Conservation at the Trailing Edge: Hybridization, Population Genomics, and Environmental Change in Quercus bicolor

Swamp white oak (Quercus bicolor Willd.) is a long-lived deciduous tree species found in eastern North America. Integrating geospatial and genomic analyses, this work presents a comprehensive study on the conservation and population genomics of this ecologically important species, with special attention to trailing-edge populations occurring in Tennessee (USA). Chapter 1 employs species distribution modeling to analyze the impacts of climate and land use change throughout the range of Q. bicolor. Projections indicate northward range shifts and significant losses of southern populations under future climate scenarios, while emphasizing the critical threats posed by existing and ongoing agricultural and urban development. Chapter 2 evaluates the central-marginal hypothesis (CMH), using RADseq genomic data to assess levels of population structure, genetic diversity, and introgression in range core populations of swamp white oak compared to trailing-edge populations. Widespread introgression across the southern range edge was uncovered, particularly from Q. lyrata, yet no introgression was detected in the range core. While the CMH’s prediction of reduced genetic diversity at range edges was not supported, increased population differentiation was observed. This study suggests that introgression may buffer range-edge populations against genetic erosion by introducing novel, adaptive alleles. Genomic cline and environmental association analyses further support adaptive introgression, identifying differentially introgressed and climate-associated genes linked to stress response. Hindcast and forecast ecological niche models suggest that a historically expansive hybrid zone could enlarge again with projected warming climates. Chapter 3 outlines the planning and implementation of genomically informed ex situ conservation for Tennessee's imperiled Q. bicolor populations. Geospatial and genomic data are leveraged to locate populations, assess introgression levels, and select genotypes that maximize genetic diversity while minimizing inbreeding for seed orchard development. Selected genotypes are cloned through whip-and-tongue grafting and will be planted into two duplicate swamp white oak seed orchards and one hybrid seed orchard to facilitate research, seed production, and the long-term preservation of these genotypes. The establishment of duplicated conservation seed orchards is a vital step for producing provenanced seed for climate-smart conservation and reforestation efforts, as well as maintaining a stable genetic repository of southern genotypes for perpetuity.