Distribution of genetic variation in five coppice growth traits among natural populations of seven North American willow (Salix) species

Genetic variation and population structure in biomass yield and coppice growth traits were assessed in seven native North American willow species (Salix amygdaloides (AMY), Salix bebbiana (BEB), Salix discolor (DIS), Salix eriocephala (ERI), Salix humilis (HUM), Salix interior (INT), and Salix nigra (NIG)) established together in common-garden field tests on two sites. Differences in biomass yield, coppice stem number, and average single-stem mass were significant at the site, species, population, and genotype (clonal) levels. There were also species x site interactions. Analyses of variance components for these traits showed that only 3%-5% of the total variation in these traits was due to site differences, whereas genetic variation at the species, population, and genotype levels accounted for approximately 10%-39%, 5%-13%, and 12%-23%, respectively. Populations were a significant source of variation in some willow species (e.g., AMY, DIS, ERI, and INT) but not in other species. Tree willows were less prolific in stem sprout production than shrub willows, and ERI coppices produced by far the highest number of stem sprouts per coppice. This multispecies investigation demonstrated strong species and clonal differences, but variation among populations within a species, although significant, was relatively small, indicating that major growth and yield gains can be made through proper species selection and clonal selection within local populations.