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 Canada. Hindcast Species Distribution Modeling suggests suitable climate conditions for larch in Alaska during the LGM corroborating our genetic data in suggesting the presence of an Alaskan LGM refugium for larch. Overall evidence from this system suggests that either suitable LGM climate, adaptation from limited standing variation, or an interplay of both likely promoted in situ persistence of larch in Alaska. Regardless of which mechanism facilitated persistence, the results from this study indicate that long distance dispersal did not play any significant role in the modern development or maintenance of the disjunct Alaskan populations of Larix laricina.
Uncovering a cluster of cryptic LGM refugia for Alnus in Alaska and adjacent Canada suggests that adaptation potentially facilitated in situ persistence for boreal taxa during rapid climate shifts. To understand the role of adaptation within the heterogeneous climate landscape of Alaska, Picea marinara (black spruce) populations were sampled from contrasted regions of local climate. Specifically, foliar samples were taken from both warmer, drier sites in the interior as well as comparatively cooler, wetter sites closer to the southern coast of Alaska. Using a combination of multivariate and univariate genotype-by-environment approaches, I investigated the relationship between genetic variability (genotyping single nucleotide polymorphism array) and downscaled climate variables for each sampling site to provide insight into possible local adaptive responses. Intersecting results from these analyses identified 9 SNPs that displayed significant associations with climate differences between these two sampling regions, which are usually interpreted as the signature of putative local adaptation to environmental selective pressure. My results also indicated long-term persistence of black spruce in interior Alaska implying that the interior populations were exposed to millennia of selective pressure for comparatively lower effective moisture than surrounding regions that also harbored black spruce during the last glacial-interglacial cycle. The genomic imprint detected in this study, consistent with ongoing adaptation to contrasted modern-day selective pressures, could also reflect long-term adaptive response to Late Quaternary environmental conditions
