DISSERTATION RESEARCH:Seeking Evidence for Glacial Refugia of North American White Spruce

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This project aims to investigate the development of the boreal forest biome from the last glacial maximum through the Holocene (the past ~20,000 years). Two long-standing hypotheses exist about the history of white spruce (Picea glauca), a dominant tree species of North American boreal forests. The first posits that refuge populations were restricted to the southeastern United States. The second hypothesis involves the existence of small populations in the ice-free areas of eastern Beringia (Alaska and adjacent Canada) during the last glacial maximum, which expanded and merged with southern populations during the Holocene. Over the past decades, numerous paleoecological studies (pollen and macrofossil analyses) attempted to test these hypotheses. Unfortunately, previous studies yielded no unambiguous evidence because of inherent limitations of conventional paleoecological approaches. Emerging DNA techniques now promise to resolve these hypotheses. Preliminary chloroplast-DNA data from this project show unique haplotypes in Alaska and no genetic-diversity gradient across western North America, supporting the second hypothesis. The proposed research is important for understanding biotic response to climatic change. The existence of a white spruce refuge in Beringia implies that small populations are capable of surviving extremely severe climatic conditions for thousands of years. It also suggests that the migration rates of white spruce in response to climatic warming are much lower than previously thought. The innovative integration of molecular biology and paleoecology offers a model for future studies of long-term biotic response to climatic forcing. Equally important, this project provides research opportunities for graduate and undergraduate students.