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Herbivores can modify the rate of shrub and treeline advance. Both direct and indirect effects of herbivory may simultaneously interact to affect the growth rates of plants at this ecotone. We investigated the effect of snowshoe hare herbivory on the height of white spruce at two treeline locations in Alaska, USA. White spruce is expanding its distribution both upwards in elevation and northward in latitude because of climate warming, and snowshoe hares are already present in areas likely to be colonized by spruce. We hypothesized that herbivory would result in browsed individuals having reduced height, suggesting herbivory is a direct, negative effect on spruce treeline advance. We found an interactive effect between browsing history and spruce age. When young (under 30 years old), individuals that were browsed tended to be taller than unbrowsed individuals. However, older seedlings (over 30 years old) that had been browsed were shorter than unbrowsed individuals of the same age. Hares suppress faster growing individuals that are initially taller by preferentially browsing them as they emerge above the winter snowpack. This reduced height, in combination with increased mortality associated with browsing, is predicted to slow the advance of both latitudinal and altitudinal treeline expansions and alter the structure of treeline forests. [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]

1 Boundaries between forest and tundra ecosystems, tree lines, are expected to advance in altitude and latitude in response to climate warming. However, varied responses to 20th century warming suggest that in addition to temperature, tree line dynamics are mediated by species-specific traits and environmental conditions at landscape and local scales. 2 We examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada. Dendroecological techniques were used to reconstruct changes in density of the dominant tree species, white spruce ( Picea glauca), and to construct static age distributions of willow ( Salix spp.), one of two dominant shrub genera. Data were analysed to identify periods and rates of establishment and mortality and to relate these to past climate. 3 Tree line elevation and stand density increased significantly during the early to mid 20th century. However, this change was not uniform across sites. Spruce advanced rapidly on south-facing slopes and tree line rose 65–85 m in elevation. Tree line did not advance on north-facing slopes, but stand density increased 40–65%. Differences observed between aspects were due primarily to the differential presence of permafrost. Additional variability among sites was related to slope and vegetation type. Results were less conclusive for willow, but evidence for an advance was found at two sites. 4 Increases in stand density were strongly correlated with summer temperatures. The period of rapid change coincided with a 30-year period of above average temperatures, beginning in 1920. The highest correlations were obtained using a forward average of 30–50 years, supporting the hypothesis that tree line dynamics are controlled more by conditions influencing recruitment than by establishment alone. 5 The changes observed at several sites are suggestive of a threshold response and challenge the notion that tree lines respond gradually to climate warming. Overall, the results provide further evidence to support the idea that the pattern and timing of change is contingent on local, landscape, and regional-scale factors, as well as species’ biology. [ABSTRACT FROM AUTHOR]

The treeline in Alaska is usually mapped as the limit of white spruce Picea glauca (Moench) Voss along the south slope of the Brooks Range and in western Alaska and as the limit of Sitka spruce Picea sitchensis (Bong.) Can. on Kodiak Island and in the Alaska Peninsula. In some localities black spruce Picea mariana (Mill.) B.S.P. and paper birch Bemia papyrifera Marsh, fonn components of the treeline community with white spruce. Of special interest are the groves of balsam poplar Populus balsamifera L. occurring on the north slope of the Brooks Range and to the west and southwest of the spruce treeline. There is some question as to whether trees survived in unglaciated portions of Alaska during the last glacial period. Spruce trees became prevalent about 10000 yr ago and may still be expanding to their climatic limits. In some areas of Alaska the treeline was more expanded during the Hypsithermal period than it is at present, but in other regions evidence for such an expansion is lacking. Treetine in some areas seems to have been stable for the last several centuries, but is has been expanding rapidly for the last 40 yr in entral and western Alaska and for a longer period on Kodiak Island. [ABSTRACT FROM AUTHOR]