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

Abstract Widespread increases in the productivity of tundra ecosystems and static trends – or even declines – in boreal ecosystems have been detected since the early 1980s using coarse-scale remote sensing. However, intermediate-scale Landsat studies have shown that these changes are heterogeneous and may be related to landscape and regional variability in climate, land cover, topography and moisture availability. In this study, a Landsat Normalized Difference Vegetation Index (NDVI) time-series (1984–2016) was examined for an area spanning the transition from sub-Arctic boreal forest to Low Arctic tundra in central Canada. This was supplemented by analyses of relationships with a suite of environmental variables and in situ measurements of bulk vegetation volume. Results show that NDVI trends were generally positive (i.e. increasing) across the study area but were smallest in the forest zone and largest in the northern tundra zone. More than one-quarter (27%) of un-masked pixels exhibited a significant (p < 0.05) trend and virtually all (99.3%) of those pixels exhibited an increasing, or "greening", trend. Greening pixels were most common in the northern tundra zone and the southern ecotone zone. Random Forest modeling of the relationship between NDVI and environmental variables indicated that the magnitude and direction of trends varied across the forest to tundra transition. Areas that experienced larger increases in NDVI include: (i) areas where summer temperatures increased; (ii) areas exhibiting predominantly shrub and forest cover; and (iii) locations closer to major drainage systems, further from major lakes, and at lower elevations. Ground validation in the central portion of the study area reveals a strong relationship (R2 = 0.79) between vegetation volume and NDVI for non-tree functional groups and that alder (Alnus crispa) shrublands and open spruce (Picea mariana and P. glauca) woodland with shrubby understories were most likely to exhibit greening. These findings indicate that the largest positive and more significant NDVI trends were associated with increased productivity in shrub-dominated environments, especially at, and north of the treeline in localities with favorable growing conditions. Smaller and less significant NDVI trends in boreal forest environments south of the treeline were likely associated with long-term successional change following disturbance rather than the variables analyzed here. Highlights • A 33-year Landsat NDVI time series was generated across a latitudinal gradient. • 25% of the study area experienced greening, particularly at higher latitudes. • Non-tree vegetation volume measurements had a strong relationship with NDVI. • Dense shrublands and open woodlands showed the highest levels of greening. • Landscape variables influenced NDVI trends differently across the study area. [ABSTRACT FROM AUTHOR]

Arctic ecosystems exhibit functional convergence in Net Ecosystem Exchange (NEE’s) of CO 2 response to light, air temperature, the Normalized Difference Vegetation Index (NDVI), and Leaf Area Index (LAI), potentially simplifying predictions of climate change impacts on the arctic C cycle over space and time. This convergence is hypothesized to be induced by tightly coupled carbon and nitrogen cycles, but has never been explicitly tested. We used model-data fusion on a mass balance model (i.e. the Coupled Carbon and Nitrogen; CCaN) to determine whether functional convergence in NEE results from tightly coupled carbon (C) and nitrogen (N) cycles. CCaN captured a majority, but not all, of NEE and NDVI observations across eight growing seasons, and MODIS NDVI observations across a tundra latitudinal gradient in Alaska. The hypothesis of temporal functional convergence was challenged by model-data disagreements for NEE during shoulder seasons and low light. The hypothesis of spatial functional convergence was challenged by the underestimation of NDVI in warm tundra. CCaN structure and parameter uncertainty analyses revealed that factors controlling leaf litter rate, the proportion of N in leaves, and net N mineralization rate are critical knowledge gaps in predicting pan-arctic NEE and NDVI in a future warmer climate. [ABSTRACT FROM AUTHOR]

Fossil-bearing deposits in the Transantarctic Mountains, Antarctica indicate that, despite the cold nature of the continent’s climate, a tundra ecosystem grew during periods of ice sheet retreat in the mid to late Neogene (17–2.5 Ma), 480 km from the South Pole. To date, palaeotemperature reconstruction has been based only on biological ranges, thereby calling for a geochemical approach to understanding continental climate and environment. There is contradictory evidence in the fossil record as to whether this flora was mixed angiosperm-conifer vegetation, or whether by this point conifers had disappeared from the continent. In order to address these questions, we have analysed, for the first time in sediments of this age, plant and bacterial biomarkers in terrestrial sediments from the Transantarctic Mountains to reconstruct past temperature and vegetation during a period of East Antarctic Ice Sheet retreat. From tetraether lipids (MBT′/CBT palaeothermometer), we conclude that the mean continental summer temperature was ca. 5 °C, in agreement with previous reconstructions. This was warm enough to have allowed woody vegetation to survive and reproduce even during the austral winter. Biomarkers from vascular plants indicate a low diversity and spatially variable flora consisting of higher plants, moss and algal mats growing in microenvironments in a glacial outwash system. Abietane-type compounds were abundant in some samples, indicating that conifers, most likely Podocarpaceae, grew on the Antarctic continent well into the Neogene. This is supported by the palynological record, but not the macrofossil record for the continent, and has implications for the evolution of vegetation on Antarctica. [ABSTRACT FROM AUTHOR]

In tundra ecosystems, global warming is expected to accelerate litter decomposition and to lead to shifts in vegetation composition. To understand these shifts, it is important to understand the interactions between global warming, vegetation composition, litter quality and decomposition in the tundra. In addition, it is important to consider root litter since roots are the major part of plant biomass in the tundra. In order to increase our understanding of decomposition, and root decomposition in particular, we performed a litter transplant experiment in northeastern Siberia, in which we measured mass loss for leaf and root litter (live and dead material) of the two dominant plant species, graminoid Eriophorum vaginatum and shrub Betula nana , in three vegetation types ( E. vaginatum or B. nana dominated and mixed vegetation) during the growing season. Our results show that although leaf decomposition did not differ between the two species, root decomposition showed significant differences. Mass loss of live roots was higher for E. vaginatum than for B. nana , but mass loss of E. vaginatum dead roots was lowest. In addition, we found evidence for home-field advantage in litter decomposition: litter of a plant decomposed faster in vegetation where it was dominant. Mass loss rates of the litter types were significantly correlated with phosphorus content, rather than nitrogen content. This indicates that phosphorus limits decomposition in this tundra site. The low decomposition rate of B. nana live roots compared to E. vaginatum live roots suggests that the acceleration of decomposition in the Arctic may be partly counteracted by the expected expansion of shrubs. However, more information on litter input rates and direct effects of climate change on decomposition rates are needed to accurately predict the effects of climate change on carbon dynamics in tundra ecosystems. [ABSTRACT FROM AUTHOR]

Coastal tundra ecosystems are relatively flat, and yet display large spatial variability in ecosystem traits. The microtopographical differences in polygonal geomorphology produce heterogeneity in permafrost depth, soil temperature, soil moisture, soil geochemistry, and plant distribution. Few measurements have been made, however, of how water fluxes vary across polygonal tundra plant types, limiting our ability to understand and model these ecosystems. Our objective was to investigate how plant distribution and geomorphological location affect actual evapotranspiration (ET). These effects are especially critical in light of the rapid change polygonal tundra systems are experiencing with Arctic warming. At a field site near Barrow, Alaska, USA, we investigated the relationships between ET and plant cover in 2014 and 2015. ET was measured at a range of spatial and temporal scales using: (1) An eddy covariance flux tower for continuous landscape-scale monitoring; (2) An automated clear surface chamber over dry vegetation in a fixed location for continuous plot-scale monitoring; and (3) Manual measurements with a clear portable chamber in approximately 60 locations across the landscape. We found that variation in environmental conditions and plant community composition, driven by microtopographical features, has significant influence on ET. Among plant types, ET from moss-covered and inundated areas was more than twice that from other plant types. ET from troughs and low polygonal centers was significantly higher than from high polygonal centers. ET varied seasonally, with peak fluxes of 0.14 mm h −1 in July. Despite 24 hours of daylight in summer, diurnal fluctuations in incoming solar radiation and plant processes produced a diurnal cycle in ET. Combining the patterns we observed with projections for the impact of permafrost degradation on polygonal structure suggests that microtopographic changes associated with permafrost thaw have the potential to alter tundra ecosystem ET. [ABSTRACT FROM AUTHOR]

Leaf wax hydrogen isotopes (δD wax ) are increasingly utilized in terrestrial paleoclimate research. Applications of this proxy must be grounded by studies of the modern controls on δD wax , including the ecophysiological controls on isotope fractionation at both the plant and landscape scales. Several calibration studies suggest a considerably smaller apparent fractionation between source water and waxes ( ε app ) at high latitudes relative to temperate or tropical locations, with major implications for paleoclimatic interpretations of sedimentary δD wax . Here we investigate apparent fractionation in the Arctic by tracing the isotopic composition of leaf waxes from production in modern plants to deposition in lake sediments using isotopic observations of precipitation, soil and plant waters, living leaf waxes, and waxes in sediment traps in the Brooks Range foothills of northern Alaska. We also analyze a lake surface sediment transect to compare present-day vegetation assemblages to ε app at the watershed scale. Source water and ε app were determined for live specimens of Eriophorum vaginatum (cottongrass) and Betula nana (dwarf birch), two dominant tundra plants in the Brooks Range foothills. The δD of these plants’ xylem water closely tracks that of surface soil water, and reflects a summer-biased precipitation source. Leaf water is enriched by 23 ± 15‰ relative to xylem water for E. vaginatum and by 41 ± 19‰ for B. nana. Evapotranspiration modeling indicates that this leaf water enrichment is consistent with the evaporative enrichment expected under the climate conditions of northern Alaska, and that 24-h photosynthesis does not cause excessive leaf water isotope enrichment. The ε app determined for our study species average −89 ± 14‰ and −106 ± 16‰ for B. nana n -alkanes and n -acids, respectively, and −182 ± 10‰ and −154 ± 26‰ for E. vaginatum n -alkanes and n -acids, which are similar to the ε app of related species in temperate and tropical regions, indicating that apparent fractionation is similar in Arctic relative to other regions, and there is no reduced fractionation in the Arctic. Sediment trap data suggest that waxes are primarily transported into lakes from local (watershed-scale) sources by overland flow during the spring freshet, and so δD wax within lakes depends on watershed-scale differences in water isotope compositions and in plant ecophysiology. As such, the large difference between our study species suggests that the relative abundance of graminoids and shrubs is potentially an important control on δD wax in lake sediments. These inferences are supported by δD wax data from surface sediments of 24 lakes where ε app , relative to δD xylem , averages −128 ± 13‰ and −130 ± 8‰ for n -acids and n -alkanes, respectively, and co-varies with vegetation type across watersheds. These new determinations of plant source water seasonality and ε app for the Arctic will improve the δD wax paleoclimate proxy at high latitudes. [ABSTRACT FROM AUTHOR]

Rhododendron ferrugineum L. is a widespread dwarf shrub species growing in high-elevation, alpine environments of the Western European Alps. For this reason, analysis of its growth rings offers unique opportunities to push current dendrochronological networks into extreme environments and way beyond the treeline. Given that different species of the same genus have been successfully used in tree-ring investigations, notably in the Himalayas where Rhododendron spp. has proven to be a reliable climate proxy, this study aims at (i) evaluating the dendroclimatological potential of R. ferrugineum and at (ii) determining the major limiting climate factor driving its growth. To this end, 154 cross-sections from 36 R. ferrugineum individuals have been sampled above local treelines and at elevations from 1800 to 2100 m asl on northwest-facing slopes of the Taillefer massif (French Alps). We illustrate a 195-year-long standard chronology based on growth-ring records from 24 R. ferrugineum individuals, and document that the series is well-replicated for almost one century (1920–2015) with an Expressed Population Signal (EPS) > 0.85. Analyses using partial and moving 3-months correlation functions further highlight that growth of R. ferrugineum is governed by temperatures during the growing season (May–July), with increasingly higher air temperatures favoring wider rings, a phenomenon which is well known from dwarf shrubs growing in circum-arctic tundra ecosystems. Similarly, the negative effect of January–February precipitation on radial growth of R. ferrugineum , already observed in the Alps on juniper shrubs, is interpreted as a result of shortened growing seasons following snowy winters. We conclude that the strong and unequivocal signals recorded in the fairly long R. ferrugineum chronologies can indeed be used for climate–growth studies as well as for the reconstruction of climatic fluctuations in Alpine regions beyond the upper limits of present-day forests. [ABSTRACT FROM AUTHOR]

Cryosols contain ~ 33% of the global soil organic carbon. Cryosol warming and permafrost degradation may enhance the CO 2 release to the atmosphere through the microbial decomposition. Despite the large carbon pool, the permafrost carbon feedback on the climate remains uncertain. In this study, we aimed at better understanding the diurnal evolution of the temperature sensitivity of CO 2 efflux in Cryosols. A Histic Cryosol and a Turbic Cryosol were instrumented in tussock tundra ecosystems near Salluit (Nunavik, Canada). Open top chambers were installed during summer 2011 and the ground temperature, the soil moisture and meteorological variables were recorded hourly while the ecosystem respiration was measured three times per day every second day with opaque and closed dynamic chambers in control and warm stations. Despite warmer conditions, the average CO 2 efflux at the control stations at the Histic site (1.29 ± 0.45 μmolCO 2 m − 2 s − 1 ) was lower than at the Turbic site (2.30 ± 0.74 μmolCO 2 m − 2 s − 1 ). The increase in CO 2 efflux with warming was greater in the Histic Cryosol (~ 39%) than in the Turbic Cryosol (~ 16%). Our study showed that the temperature sensitivity of the ecosystem respiration evolved during the day and decreased with the experimental warming. Both sites exhibited diurnal hysteresis loops between CO 2 efflux and the soil surface temperature. The width of hysteresis loops increased with the solar radiation and decreased along the growing season. We developed simple linear models that took into account the diurnal evolution of the temperature sensitivity of CO 2 efflux and we estimated the seasonal cumulative carbon release to the atmosphere. The calculation using solely diurnal measurements significantly differed from the seasonal carbon release modelled hourly. Our study highlighted that the time of the day when measurements are performed should be taken into account to accurately estimate the seasonal carbon release from tundra ecosystems. [ABSTRACT FROM AUTHOR]

Soil is a significant source of CO 2 emission to the atmosphere, and this process is accelerating at high latitudes due to rapidly changing climates. To investigate the sensitivity of soil CO 2 emissions to high temporal frequency variations in climate, we performed continuous monitoring of soil CO 2 efflux using Forced Diffusion (FD) chambers at half-hour intervals, across three representative Alaskan soil cover types with underlying permafrost. These sites were established during the growing season of 2015, on the Seward Peninsula of western Alaska. Our chamber system is conceptually similar to a dynamic chamber, though FD is more durable and water-resistant and consumes less power, lending itself to remote deployments. We first conducted methodological tests, testing different frequencies of measurement, and did not observe a significant difference between collecting data at 30-min and 10-min measurement intervals (averaged half-hourly) ( p < 0.001). Temperature and thaw depth, meanwhile, are important parameters in influencing soil carbon emission. At the study sites, we observed cumulative soil CO 2 emissions of 62.0, 126.3, and 133.5 gC m − 2 for the growing period, in sphagnum, lichen, and tussock, respectively, corresponding to 83.8, 63.7, and 79.6% of annual carbon emissions. Growing season soil carbon emissions extrapolated over the region equated to 0.17 ± 0.06 MgC over the measurement period. This was 47% higher than previous estimates from coarse-resolution manual chamber sampling, presumably because it better captured high efflux events. This finding demonstrates how differences in measurement method and frequency can impact interpretations of seasonal and annual soil carbon budgets. We conclude that annual CO 2 efflux-measurements using FD chamber networks would be an effective means for quantifying growing and non-growing season soil carbon budgets, with optimal pairing with time-lapse imagery for tracking local and regional changes in environment and climate in a warming Arctic. [ABSTRACT FROM AUTHOR]

Accurate monitoring of climate-driven expansion of low-stature shrubs in Arctic tundra requires high-resolution maps of shrub biomass that can accurately quantify the current baseline over relevant spatial and temporal extents. In this study, our goal was to use airborne lidar and imagery to build accurate high-resolution shrub biomass maps for an important research landscape in the American Arctic. In a leave-one-out cross-validation analysis, optimized lidar-derived canopy volume was a good single predictor of harvested shrub biomass (R 2 = 0.62; RMSD = 219 g m − 2 ; slope = 1.08). However, model accuracy was improved by incorporating additional lidar-derived canopy metrics and airborne spectral metrics in a Random Forest regression approach (pseudo R 2 = 0.71; RMSD = 197 g m − 2 ; slope = 1.02). The best Random Forest model was used to map shrub biomass at 0.80 m resolution across three lidar collection footprints (~ 12.5 km 2 total) near Toolik Field Station on Alaska's North Slope. We characterized model uncertainty by creating corresponding maps of the coefficient of variation in Random Forest shrub biomass estimates. We also explore potential benefits of incorporating lidar-derived topographic metrics, and consider tradeoffs inherent in employing different data sources for high-resolution vegetation mapping efforts. This study yielded maps that provide valuable, high-resolution spatial estimates of aboveground shrub biomass and canopy volume in a rapidly changing tundra ecosystem. [ABSTRACT FROM AUTHOR]

Ecosystem respiration (ER) is a significant source in estimating terrestrial carbon budget under climate change. Here, we report on the assessment of spatial characteristics of ER, using manual chamber over three tundra ecosystems of Alaska. Annual simulated ER was 254–307 g CO 2 m −2 based on in-situ air temperature and 212–305 g CO 2 m −2 based on soil temperature, at Council, North Slope, and Arctic National Wildlife Refuge (ANWR) sites of Alaska. Growing-season ERs correspond to 79–92% (air temperature) and 81–86% (soil temperature) of simulated annual ER. Hence, soil temperature is a significant driver in modulating ER over tundra, suggesting soil temperature elucidates more than 80% of air temperature. At Council, between 31 and 84 sampling points during the growing season were required to attain spatial representativeness for ER, falling within ±10% of the full sample mean, with a 95% confidence level. At North Slope and ANWR sites, the number of sampling points was chosen to yield results within at least ±20%, with a 90% confidence level. These findings suggest that larger-size chamber and its measurement frequency can overcome logistical constraints and determine mean ER at tundra sites for the quantitative assessment of the tundra carbon budget in response to drastically changing Arctic environment and climate. [ABSTRACT FROM AUTHOR]

Arctic ecosystems are threatened by pollution from recently detected extreme atmospheric nitrogen (N) deposition events in which up to 90% of the annual N deposition can occur in just a few days. We undertook the first assessment of the fate of N from extreme deposition in High Arctic tundra and are presenting the results from the whole ecosystem 15 N labelling experiment. In 2010, we simulated N depositions at rates of 0, 0.04, 0.4 and 1.2 g N m − 2 yr − 1 , applied as 15 NH 4 15 NO 3 in Svalbard (79 ° N), during the summer. Separate applications of 15 NO 3 − and 15 NH 4 + were also made to determine the importance of N form in their retention. More than 95% of the total 15 N applied was recovered after one growing season (~ 90% after two), demonstrating a considerable capacity of Arctic tundra to retain N from these deposition events. Important sinks for the deposited N, regardless of its application rate or form, were non-vascular plants > vascular plants > organic soil > litter > mineral soil, suggesting that non-vascular plants could be the primary component of this ecosystem to undergo measurable changes due to N enrichment from extreme deposition events. Substantial retention of N by soil microbial biomass (70% and 39% of 15 N in organic and mineral horizon, respectively) during the initial partitioning demonstrated their capacity to act as effective buffers for N leaching. Between the two N forms, vascular plants ( Salix polaris ) in particular showed difference in their N recovery, incorporating four times greater 15 NO 3 − than 15 NH 4 + , suggesting deposition rich in nitrate will impact them more. Overall, these findings show that despite the deposition rates being extreme in statistical terms, biologically they do not exceed the capacity of tundra to sequester pollutant N during the growing season. Therefore, current and future extreme events may represent a major source of eutrophication. [ABSTRACT FROM AUTHOR]

The latitudinal gradient of the start of the growing season (SOS) and the end of the growing season (EOS) were quantified in Alaska (61°N to 71°N) using satellite-based and ground-based datasets. The Alaskan evergreen needleleaf forests are sparse and the understory vegetation has a substantial impact on the satellite signal. We evaluated SOS and EOS of understory and tundra vegetation using time-lapse camera images. From the comparison of three SOS algorithms for determining SOS from two satellite datasets (SPOT-VEGETATION and Terra-MODIS), we found that the satellite-based SOS timing was consistent with the leaf emergence of the forest understory and tundra vegetation. The ensemble average of SOS over all satellite algorithms can be used as a measure of spring leaf emergence for understory and tundra vegetation. In contrast, the relationship between the ground-based and satellite-based EOSs was not as strong as that of SOS both for boreal forest and tundra sites because of the large biases between those two EOSs (19 to 26 days). The satellite-based EOS was more relevant to snowfall events than the senescence of understory or tundra. The plant canopy radiative transfer simulation suggested that 84–86% of the NDVI seasonal amplitude could be a reasonable threshold for the EOS determination. The latitudinal gradients of SOS and EOS evaluated by the satellite and ground data were consistent and the satellite-derived SOS and EOS were 3.5 to 5.7 days degree − 1 and − 2.3 to − 2.7 days degree − 1 , which corresponded to the spring (May) temperature sensitivity of − 2.5 to − 3.9 days °C − 1 in SOS and the autumn (August and September) temperature sensitivity of 3.0 to 4.6 days °C − 1 in EOS. This demonstrates the possible impact of phenology in spruce forest understory and tundra ecosystems in response to climate change in the warming Artic and sub-Arctic regions. [ABSTRACT FROM AUTHOR]

Non-growing season soil CO 2 emissions are very important part of the annual C balance in the Arctic tundra ecosystems, but very limited information is available for the permafrost region of the Qinghai-Tibet Plateau, which hold great quantities of C. We conducted a full year measurement of soil CO 2 flux in an alpine meadow and swamp meadow to quantify seasonal dynamics of non-growing season (include winter and initial thaw and freeze period (TFP)) soil CO 2 flux and its contribution to annual soil CO 2 emissions. Mean soil CO 2 flux was higher in TFP (0.38–0.54 μmol m −2 s −1 ) than in winter (0.11–0.23 μmol m −2 s −1 ), with significant higher in swamp meadow than alpine meadow in the non-growing season. Soil temperature explained 76–85% of the yearly variation in the soil CO 2 flux, with higher temperature sensitivity ( Q 10 ) in the TFP (5.67–9.43) than in other seasons (2.65–2.99). The cumulative non-growing season soil CO 2 emission was 228–358 g CO 2 m −2 , accounted for 25–26% of annual emissions. The integrated contribution of non-growing season to annual emission based on season-specific, annual and growing season Q 10 model were 26–27%, 32–34% and 44–45%, respectively, season-specific model may improve the accuracy of estimating the soil CO 2 emissions. Our results indicate that non-growing season contributes a large portion of the annual soil CO 2 emission in the permafrost region of the Qinghai-Tibet Plateau. Hence, estimating of annual C balance must consider the non-growing season soil CO 2 emission, and should not neglect the soil CO 2 emission of swamp meadow. [ABSTRACT FROM AUTHOR]

Tundra wetlands receive primary treated municipal wastewater in many communities in the Canadian Arctic and have demonstrated the ability to improve water quality. The kinetics of treatment within these wetlands has not been previously characterized which contributes to uncertainties in performance expectations. A case study was conducted on a tundra wetland area receiving primary treated municipal wastewater in Coral Harbour, Nunavut, Canada. The objectives of the study were to: (i) develop a tanks-in-series (TIS) performance model to determine first order areal rate constants ( k ) for a tundra wetland treatment area; and (ii) compare the rate constants to published literature on treatment wetlands operating in more temperate climates. This study serves as a proof of concept and initial derivation of rate constants characteristic of a tundra wetland treatment area. A TIS performance model was modified to account for external hydrologic contributions. Tracer studies, hydraulic measurements, and water quality data were used to parameterize the model and fit first order rate constants for several wastewater parameters. Dilution from the external hydrologic contributions from the watershed of the wetland accounted for approximately 33% of the contaminant reductions observed. The fitted k values normalized to 20 °C, and percentiles compared to literature, ranged from: 4.4–120 m/y (5th-70th) for CBOD 5 ; 78 –887 m/y (40th–90th) for E. coli ; 2.7 20 m/y (10th–60th) for TN; and 12–64 m/y (40th–80th) for TAN. In general, rate constants for this arctic tundra wetland were comparable to low rate constants derived from wetlands operating in non-arctic climates. [ABSTRACT FROM AUTHOR]

Changes in CO 2 and CH 4 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 CO 2 and CH 4 , 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 CH 4 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, CO 2 emission in the dry section was 3.9-fold higher than in others. CH 4 emission tends to increase with deeper thaw depth, which strongly depends on the water table; however, CO 2 emission is not related to thaw depth. Quotients of global warming potential (GWPCO 2 ) (dry/control) and GWPCH 4 (wet/control) increased by 464 and 148%, respectively, and GWPCH 4 (dry/control) declined by 66%. This suggests that CO 2 emission in a drained section is enhanced by soil and ecosystem respiration, and CH 4 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 CO 2 and CH 4 , 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 CH 4 release, and the disappearance of the lakes causes the stimulated CO 2 production in response to the Arctic climate change. [ABSTRACT FROM AUTHOR]

Shrubs have increased in abundance and dominance in arctic and alpine regions in recent decades. This often dramatic change, likely due to climate warming, has the potential to alter both the structure and function of tundra ecosystems. The analysis of shrub growth is improving our understanding of tundra vegetation dynamics and environmental changes. However, dendrochronological methods developed for trees, need to be adapted for the morphology and growth eccentricity of shrubs. Here, we review current and developing methods to measure radial and axial growth, estimate age, and assess growth dynamics in relation to environmental variables. Recent advances in sampling methods, analysis and applications have improved our ability to investigate growth and recruitment dynamics of shrubs. However, to extrapolate findings to the biome scale, future dendroecological work will require improved approaches that better address variation in growth within parts of the plant, among individuals within populations and between species. [ABSTRACT FROM AUTHOR]

Currently, tundra wetlands are used to treat municipal wastewater in many Arctic communities, typically following primary treatment from a wastewater stabilization pond. Inclusion of tundra wetlands as part of the municipal wastewater treatment strategy for the Canadian Arctic is an option to meet upcoming federal Wastewater Systems Effluent Regulations (WSER). Water quality improvement has been demonstrated in tundra wastewater treatment wetlands; however, the mechanisms responsible for this observed treatment are not well understood. In this study, we conducted a detailed physical, hydraulic, and biogeochemical analysis of a tundra wetland receiving primary treated municipal wastewater in Coral Harbour, Nunavut. The primary objective was to gain a better understanding of the processes which contribute to water quality improvement within these types of environmental systems. Data were collected during four study periods throughout the treatment season, between mid-June 2011 and early-September 2012. Study results illustrated seasonal variability in the treatment performance and hydraulic characteristics of the wetland. Hydraulic residence times (HRTs) as short as 11 h were observed during the spring freshet, and as long as 14 days in mid-summer. Short HRTs in the spring freshet suggested the observed reduction of wastewater constituents was primarily by dilution from watershed contributions. Biogeochemical data showed seasonal increases in pH and DO, which were driven by algae growth, which corresponded to increases in TSS, and decreases in E. coli and nitrogen species. During the entire treatment season, the wetland effluent met the WSER, which does not currently include Nunavut, Canada. Contaminant concentration reductions were variable over the treatment season with the minimum reductions observed during the spring freshet (92.5% of BOD 5 , 3.7 log of E. coli , 95.4% of TSS, 77.3% of TN, 73.7% of TAN, 46.2% of NH 3 –N, and 78.9% of TP). Requisites for the responsible use of a wetland for wastewater treatment in an arctic setting should include the characterization of hydrological setting to quantify how external hydrologic inputs will influence treatment performance. [ABSTRACT FROM AUTHOR]

Soil microorganisms are critical to carbon and nutrient fluxes in terrestrial ecosystems. Understanding the annual pattern of soil microbial community structure and how it corresponds to soil nutrient availability and plant production is a fundamental first step towards being able to predict impacts of environmental change on ecosystem functioning. We investigated the composition, structure and nutrient stoichiometry of the soil microbial community in mesic arctic tundra on 9 sample dates in 6 months from winter to fall using phospholipid fatty acid analysis (PLFA), quantitative polymerase chain reaction (qPCR), epifluorescent microscopy and chloroform-fumigation–extraction (CFE). PLFA analysis indicates that the winter microbial community was fungal-dominated, cold-adapted and associated with high C, N and P in the soil solution and microbial biomass. The microscopy data suggest that both bacteria and fungi were active and growing in soils between −5 °C and 0 °C. A significant shift occurred in the PLFA data, qPCR patterns, microscopy and microbial biogeochemistry after the thaw period, resulting in a distinct community that persisted through our spring, summer and fall sample dates, despite large changes in plant productivity. This shift was characterised by increasing relative abundances of certain bacteria (especially Gram +ves) as well as a decline in fungal biomass, and corresponded with decreasing C, N and P in the soil solution. The summer period of low substrate availability (plant–microbe competition) was associated with microbial indicators of nutritional stress. Overall, our results indicate that tundra microbial communities are clearly differentiated according to the changes in soil nutrient status and environmental conditions that occur between winter and post-thaw, and that those changes reflect functionally important adaptations to those conditions. [ABSTRACT FROM AUTHOR]

Abstract: The size and composition of soil microbial communities have important influences on terrestrial ecosystem processes such as soil decomposition. However, compared with studies of aboveground plant communities, there are relatively few studies on belowground microbial communities and their interactions with aboveground vegetations in the arctic region. In this study, we conducted the first investigation of the abundance and composition of prokaryotic communities along small-scale vegetation gradients (ca. 1–3 m) in a dry arctic tundra ecosystem in Northern Sweden using fluorescent in situ hybridization (FISH) coupled with catalyzed reporter deposition (CARD). The number of prokaryotic cells increased with increasing vegetation cover along vegetation gradients, mainly as a function of increased amounts of soil carbon and moisture. Eubacteria and Archaea constituted approximately 59.7% and 33.4% of DAPI-positive cells, respectively. Among the analyzed bacterial phyla and sub-phyla, Acidobacteria and α-proteobacteria were the most dominant groups, constituting approximately 13.5% and 10.7% of DAPI-positive cells, respectively. Interestingly, the soil prokaryotic community composition was relatively unaffected by the dramatic changes in the aboveground vegetation community. Multivariate analyses suggested that the prokaryotic community composition depended on soil pH rather than on aboveground vegetation. Surface plants are weak predictors of the composition of the soil microbial community in the studied soil system and the size of the community is constrained by carbon and water availability. In addition, our study demonstrated that CARD-FISH, which is still a rarely-used technique in soil ecology, is effective for quantifying soil microbes. [Copyright &y& Elsevier]

Arctic ecosystems play a key role in the terrestrial carbon cycle. Our aim was to combine satellite-based normalized difference vegetation index (NDVI) with field measurements of CO2 fluxes to investigate changes in gross primary production (GPP) for the peak growing seasons 1992–2008 in Rylekærene, a wet tundra ecosystem in the Zackenberg valley, north-eastern Greenland. A method to incorporate controls on GPP through satellite data is the light use efficiency (LUE) model, here expressed as GPP= ɛ peak ×PARin ×FAPARgreen_peak; where ɛ peak was peak growing season light use efficiency of the vegetation, PARin was incoming photosynthetically active radiation, and FAPARgreen_peak was peak growing season fraction of PAR absorbed by the green vegetation. The ɛ peak was measured for seven different high-Arctic plant communities in the field, and it was on average 1.63g CO2 MJ−1. We found a significant linear relationship between FAPARgreen_peak measured in the field and satellite-based NDVI. The linear regression was applied to peak growing season NDVI 1992–2008 and derived FAPARgreen_peak was entered into the LUE-model. It was shown that when several empirical models are combined, propagation errors are introduced, which results in considerable model uncertainties. The LUE-model was evaluated against field-measured GPP and the model captured field-measured GPP well (RMSE was 192mg CO2 m−2 h−1). The model showed an increase in peak growing season GPP of 42mg CO2 m−2 h−1 y−1 in Rylekærene 1992–2008. There was also a strong increase in air temperature (0.15°Cy−1), indicating that the GPP trend may have been climate driven. [ABSTRACT FROM AUTHOR]

Abstract: The effect of ongoing climate change on sub-arctic and alpine forests has led to increased interest in monitoring potential changes in the forest–tundra ecotone. In addition to climate change, insect damage, browsing pressure by herbivores such as moose and reindeer, as well as anthropogenic impacts will contribute to changes in the forest–tundra ecotone. These changes are difficult to monitor with manual methods because of the complex mosaic pattern of the ecotone. In this study, the possibility to predict maximum tree height, above ground tree biomass and canopy cover with airborne laser scanning (ALS) was therefore tested at a forest–tundra ecotone site near Abisko in northern Sweden (Lat. N 68°20′, Long. E 19°01′, 420-700ma.s.l.). The forest in the area is dominated by mountain birch (Betula pubescens ssp. czerepanovii), which has highly irregular stem and canopy forms. Predictions from two different laser data acquisitions were compared. The first laser data set had 6.1pointsm−2 and was obtained in 2008 with a TopEye MKII scanner carried by a helicopter flown at 500m a.g.l. The second laser data set had 1.4pointsm−2 and was obtained in 2010 with an Optech ALTM Gemini scanner carried by a fixed-wing aircraft flown at 1740m a.g.l. Linear regression models were developed for the predictions using data from 73 sample plots with ten meter radius surveyed in 2009 and 2010. The relative RMSEs obtained for the TopEye and Optech data after leave-one-out cross-validation were, respectively, 8.8% and 9.5% for maximum tree height; 18.7% and 21.2% for above ground tree biomass; and, 16.8% and 18.7% for vertical canopy cover on plot level. The results were clearly improved by introducing a new procedure to compensate for unevenly distributed laser points. In conclusion, ALS has strong potential as a data source to map mountain birch biomass in the forest–tundra ecotone, even when using sparse point density ALS data. [Copyright &y& Elsevier]

Abstract: Due to the climate change, vegetation of tundra ecosystems is predicted to shift toward shrub and tree dominance, and this change may influence bryophytes. To estimate how changes in growing environment and the dominance of vascular plants influence bryophyte abundance, we compared the relationship of occurrence of bryophytes among other plant types in a five-year experiment of warming (T), fertilization (F) and T+F in two vegetation types, heath and meadow, in a subarctic–alpine ecosystem. We compared individual leaf area among shrub species to confirm that deciduous shrubs might cause severe shading effect. Effects of neighboring functional types on the performance of Hylocomium splendens was also analyzed. Results show that F and T+F treatments significantly influenced bryophyte abundance negatively. Under natural conditions, bryophytes in the heath site were negatively related to the abundance of shrubs and lichens and the relationship between lichens and bryophytes strengthened after the experimental period. After five years of experimental treatments in the meadow, a positive abundance relationship emerged between bryophytes and deciduous shrubs, evergreen shrubs and forbs. This relationship was not found in the heath site. Our study therefore shows that the abundance relationships between bryophytes and plants in two vegetation types within the same area can be different. Deciduous shrubs had larger leaf area than evergreen shrubs but did not show any shading effect on H. splendens. [Copyright &y& Elsevier]

Abstract: Industrial development, expansion of human populations and climate change increasingly affect habitats of migratory species. Effective protection of critical habitats is urgently required because several large migratory species have declined over the last decades. Protection of critical habitats, such as the calving grounds of migratory ungulates, may however require consideration of temporal shifts in spatial location. We assessed changes in the location of calving grounds used by migratory caribou over 35years by the Rivière-George (RG) herd and 15years by the Rivière-aux-Feuilles (RAF) herd, in Northern Québec and Labrador, Canada. We also evaluated the proportion of annual calving ground within protected Wildlife Habitats established to protect caribou calving grounds. The annual size and location of calving grounds changed substantially over time: calving ground size remained relatively stable for the RAF but it declined over 85% for the RG. Calving grounds moved 300km northward in the Ungava peninsula for the RAF and shifted over 230km back and forth to the Labrador coast for the RG. Despite recent modifications, legally designated Wildlife Habitats in Québec protected less than 20% of the RG and RAF calving grounds. Protection of calving grounds of migratory caribou must consider the dynamic use of space by adult females. We present recommendations on the use of available monitoring data to better protect calving grounds of migratory caribou and critical habitats of large migratory species. [Copyright &y& Elsevier]

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Abstract: Juvenile north-temperate and Arctic fishes are faced with trade-offs between energy allocation to growth and energy storage (primarily lipids) prior to over-wintering. We determined classical morphometric (fork length, body weight and condition factor) and biochemical (whole body triglycerides, muscle RNA/DNA ratio, muscle proteins) measures of growth and condition in individual young-of-the-year (YOY) Arctic grayling (Thymallus arcticus). Grayling were collected just prior to over-wintering in late August (approximately 50 days after swim-up) from two natural streams and five locations within a 3.4km long artificial stream constructed as a fish habitat compensation project and diversion channel for the diamond mining industry in Northwest Territories, Canada (64°45′N). Fork lengths, body weights and whole body triglyceride levels in grayling collected from all sites along the artificial stream were significantly lower than fish collected from one of the natural streams. Condition factor (weight-at-length) was not different among grayling collected from natural and artificial streams. Muscle proteins were lower in grayling collected from four sites along the artificial stream compared to the natural streams. In contrast, muscle RNA/DNA ratios were greater in grayling collected from two sites in the artificial stream compared to natural streams. There were no consistent differences in any variable among grayling collected at the five artificial stream sites or among grayling collected from the two natural streams. The higher RNA/DNA ratios and lower fork lengths, whole body triglycerides and muscle proteins in grayling inhabiting the artificial stream are consistent with energy still being primarily allocated to growth in these fish at this late stage of summer. Individuals that are both larger and possess greater energy storage in the form of triglycerides are more likely to survive the long over-wintering period at this latitude. Our results suggest that YOY grayling using the artificial stream as nursery habitat will likely face increased over-winter mortality, thus raising concerns over the use of fish presence, spawning and rearing as criteria for the initial success of artificial streams as habitat compensation measures in Arctic tundra regions. Further research is needed to determine the potential consequences of reduced size and energy storage in juvenile fishes in order to assess the viability of stream fish habitat compensation and restoration projects associated with industrial development in Arctic tundra regions. [Copyright &y& Elsevier]

Abstract: The surface temperature of permafrost soils in remote arctic areas is accessible by satellite land surface temperature (LST) detection. However, the spatial resolution of satellite measurements such as the MODIS LST products is limited and does not detect the heterogeneities of the wet polygonal tundra landscape where surface wetness varies over distances of several meters. This paper examines the spatial and temporal variability of summer surface temperatures of a polygonal tundra site in northern Siberia using a ground based high resolution thermal imaging system. Thermal infrared images were taken of a 1000m2 polygonal tundra area in 10min intervals from July to September 2008. Under clear sky conditions, the individual measurements indicate temperature differences of up to 6K between dry and wet tundra surfaces and which can exceed 12K when dry tundra and water surfaces are compared. These differences disappear when temperature averages are considered for intervals longer than the diurnal cycle; for weekly averages the spatial temperature variability decreases below 1K. The exception is the free water surface of a shallow polygonal pond where weekly averaged temperature differences of 2.5K are sustained compared to the tundra surface. The ground based thermal infrared images are upscaled to MODIS sized pixels and compared to available MODIS LST data for individual measurements and weekly averages. The comparisons show generally good agreement for the individual measurements under clear sky conditions, which exist during 20% of the studied time period. However, several erroneous measurements and large data gaps occur in the MODIS LST data during cloudy conditions, leading to biased weekly temperature averages inferred from the satellite observations. Based on these results the following recommendations are given for future permafrost temperature monitoring based on MODIS LST products: (i) high resolution surface water masks for the quality assessment in landscapes where lakes and ponds are frequent and (ii) reliable cloud cover detection in conjunction with a gap filling procedure for accurate temporal averages. [Copyright &y& Elsevier]

Abstract: Airborne and satellite brightness temperature (T B) measurements were combined with intensive field observations of sub-Arctic tundra snow cover to develop the framework for a new tundra-specific passive microwave snow water equivalent (SWE) retrieval algorithm. The dense snowpack and high sub-grid lake fraction across the tundra mean that conventional brightness temperature difference approaches (such as the commonly used 37GHz–19GHz) are not appropriate across the sub-Arctic. Airborne radiometer measurements (with footprint dimensions of approximately 70×120m) acquired across sub-Arctic Canada during three field campaigns during the 2008 winter season were utilized to illustrate a slope reversal in the 37GHz T B versus SWE relationship. Scattering by the tundra snowpack drives a negative relationship until a threshold SWE value is reached near 130mm at which point emission from the snowpack creates a positive but noisier relationship between 37GHz T B and SWE. The change from snowpack scattering to emission was also evident in the temporal evolution of 37GHz T B observed from satellite measurements. AMSR-E brightness temperatures (2002/03–2006/07) consistently exhibited decreases through the winter before reaching a minimum in February or March, followed by an increase for weeks or months before melt. The cumulative absolute change (Σ|Δ37V|) in vertically polarized 37GHz T B was computed at both monthly and pentad intervals from a January 1 start date and compared to ground measured SWE from intensive and regional snow survey campaigns, and climate station observations. A greater (lower) cumulative change in |Δ37V| was significantly related to greater (lower) ground measured SWE (r 2 =0.77 with monthly averages; r 2 =0.67 with pentad averages). Σ|Δ37V| was only weakly correlated with lake fraction: monthly r 2 values calculated for January through April 2003–2007 were largely less than 0.2. These results indicate that this is a computationally straightforward and viable algorithmic framework for producing tundra-specific SWE datasets from the complete satellite passive microwave record (1979 to present). [Copyright &y& Elsevier]

Abstract: Diatom assemblage changes over the Holocene were examined from a 14C-dated sediment core retrieved from Lake TK-2, a small low Arctic lake located ca. 200km north of the forest–tundra ecotone in mainland Nunavut, Canada. Notable changes in the diatom assemblages were recorded throughout the core, suggesting that the Holocene epoch in this region has been environmentally and climatically dynamic. The earliest diatom assemblages (ca. 9000cal yr BP) were dominated by taxa that are atypical of post-glacial assemblages commonly recorded throughout the Arctic, and may suggest that early Holocene conditions at Lake TK-2 were relatively warmer. A shift to dominance by small, benthic, opportunistic Fragilaria taxa followed (after ca. 8550cal. yr BP), more typical of Arctic assemblages during initial lake ontogeny, suggesting the onset of cooler, more alkaline conditions. An abrupt and short-lived marked decrease in diatoms between ca. 8550 and 8500cal. yr BP, with corresponding changes in physical and chemical indicators (e.g., sedimentation rate, siliciclastic content, % organic matter content), provides potential evidence for the 8.2k cooling anomaly, an event rarely recognized from other paleolimnological studies in the Canadian north. Following ca. 7000cal. yr BP, a substantial shift occurred to a more complex and diverse diatom assemblage that now included more acidophilic taxa. This compositional change is likely indicative of a natural, long-term loss of alkalinity in the lake, and marks the onset of the Holocene Thermal Maximum, consistent with the timing of this warm period for this region. The relatively stable diatom assemblage composition during the Neoglacial period was punctuated by fluctuations in key species potentially correlative with the so-called Medieval Warm Period and Little Ice Age. In the most recent sediments, the post-industrial expansion of the small, planktonic Cyclotella stelligera complex, and a concurrent decline in the heavily-silicified Aulacoseira lirata complex, are similar to shifts that are increasingly being recognized as geographically widespread diatom responses to recent climate warming. The Lake TK-2 diatom record provides important insights into the Holocene environmental history of this understudied region of the Canadian Arctic. Furthermore, it is one of the few Arctic lakes in which the 8.2k cold event is possibly expressed. [Copyright &y& Elsevier]

Abstract: To develop a model describing the dependence of annual soil respiration on climate and soil properties, we compiled 657 published annual soil respiration (R s ) measurements that were assembled from 147 sites globally, representing croplands, grasslands, forests and tundra ecosystems. Each of these annual soil respiration data was then aggregated with the appropriate mean air temperature (T) and annual precipitation (P) data derived from geographically referenced datasets and with soil properties gathered from the original literature. Partial correlation analyses showed that global annual R s significantly related to annual mean temperature, annual precipitation, and topsoil (0–20cm) organic carbon (SOC) storage, while topsoil total nitrogen (SN) and pH did not show a direct and clear relationship with R s across ecosystems. While we employed the T&P-model that used temperature and annual precipitation to globally predict annual soil respiration, it was able to explain 41%, 57%, and 31% of the variability of soil respiration for croplands, grasslands and forests, respectively. However, the residuals were significantly related to SOC for croplands and grasslands. Thus, we developed a T&P&C-model that includes SOC as an additional predictor of annual R s . This extended but still simple model performed better than the T&P-model and explained 69%, 89%, and 47% of the interannual and intersite variability of R s with a mean absolute error of 0.11, 0.18 and 0.28kgCm−2 yr−1 for croplands, grasslands and forests, respectively. Overall, the modeling efficiency of the T&P&C-model was nearly 60% across ecosystems. Globally, the mean turnover time of topsoil carbon (SOC/R s ) was highly comparable among croplands, grasslands and forests, equivalent to 6.1–6.3 years. Therefore, better estimates of global annual soil respiration would be obtained with the new model driven by climate and soil properties together. We expect significant improvements of global annual soil respiration predictions given that measurements of soil respiration coupling with soil properties and site productivities are widely taken across ecosystems over the world. [Copyright &y& Elsevier]

Abstract: Satellite observations have shown greening trends in tundra in response to climate change, suggesting increases in productivity. To better understand the ability of remote sensing to detect climate impacts on tundra vegetation productivity, we applied a photosynthetic light use efficiency model to simulated climate change treatments of tundra vegetation. We examined changes in the Normalized Difference Vegetation Index (NDVI) and photosynthetic light use efficiency (ε) in experimental warming and moisture treatments designed to simulate climate change in northern Alaska. Plots were warmed either passively, using Open Top Chambers, or actively using electric heaters in the soil. In one set of plots water table depth was actively altered, while other plots were established in locations that were naturally wet or dry. Over two growing seasons, plot-level carbon flux and spectral reflectance measurements were collected, and the results were used to derive a light use efficiency model that could explore the effects of moisture and temperature treatments using remote sensing. Warming increased values of canopy greenness (NDVI) relative to control plots, this effect being more pronounced in wet plots than in dry plots. Light use efficiency (LUE), the relationship between absorbed photosynthetically active radiation (PAR) and gross ecosystem production (GEP), was consistent across warming treatments, growing season, subsequent years, and sites. However, LUE was affected by vegetation type, which varied with moisture; plots in naturally dry locations showed reduced light use efficiency relative to moist plots. Additionally moss exhibited reduced LUE relative to vascular plants. Understory moss production, not accounted for by the usual definition of the fraction of absorbed PAR (f APAR), was found to be a significant part of total GEP, particularly in areas with low vascular plant cover. These results support the use of light use efficiency models driven by spectral reflectance for estimating GEP in tundra vegetation, provided effects of vegetation functional type (e.g. mosses versus vascular plants) and microtopography are considered. [Copyright &y& Elsevier]

Abstract: A validation of the 2005 500m MODIS vegetation continuous fields (VCF) tree cover product in the circumpolar taiga–tundra ecotone was performed using high resolution Quickbird imagery. Assessing the VCF''s performance near the northern limits of the boreal forest can help quantify the accuracy of the product within this vegetation transition area. The circumpolar region was divided into 7 longitudinal zones and validation sites were selected in areas of varying tree cover where Quickbird imagery is available in Google Earth. Each site was linked to the corresponding VCF pixel and overlaid with a regular dot grid within the VCF pixel''s boundary to estimate percent tree crown cover in the area. Percent tree crown cover was estimated using Quickbird imagery for 396 sites throughout the circumpolar region and related to the VCF''s estimates of canopy cover for 2000–2005. Regression results of VCF inter-annual comparisons (2000–2005) and VCF-Quickbird image-interpreted estimates indicate that: (1) Pixel-level, inter-annual comparisons of VCF estimates of percent canopy cover were linearly related (mean R 2 =0.77) and exhibited an average root mean square error (RMSE) of 10.1% and an average root mean square difference (RMSD) of 7.3%. (2) A comparison of image-interpreted percent tree crown cover estimates based on dot counts on Quickbird color images by two different interpreters were more variable (R 2 =0.73, RMSE=14.8%, RMSD=18.7%) than VCF inter-annual comparisons. (3) Across the circumpolar boreal region, 2005 VCF-Quickbird comparisons were linearly related, with an R 2 =0.57, a RMSE=13.4% and a RMSD=21.3%, with a tendency to over-estimate areas of low percent tree cover and anomalous VCF results in Scandinavia. The relationship of the VCF estimates and ground reference indicate to potential users that the VCF''s tree cover values for individual pixels, particularly those below 20% tree cover, may not be precise enough to monitor 500m pixel-level tree cover in the taiga–tundra transition zone. [Copyright &y& Elsevier]

Abstract: Eddy covariance measurements of evapotranspiration at a wetland tundra sedge fen near Churchill, Manitoba, Canada, from 15 June to 21 August, 2007, were used to calculate hourly resistance values for the peat component of the fen’s surface. Inherent in the calculations were the assumptions that evaporation from the open water and transpiration from the living sedge components of the fen’s surface, as well as the aerodynamic resistance, could be reasonably well modeled. Fitting a curve to these results provided a regression model for the daytime variation of the hourly surface resistance to evaporation from the peat. It was concluded that the surface resistance, which was relatively low during the morning hours and increased during the afternoon hours, reflected, in part, a diurnal wetting and drying of the near surface layer. This insight into the daytime variation of resistance to evaporation from the peat surface of a wetland tundra sedge fen may be of assistance in the formulation of land-surface schemes. [Copyright &y& Elsevier]

Abstract: Pollen, chironomid, and ostracode records from a lake located at alpine treeline provide regional paleoclimate reconstructions from the southwest Yukon Territory, Canada. The pollen spectra indicate herbaceous tundra existed on the landscape from 13.6–11ka followed by birch shrub tundra until 10ka. Although Picea pollen dominated the assemblages after 10ka, low pollen accumulation rates and Picea percentages indicate minimal treeline movement through the Holocene. Chironomid accumulation rates provide evidence of millennial-scale climate variability, and the chironomid community responded to rapid climate changes. Ostracodes were found in the late glacial and early Holocene, but disappeared due to chemical changes of the lake associated with changes in vegetation on the landscape. Inferred mean July air temperature, total annual precipitation, and water depth indicate a long-term cooling with increasing moisture from the late glacial through the Holocene. During the Younger Dryas (12.9–11.2ka), cold and dry conditions prevailed. The early and mid-Holocene were warm and dry, with cool, wet conditions after 4ka, and warm, dry conditions since the end of the Little Ice Age. [Copyright &y& Elsevier]

Abstract: Ecotones are gradual transitions between two adjacent ecological systems. Ecotones are characterized by their spatial properties which are reflected in an ecotone width and location and the temporal variation across time during succession or environmental change on both a local and a global scale. If only one main environmental factor drives this gradual change the shape of the ecotone is evident as a sigmoid wave. In this paper we explore a two-dimensional sigmoid wave curve fitting algorithm that describes an ecotone and validate it on classified remote sensing data of a forest-tundra ecotone in the Northwest Territories of Canada. The estimated location and width of the forest-tundra ecotone are validated with digital land cover data. We conclude that the algorithm was able to delineate the forest-tundra ecotone based upon the classified remote sensing image and is robust for various algorithm parameter settings. [Copyright &y& Elsevier]

Abstract: The Bering Land Bridge (BLB) connected the two principal arctic biological refugia, Western and Eastern Beringia, during intervals of lowered sea level in the Pleistocene. Fossil evidence from lowland BLB organic deposits dating to the Last Glaciation indicates that this broad region was dominated by shrub tundra vegetation, and had a mesic climate. The dominant ecosystem in Western Beringia and the interior regions of Eastern Beringia was steppe–tundra, with herbaceous plant communities and arid climate. Although Western and Eastern Beringia shared many species in common during the Late Pleistocene, there were a number of species that were restricted to only one side of the BLB. Among the vertebrate fauna, the woolly rhinoceros was found only to the west of the BLB, North American camels, bonnet-horned musk-oxen and some horse species were found only to the east of the land bridge. These were all steppe–tundra inhabitants, adapted to grazing. The same phenomenon can be seen in the insect faunas of the Western and Eastern Beringia. The steppe–tundra beetle fauna of Western Beringia was dominated by weevils of the genus Stephanocleonus, a group that was virtually absent from Eastern Beringia. The dry-adapted weevils, Lepidophorus lineaticollis and Vitavitus thulius were important members of steppe–tundra communities in Eastern Beringia, but were either absent or rare in Western Beringia. The leaf beetles Chrysolina arctica, C. brunnicornis bermani, and Galeruca interrupta circumdata were typical members of the Pleistocene steppe–tundra communities of Western Beringia, but absent from Eastern Beringia. On the other hand, some steppe tundra-adapted leaf beetles managed to occupy both sides of the BLB, such as Phaedon armoraciae. Much of the BLB remains unstudied, but on biogeographic grounds, it appears that there was some kind of biological filter that blocked the movements of some steppe–tundra plants and animals across the BLB. [Copyright &y& Elsevier]

Abstract: Sediments and vegetation dated 21,570 cal yr BP were buried under tephra on the northern Seward Peninsula. This buried surface has yielded plant macrofossils in growth position, as well as numerous insect fossils, excellently preserved in permafrost. It appears that many of the insects were buried alive by the volcanic ash. The species composition and ecological affinities of this fossil fauna are typical of Alaskan Late Pleistocene steppe–tundra environments. The assemblages are dominated by the weevil Lepidophorus lineaticollis, one of the most common species in Eastern Beringian Pleistocene fossil assemblages. Many other members of the ancient steppe–tundra insect community are preserved in these assemblages, including the pill beetle Morychus sp. and weevils of the genus Coniocleonus. In Alaska, most of these species (but not all of them) survived the Pleistocene/Holocene environmental transition, but are restricted today to relict patches of steppe-like vegetation. Faunal diversity is low, in spite of the recovery of more than 1000 individual insects and mites including more than 600 beetles. This reflects the small number of species adapted to the cold, dry environments of the LGM in Eastern Beringia. They represent an ecosystem which no longer exists. [Copyright &y& Elsevier]

Mercury (Hg) contamination in tundra region has raised substantial concerns, especially since the first report of atmospheric mercury depletion events (AMDEs) in the Polar Regions. During the past decade, steady progress has been made in the research of Hg cycling in the Polar Regions. This has generated a unique opportunity to survey the whole Arctic in respect to Hg issue and to find out new discoveries. However, there are still considerable knowledge gaps and debates on the fate of Hg in the Arctic and Antarctica, especially regarding the importance and significance of AMDEs vs. net Hg loadings and other processes that burden Hg in the Arctic. Some studies argued that climate warming since the last century has exerted profound effects on the limnology of High Arctic lakes, including substantial increases in autochthonous primary productivity which increased in sedimentary Hg, whereas some others pointed out the importance of the formation and postdeposition crystallographic history of the snow and ice crystals in determining the fate and concentration of mercury in the cryosphere in addition to AMDEs. Is mercury re-emitted back to the atmosphere after AMDEs? Is Hg methylation effective in the Arctic tundra? Where the sources of MeHg are? What is its fate? Is this stimulated by human made? This paper presents a critical review about the fate of Hg in the Arctic tundra, such as pathways and process of Hg delivery into the Arctic ecosystem; Hg concentrations in freshwater and marine ecosystems; Hg concentrations in terrestrial biota; trophic transfer of Hg and bioaccumulation of Hg through food chain. This critical review of mercury fates and contamination in the Arctic tundra ecosystem is assessing the impacts and potential risks of Hg contamination on the health of Arctic people and the global northern environment by highlighting and “perspectiving” the various mercury processes and concentrations found in the Arctic tundra. [Copyright &y& Elsevier]

Abstract: The need for improved processes understanding to develop enhanced knowledge and modelling strategies is a central issue within the Prediction in Ungauged Basins (PUB) initiative. Prediction of snow-cover depletion and spring-melt runoff in arctic basins is particularly challenging due to the spatial heterogeneity of the snow-cover as a result of topographic and vegetation effects on snow accumulation, wind redistribution, and ablation processes. Additionally the remote location and winter inaccessibility of Arctic basins contribute to the lack of proper data to model the hydrology of these sites. This study was conducted in two topographically distinct cold regions research basins in the north-west of Canada characterised as tundra environments, one an alpine mountain tundra at 60°N and the other a rolling planar tundra at approximately 70°N. This paper examines the transference of parameters of a distributed physically based land surface hydrological model from one site to another across 1350km using a step-wise calibration procedure. The transference methodology uses a physiographic similarity criterion to identify similar landscapes units and makes use of landcover-based parameters rather than the usual relationships between parameters and basin characteristics. Results showed that simulations using regionalised vegetation parameters performed better than those using default parameters and accurately described both snow-cover ablation and snowmelt runoff in both basins. [Copyright &y& Elsevier]

Abstract: We studied the vertical patterns of δ 15nitrogen in total N and exchangeable NH4 +-N through soil profiles in diverse alpine and tundra ecosystems. Soil samples were analyzed from 11 sites located in three mountain areas: NW Caucasus (Russia), the Khibiny Mountains (NW Russia) and Abisko region (N Sweden). Despite differences in the profile patterns of organic matter, nitrogen accumulation and nitrogen availability, we found consistent patterns of 15N distribution through all studied soil profiles. The δ 15N values of total N were in general about zero or positive in the surface horizon and increased with soil depth. In contrast with total N, the δ 15N values of exchangeable NH4 +-N were in general about zero or negative in the surface horizons and decreased with soil depth. NH4 +-N was significantly 15N-depleted compared with total N in all mineral horizons, while in the surface organic horizons differences between isotopic composition of total N and NH4 +-N were mostly not significant. We do not know the exact mechanism responsible for 15N depletion of NH4 +-N with soil depth and further research needs to evaluate the contributions of natural processes (higher nitrification activity and biological immobilization of “lighter” NH4 +-N near the soil surface) or artifacts of methodological procedure (contribution of the 15N-enriched microbial N and dissolved organic N near the soil surface). Nevertheless, our finding gives a new possibility to interpret variability in foliar δ 15N values of plant species with different rooting depth in alpine and tundra ecosystems, because plants with deeper root systems can probably consume “lighter” rather than “heavier” NH4 +-N. [Copyright &y& Elsevier]

Plant species, forming important components of Arctic food chains and of interest from a monitoring perspective, were studied at 36 plots representing flood plain and terrace landscapes of the Yenisey River and Estuary from its upper delta to the gulf. 137Cs contamination densities at the plots varied from 0.35kBq/m2 (central delta, sandy riverside plot) to 88kBq/m2 (the upper delta plot) indicating both global and regional sources of anthropogenic pollution. Cs-137 levels in plants were within the range expected from global fallout inputs and varied from 31 to 140Bq/kg d.w. increasing in dominant groups in the order: grasses137Cs Tag values (0.03–0.1m2/kg) comparable to those found on swampy areas contaminated by Chernobyl fallout. The frontal delta island seems to act as a barrier, contributing to local accumulation of 137Cs as reflected in higher inventories in soil and biomass. Horsetail and willow leaves were noted for higher 40K content and lower 137Cs Tag compared to mosses and alder, respectively. This makes the latter plant types more appropriate indicators of 137Cs contamination. [Copyright &y& Elsevier]

Abstract: This study examines the carbon and nitrogen stocks of soils and vegetation in different frost-boil tundra microsites (rims, troughs and bare soil patches) and aims at elucidating differences in controls of organic matter turnover. Troughs, the wettest microsite, stored the greatest part of soil C and N (23.9 and 1.7kgm−2, respectively), while drier rims held only 50% and bare soil patches only about 17% of the C and N found in troughs. Both C and N mineralization rates were higher in rims compared to troughs, suggesting a higher turnover of organic matter in rims. On an areal basis N was predominantly mineralized in mineral horizons, while C mineralization was more or less equally distributed between organic and mineral horizons. Thus, atmospheric warming, which has a stronger effect on the upper soil layers, may increase C mineralization to a higher extent than N mineralization (mainly located in lower soil layers). This suggests that frost-boil tundra ecosystems may be (at least in the short-term) sources of CO2 to the atmosphere. Furthermore, the combined results of gross N mineralization and net N mineralization measurements showed a higher microbial sink strength for N in rims compared to troughs, suggesting that decomposition of organic material in rims is controlled mainly by N availability, while the main factor constraining decomposition in troughs may be unfavourable hydrothermal conditions. This may lead to differential responses of frost-boil tundra microsites to changing climatic conditions. [Copyright &y& Elsevier]

Monitoring the dynamics of the circumpolar boreal forest (taiga) and Arctic tundra boundary is important for understanding the causes and consequences of changes observed in these areas. This ecotone, the world''s largest, stretches for over 13,400 km and marks the transition between the northern limits of forests and the southern margin of the tundra. Because of the inaccessibility and large extent of this zone, remote sensing data can play an important role for mapping the characteristics and monitoring the dynamics. Basic understanding of the capabilities of existing space borne instruments for these purposes is required. In this study we examined the use of several remote sensing techniques for characterizing the existing tundra–taiga ecotone. These include Landsat-7, MISR, MODIS and RADARSAT data. Historical cover maps, recent forest stand measurements and high-resolution IKONOS images were used for local ground truth.It was found that a tundra–taiga transitional area can be characterized using multi-spectral Landsat ETM+ summer images, multi-angle MISR red band reflectance images, RADARSAT images with larger incidence angle, or multi-temporal and multi-spectral MODIS data. Because of different resolutions and spectral regions covered, the transition zone maps derived from different data types were not identical, but the general patterns were consistent. [Copyright &y& Elsevier]

The objective of this paper is to review research conducted over the past decade on the application of multi-temporal remote sensing for monitoring changes of Arctic tundra lands. Emphasis is placed on results from the National Science Foundation Land–Air–Ice Interactions (LAII) program and on optical remote sensing techniques. Case studies demonstrate that ground-level sensors on stationary or moving track platforms and wide-swath imaging sensors on polar orbiting satellites are particularly useful for capturing optical remote sensing data at sufficient frequency to study tundra vegetation dynamics and changes for the cloud prone Arctic. Less frequent imaging with high spatial resolution instruments on aircraft and lower orbiting satellites enable more detailed analyses of land cover change and calibration/validation of coarser resolution observations.The strongest signals of ecosystem change detected thus far appear to correspond to expansion of tundra shrubs and changes in the amount and extent of thaw lakes and ponds. Changes in shrub cover and extent have been documented by modern repeat imaging that matches archived historical aerial photography. NOAA Advanced Very High Resolution Radiometer (AVHRR) time series provide a 20-year record for determining changes in greenness that relates to photosynthetic activity, net primary production, and growing season length. The strong contrast between land materials and surface waters enables changes in lake and pond extent to be readily measured and monitored. [Copyright &y& Elsevier]

Carbon assimilation and allocation were studied in a tundra ecosystem in northern Scandinavia. Seasonal variation in the below-ground carbon allocation to dissolved organic carbon (DOC), coarse-, fine-, and hair roots was investigated using in situ 14C pulse-labelling, adding 2–3 MBq 14CO2 dm−2 to the above-ground vegetation. Combining the allocation data with regression models of the seasonal carbon flux made it possible to estimate a temporally explicit ecosystem carbon allocation budget.The ecosystem was a net source of CO2, losing on average 0.97 g C m−2 d−1 to the atmosphere, with little variation through the season. There was, however, significant temporal variation in partitioning of recently assimilated carbon. Allocation to below-ground compartments over 32 days following labelling increased from 18% in June to 55% in September. Above-ground allocation showed the opposite trend. Hair roots and DOC were strong sinks in the autumn. Transport of newly assimilated carbon occurred rapidly throughout the season, 14C appearing in all sampled pools within 4 h of labelling.The seasonal variation in carbon partitioning observed in this study has implications for the residence time of assimilated carbon in the ecosystem. A relatively greater allocation to rapidly decomposing pools, such as hair roots and DOC, would tend to reduce incorporation into woody tissue, increasing the overall rate of carbon cycling and decreasing ecosystem storage. The results of this study will be of value for building and validating mechanistic models of ecosystem carbon flow in tundra and subarctic ecosystems. [Copyright &y& Elsevier]

Wolverines (Gulo gulo) are circumpolar omnivores that live throughout the alpine and arctic tundra ecosystem. Wolverine livers were collected at Kugluktuk (Coppermine), NU (n=12) in the western Canadian Arctic to report, for the first time, the residue patterns of persistent organochlorine contaminants (OCs) in this species. The enantiomer fractions (EFs) of several chiral OCs, including PCB atropisomers, in wolverines were also determined. Results were compared to OC concentrations and EFs of chiral contaminants in arctic fox (Alopex lagopus) from Ulukhaqtuuq (Holman), NT (n=20); a closely related species that scavenges the marine and terrestrial arctic environment. The rank order of hepatic concentrations for sum (∑) OC groups in wolverines were polychlorinated biphenyls (∑PCB) > chlordane-related components (∑CHLOR) > DDT-related compounds (∑DDT) > hexachlorocyclohexane isomers (∑HCHs). The most abundant OC analytes detected in wolverine liver were PCB-153, PCB-180, and oxychlordane (OXY). Wolverine age and gender did not influence OC concentrations, which were comparable to lipid-normalized values in arctic fox. The EFs of several chiral OCs (α-HCH, cis- and trans-chlordane, OXY, heptachlor exo-epoxide) and PCB atropisomers (PCB-136, 149) were nonracemic in arctic fox and wolverine liver and similar to those previously calculated in arctic fox and polar bears from Iceland and the Canadian Arctic. Results suggest that these species have similar ability to biotransform OCs. As well, contaminant profiles suggest that terrestrial mammals do not represent the major source of OC exposure to wolverines and that wolverines are scavenging more contaminated prey items, such as marine mammals. While ∑PCB did not exceed the concentrations associated with mammalian reproductive impairment, future research is required to properly evaluate the potential affect of other OCs on the overall health of wolverines. [Copyright &y& Elsevier]

Loess is one of the most widespread subaerial deposits in Alaska and adjacent Yukon Territory and may have a history that goes back 3 Ma. Based on mineralogy and major and trace element chemistry, central Alaskan loess has a composition that is distinctive from other loess bodies of the world, although it is quartz-dominated. Central Alaskan loess was probably derived from a variety of rock types, including granites, metabasalts and schists. Detailed stratigraphic data and pedologic criteria indicate that, contrary to early studies, many palaeosols are present in central Alaskan loess sections. The buried soils indicate that loess sedimentation was episodic, or at least rates of deposition decreased to the point where pedogenesis could keep ahead of aeolian input. As in China, loess deposition and pedogenesis are likely competing processes and neither stops completely during either phase of the loess/soil formation cycle. Loess deposition in central Alaska took place before, and probably during the last interglacial period, during stadials of the mid-Wisconsin period, during the last glacial period and during the Holocene. An unexpected result of our geochronological studies is that only moderate loess deposition took place during the last glacial period. Our studies lead us to conclude that vegetation plays a key role in loess accumulation in Alaska. Factors favouring loess production are enhanced during glacial periods but factors that favour loess accumulation are diminished during glacial periods. The most important of these is vegetation; boreal forest serves as an effective loess trap, but sparsely distributed herb tundra does not. Thus, thick accumulations of loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in central Alaska. Much of the stratigraphic diversity of North American loess, including that found in the Central Lowlands, the Great Plains, and Alaska is explained by a new model that emphasizes the relative importance of loess production factors versus loess accumulation factors. [Copyright &y& Elsevier]

Reports on the results of a carbon ([sup14]C) pulse-labeling experiment conducted under field conditions in arctic Alaska. Movement of carbon through arctic wetland tundra ecosystems; Measurement of the contribution of recent plant photosynthates to net methane (CH[sub4]) emission; Gas exchange and soil porewater measurements and analysis; Mesocosm characteristics.