Height growth response of tree line black spruce to recent climate warming across the forest-tundra of eastern Canada

1 The northward expansion of the boreal forest vegetation zone is generally predicted under a warmer doubled CO2, but the delay associated with vegetation development processes often has been overlooked. In the subarctic forest-tundra of northern Quebec, 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, wind-exposed 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 forest-tundra, reforestation of tundra hilltops and northward expansion of the boreal forest predicted under doubled CO2 conditions could be delayed, as suggested by suppressed height growth of spruce above tree line.