depolymerization (16% of organic horizons) which is considered the rate-limiting step in soil N cycling.

Recovery of ecosystem carbon fluxes and storage from herbivory

The carbon (C) sink strength of arctic tundra is under pressure from increasing populations of arctic breeding geese. In this study we examined how CO₂ and CH₄ fluxes, plant biomass and soil C responded to the removal of vertebrate herbivores in a high arctic wet moss meadow that has been intensively used by barnacle geese (Branta leucopsis) for ca. 20 years. We used 4 and 9 years old grazing exclosures to investigate the potential for recovery of ecosystem function during the growing season (July 2007). The results show greater above- and below-ground vascular plant biomass within the grazing exclosures with graminoid biomass being most responsive to the removal of herbivory whilst moss biomass remained unchanged. The changes in biomass switched the system from net emission to net uptake of CO₂ (0.47 and -0.77 μmol m^-2 s^-1 in grazed and exclosure plots, respectively) during the growing season and doubled the C storage in live biomass. In contrast, the treatment had no impact on the CH₄ fluxes, the total litter C pool or the soil C concentration. The rapid recovery of the above ground biomass and CO₂ fluxes demonstrates the plasticity of this high arctic ecosystem in terms of response to changing herbivore pressure.

Foliar and soil nutrients in tundra on glacial landscapes of contrasting ages in northern Alaska

We compared foliar and soil nutrients in tundra between two different landscapes in the foothills of the Brooks Range, Alaska, that were deglaciated >50,000 and >11,500 years ago, respectively. Our goal was to determine whether foliar nutrients reflect differences in soil nutrient availability, or whether species and/or growth forms have characteristic foliar nutrient concentrations regardless of soil nutrient availability. Sites are located less than 2 km from one another, and both are dominated by moist tussock tundra. However, forbs are less common and deciduous and evergreen shrubs more common at the older site. Soils at the older site had higher net nitrogen (N) mineralization rates, but lower pH, cation exchange capacity, percent base saturation, exchangeable calcium (Ca) and magnesium, total Ca, and inorganic N and phosphorus than the younger site. Foliar nutrients generally reflected differences in soil net N mineralization rates and exchangeable base cations, with higher foliar N at the older site, and higher foliar Ca at the younger site. However, large differences in foliar nutrients also occurred among growth forms, and the magnitude of the site differences in foliar nutrients between sites was growth form-dependent. In general, species with short leaf lifespans (deciduous shrubs, sedges, and forbs) had higher foliar nutrient concentrations than evergreen species. Thus, foliar nutrients were a function both of underlying variation in soil nutrient availability and of species and growth form composition.

Foliar and soil nutrients in tundra on glacial landscapes of contrasting ages in northern Alaska

We compared foliar and soil nutrients in tundra between two different landscapes in the foothills of the Brooks Range, Alaska, that were deglaciated >50,000 and >11,500 years ago, respectively. Our goal was to determine whether foliar nutrients reflect differences in soil nutrient availability, or whether species and/or growth forms have characteristic foliar nutrient concentrations regardless of soil nutrient availability. Sites are located less than 2 km from one another, and both are dominated by moist tussock tundra. However, forbs are less common and deciduous and evergreen shrubs more common at the older site. Soils at the older site had higher net nitrogen (N) mineralization rates, but lower pH, cation exchange capacity, percent base saturation, exchangeable calcium (Ca) and magnesium, total Ca, and inorganic N and phosphorus than the younger site. Foliar nutrients generally reflected differences in soil net N mineralization rates and exchangeable base cations, with higher foliar N at the older site, and higher foliar Ca at the younger site. However, large differences in foliar nutrients also occurred among growth forms, and the magnitude of the site differences in foliar nutrients between sites was growth form-dependent. In general, species with short leaf lifespans (deciduous shrubs, sedges, and forbs) had higher foliar nutrient concentrations than evergreen species. Thus, foliar nutrients were a function both of underlying variation in soil nutrient availability and of species and growth form composition.

Competition intensity and its importance: results of field experiments with Anthoxanthum odoratum

The effect of community productivity on competition was studied in 82 permanent plots using two removal experiments with the rhizomatous perennial grass Anthoxanthum odoratum. The removal of neighbouring plants had a positive effect on the number of shoots and total above-ground biomass of Anthoxanthum but no significant effect on mean shoot biomass. The relative competition intensity coefficient (RCI) calculated from these data showed that competition intensity increased with increasing community productivity. Similarly, the importance of competition and the difference between local maximum and local average population density increased with increasing community productivity. We concluded that for Anthoxanthum the impact of competition is greater in high-productivity areas and that competition reduces population density. No evidence was found supporting the importance of positive interactions between plants in tundra areas.

Distribution patterns of some criconematinae in different forest associations

A total of 243 samples from Hemlock-Hardwood, Boreal Forest, and Alpine Tundra associations of New York, Vermont, New Hampshire, and Maine were analyzed for species of Bakernema, Criconema, and Criconemoides and for selected edaphic factors. The Hemlock-Hardwood formation contained 13 species of these genera, but the Boreal Forest and Alpine Tundra contained only Criconema menzeli and Criconemoides sphagni. Criconemoides axeste, C. rusticum, and C. xenoplax were associated primarily with mineral soils that have high pH, low moisture after drainage, and organic matter content of less than 15%. Criconemoides sphagni was associated with organic soils that had low pH, high moisture after drainage, and organic matter content greater than 15%.