Vessel and fiber properties, diameter growth, and chemical compositions were investigated for transgenic Populus trichocarpa (black cottonwood) trees harvested after six months of growth in a greenhouse. Genetic modifications were cinnamyl alcohol dehydrogenase (CAD), cinnamate 3-hydroxylase (C3H), or cinnamate 4-hydroxylase (C4H), which resulted in modified lignin composition or content, and changed the syringyl-to-guaiacyl ratio. Comparing the genetic groups to the wild-type as the control, trees with reduced lignin content showed different results for vessel and fiber properties measured. Genetic groups with reduced PtrC3H3 and PtrC4H1&2 (with lower lignin content and higher S/G ratio than the control) exhibited splitting perpendicular to the rays, yet had the same fiber lumen diameter and the same fiber cell wall thickness as the control. Changes in lignin structure from modifications to PtrCAD resulted in reductions to the number of vessels, increases in vessel and fiber diameters, and had no consistent impact on stem diameter.