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Aim A critical concern for boreal ecosystems centres on broad-scale responses to warming, i.e. declining growth and mortality, or enhanced growth and greater productivity. However, few studies have synthesized tree growth along biogeographic gradients to address this issue. This study develops a broader understanding of how growth of a dominant conifer has responded to recent warming near the western forest margin of Alaska. Location Alaska, United States. Methods Thirty Picea glauca sites in south-west Alaska (1216 trees ≥ 4 cm dbh) were evaluated for growth differences from the southern boreal forest to the western forest-tundra margin across low-elevation forests and woodlands, and altitudinal tree line. Regional climate records were used to evaluate (1) whether tree growth near western tree line showed greater sensitivity to temperature and/or precipitation than southern boreal sites, (2) if the climate-growth response varied through time, across ecoregions and ecosystems, with spruce beetle disturbance, and by tree age, and (3) if there was a temperature threshold that limited growth. Results Positive growth trends since the 1980s in many open stands were consistent with the predicted expansion of western and altitudinal tree line. However, growth levelled off with temperatures ≥ 13°C at all but altitudinal tree line. An increasingly positive effect of precipitation on growth occurred after 1985, particularly at boreal tree line and closed forest sites. Closed-canopy forests showed lower rates of growth, greater spruce beetle activity and less potential for resiliency to warming than other ecosystem types. Main Conclusions Warming has led to markedly different growth responses according to ecosystem type and ecoregion near the forest-tundra margin. Altitudinal tree line sites showed consistently positive growth with warmer temperatures in recent decades, whereas low-elevation forests had reduced growth and greater beetle mortality. Strong positive correlations between growth and summer-fall precipitation since the mid-1980s suggest that precipitation will become an increasingly important factor with further warming. [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]

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]

The influence of dry climates on white spruce (Picea glauca (Moench)Voss)) regeneration was examined by conducting surveys of seedlings and small trees that had regenerated naturally at 100 farm shelterbelts and plantations in southern Saskatchewan, Canada. The sites surveyed were located along a climate moisture gradient extending from the relatively moist boreal forest, across the aspen parkland, to the semi-arid prairie grasslands. Natural regeneration was greatest at sitesin the boreal forest and northern aspen parkland, decreased in the southern aspen parkland, and was negligible in the grassland zone. Furthermore, the few seedlings found in the drier zones were usually in poor condition. Similar results were obtained for the introduced Colorado spruce (Picea pungens Engelm.) and Scots pine (Pinus sylvestris L.). It is concluded that the present climate of the southern parkland and grassland is too dry to permit natural regeneration of white spruce and other conifers. If increases in atmospheric CO2 levels lead to a drier future climate in the southern boreal forest ofwestern Canada, the ability of conifers to regenerate naturally may be significantly reduced. [ABSTRACT FROM AUTHOR]