| The boundary between forest and tundra in mountainous areas, alpine treeline, is expected to advance as climate warming continues and change is likely to be pronounced in northern latitudes. I studied the white spruce ( Picea glauca (Moench) Voss) treeline in the Kluane region of southwest Yukon, Canada, with the objectives of: (i) characterizing its responses to past climate change, and (ii) assessing the influence of different environmental variables on spruce growth and treeline dynamics. Four investigations were conducted, each employing fundamentally different methodologies and occupying distinct levels within a hierarchical, scale-based, analytical framework. At the landscape scale, I mapped spruce distribution and abundance from a QuickBird2 multispectral satellite image. Decision tree analysis was used to characterize relationships with topoclimatic variables. Results demonstrated the primary role of temperature in limiting spruce distribution, but also identified substantial fine-scale variability. At the population level, dendroecology was used to reconstruct recent dynamics of spruce and grayleaf willow ( Salix glauca L.). Evidence of a rapid increase in treeline elevation or spruce density during the early-mid 20th century was found. There was a significant correlation between recruitment and reconstructed summer temperatures, indicating that climate was the cause of change. Comparison of aerial photographs from 1947 and 1989 verified stand reconstructions and showed that the change was widespread. At the organism level, I warmed 40 young spruce at treeline for 4-years using open-top chambers. Individuals responded positively to the treatment in both growth and physiology. There were also significant differences between individuals growing on north and south aspects. Collectively, my results suggest a significant potential for subarctic alpine treeline to advance in response to future climate warming. However, the rate, pattern, and timing of advance will be contingent on heterogeneity of environmental variables at finer scales, especially direct solar radiation and permafrost distribution. |
