RECENT FLUCTUATIONS OF THE LICHEN SPRUCE FOREST LIMIT IN SUB-ARCTIC QUEBEC

1 Black spruce (Picea mariana) experienced increased radial growth and stem height over the last 100 years at treeline in subarctic Quebec, suggesting warmer and snowier conditions. We tested if recent climate change also induced a shift from forest to krummholz during the Little Ice Age and stand reversion from krummholz to forest during the 1900s. Whether the shifts caused measurable displacements of the forest limit were also examined. 2 Growth forms of living and dead spruces were compared in five lichen-spruce stands located 0, 2, 4, 6 and 8 km south of the krummholz limit. The age structure of supranival shoots (stems standing above the snow cover) at each site was also determined. 3 A reversion from lichen-spruce forest to krummholz probably occurred during the mid-1800s. Since the late 1800s, the forest limit moved 4 km northward most likely in response to milder winter conditions. Krummholz changed progressively to forest as spruce height and frequency of the tree growth form increased. Thus the northward advance of the forest limit resulted from structural changes of pre-established spruces, whereas there was no evidence for a recent spruce establishment in the tundra.