About 38% of indigenous vascular plants in Alaska and Yukon (609 of 1,620 taxa) were likely postglacial migrants from the Last Glacial Maximum (LGM) refugium that occurred in the United States. Their possible source areas were assessed using present-day plant presence and absence data, based on the assumption that members of the LGM flora still occur within the former refugial area. Present-day occurrences indicate that 42% of migrant taxa in Alaska-Yukon could have originated from anywhere within the Pacific Coast, Rocky Mountains, or Midwest portions of the refugium. Another 34% were only found in the two western areas, and 64-82% of all postglacial migrants and the majority of taxa unique to each source area were concentrated in westernmost Washington and Oregon and the northern Rocky Mountains (Montana, Wyoming and Idaho). The veracity of these specific refugial locations was supported by LGM (22,000-19,000 cal bp) archival phylogenetic tree data and pollen evidence. Macrofossil evidence was also available for westernmost Washington-Oregon. Alaska-Yukon taxa which originated from westernmost Washington-Oregon likely migrated northwards along coastal British Columbia to southern Alaska. Taxa from the Rocky Mountains probably moved northwards through both inland British Columbia and along the lower eastern slopes of the Rocky Mountains after glaciers had receded sufficiently to allow their passage. Migrant taxa from the Midwest likely followed the eastern slopes route to Yukon. The purported LGM occurrence of Picea mariana (black spruce) in eastern Oregon was questioned, based on its present-day absence, no evidence of its past presence, and what would have been an uncharacteristic botanical association with non-boreal species.
