Genetic variation and productivity of Populustrichocarpa and its hybrids. I. Morphology and phenology of 50 native clones

Morphological and phenological variation among five black cottonwood (Populustrichocarpa Torr. & Gray) clones from each of 10 natural populations were studied at one plantation site in western Washington. Source populations are located in major river valleys, west of the Cascade Mountains, between central Oregon (latitude 44°44′ N) and southern British Columbia (latitude 49°05′ N). Populations and individual clones were selected for superior form and growth rate. Tested at a spacing of 1.2 × 1.2 m, the 50 clones displayed a large range of variation in 15 leaf, branch, and phenology characters studied. Clones and populations differed significantly in 14 and 10 characters, respectively. Population means for seven characters and a character association (principal component) varied clinally with source latitude, longitude, and (or) elevation. The most consistent genetic cline follows a geographic gradient from southwest to northeast: southwestern clones develop smaller leaves on more erect branches, and conti...