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Summary 1The northward expansion of the boreal forest vegetation zone is generally predicted under a warmer doubled CO 2 , but the delay associated with vegetation development processes often has been overlooked. In the subarctic forest-tundra of northern Québec, reforestation of tundra uplands appears currently limited by the poor reproductive capacity of shrubby black spruce ( Picea mariana ), and the development of erect stems through accelerated height growth should be the first registered response to 20th century climate warming. The subarctic forest-tundra is characterized by small-and large-scale heterogeneity in topography, vegetation structure and climate. This spatial heterogeneity, added to the complexity of tree growth-climate relationships, can cause various growth responses of subarctic tree line black spruce to 20th century climate change. 2 Twenty spruce populations at subarctic tree lines and seven isolated clones at the species limit were sampled along a > 300-km latitudinal transect from the southern forest-tundra to the shrub tundra. Height growth patterns of black spruce at tree line and above tree line were examined (i) over their life span, using dendrochronological dating of stem cross-sections, and (ii) for the recent decades, using leader shoot elongation measurements. Indexed elongation chronologies were compared with regional climate data. 3 Height growth of tree line trees generally decreased with increasing latitude. However, tree line trees in the northern forest-tundra have experienced an acceleration of height growth since the 1970s, with their growth comparable to that of trees in the southern forest-tundra. Height growth response of spruce trees appeared increasingly delayed from the northern forest-tundra to the species limit. Above the subarctic tree line, windexposed conditions obscured the decrease in height growth with latitude observed for tree line trees. 4 Leader shoot elongation of spruce trees established on tundra hilltops appeared more controlled by summer heat sums than those at tree line all over the forest-tundra, except at the arctic tree line. Winter precipitation also was linked to leader shoot elongation in some forest-tundra sites. The increasing snow cover associated with recent warming appeared to have reduced the shoot elongation of spruce at forest margins showing the steepest slopes, hence subjected to snow overloading. 5 In the northern forest-tundra sites, the recent increase in height growth and positive trend in leader shoot elongation, consistent with a 1990s' increase in heat sums, point to the development of spruce krummholz into erect growth forms. In the southern foresttundra, reforestation of tundra hilltops and northward expansion of the boreal forest predicted under doubled CO 2 conditions could be delayed, as suggested by suppressed height growth of spruce above tree line.

Aim The predictions from biogeographical models of poleward expansion of biomes under a warmer 2 · CO2 scenario might not be warranted, given the non-climatic influences on vegetation dynamics. Milder climatic conditions have occurred in northern Québec, Canada, in the 20th century. The purpose of this study was to document the early signs of a northward expansion of the boreal forest into the subarctic forest-tundra, a vast heterogeneous ecotone. Colonization of upland tundra sites by black spruce (Picea mariana (Mill.) BSP.) forming local subarctic tree lines was quantified at the biome scale. Because it was previously shown that the regenerative potential of spruce is reduced with increasing latitude, we predicted that tree line advances and recent establishment of seedlings above tree lines will also decrease northwards.Location Black spruce regeneration patterns were surveyed across a > 300-km latitudinal transect spanning the forest-tundra of northern Québec, Canada (55°29¢-58°27¢ N).Methods Elevational transects were positioned at forest-tundra interfaces in two regions from the southern forest-tundra and two regions from the northern forest-tundra, including the arctic tree line. The surroundings of stunted black spruce, forming the species limit in the shrub tundra, were also examined. Position, total height and origin (seed or layer) of all black spruce stems established in the elevational transects were determined. Dendrochronological and topographical data allowed recent subarctic tree line advances to be estimated. Age structures of spruce recently established from seed (< 2.5 m high) were constructed and compared between forest-tundra regions. Five to 20-year heat sum (growing degree-days, > 5°C) and precipitation fluctuations were computed from regional climatic data, and compared with seedling recruitment patterns.Results During the 20th century, all tree lines from the southern forest-tundra rose slightly through establishment of seed-origin spruce, while some tree lines in the northern forest-tundra rose through height growth of stunted spruce already established on the tundra hilltops. However, the rate of rise in tree lines did not slow down with latitude. The density of < 2.5-m spruce established by seed declined exponentially with latitude. While the majority of < 2.5-m spruce has established since the late 1970s on the southernmost tundra hilltops, the regeneration pool was mainly composed of old, suppressed individuals in the northern forest-tundra. Spruce age generally decreased with increasing elevation in the southern forest-tundra stands, therefore indicating current colonization of tundra hilltops. Although spruce reproductive success has improved over the twentieth century in the southern forest-tundra, there was hardly any evidence Journal of Biogeography (J. Biogeogr.) (2005) 32, 849-862 ª 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi