To mark the 50^th anniversary of the Centre d'études nordiques (CEN: Centre for Northern Studies), this commentary presents a history of CEN over the last 5 decades. We first address the socio-political context at the time that CEN was founded and the development of the centre over the first two decades of its existence, a period of emerging national and international interest in northern research. The subsequent evolution of CEN in terms of its research orientation is then described, showing the relationship with research priorities in Canada and Quebec over this period. The history of CEN underscores the ongoing importance of maintaining and developing field and laboratory infrastructure over the long term for research and training. This commentary also includes elements of personal reflection based on the experience of the first author, who is a long-time member and former director of CEN.

A full English language translation of this commentary is available online ( www.ecoscience.ulaval.ca).

The Great Whale River region on the eastern shore of Hudson Bay, Canada, encompasses the villages of Whapmagoostui (Cree First Nation) and Kuujjuarapik (Inuit) and surrounding areas. The principal field station of Centre d'études nordiques (CEN: Centre for Northern Studies) has operated at Whapmagoostui-Kuujjuarapik (W-K; 55° 15′ N, 77° 45′ w) since the 1970s, with diverse research projects on past and present environments. The climate at W-K is strongly influenced by the proximity of Hudson Bay, and the recent pronounced loss of sea ice in this sector of northern Canada has been accompanied by large increases in air temperature. Discontinuous or scattered permafrost occurs throughout the region and is degrading rapidly. The W-K region continues to experience particularly rapid isostatic uplift in response to the retreat of the Laurentide Ice Sheet. Parabolic dunes occur along the coast and are strongly influenced by the plant cover. Paleoecological studies have documented the Holocene evolution of landscapes, including lakes, wetlands, and forests. The vegetation type is coastal forest tundra, with some 400 recorded species. Studies on certain insect groups provide a baseline for assessing future ecological change. The first signs of human occupation in the W-K region have been dated at 3800 BP. The arrival of the Hudson's Bay Company in the 18^th century marked the onset of continuous occupation. Rapid social, economic, and environmental change initiated in the mid-20th century continues to this day.

This review provides a synthesis of limnological data and conclusions from studies on ponds and small lakes at our research sites in Subarctic and Arctic Canada, Alaska, northern Scandinavia, and Greenland. Many of these water bodies contain large standing stocks of benthic microbial mats that grow in relatively nutrient-rich conditions, while the overlying water column is nutrient-poor and supports only low concentrations of phytoplankton. Zooplankton biomass can, however, be substantial and is supported by grazing on the microbial mats as well as detrital inputs, algae, and other plankton. In addition to large annual temperature fluctuations, a short growing season, and freeze-up and desiccation stress in winter, these ecosystems are strongly regulated by the supply of organic matter and its optical and biogeochemical properties. Dissolved organic carbon affects bacterial diversity and production, the ratio between pelagic and benthic primary productivity via light attenuation, and the exposure and photoprotection responses of organisms to solar ultraviolet radiation. Climate warming is likely to result in reduced duration of ice-cover, warmer water temperatures, and increased nutrient supplies from the more biogeochemically active catchments, which in turn may cause greater planktonic production. Predicted changes in the amount and origin of dissolved organic matter may favour increased microbial activity in the water column and decreased light availability for the phytobenthos, with effects on biodiversity at all trophic levels, and increased channelling of terrestrial carbon to the atmosphere in the form of greenhouse gases.

It is increasingly recognized that ecosystems are not closed systems and that exchanges of resources across ecosystem boundaries can have repercussions on food webs, especially in low productivity systems such as the terrestrial Arctic. However, because these exchanges can take multiple forms, assessing their significance in the functioning of the tundra food web is difficult. In this paper, we first review some important concepts related to resource exchanges between ecosystems and examine their relevance to the study of trophic interactions in the arctic tundra. An analysis of the Bylot Island food web in the Canadian Arctic using a mass-balance trophic model suggests that predators are the dominant force controlling this food web. However, an important feature of this ecosystem is that several top predators benefit from allochthonous inputs, either through the presence of migratory birds during the summer or the use of the marine environment as a foraging ground in winter. We also show that migratory birds may act as autochthonous resource exporters for lower trophic levels, for instance by removing nitrogen from the nutrient pool when young produced locally migrate south and die away from the system. Although these resource exchanges may be a general feature of several arctic terrestrial ecosystems, their importance in the functioning of the tundra food web remains to be determined. Through long-term monitoring, we found that primary production in wetlands of Bylot Island increased by 85% over a 20-y period, likely a consequence of the warming trend observed in the area. However, we have not detected any changes at higher trophic levels, which is consistent with a top-down control of this food web. Given the importance of resource exchanges between ecosystems in the dynamics of the tundra food web, a full investigation of the effects of climate change will require a broader cross-ecosystem perspective.

Global circulation models predict that the strongest and most rapid effects of global warming will take place at the highest latitudes of the Northern Hemisphere. Consistent with this prediction, the Ward Hunt Island region at the northern terrestrial limit of Arctic Canada is experiencing the onset of major environmental changes. This article provides a synthesis of research including new observations on the diverse geosystems/ecosystems of this coastal region of northern Ellesmere Island that extends to latitude 83.11° N (Cape Aldrich). The climate is extreme, with an average annual air temperature of -17.2 °C, similar to Antarctic regions such as the McMurdo Dry Valleys. The region is geologically distinct (the Pearya Terrane) and contains steep mountainous terrain intersected by deep fiords and fluvial valleys. Numerous glaciers flow into the valleys, fiords, and bays, and thick multi-year sea ice and ice shelves occur along the coast. These extreme ice features are currently undergoing rapid attrition. The polar desert landscape contains sparse, discontinuous patches of vegetation, including dense stands of the prostrate shrub Salix arctica (Artic willow) at some sites, and 37 species of vascular plants on Ward Hunt Island. Diverse aquatic ecosystems occur throughout the area, including meromictic, epishelf, and perennially ice-covered lakes. Many of these have responded strongly to climate shifts in the past and like other geosystems/ecosystems of the region are now sentinels of ongoing global climate change.

In many areas of North America and Europe, population densities of large herbivores are increasing and strongly affecting species composition and structure of plant communities. Although reduced resources associated with increasing density affect life history traits of large herbivores, their effects on foraging behaviour have received little attention. We experimentally controlled population density in large enclosures to assess how increasing density affected white-tailed deer (Odocoileus virginianus) space use in relation to forage biomass and cover at a fine scale. We quantified space use in 3 blocks, each with 2 enclosures, one containing deer at a density of 7.5 deer·km^-2 (low density) and the other containing deer at a density of 15 deer·km^-2 (high density). We interpolated forage biomass, lateral cover, and canopy cover in space by kriging and divided deer observations (radiolocations) into 3 diel-periods: dawn/dusk, day, and night. Deer space use was positively related to forage biomass and negatively related to lateral cover at both densities, but it was not affected by the diel-period. Deer increased the use of areas with dense canopy cover at low density, but not at high density. Population density thus modified deer resource use by constraining deer at high density to forage where canopy cover is lower but forage biomass higher. Our results provide evidence of density dependence in foraging decisions, as deer space use patterns appeared to be based more strongly on forage biomass than on cover, particularly when population density was high.

In the James Bay region of northwestern Quebec, earlier pollen data suggests that regional expansion of jack pine occurred around 4500–4000 cal BP, a period for which the species' pollen curve shows a marked increase in pollen diagrams. New macrofossil data from charred dune paleosols in the Radisson area indicates that the species has been present in this region since at least 6300 cal BP, despite a representation of only about 1% in pollen assemblages. The postglacial migration of jack pine is an example of low-density migration. The first individuals were probably restricted to dry sites such as eolian deposits and river terraces, upon which more frequent fires had a greater impact.

Lichen—spruce woodlands occur in the closed-crown forest zone as a divergent type of the spruce—moss forest because of regeneration failure caused by compounded disturbances (fire, insect outbreaks, and logging). From the southern limit of distribution of lichen woodlands (47° 30′ N) to the northern limit of the closed-crown forest zone (52° 40′ N), 53 lichen woodlands were sampled for a detailed dendroecological analysis. Radial, height, and volumetric growth of black spruce (Picea mariana) trees among the woodlands varied significantly according to stand age. Growth rates were similar for all even-aged spruce trees, whereas growth rates of trees < 100 y old were significantly greater than those of trees > 100 y old. No significant differences were found in growth rates of spruces distributed along the latitudinal gradient. Spruce trees were generally small in all the woodlands studied, with only a few trees taller than 12 m. Although spruce budworm may have affected all stands, the timing and intensity of the infestations were not necessarily synchronous among the studied stands. Partial least square response (PLS) functions were calculated to evaluate the impact of climate on tree-rings with latitude. Along the 600-km transect, PLS response functions indicated that growth was strongly influenced by current and previous year climatic conditions, particularly precipitation.

Monitoring the body condition of ungulates is often considered an efficient way to assess habitat quality. It is therefore essential to select adequate measures to describe individual body condition. Because there is no consensus on which measurement(s) can best describe individual variability in body condition, field biologists often measure several variables, increasing processing time. From 2007 to 2009, we assessed body condition of female-calf pairs in 2 herds of migratory caribou in Northern Quebec/Labrador, Canada, using multiple measurements of size, mass, and fat depth. We sought to identify, using multivariate analysis, which measurement(s) had the greatest influence on a composite measure of body condition of females and calves at calving and weaning. Our results indicate that adult females are best described with a body bulkiness index opposing heavy and long/round-bodied females with high body protein reserves to light and short/ slender-bodied females with low body protein reserves. At weaning, adult females can also be differentiated by a body fat index opposing fat to lean females. Calf body condition is best described by mass at birth and by a combination of mass and size measurements at weaning, opposing heavy and tall individuals with high protein reserves to light and short ones with low protein reserves. Overall, body mass appears to be the measurement that best describes individual variability in body condition of females and calves at calving and weaning. Our systematic comparison of body condition measurements will provide field biologists with guidance for future data collection.

Milne Fiord in the Canadian High Arctic contains the last known ice-dammed fiord lake (epishelf lake) in the Northern Hemisphere. This freshwater ecosystem is retained by the Milne Ice Shelf and is underlain by sea water that is connected to the Arctic Ocean. Using microscopy, photosynthetic pigment analyses, and molecular techniques we examined the planktonic communities present in Milne Fiord to determine the biotic characteristics of the epishelf lake and the sea water below. Net sampling of the water column of Milne Fiord revealed a mixture of marine, freshwater, and brackish Zooplankton taxa, and high performance liquid chromatography (HPLC) pigment analysis showed pronounced differences in phytoplankton composition through the highly stratified water column. Chlorophytes dominated in the epishelf lake, prasinophytes prevailed in the halocline, and the bottom layer harboured mainly fucoxanthin-containing groups. Clone libraries of a dark-incubated, concentrated sample from below the halocline (30 m depth) yielded marine Archaea (mainly Crenarchaeota) and known bacterial taxa from the Pacific and Arctic oceans (e.g., Roseobacter, Oleispira, Colwellia). An equivalent sample from the epishelf lake (5 m depth) yielded many bacterial taxa that are characteristic of cold, freshwater habitats (e.g., Polynucleobacter, Variovorax, Flavobacterium), the euryhaline genus Polaromonas, and freshwater eukaryotes, notably ciliates. Similarly, denaturing gradient gel electrophoresis (DGGE) analyses of T4-like bacteriophages showed different viral assemblages in the upper and lower water column. This diverse, stratified ecosystem is dependent on the integrity of the bounding ice shelf and is therefore vulnerable to the ongoing effects of climate change in this region.