Fossil and genetic evidence of glacial refugia for the boreal-forest species of North America

Background/Question/Methods

Glacial-interglacial cycles have resulted in large-scale shifts in species distributional ranges and provide a unique opportunity to study biotic responses to large, rapid climatic perturbations. Pollen and macrofossil records from lake, fluvial, and permafrost sediments provide evidence for the spatial and temporal dynamics of major boreal-forest taxa over the past 20,000 years. Phylogeographic surveys have further uncovered cryptic refugia difficult to detect with paleoecological analyses. In this presentation, we synthesize recent advances in this area of research and highlight the strength of combining fossil and genetic data to shed light on the whereabouts of glacial refugia and postglacial colonization histories of several key boreal tree species in North America.

Results/Conclusions

Recent re-analysis of pollen data and new macrofossil evidence support the existence of multiple glacial refugia for most North American boreal tree taxa, including Picea glauca (white spruce), Picea mariana (black spruce), Pinus banksiana (jack pine), Populus balsamifera (balsam poplar), and Betula (birch) tree species. Studies utilizing neutral variation in organellar and nuclear genomes revealed refugia south of the continental ice sheets for P. glauca, P. mariana, and P. banksiana. In addition to a well-known large refuge in the southeastern U.S., refuge populations likely existed in the Intermountain West and central U.S. regions, and on submerged continental margins off the coast of Nova Scotia and Newfoundland. Spatial patterns of chloroplast, mitochondrial, and/or microsatellite DNA markers also indicate glacial refugia in eastern Beringia north of the ice sheets for P. glauca and P. mariana. However, current genetic data suggest that different refuges had grossly varying post-glacial colonization successes, possibly related to glacial population sizes and geographic barriers. For example, although fossil evidence suggests that Populus trees survived the glaciation in Beringia, a chloroplast-DNA survey of P. balsamifera did not detect unique alleles or higher diversity distinct from populations in boreal Canada, potentially indicating extensive admixture of alleles from central Canada. These patterns are reminiscent of those in P. glauca and P. mariana, where surviving “Beringian alleles” are rarer than those putatively introduced from Canadian populations. However, existing data do not offer clear information on the number and population sizes of cryptic glacial refugia. Next generation genomic tools, such as Illumina sequencing of restriction-site associated DNA markers (RADseq), can yield an unprecedented number of genetic markers to help pinpoint refugia and postglacial colonization histories. Such data are necessary for testing ecological hypotheses for the differential colonization success of refugial populations.