Wood logs of native aspen were obtained from natural stands growing in northern Wisconsin, USA. Aspen was tested because it is a native species with very low recalcitrance shown by our previ- ous work [19,20]. In addition, fourteen-year-old trees of poplar clones NE222, DN5, and NM6 were harvested from the US Forest Service, Hugo Sauer Nursery in Rhinelander, WI. Specific trees were selected based on size, tree form, and overall health (e.g., lack of
disease). The genotypes were selected because they belonged to the most common genomic groups (P. deltoides