A process geomorphological investigation was started in 1999 to study present denudation rates and the mutual relationship of chemical and mechanical fluvial denudation in periglacial environments. Latnjavagge (9 km2; 950-1440 m a.s.l.; 68°20'N, 18°30'E) was chosen as a representative drainage basin of the arctic-oceanic mountain area in northernmost Swedish Lapland. Atmospheric solute inputs, chemical denudation, and mechanical fluvial denudation were analyzed. During the arctic summer field seasons of 2000, 2001, and 2002 measurements of daily precipitation, solute concentrations in precipitation, and in melted snow cores, taken before snowmelt, were recorded. In addition, solute and suspended sediment concentrations in creeks were analyzed, and bedload tracer movements were registered during the entire summer seasons (end of May until beginning of September). Results show a mean annual chemical denudation net rate of 5.4 t km-2 yr-1 in the entire catchment. Chemical denudation in Latnjavagge is less than one third of chemical denudation rates reported for Kärkevagge (Swedish Lapland) but seems to be at a similar level as in a number of other subarctic, arctic, and alpine environments. Mechanical fluvial denudation is lower than chemical denudation. Most sediment transport in channels occurs in the early summer season during a few days with snowmelt generated runoff peaks. The main sediment sources in the drainage basin are mobilized channel bed pavements exposing fines, ice patches/fields, and material mobilized by slush flows. The calculated mean mechanical fluvial denudation rate is 2.3 t km-2 yr-1 at the inlet of lake Latnjajaure, situated in Latnjavagge close to the outlet of the valley. A very stable vegetation cover and rhyzosphere in this environment mainly explain the low value. The mean mechanical fluvial denudation rate at the outlet of the entire Latnjavagge drainage basin, below lake Latnjajaure, is only 0.8 t km-2 yr-1. Both chemical and mechanical fluvial denudation show low intensity. The results from Latnjavagge support the contention that chemical denudation is a somewhat important denudational process in periglacial environments.

The ecosystems of alpine snowbed habitats are reviewed with emphasis on ecosystem functioning and capability to adapt to current and predicted global change. Snowbeds form in topographic depressions that accumulate large amounts of snow during the winter months, and the final snowmelt does not occur until late in the growing season. Many species preferentially grow in snowbed habitats and some of these are even restricted to these habitats. In this review we identify several ecosystem services which snowbeds provide to the alpine landscape. For instance, snowbeds provide a steady water and nutrient supply to adjacent plant communities and offer newly emerged high-quality food for herbivores late in the growing season. We also propose that alpine snowbeds are much more productive than earlier thought, especially when the very short growing season and often high grazing pressure are taken fully into account. Furthermore, we propose that bryophytes and graminoids (grasses, sedges, and rushes) probably will be most negatively impacted by global change, and the snowbed plant communities will be invaded by species from adjacent plant communities, especially by shrubs and boreal species. As snowbed plants have special growth conditions, their sensitivity and ability to respond rapidly to changes in annual snowfall patterns make snowbed communities particularly vulnerable in a warmer climate, and thereby sensitive indicators of global change.

Respiration rates, thermal sensitivity, and thermal acclimation potential of root respiration were investigated in Ranunculus from the Arctic. Comparisons of three species (R. glacialis, R. nivalis and R. acris subsp. pumilus) used plants grown on a mountain or in a glasshouse for 6 wk at contrasting soil temperatures (5.4 and 14.5 degreesC, respectively). Northern and southern ecotypes of two species of Ranunculus (R. pygmaeus, and R. acris subsp. acris), together ranging from Svalbard (79 degreesN) to Scotland (56 degreesN), were similarly compared after 2 wk in a growth cabinet at 5 and 15 degreesC. Respiration rates varied at standard measurement temperatures; R. nivalis and R. pygmaeus grown on the mountain or at 5 degreesC had the highest respiration, followed by other alpine snowbed species (R. glacialis and R. acris subsp. pumilus) and R. acris subsp. acris from the arctic lowland; R. acris subsp. acris from Scotland had lowest rates. Respiration was temperature sensitive for all populations, increasing progressively between 5 and 20 degreesC (Q(10) ((5-15)): 1.2-2.4). Extent and type of acclimation of root respiration varied with no clear latitudinal pattern emerging. Acclimation to a 10 degreesC increase in growth temperature was achieved through: change in temperature sensitivity (shown by changes in Q(10 (5-15)) values) (R. acris subsp. pumilus); or reduction in absolute rates (R. pygmaeus from Svalbard, R. acris subsp. pumilus and R. nivalis). Complete acclimation occurred in R. acris subsp. pumilus and R. pygmaeus, whereas R. acris subsp. acris from Scotland and R. glacialis did not acclimate. Plants that adjust root respiration (e.g., R. pygmaeus from Svalbard and R. acris subsp. pumilus) to maintain a positive carbon balance, may tolerate predicted temperature increases in arctic regions. Plants with high rates of root respiration and/or high sensitivity to temperature as well as poor acclimation potential, (e.g., R. glacialis) may only persist in cold microhabitats.

Age structure, tree characteristics, and environmental data were used to analyze the status of the birch treeline in three regions along the Scandes Mountains from 62 degrees 10’N to 69 degrees 50’N. Aspect and estimated relative radiation explained most of the treeline altitude across studied regions, but not all variation. Main tree establishment occurred during the 1940s in the southern and northernmost regions, and during the 1960s in the middle region. Age distribution patterns at 2 m (tree size), however, showed stable or possibly progressive treelines in the southern and middle regions but recent recession in the north. Growth rates varied through time and between regions, with an apparent decrease in the north since the 1940s. Weak negative correlations between attitude and age in the south indicate recent changes favoring tree growth or increased turnover at higher, more exposed altitudes. Although Scandinavian treelines are expected to advance in response to climate warming, this was not evident as a general pattern for all regions. Seasonally different climate patterns, browsing, and abrasion are mechanisms involved in this, These regionally different patterns have to be taken into account in predictions of future responses to avoid overestimation of, e.g., ecosystem change, carbon uptake capacity, and feedbacks to climate systems.

Karkevagge, an alpine ecosystem in the subarctic, has been the subject of scientific study for half a century. We investigated the relationship of its vegetation to soil properties. At the lowest elevations, on stable portions of the landscape dominated by Betula and Empetrum, are found Spodosols developed in glacio-fluvial sediments. Cryosaprists occur in scattered bedrock depressions with bog-type vegetation. Cryofluvents are found in flood-prone areas covered by dwarf-willow thickets. Intermediate elevations have meadow-type vegetation with Cryofluvents on floodplains and on lower colluvial slopes, Cryaquents in wetter areas, and Cryorthents on steeper soliflucted slopes. Above that, on steep, west-facing Dryas-covered colluvial slopes, soils are Ca-rich Eutrocryepts and Haplocryolls. The highest-elevation soils are infertile with poor horizonation, despite their possible antiquity, and vegetation that is largely cryptogams. Dystrocryepts occur on more stable alpine locations; Cryorthents on soliflucted areas; and Haploturbels, with shallow permafrost, above 1400 m. Measured annual soil temperatures range from +2.4degreesC at a Dryas site to -3.4degreesC at 1585 to at an alpine cryptogam site. Vegetation distribution in Karkevagge is related to climatic factors, which are controlled by elevation and landscape position, and edaphic factors, which control soil moisture and nutrient availability.

Remote sensing provides a viable alternative for mapping vegetation in the Arctic because it allows for the mapping of discontinuous distribution of cover types over different spatial scales. In this paper we present a statistical method to map the distribution of important cover types for the reindeer Rangifer tarandus during summer in northernmost Sweden using IRS 1D-LISS satellite imagery. We exemplify our method with modeling of the distribution of snowbed vegetation, the cover type used most intensively by the reindeer in the study area. An autologistic regression model that incorporates the spatial structure of the data is used to combine the field data and the satellite image data. The terrain effects in the satellite image are accounted for in the regressions using a digital elevation model (DEM). We produced a fine-scaled coverage depicting the probability of occurrence of snowbed vegetation as a continuous variable at the pixel level. The accuracy of mapping snowbed vegetation was 69-77%, depending on the data used. We conclude that small-scale, pixel-wise classification modeling may be useful for depicting sparsely occurring cover types, some of which may be important determinants of range quality for reindeer.

Carbon dioxide fluxes in a dry subarctic heath were examined after 10 and 11 yr of experimental manipulations of temperature, light, and nutrients. The aim was to investigate how growing season carbon (C) balance was affected by the major climatic factors that are expected to change in the future. Carbon flux was measured in closed chambers as uptake through gross ecosystem production (GP), release through ecosystem respiration (ER), and as net ecosystem production (NEP). Diurnal NEP through a day with clear skies at peak growing season was consistently negative through all treatments the first year of measurement, and day-time NEP varied around zero at eight days across the growing season the second year, implying that a net release of C from the ecosystem to the atmosphere may take place during the growing season. Our results suggest that respiration was the main determinant of C balance, and that variations in light levels and temperature could alter the balance between C uptake and C loss. Fertilization strongly enhanced both ER and GP whereas temperature enhancement changed neither ER nor GP. Shading decreased both ER and GP. After harvest of the aboveground plant biomass, the belowground respiration was 72 to 93% of the ER before harvest. The significant treatment effects on belowground respiration after harvest were similar to the effects on ER before harvest. These results suggest that the ER were mainly from belowground respiration, and that the treatments affected the belowground respiration more than the respiration above ground.

The subarctic landscape is composed of a complex mosaic of vegetation, geology and topography, which control both the hydrology and biogeochemistry of streams across space and time. We present a synoptic sampling campaign that aimed to estimate dissolved C export variability under low-flow conditions from a subarctic landscape. The results included measurements of stream discharge and concentrations of both dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and carbon dioxide (CO₂) for 32 subcatchments of the Abiskojokka catchment in northern Sweden. For these subarctic headwater streams, we found that DOC, DIC and CO₂ concentrations showed significant variability (p < 0.05) relative to catchment size, discharge, specific discharge, lithology, electrical conductivity, weathering products, and the estimated travel time of water through the subcatchment. Our results indicate that neither vegetation cover nor lithology alone could explain the concentrations and mass flux rates of DOC and DIC. Instead, we found that mass flux rates of DOC, DIC, and CO₂ depended mainly on specific discharge and water travel time. Furthermore, our results demonstrate the importance of studying lateral carbon transport in combination with hydrological flow paths at small scales to establish a knowledge foundation applicable for expected carbon cycle and hydroclimatic shifts due to climate change.

Calving events of Petermann Glacier, northwest Greenland, in 2010 and 2012 reduced the length of its ice tongue by c. 25 km, allowing exploration of newly uncovered seafloor during the Petermann 2015 Expedition. This article presents the results of foraminiferal analysis and environmental data from thirteen surface sediment samples in northern Nares Strait and Petermann Fjord, including beneath the modern ice tongue. This is the first study of living foraminifera beneath an arctic ice tongue and the first modern foraminiferal data from this area. Modern assemblages were studied to constrain species environmental preferences and to improve paleoenvironmental interpretations of foraminiferal assemblages. Sub-ice tongue assemblages differed greatly from those at all other sites, with very low faunal abundances and being dominated by agglutinated fauna, likely reflecting low food supply under the ice tongue. Fjord fauna were comprised of 80 percent or more calcareous species. Notably, Elphidium clavatum is absent beneath the ice tongue although it is dominant in the fjord. Increasing primary productivity associated with the transition to mobile sea ice, diminishing influence of the Petermann Glacier meltwater with distance from the grounding line, and increased influence of south-flowing currents in Nares Strait are the important controls on the faunal assemblages.

In the nineteenth century, numerous settlements were established in the alpine region of Fennoscandia (the Scandes), an area that later became a major international scene for Arctic research. Here we raise awareness of this era and show that earthworm-driven bioturbation in “pristine” soils around contemporary Arctic research infrastructure is caused by soil fauna left behind during early land use. We use soil preserved under an alpine settlement to highlight that soils were not bioturbated when the first house was built at a site where bioturbation is now widespread. A review of archived material with unique site-specific chronology constrained the onset of bioturbation to the post-1871 era. Our results suggest that small-scale land use introduced earthworms that now thrive far beyond the realms of former cultivated fields. The legacy of soil fauna from this example of “ecological imperialism” still lingers and should be considered when studying soils of the Scandes.

Parteglaciaren in northern Sweden has a response time of similar to 200 years, demonstrating a long response time for a continentally located glacier. Paeteglaciaren is a polythermal valley glacier presently covering an area of 10 km(2). Its size will be reduced another 60-70% if the present climate persists and will then only have similar to 30% of its Little Ice Age maximum volume left. Future global warming will of course enhance melt rates, and the relative size and volume reduction will probably be even larger. Photogrammetric studies between 1963 and 1992 show a general thinning of the entire glacier except for the center one of the three cirques in the accumulation area, which seems to have a surface elevation in balance with present climate. Balanced flow studies performed using GPS and Ground Penetrating Radar at the outlet of the cirques gave negative values for two cirques and a positive value for the center cirque. The future Parteglaciaren will split up into three small glaciers, and only the center one will extend beyond its cirque.

Plant-associated fungi have elementary roles in ecosystem productivity. There is little information on the interactions between arbuscular mycorrhizal (AM) fungal symbiosis, fine endophytic (FE) and dark septate endophytic (DSE) fungi, and their host plants in cold climate systems. In particular, the environmental filters potentially driving the relative abundance of these root symbionts remain unknown. We investigated the interlinkage of plant and belowground fungal responses to simulated herbivory (clipping, fertilization, and trampling) in a subarctic meadow system. AM and FE frequency in the two target plant roots, Potentilla crantzii and Saussurea alpina, was unaffected by simulated herbivory, highlighting the importance and resilience of arbuscule forming mycorrhizas in a range of environmental conditions. Fertilization and trampling increased DSE colonization in P. crantzii roots although generally P. crantzii performance was reduced in these plots. The idiosyncratic responses by DSE fungal frequency in the two host plants in our experiment indicate that the host plant identity has a pivotal role in the DSE fungus–plant outcome. DSE fungal frequency did not respond to environmental manipulations in a manner similar to arbuscular mycorrhizas, suggesting that they have a different role in plant ecology.

Recent evidence suggests that biogeochemical processes in the Arctic during late winter and spring-thaw strongly affect the annual cycling of carbon and nutrients, indicating high susceptibility to climate change. We therefore examined the carbon and nutrient dynamics in a sub-arctic heath and a birch forest with high temporal resolution from March until snowmelt at both ambient and experimentally increased snow depths. Ecosystem respiration (ER) from mid-March to snowmelt at ambient snow was high, reaching 99 +/- 19 (birch) and 67 +/- 1.4 g C m(-2) (heath). Enhanced snow depth by about 20-30 cm increased ER by 77-157% during late winter but had no effects during spring-thaw. ER rates at the birch site were poorly described by classic first-order exponential models (R-2 = 0.06-0.10) with temperature as a single variable, but model fit improved considerably by including the supply of dissolved organic carbon (DOC) or nitrogen (DON) in the model (R-2 = 0.40-0.47). At the heath, model fit with temperature as the single variable was better (R-2 = 0.38-0.52), yet it improved when the supply of DOC or DON was included (R-2 = 0.65-0.72). Microbial carbon decreased by 43% within a few days after the first soil freeze-thaw event, while microbial nitrogen and phosphorus decreased more slowly. Because soil inorganic nitrogen and phosphorus concentrations were low, nutrients released from lysed microbial cells may have been sequestered by surviving microbes or by plants resuming growth. The fast change in microbial biomass and the dependence of ER on substrate availability stress the need for high temporal resolution in future research on ecosystem carbon and nutrient dynamics at snowmelt in order to make robust models of their turnover.

Relationships were determined between methane (CH4) production and in situ conditions within the permafrost active layer during a single melt season at Stordalen, Sweden, with a specific emphasis on temperature sensitivity of methanogenesis. In situ temperature, moisture, pH, dissolved organic carbon, and CH4 concentration data were measured at three contrasting active layer sites (sedge mire, Sphagnum mire, and ombrotrophic bog), and laboratory incubations of active layer material were subsequently employed to determine the sensitivity of CH4 production to temperature. Q(10) values, describing the CH4 production response of peat to a temperature change of 10 degrees C, ranged from 1.9 to 3.5 and 2.4 to 5.8 for the sedge and Sphagnum mire sites, respectively. A wider review of the literature on Q(10) responses of methanogenesis in northern peatlands shows similar features to the temperature response of CH4 production in the active layer at Stordalen. In general, Q(10) to values are not significantly different in Arctic permafrost wetlands than non-Arctic northern wetlands; however, Sphagnum sites display Q(10) responses (mean Q(10) = 8) that are notably greater than that of wetter minerotrophic-sedge environments (mean Q(10) = 4.3). This finding has implications for the parameterization of Q(10) factors in numerical carbon cycling models, and suggests that the use of spatially variable Q(10) values could be a useful approach for more accurate modeling of CH4 fluxes front northern wetlands under different climatic change scenarios.

This empirical study explores shifts in stable water isotopic composition for a subarctic catchment located in northern Sweden as it transitions from spring freshet to summer low flows. Relative changes in the isotopic composition of streamflow across the main catchment and fifteen nested subcatchments are characterized in relation to the isotopic composition of precipitation. With our sampling campaign, we explore the variability in stream-water isotopic composition that originates from precipitation as the input shifts from snow to rain and as landscape flow pathways change across scales. The isotopic similarity of high-elevation snowpack water and early season rainfall water seen through our sampling scheme made it difficult to truly isolate the impact of seasonal precipitation phase change on stream-water isotopic response. This highlights the need to explicitly consider the complexity of arctic and alpine landscapes when designing sampling strategies to characterize hydrological variability via stable water isotopes. Results show a potential influence of evaporation and source water mixing both spatially (variations with elevation) and temporally (variations from post-freshet to summer flows) on the composition of stream water across Miellajokka. As such, the data collected in this empirical study allow for initial conceptualization of the relative importance of, for example, hydrological connectivity within this mountainous, subarctic landscape.

Litter decomposition, a key process by which recently fixed carbon is lost from ecosystems, is a function of environmental conditions and plant community characteristics. In ice-rich peatlands, permafrost thaw introduces high variability in both abiotic and biotic factors, both of which may affect litter decomposition rates in different ways. Can the existing conceptual frameworks of litter decomposition and its controls be applied across a structurally heterogeneous thaw gradient? We investigated the variability in litter decomposition and its predictors at the Stordalen subarctic peatland in northern Sweden. We measured in situ decomposition of representative litter and environments using litter bags throughout two years. We found highly variable litter decomposition rates with turnover times ranging from five months to four years. Surface elevation was a strong correlate of litter decomposition across the landscape, likely as it integrates multiple environmental and plant community changes brought about by thaw. There was faster decomposition but also more mass remaining after two years in thawed areas relative to permafrost areas, suggesting faster initial loss of carbon but more storage into the slow-decomposing carbon pool. Our results highlight mechanisms and predictors of carbon cycle changes in ice-rich peatlands following permafrost thaw.

Reindeer, Rangifer tarandus, live in subarctic and alpine environments with spatially and temporally heterogeneous resource distribution. In this study, we used a hierarchical approach to test whether reindeer responded to spatial heterogeneity during the plant growing season (divided into three distinct periods) in a mountainous subarctic environment in northern Sweden. A reindeer herd in northern Sweden was surveyed using radio-telemetry (8 female reindeer) and the selection of feeding habitats by observing individuals/groups (135 observations) using laser range-finding binoculars. Reindeer selected feeding areas (evaluated at 5-km grid size), as well as feeding habitats (evaluated at 0.5- and 1-km grid size) during spring, in response to high terrain ruggedness and habitat heterogeneity. Reindeer switched during summer to select against terrain ruggedness and habitat heterogeneity at the level of feeding habitats, while preferring southward facing habitats. During autumn, a broader spectrum of feeding habitats was used. We conclude that reindeer seem to adopt a hierarchical strategy in agreement with general foraging theory, and are capable of responding to seasonal changes in resource distribution occurring across spatial scales. Furthermore, our results support the idea that spatial heterogeneity is an important factor to large-sized herbivores at high and intermediate levels of habitat selection. Conservation of large continuous and undeveloped landscapes is an important management goal, as they provide a wide range of habitats necessary for animals such as reindeer that use large territories.

AbstractBurrowing mammals often have considerable geomorphological impacts, and their tunneling activities may decrease the stability of landforms. We document the spatial distribution of Norwegian lemming burrows in a subarctic alpine meadow to determine the preferred locations for burrow entrances and to examine the potential for burrowing to decrease the stability of periglacial landforms at the site. Burrow entrances were disproportionately common into the base and sides of landforms (>68% of burrows), probably reflecting the lower energetic cost of moving soil horizontally, rather than vertically, out of burrows. Most burrow entrances (>60%) were also located under large rocks, which probably improve burrow stability by providing a firm ceiling to the entrance. Field observations show that these burrows are relatively stable, as only 3% were associated with any signs of increased erosion or landform instability. Therefore, in contrast to some previous studies, and despite burrowing being concentrated on landforms, we suggest that these rodents have little direct impact on landform integrity at this site.

The ectomycorrhizal communities in alpine habitats have been relatively little studied. As global change is predicted to have a large impact in Arctic and alpine environments, it is important to document the fungi of these climatic regions to monitor changes and to understand upcoming successions. This study investigates the ectomycorrhizal community of Dryas octopetala and Salix reticulata on cliff ledges in a mid-alpine setting using the internal transcribed spacer region of nuclear ribosomal DNA for the identification of the fungal component of ectomycorrhizal root tips. It is shown that the community is relatively species rich, with 74 molecular operational taxonomic units (MOTUs)/species, and that it is dominated by Cenococcum geophilum, Thelephoraceae spp., Cortinarius spp., and Sebacinales spp. Furthermore, the dominating species have low specificity regarding the tested hosts and seem likely to be able to facilitate the succession of the alpine tundra to subalpine forest by serving as mycorrhizal partners for establishing pioneer trees.

Sulfate and nitrate records from 5 ice cores spread across Svalbard were compared and revealed strong temporal similarities with previously published global estimates of SO2 and NOx anthropogenic emissions during the 20th century. A significant departure from the early century sulfate and nitrate levels was evident at all drilling sites starting from the mid-1940s. A steady increase was observed in both sulfate and nitrate profiles at most sites until the late 1960s, when the annual concentrations started to increase at a higher rate. This peak activity lasted for about a decade, and was observed to decrease steadily from the early 1980s on, when sulfate levels declined significantly and when nitrate levels finally reached sulfate levels for the first time in 20th century. The timing of these trends in Svalbard with global SO2 and NOx concentration profiles was best appraised when considering composite concentration profiles of all Svalbard ice cores for sulfate and nitrate, respectively. Composite profiles were also found to provide a convenient mean for distinguishing between the most important world source regions. Based on correlation analysis, the major pollutant sources appeared to be Western Europe and North America for both sulfate and nitrate, followed by Central Europe and former U.S.S.R. in generally similar proportions.

Nitrogen availability is considered limiting for plant growth at the forest-tundra ecotone, and it might modulate ecosystem response to climate warming. The aim of this research was to compare the impact of climate, vegetation cover, and soil organic matter (SOM) chemistry on N mineralization rates at the forest-tundra ecotone. We therefore estimated N mineralization in mountain birch (Betula pubescens Ehrh. ssp. czerepanovii) forest and tundra soil across a broad-scale latitudinal gradient in Fennoscandia, which incorporated 4 research sites (Dovrefjell, Vassijaure, Abisko, and Joatka). During the summer period, ammonium was the dominant form of mineralized nitrogen in forest soils, while nitrate mineralization rates were higher at tundra sites during the winter. A negative regression relationship between an index of climatic continentality and N mineralization was found. Further, summer NH4+ mineralization rates increased with total N content in soils, while NO3- mineralization seemed to be associated with C availability. Our study showed markedly contrasting inorganic N release in forest and tundra soil, and that, although mineralization rates differed between the summer and winter period, the winter activity was relatively high and should not be ignored. We conclude that a shift in the forest-tundra ecotone in response to climate wan-ning will have stronger effects on nitrogen availability at these sites than the direct effects of warming.

Methanotrophy (the bacterial oxidation of CH4) in soils is the major biological sink for atmospheric CH4. Here we present results from a study designed to quantify the role of the physical diffusion barrier to CH4, through surface soils, as a factor affecting methanotrophy. We used the mountain birch forest-tundra heath ecotone in subarctic northern Sweden as our study system. Our results show that, although CH4 fluxes were generally low (around -20 mu mol m(-2) h(-1); a net flux from atmosphere to soil), the two adjacent communities responded in contrasting ways to in situ experimental reduction of the diffusion barrier (removal of the top 50 mm of soil): Uptake increased by 40% in forest soil in association with the removal, whereas it decreased marginally (by 10%) in tundra heath. Investigations of the depth-distribution of CH4 oxidation in vitro revealed maximum rates at the top of the mineral soil for the forest site, whereas at the tundra heath this was more evenly spread throughout the organic horizon. The contrasting physicochemical properties and methanotroph activity in the organic horizons together explain the contrasting responses to the removal treatment. They also illustrate the potential role of vegetation for methane oxidation around this ecotone, exerted through its influence on the depth and properties of the organic horizons in these subarctic soils.

Nitrogen (N) fixation, denitrification, and ecosystem pools of nitrogen were measured in three subarctic ecosystem types differing in soil frost-heaving activity and vegetation cover. N-2-fixation was measured by the acetylene reduction assay and convened to absolute N ecosystem input by estimates of conversion factors between acetylene reduction and N-15 incorporation. One aim was to relate nitrogen fluxes and nitrogen pools to the mosaic of ecosystem types of different stability common in areas of soil frost movements. A second aim was to identify abiotic controls on N2-fixation by simultaneous measurements of temperature, light, and soil moisture. Nitrogen fixation rate was high with seasonal input estimated at 1.1 g N m(-2) on frost-heaved sorted circles, which was higher than the total plant N content and exceeded estimated annual plant N uptake several-fold but was lower than the microbial N content. Seasonal fixation decreased to 0.88 g N m(-2) on frost-heaved moss-covered surfaces and to 0.25 g N m(-2) in stable heath vegetation, both lower than the plant and microbial N content. Yet fixation was estimated to correspond to about 2.7 times the annual plant N demand on the moss-covered surfaces but less than the plants’ demand on the heath. Surprisingly, we found no denitrification on any surface. Climatic changes in the Arctic will generate a warmer climate and change precipitation patterns. A warmer, drier environment will decrease N2-fixation and thereby N availability to plants and microorganisms, while wetter conditions probably will increase N2-fixation and thereby N supply to the surroundings.

Extrapolating biosphere-atmosphere CO2 flux observations to larger scales in space, part of the so-called "upscaling" problem, is a central challenge for surface-atmosphere exchange research. Upscaling CO2 flux in tundra is complicated by the pronounced spatial variability of vegetation cover. We demonstrate that a simple model based on chamber observations with a pan-Arctic parameterization accurately describes up to 75% of the observed temporal variability of eddy covariance?measured net ecosystem exchange (NEE) during the growing season in an Abisko, Sweden, subarctic tundra ecosystem, and differed from NEE observations by less than 4% for the month of June. These results contrast with previous studies that found a 60% discrepancy between upscaled chamber and eddy covariance NEE sums. Sampling an aircraft-measured normalized difference vegetation index (NDVI) map for leaf area index (L) estimates using a dynamic flux footprint model explained less of the variability of NEE across the late June to mid-September period, but resulted in a lower root mean squared error and better replicated large flux events. Findings suggest that ecosystem structure via L is a critical input for modeling CO2 flux in tundra during the growing season. Future research should focus on quantifying microclimate, namely photosynthetically active radiation and air temperature, as well as ecosystem structure via L, to accurately model growing season tundra CO2 flux at chamber and plot scales.

Subarctic peatlands are rich sources of organic carbon for freshwater ecosystems. Where those peatlands are underlain by permafrost, permafrost thaw may cause an initial release of bioavailable dissolved organic carbon (DOC) to surrounding freshwaters. In this study, we measured icebound and potentially leachable (extracted) DOC quantities and indices of DOC quality in active layer and permafrost layers from two subarctic peat mires, Stord-alen and Storflaket. Most of the permafrost layers did not contain more organic matter or exportable DOC (as g kg(-1) dry soil) than the overlying active layer, and there was no difference in aromaticity, molecular weight, or the ratio between labile and recalcitrant DOC extracted from the permafrost and active layer. However, DOC held in segregated ice of the near-surface permafrost had relatively low aromaticity compared to extracted DOC from the same depth. Total icebound and potentially leachable DOC in the Stordalen mire permafrost that is predicted to experience active layer deepening during each of the next 50 years corresponded to about 0.1% of the current annual aquatic export of DOC from the mire. We conclude that the pool of potentially leachable DOC currently stored in permafrost layers is small. We also highlight differences in permafrost organic material between the two studied mire systems, which has an effect on the pool and properties of leachable DOC that is potentially available for export during thaw. Moreover, the geomorphological form of permafrost thaw will influence future hydrological connectedness and DOC production, in turn determining future DOC export from the mires.

Abstract In subarctic Sweden, recent decadal colonization and expansion of aspen (Populus tremula L.) were recorded. Over the past 100 years, aspen became c. 16 times more abundant, mainly as a result of increased sexual regeneration. Moreover, aspen now reach tree-size (>2 m) at the alpine treeline, an ecotone that has been dominated by mountain birch (Betula pubescens ssp. czerepanovii) for at least the past 4000 years. We found that sexual regeneration in aspen probably occurred seven times or more within the last century. Whereas sexual regeneration occurred during moist years following a year with an exceptionally high June?July temperature, asexual regeneration was favored by warm and dry summers. Disturbance to the birch forest by cyclic moth population outbreaks was critical in aspen establishment in the subalpine area. At the treeline, aspen colonization was less determined by these moth outbreaks, and was mainly restricted by summer temperature. If summer warming persists, aspen spread may continue in subarctic Sweden, particularly at the treeline. However, changing disturbance regimes, future herbivore population dynamics and the responses of aspen's competitors birch and pine to a changing climate may result in different outcomes.

Soil nutrient supply is likely to change in the Arctic due to altered process rates associated with climate change. Here, we compare the responses of herbaceous tundra and birch forest understory to fertilization, considering both above-and below-ground responses. We added nitrogen and phosphorus to plots in both vegetation types for three years near Abisko, northern Sweden, and measured the effect on above-and below-ground plant community properties and soil characteristics. Fertilization increased ground-layer shoot mass, the cover of grasses, and tended to enhance total root length below-ground, while it reduced the cover of low statured deciduous dwarf-shrubs. The only statistically significant interaction between vegetation type and fertilization was for grass cover, which increased twofold in forest understory but sixfold in tundra following fertilization. The lack of interactions for other variables suggests that the ground layers in these contrasting vegetation types have similar responses to fertilization. The nutrient-driven increase in grass cover and species-specific differences in productivity and root characters may alter ecosystem dynamics and C cycling in the long-term, but our study indicates that the response of birch forest understory and tundra vegetation may be consistent.

Two different biostratigraphic approaches are used to identify Marine Isotope Stage 11 (MIS 11) in Arctic Ocean sediments. On the Lomonosov Ridge, globally calibrated nannofossil bioevents constrain the age of sediments back to MIS 13 (Core LOMROG12-3PC). In the Amerasian Basin the unique occurrence of the planktonic foraminifer Turborotalita egelida is increasingly used as a marker for MIS 11. However, the T. egelida horizon has only been dated using cyclostratigraphy. Here we bridge these approaches through investigation of a new core (AO16-8GC) from the Alpha Ridge, Amerasian Basin. AO16-8GC is easily correlated to LOMROG12-3PC and contains the T. egelida horizon, allowing the first comparison between the biostratigraphy of both regions. Based on the nannofossil biochronology of LOMROG12-3PC, the most convincing lithologic correlation between the Alpha and Lomonosov Ridge cores places the T. egelida horizon between MIS 15 and MIS 17. This potentially older age for the T. egelida biohorizon emphasizes the need for continued caution in interpreting paleoceanographic records predating MIS 6, until further work can reconcile the nanno- and microfossil biostratigraphies that are emerging for middle Pleistocene sediments of the central Arctic Ocean.

The alpine treeline in northern Fennoscandia is composed primarily of mountain birch (Betula pubescens ssp. czerepanovii), a deciduous tree that experiences episodic defoliation due to outbreaks of the autumnal moth (Epirrita autumnata) and winter moth (Operophtera brumata). Here, we use an extensive dendroecological data set to reconstruct historic defoliating outbreaks and relate them to climatic conditions. Our data are from 25 sites in eight valleys in northern Sweden. We used the computer program OUTBREAK to reconstruct moth outbreaks. The reconstructed outbreak record matches the historical record well. There is a significant, but weak relationship between the outbreak severity and temperatures in February, April, July, and August of the outbreak year. Temperatures in the previous May and November were also positively correlated with outbreak severity. For seasonally aggregated temperatures, only autumn temperatures are correlated with outbreak severity. There was no significant correlation between NAO index and outbreak severity. A spatiotemporal semivariogram analysis showed that sites within approximately 100 km of each other show similar patterns in outbreak severity. Our analyses suggest that moths are affected by climatic variations. The influence of climate on outbreaks is weak because background climatic conditions alone cannot induce an outbreak. Outbreaks also depend on nonclimatic factors, such as tree age, and the outbreak status of neighboring areas.