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Ecosystem-scale methane (CH4) flux (FCH4) over a subarctic fen at Churchill, Manitoba, Canada was measured to understand the magnitude of emissions during spring and fall shoulder seasons, and the growing season in relation to physical and biological conditions. FCH4 was measured using eddy covariance with a closed-path analyzer in four years (2008-2011). Cumulative measured annual FCH4 (shoulder plus growing seasons) ranged from 3.0 to 9.6 g CH4 m-2 yr-1 among the four study years, with a mean of 6.5 to 7.1 g CH4 m-2 yr-1 depending upon gap-filling method. Soil temperatures to depths of 50 cm and air temperature were highly correlated with FCH4, with near surface soil temperature at 5 cm most correlated across spring, fall, and the whole season. The response of FCH4 to soil temperature at the 5 cm depth and air temperature was more than double in spring to that of fall. Emission episodes were generally not observed during spring thaw. Growing season emissions also depended upon soil and air temperatures but water table also exerted influence with FCH4 highest when water was 2-13 cm below and least when it was at or above the mean peat surface. [ABSTRACT FROM AUTHOR]

The variation of polar sea ice is an indicator of polar environmental change, which plays an important role in the study of regional and global climate change. In this paper, the latest sea ice data sets from the European Space Agency Climate Change Initiative were firstly combined to comprehensively analyze the spatial and temporal variation of Arctic sea ice concentration (SIC) and sea ice thickness (SIT) from 2002 to 2017. The results show that during this period, the SIC of the Kara Sea and the Barents Sea decreases the most significantly, reaching −1.11%·year−1. The Arctic annual average sea ice extent (SIE) and sea ice area (SIA) exhibit noticeable decreasing trends, reaching −0.0592 × 106 km·year−1 and −0.0628 × 106 km·year−1, respectively. Both of Arctic SIE and SIA vary seasonally, and they reach the minimum values of 4.97 × 106 km2 and 3.96 × 106 km2 in September and the maximum values of 14.58 × 106 km2 and 13.35 × 106 km2 in March, respectively. Moreover, the Arctic SIE and SIA anomaly series from 2002 to 2017 were analyzed based on the linear regression method. It is found the most noticeable decreasing trends of the Arctic SIE and SIA are from July to October. The maximum variation of the SIE is −1.06 × 106 km2(10a)−1, and the maximum variation of the SIA is −1.09 × 106 km2(10a)−1. The Arctic SIE and SIA present the smallest decreasing trends from April to May, which are −0.19 × 106 km2(10a)−1 and −0.2 × 106 km2(10a)−1, respectively. The Arctic SIT also displays a pronounced decreasing trend with −0.015 m·year−1 in autumn and −0.018 m·year−1 in winter. The annual average SIT reaches a minimum of 1.28 m in 2013 and a maximum of 1.66 m in 2005. Particularly, the SIT of the Hudson Bay decreases the most significantly, reaching −0.051 m·year−1. Finally, the Arctic average SIT was tested by the Mann‐Kendall method. It is found that the SIT has a sudden change in 2007, from a slight increase in 2003–2007 to a significant decrease in 2007–2017. Overall, the decreasing trend of the Arctic sea ice (both SIC and SIT) is significant in the past decade. These findings can contribute to future research on polar regions and global climate change. Key Points: We analyzed the whole and regional variation of Arctic sea ice by combining use of the latest sea ice concentration and sea ice thickness data sets from European Space AgencyThe sea ice concentration in the Barents Sea and the Kara Sea and the sea ice thickness in the Hudson Bay decreased the most obviouslyThe sea ice thickness has a sudden change in 2007 based on Mann‐Kendall mutation test [ABSTRACT FROM AUTHOR]

Abstract: We address the three main issues raised by Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity]: (1) evidence of the role of climate warming in affecting the western Hudson Bay polar bear population, (2) responses to suggested importance of human–polar bear interactions, and (3) limitations on polar bear adaptation to projected climate change. We assert that our original paper did not provide any “alternative explanations [that] are largely unsupported by the data” or misrepresent the original claims by Stirling et al. [Stirling, I., Lunn, N.J., Iacozza, I., 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climate change. Arctic 52, 294–306], Derocher et al. [Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar bears in a warming climate. Integr. Comp. Biol. 44, 163–176], and other peer-approved papers authored by Stirling and colleagues. In sharp contrast, we show that the conclusion of Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity] – suggesting warming temperatures (and other related climatic changes) are the predominant determinant of polar bear population status, not only in western Hudson (WH) Bay but also for populations elsewhere in the Arctic – is unsupportable by the current scientific evidence. The commentary by Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity] is an example of uni-dimensional, or reductionist thinking, which is not useful when assessing effects of climate change on complex ecosystems. Polar bears of WH are exposed to a multitude of environmental perturbations including human interference and factors (e.g., unknown seal population size, possible competition with polar bears from other populations) such that isolation of any single variable as the certain root cause (i.e., climate change in the form of warming spring air temperatures), without recognizing confounding interactions, is imprudent, unjustified and of questionable scientific utility. Dyck et al. [Dyck, M.G., Soon, W., Baydack, R.K., Legates, D.R., Baliunas, S., Ball, T.F., Hancock, L.O., 2007. Polar bears of western Hudson Bay and climate change: Are warming spring air temperatures the “ultimate” survival control factor? Ecol. Complexity, 4, 73–84. doi:10.1016/j.ecocom.2007.03.002] agree that some polar bear populations may be negatively impacted by future environmental changes; but an oversimplification of the complex ecosystem interactions (of which humans are a part) may not be beneficial in studying external effects on polar bears. Science evolves through questioning and proposing hypotheses that can be critically tested, in the absence of which, as Krebs and Borteaux [Krebs, C.J., Berteaux, D., 2006. Problems and pitfalls in relating climate variability to population dynamics. Clim. Res. 32, 143–149] observe, “we will be little more than storytellers.” [Copyright &y& Elsevier]

Little is known about the microbial diversity of rivers that flow across the changing subarctic landscape. Using amplicon sequencing (rRNA and rRNA genes) combined with HPLC pigment analysis and physicochemical measurements, we investigated the diversity of two size fractions of planktonic Bacteria, Archaea and microbial eukaryotes along environmental gradients in the Great Whale River (GWR), Canada. This large subarctic river drains an extensive watershed that includes areas of thawing permafrost, and discharges into southeastern Hudson Bay as an extensive plume that gradually mixes with the coastal marine waters. The microbial communities differed by size-fraction (separated with a 3-μm filter), and clustered into three distinct environmental groups: (1) the GWR sites throughout a 150-km sampling transect; (2) the GWR plume in Hudson Bay; and (3) small rivers that flow through degraded permafrost landscapes. There was a downstream increase in taxonomic richness along the GWR, suggesting that sub-catchment inputs influence microbial community structure in the absence of sharp environmental gradients. Microbial community structure shifted across the salinity gradient within the plume, with changes in taxonomic composition and diversity. Rivers flowing through degraded permafrost had distinct physicochemical and microbiome characteristics, with allochthonous dissolved organic carbon explaining part of the variation in community structure. Finally, our analyses of the core microbiome indicated that while a substantial part of all communities consisted of generalists, most taxa had a more limited environmental range and may therefore be sensitive to ongoing change. [ABSTRACT FROM AUTHOR]

Abstract: Long-term warming of late spring (April–June) air temperatures has been proposed by Stirling et al. [Stirling, I., Lunn, N.J., Iacozza, J., 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climatic change. Arctic 52, 294–306] as the “ultimate” factor causing earlier sea-ice break-up around western Hudson Bay (WH) that has, in turn, led to the poorer physical and reproductive characteristics of polar bears occupying this region. Derocher et al. [Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar bears in a warming climate. Integr. Comp. Biol. 44, 163–176] expanded the discussion to the whole circumpolar Arctic and concluded that polar bears will unlikely survive as a species should the computer-predicted scenarios for total disappearance of sea-ice in the Arctic come true. We found that spring air temperatures around the Hudson Bay basin for the past 70 years (1932–2002) show no significant warming trend and are more likely identified with the large-amplitude, natural climatic variability that is characteristic of the Arctic. Any role of external forcing by anthropogenic greenhouse gases remains difficult to identify. We argue, therefore, that the extrapolation of polar bear disappearance is highly premature. Climate models are simply not skilful for the projection of regional sea-ice changes in Hudson Bay or the whole Arctic. Alternative factors, such as increased human–bear interaction, must be taken into account in a more realistic study and explanation of the population ecology of WH polar bears. Both scientific papers and public discussion that continue to fail to recognize the inherent complexity in the adaptive interaction of polar bears with both human and nature will not likely offer any useful, science-based, preservation and management strategies for the species. [Copyright &y& Elsevier]

Migration is predicted to change both spatially and temporally as climate change alters seasonal resource availability. Species in extreme environments are especially susceptible to climate change; hence, it is important to determine environmental and biological variables that influence their migration. Polar bears (Ursus maritimus) are an Arctic apex carnivore whose migration phenology has been affected by climate change and is vulnerable to future changes. Here, we used satellite-linked telemetry collar data from adult female polar bears in western Hudson Bay from 2004 to 2016 and multivariate response regression models to demonstrate that 1) spatial and temporal migration metrics are correlated, 2) ice concentration and wind are important environmental variables that influence polar bear migration in seasonal ice areas, and 3) migration did not vary across the years of our study, highlighting the importance of continued monitoring. Specifically, we found that ice concentration, wind speed, and wind direction affected polar bear migration onto ice during freeze-up and ice concentration and wind direction affected migration onto land during breakup. Bears departed from land earlier with increased wind speed and the effect of wind direction on migration may be linked to prey searching and ice drift. Low ice concentration was associated with higher movement during freeze-up and breakup. Our findings suggest that migration movement may increase in response to climate change as ice concentration and access to prey declines, potentially increasing nutritional stress on bears. [ABSTRACT FROM AUTHOR]

Heinrich events are among the dominant modes of glacial climate variability. During these events, massive iceberg armadas were released by the Laurentide Ice Sheet and sailed across the Atlantic where they melted and released freshwater, as well as detritus, that formed characteristic layers on the seafloor. Heinrich events are known for cold climates in the North Atlantic region and global climate changes. We study these events in a fully coupled complex ice sheet–climate model with synchronous coupling between ice sheets and oceans. The ice discharges occur as an internal variability of the model with a recurrence period of 5 kyr, an event duration of 1–1.5 kyr, and a peak discharge rate of about 50 mSv, roughly consistent with reconstructions. The climate response shows a two-stage behavior, with freshwater release effects dominating the surge phase and ice sheet elevation effects dominating the post-surge phase. As a direct response to the freshwater discharge during the surge phase, deepwater formation in the North Atlantic decreases and the North Atlantic deepwater cell weakens by 3.5 Sv. With the reduced oceanic heat transport, the surface temperatures across the North Atlantic decrease, and the associated reduction in evaporation causes a drying in Europe. The ice discharge lowers the surface elevation in the Hudson Bay area and thus leads to increased precipitation and accelerated ice sheet regrowth in the post-surge phase. Furthermore, the jet stream widens to the north, which contributes to a weakening of the subpolar gyre and a continued cooling over Europe even after the ice discharge. This two-stage behavior can explain previously contradicting model results and understandings of Heinrich events. [ABSTRACT FROM AUTHOR]

Previous work has found Hudson Bay seasonal sea ice particularly sensitive to climate change with a strong signal of earlier breakup dates. This work extends the previous analysis by including eight additional years of recent sea ice data. The expanded sea ice record, 1971 to 2011, revealed stronger and more statistically significant trends than the earlier work, most strikingly for the later freeze up. The average magnitude of the temporal trend for all 36 locations studied is 0.50 days/year for earlier breakup, 0.46 days/year for the later freeze-up, and 0.91 days/year for longer ice-free period. Of the 36 points, 12 points for the breakup period, 30 points for the freeze-up period, and 22 points for the ice-free season have accelerating temporal trends during the past decade. [ABSTRACT FROM AUTHOR]

This study used air temperatures from a suite of regional climate models participating in the North American Climate Change Assessment Program (NARCCAP) together with two atmospheric reanalysis datasets to investigate changes in freezing days (defined as days with daily average temperature below freezing) likely to occur between 30-yr baseline (1971-2000) and midcentury (2041-70) periods across most of North America. Changes in NARCCAP ensemble mean winter temperature show a strong gradient with latitude, with warming of over 4°C near Hudson Bay. The decline in freezing days ranges from less than 10 days across north-central Canada to nearly 90 days in the warmest areas of the continent that currently undergo seasonally freezing conditions. The area experiencing freezing days contracts by 0.9-1.0 × 106 km2 (5.7%-6.4% of the total area). Areas with mean annual temperature between 2° and 6°C and a relatively low rate of change in climatological daily temperatures (<0.2°C day−) near the time of spring thaw will encounter the greatest decreases in freezing days. Advances in the timing of spring thaw will exceed the delay in fall freeze across much of the United States, with the reverse pattern likely over most of Canada. [ABSTRACT FROM AUTHOR]

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The climatological conditions for the presence of palsas in the Hudson Bay Lowlands (HBL) in Ontario, Canada, are examined using data from four climate stations: Big Trout Lake, Lansdowne House, Peawanuck, and Fort Severn. These stations sandwich the existing region where palsas occur. The criteria for the formation and occurrence of palsas that were taken from the literature on Fennoscandian and neighboring Québec palsas were applied to the HBL. Thermal thresholds set at -2 °C and 0 °C mean annual air temperature, and number of days below -10 °C per year were met for the two more northerly locations; the two southerly locations were on the edge of the thresholds. Climate projections from two models under two emission scenarios for the 2020s, 2050s, and 2080s indicated that by the 2080s all four locations would fail the -2 °C threshold for palsa formation but at three locations the 0 °C threshold for palsa presence was met for some projection scenarios. Over the next century, it is likely that the climate conditions will continue to be capable of supporting existing palsas; however, by the end of the century the threshold criteria for new palsa formation will not be met for most of the HBL. [ABSTRACT FROM AUTHOR]

The Hudson Bay Lowlands (HBL) is the largest peatland complex in North America. More than 75% of the HBL occurs in Ontario, where the provincial government mandates that ecosystem carbon storage and sequestration be considered in land-use planning. Accomplishing this task requires identifying carbon indicators and assessing their responses to changing ecosystem processes, such as succession, permafrost thaw, and evapotranspiration (ET). Therefore, we synthesized information on peat carbon indicators and ecosystem process from the literature. Findings indicate that the long-term carbon accumulation, carbon dioxide (CO2) sequestration, peat depth, and peatland age were similar ( p > 0.10) between dry and wet peatland features. Furthermore, CO2 sequestration displayed the highest variability and ponds were net CO2 emitters. Recent carbon accumulation, CH4 emission, and ET were highest ( p < 0.01) in wet features, with CH4 emission displaying wide variation. Increased active layer thickness (105 ± 92 cm per 100 years) in permafrost was the most variable ecosystem process analyzed in this study, while variation in permafrost loss (53 ± 23% per 100 years) was similar to that of carbon accumulation and ET rates. Processes creating wet and pond conditions may increase landscape-scale CO2 and CH4 emissions to the atmosphere, weakening peatland carbon sinks. Dry conditions may reduce CH4 emissions but potentially increase peatland susceptibility to fire. Knowledge of these changes should be useful for climate change vulnerability and adaptation assessments for large landscapes. However, better understanding of variability in CO2 sequestration, CH4 emission, and permafrost dynamics is required to design such assessments for small landscapes. [ABSTRACT FROM AUTHOR]

There is concern over the fate of surface water bodies at high latitudes as a consequence of rising global temperatures. The goal of this study is to characterize climatic change that has occurred in the northern Hudson Bay Lowlands (HBL), Canada, from 1943 to 2009, to determine if this has resulted in a change to pond surface areas and to predict if changes may continue in the future. Climate change and changes to pond volume and size over the past ∼60 years were examined using a combination of field methods/instrumental records (1943-2009), modeling (1953-2009; 1961-2100), and remote sensing/imagery analyses (1947-2008). Results demonstrate that temperatures are warming and breakup dates are earlier, but this has not significantly increased the duration of the open-water period or pond evaporation rates, which can be highly variable from year to year. Annual precipitation, primarily summer rainfall, has increased, lessening the summer moisture deficit and leading to wetter conditions. The observed changes of a smaller summer moisture deficit are predicted to continue in future, although there is less certainty with predictions of future precipitation than there is with predictions of air temperature. Thus, ponds are likely not at risk for drying and instead may be at risk for expansion. Despite the increases in summer rainfall, imagery analysis of 100 ponds shows that pond surface areas have fluctuated over the study period but have not increased in size. [ABSTRACT FROM AUTHOR]

In this study we tested the ability of a mechanistic model (Niche Mapper™) to accurately model adult, non-denning polar bear (Ursus maritimus) energetics while fasting during the ice-free season in the western Hudson Bay. The model uses a steady state heat balance approach, which calculates the metabolic rate that will allow an animal to maintain its core temperature in its particular microclimate conditions. Predicted weight loss for a 120 day fast typical of the 1990s was comparable to empirical studies of the population, and the model was able to reach a heat balance at the target metabolic rate for the entire fast, supporting use of the model to explore the impacts of climate change on polar bears. Niche Mapper predicted that all but the poorest condition bears would survive a 120 day fast under current climate conditions. When the fast extended to 180 days, Niche Mapper predicted mortality of up to 18% for males. Our results illustrate how environmental conditions, variation in animal properties, and thermoregulation processes may impact survival during extended fasts because polar bears were predicted to require additional energetic expenditure for thermoregulation during a 180 day fast. A uniform 3°C temperature increase reduced male mortality during a 180 day fast from 18% to 15%. Niche Mapper explicitly links an animal’s energetics to environmental conditions and thus can be a valuable tool to help inform predictions of climate-related population changes. Since Niche Mapper is a generic model, it can make energetic predictions for other species threatened by climate change. [ABSTRACT FROM AUTHOR]

Aim The objectives of the study are: (1) to evaluate the dynamics of the maritime tree line and forest limit of white spruce, Picea glauca, within the dual framework of primary succession induced by the rapid post-glacial land emergence on the eastern coast of Hudson Bay and the impacts of recent and past climate changes; and (2) to determine the time lapse between land emergence and seedling, tree, and forest establishment in the context of the primary chronosequence occurring on rising, well-drained sandy beaches and terraces. Location The study area was located on the eastern coast of Hudson Bay (56°20′ N, 76°32′ W) in northern Québec, Canada. Methods We evaluated the colonization dynamics of white spruce as seedlings, tree-line trees and primary-forest trees at eight sites distributed along a 200-km latitudinal gradient based on a mean land emergence rate of 1.2 m century−1. A 30-m wide by 140–300-m long quadrat was positioned at random at the centre of each site. The elevation above sea level, position and age of all individuals of spruce present in the quadrat areas were determined, and the soils of each chronosequence were described. Results The main stages of primary succession along the emerging coast were common to all the sites, regardless of latitude, but occurred at different elevations above sea level (a.s.l.). White spruce seedlings colonized near-shore beaches 2 m a.s.l., whereas the tree line and forest limit tended to form only at about 3–4 m and 4–8 m a.s.l., corresponding approximately to 180–825 years and 310–1615 years after land emersion, respectively. White spruce establishment at the tree line occurred about 50 years ago. Climatic conditions at this time were probably more favourable to tree colonization than when the species established at the forest limit. Soil formation was influenced primarily by distance from the seashore and elevation above sea level, with podzolization being accelerated by white spruce cover. Main conclusions The current tree-line and forest-limit positions on the rising coast of eastern Hudson Bay correspond to ecological limits established during the course of primary succession within a context of changing climatic conditions. The recent establishment of trees at the tree line and forest limit at relatively old coastal sites is associated with warmer conditions over the last 100 years. Although white spruce was present nearby, coastal sites were devoid of trees before the 20th century. [ABSTRACT FROM AUTHOR]

• The past and present occurrence of insect disturbance on white spruce ( Picea glauca) trees was evaluated at their northern range limit on the eastern coast of Hudson Bay, and its effects on tree growth and population dynamics studied. • Three sites were sampled along an altitudinal gradient. Ring-width chronologies and stem analysis were used to evaluate tree growth. The occurrence of holes in the bark, of resin pockets and blue-stain fungi, and ring-width evidence for growth releases were used to assess the impact of bark beetle. • The white spruce population was established at these sites in the 17th century. Since their establishment, the spruce trees have developed a tree growth form, except at the uppermost site, where severe growth suppression occurred in the 19th century. Bark beetle and blue-stain fungi occurred with different timing and intensity. Their highest occurrence, associated with high mortality rates, was at the lowest site in the late 20th century. In the uppermost sites, biotic disturbance has occurred since the 18th century, associated with evidence for mechanical disturbance. • The simultaneous arrival of white spruce in the area resulted in a synchronous onset of spruce beetle activity driven by tree ageing. Unfavourable climatic conditions affected tree growth severely in the most exposed sites. [ABSTRACT FROM AUTHOR]

Some of the most pronounced ecological responses to climatic warming are expected to occur in polar marine regions, where temperature increases have been the greatest and sea ice provides a sensitive mechanism by which climatic conditions affect sympagic (i.e., with ice) species. Population-level effects of climatic change, however, remain difficult to quantify. We used a flexible extension of Cormack-Jolly-Seber capture-recapture models to estimate population size and survival for polar bears (Ursus maritimus), one of the most ice-dependent of Arctic marine mammals. We analyzed data for polar bears captured from 1984 to 2004 along the western coast of Hudson Bay and in the community of Churchill, Manitoba, Canada. The Western Hudson Bay polar bear population decfined from 1,194 (95% CI = 1,020-1,368) in 1987 to 935 (95% CI = 794-1,076) in 2004. Total apparent survival of prime-adult polar bears (5-19 yr) was stable for females (0.93; 95% CI = 0.91-0.94) and males (0.90; 95% CI = 0.88-0.91). Survival of juvenile, subadult, and senescent-adult polar bears was correlated with spring sea ice breakup date, which was variable among years and occurred approximately 3 weeks earlier in 2004 than in 1984. We propose that this correlation provides evidence for a causal association between earlier sea ice breakup (due to climatic warming) and decreased polar bear survival. It may also explain why Churchill, like other communities along the western coast of Hudson Bay, has experienced an increase in human-polar bear interactions in recent years. Earlier sea ice breakup may have resulted in a larger number of nutritionally stressed polar bears, which are encroaching on human habitations in search of supplemental food. Because western Hudson Bay is near the southern limit of the species' range, our findings may foreshadow the demographic responses and management challenges that more northerly polar bear populations will experience if climatic warming in the Arctic continues as projected. [ABSTRACT FROM AUTHOR]

Aim The species-specific response of tree-line species to climatic forcing is a crucial topic in modelling climate-driven ecosystem dynamics. In northern Québec, Canada, black spruce ( Picea mariana) is the dominant species at the tree line, but white spruce ( Picea glauca) also occurs along the maritime coast of Hudson Bay, and is expanding along the coast and on lands that have recently emerged because of isostatic uplift. Here we outline the present distribution, structure, dynamics and recent spread of white spruce from the tree line up to its northernmost position in the shrub tundra along the Hudson Bay coast. We aimed to obtain a minimum date of the arrival of the species in the area and to evaluate its dynamics relative to recent climate changes. Location White spruce populations and individuals were sampled along a latitudinal transect from the tree line to the northernmost individual in the shrub tundra along the Hudson Bay coast and in the Nastapoka archipelago in northern Québec and Nunavut, Canada (56°06′–56°32′ N). Methods White spruce populations were mapped, and the position, dimension, growth form and origin (seed or layering) of every individual recorded. Tree-ring analyses of living and dead trees allowed an estimation of the population structure, past recruitment, growth trends and growth rate of the species. A macrofossil analysis was performed of the organic horizon of the northernmost white spruce stands and individuals. Radiocarbon dates of white spruce remains and organic matter were obtained. The rate of isostatic uplift was assessed by radiocarbon dating of drifted wood fragments. Results The first recorded establishment of white spruce was almost synchronous at all sites and occurred aroundad 1660. Spruce recruitment was rather continuous at the tree line, while it showed a gap in the northern shrub tundra during the first decades of the 19th century. A vigorous, recent establishment of seedlings was observed in the shrub tundra; only wind-exposed, low krummholz (stunted individuals) did not show any sexual regeneration. A period of suppressed growth occurred from the 1810s to the 1850s in most sites. A growth increase was evident from the second half of the 19th century and peaked in the 1880s and the 20th century. A shift from stunted to tree growth form has occurred since the mid-19th century. No sample associated with white spruce remains gave a date older than 300 14C yearsbp [calibrated age (cal.)ad 1430–1690]. Main conclusions White spruce probably arrived recently in the coastal tundra of Hudson Bay due to a delayed post-glacial spread. The arrival of the species probably occurred during the Little Ice Age. The established individuals survived by layering during unfavourable periods, but acted as nuclei for sexual recruitment almost continuously, except in the northernmost and most exposed sites. Warmer periods were marked by strong seedling recruitment and a shift to tree growth form. Unlike white spruce, black spruce showed no evidence of an ongoing change in growth form and sexual recruitment. Ecological requirements and recent history of tree-line species should be taken into account in order to understand the present dynamics of high-latitude ecosystems. [ABSTRACT FROM AUTHOR]

Examines the impact of model tuning on modelled climate change response in Hudson Bay sea ice. Pairs of values for the thermal conductivity of sea ice and thermal diffusivity of sea water; Identification on the diffusion of upstream scheme using a proxy model; Relation of ice thickness to the choice of pairs of values and the seasonal distribution of the warming.