Genetic population structure and adaptation to climate across the range of eastern white pine (Pinus strobus L.) and western white pine (Pinus monticola Douglas ex D. Don).

Under rapid global warming, it is critical to better understand the capacity of forest trees to adapt to a changing climate. Western white pine (Pinus monticola Douglas ex D. Don) and eastern white pine (P. strobus L.) are species at higher risk to climate change, as both have fragmented ranges and have suffered declines due to harvesting, fire suppression, and the white pine blister rust. We identified and compared patterns of genetic diversity and adaptation to climate in both species using a set of 267 orthologous genes. These genes included candidates for growth, bud phenology, and resistance to biotic and abiotic stresses. Genotyping resulted in 158 and 153 successful SNPs for P. monticola and P. strobus, respectively. Each set of SNPs was genotyped on range-wide samples of 362 P. monticola individuals (61 populations) and 840 P. strobus individuals (133 populations). Analyses were conducted separately in each species. STRUCTURE analyses identified two genetic clusters in each species, corresponding to north-south genetic discontinuities, as well as weak hierarchical sub-structure within each of those groups. We found evidence of local adaptation in both species. FST outlier analyses revealed that ∼7 percent and ∼10 percent of SNPs were under selection in P. monticola and P. strobus, respectively. Environmental association methods identified that ∼38 percent of P. monticola SNPs and ∼47 percent of P. strobus SNPs were correlated with climate. Strong candidate genes for future adaptation studies were identified: 7 genes in each species were detected by at least 2 methods and 22 candidate genes were common to both species. These genes were involved in growth, bud phenology, and response to abiotic and biotic stress. The implications of these findings for the conservation of white pine populations under climate change are discussed.Forestry, Faculty ofGraduat