Variability, contingency and rapid change in recent subarctic alpine tree line dynamics

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