DISSERTATION RESEARCH: A hypothesis test for cryptic northern refugia in bitternut and shagbark hickory, with implications for migration and adaptation

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During the height of the ice age, glaciers covered large portions of North America. Temperate forests persisted in areas known as glacial refugia, such as the coastal plains of the southern United States. Some species may also have existed much farther north than previously thought possible, but evidence for these cryptic northern refugia is controversial. The proposed research will test whether two hickory species survived glaciation in cryptic refugia, possibly located in the southern Appalachians or Ozarks. Genetic variation will be assessed in populations of the two species from across the eastern United States, and patterns of variation will be statistically compared to patterns generated under computer simulations according to a variety of historical scenarios. For the first time, this will provide a measure of confidence in how likely it is that cryptic northern refugia existed. This aspect of forest history is important for understanding how tree populations have migrated and adapted to climate change in the past. The patterns of genetic variation detected will have implications for forestry and conservation biology, and as a resource for future research. The research itself promotes the progress of science by combining genetic analysis and paleogeographical reconstruction, and it has implications for national welfare, given the potential impacts a changing climate; the project also supports education and diversity.The survival of temperate species in southern refugia during the Last Glacial Maximum (21.5 Ka) is a classic phylogeographic paradigm, yet this paradigm has been challenged by new but inconclusive evidence suggesting the additional existence of cryptic northern refugia. Using two eastern North American hickory species, the research will analyze a large genetic dataset of single nucleotide polymorphisms (SNPs) using computer simulations and Approximate Bayesian Computation to test whether spatially-explicit demographic models containing both cryptic northern refugia and southern refugia better explain the data than models containing only traditional southern refugia. The rangewide SNP dataset will also allow exploratory analysis of patterns of local adaptation. This project will be the first to assess statistical support for cryptic northern refugia in any species, and will establish a genomic resource for future studies. Results will aid in better understanding past responses to climate change, including historical migration rates, patterns of adaptation, and geographic distributions of genetic diversity. The relevance to applied biology will be highlighted in a brochure distributed to a wide network of foresters and conservation biologists established through fieldwork. An existing University of Michigan program will also serve as a platform for discussing the project and its relevance to climate change with local high school students.