| Summary: | Evidence has been accumulating that suggests some arcto-boreal plant taxa persisted through the Last Glacial Maximum (LGM) in Alaska and adjacent Canada. In part, my dissertation investigated the spatial patterns of glacial persistence and postglacial colonization of an alder (Alnus) species complex (n = 3 taxa) from this vast region. Using high-throughput DNA sequencing, hindcast Species Distribution Modeling, and a reassessment of pollen records, I found evidence that Alnus expanded from several population nuclei (i.e. refugia) that existed during the LGM and coalesced during the Holocene to form its present range. These results challenge the unidirectional model for postglacial vegetation expansion based on several decades of palynological studies, implying that climate buffering associated with landscape heterogeneity and adaptation to millennial-scale environmental variability played important roles in driving late-Quaternary population dynamics. Results from the sampled alder species complex support the conclusion that numerous plant taxa seem to have persisted in northern refugia within Alaska despite concerns of an adverse regional climate during the LGM. Another taxa, Tamarack larch (Larix laricina), presents as another plant species that might have survived within Alaska during this period of climatic upheaval, but sparse pollen data and a lack of genetic sampling have thus far obscured any solid insights. Unlike alder, larch has a transcontinental distribution across North America with a prominent disjunction in the Yukon resulting in the isolation of Alaskan populations from the primary distribution in Canada. Range disjunctions, such as the one sampled here, serve as natural laboratories that allow us to assess the interplay of long-distance migration versus refugial persistence in biome development since the LGM. In this case, genetic analysis of chloroplast microsatellites from sampling populations on both sides of the disjunction revealed a long-standing isolation between larch in Alaska and ...
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