Harnessing natural variation in Canadian shrub willow

Shrub willows (Salix spp.) are fast-growing perennial woody plants that have long been promoted as renewable biomass feedstocks in Canada. Although willow breeding programs in Europe and the United States have advanced the development of improved cultivars, Canadian shrub willow remains comparatively under-studied. This dissertation explores patterns of natural genetic and phenotypic variation in two native shrub willow species, Salix famelica and Salix eriocephala. In this work, a range-wide collection of 324 genotypes originating from 33 sites was analysed using a genotyping-by-sequencing approach that generated 26,016 single nucleotide polymorphisms. These data were subsequently used to assess population structure and genetic diversity, revealing five primary genetic clusters that were largely structured by geography and geoclimatic variables, including mean annual precipitation and number of frost-free days. These findings provide genetic support for previously reported species circumscriptions that were based primarily on morphology. Cell wall composition, wood ultrastructure, and saccharification potential were assessed in 338 xylem samples from the same collection. This analysis revealed substantial variation in wood chemistry and ultrastructure, and highlighted lignin content and syringyl-to-guaiacyl lignin ratio as important predictors of glucose release following mild alkali pretreatment and enzymatic digestion. Finally, genome-wide association studies identified marker–trait associations related to growth and development, biotic and abiotic stress resilience, cell wall composition, and bioenergy potential. These marker–trait associations provide an array of future breeding targets for the genetic improvement of willow. This work demonstrates that Canadian shrub willow contains substantial natural variation and establishes a foundation for the evaluation, selection, and development of locally adapted genotypes.