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Variability in the sensitivity of arctic and alpine treeline to climate change.
Lloyd, Andrea1, Fastie, Christopher1, 1
ABSTRACT- Although it is widely expected that boreal forest taxa will advance into tundra communities as climate warms, the transient dynamics of the forest-tundra ecotone remain uncertain. High-resolution reconstructions of forest history can be used to assess transient community responses to past changes in climate, and can thus be useful in projecting scenarios of future ecological change. As an example of how paleoecological studies can inform our understanding of transient ecosystem responses to climate change, we present reconstructions of recent forest-tundra ecotone dynamics at nine sites in Alaska. White spruce advanced into tundra at eight of the nine study sites, but spatio-temporal dynamics of spruce expansion differed among sites. Spruce expansion at alpine treeline began later, affected a smaller area, and was more heavily influenced by changes in spruce growth form than at arctic treeline. Disturbance played two opposing roles in mediating the sensitivity of the forest-tundra ecotone to climate. At the arctic treeline, spruce expansion into tussock tundra communities with poorly drained, permafrost-affected soils appeared to be contingent upon some degree of permafrost melting. At alpine treeline, disturbance by fire may reduce forest responsiveness to climate. This retrospective analysis of the dynamics of the forest-tundra ecotone thus suggests that highly lagged responses to warming are unlikely, but that the sensitivity of the ecotone to climate change has varied according to landscape context and disturbance history.
KEY WORDS: arctic treeline, climate change, boreal forest, Picea glauca