Genetic variation and productivity of Populustrichocarpa and its hybrids. III. Structure and pattern of variation in a 3-year field test

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Abstract
Growth and morphology of 308 black cottonwood (Populustrichocarpa Torr & Gray) clones were studied at one plantation site in northwestern Washington. The material was derived from both open-pollinated seed and branch cuttings from 10 natural populations in major river valleys west of the Cascade Range, between central Oregon (latitude 44°44′) and southern British Columbia (latitude 49°05′). Most of the clones (258) were derived from open-pollinated seedlings, with approximately six from each of two to five selected parents per population. Fifty of the clones were ramets from five field-selected ortets per population. Each clone was represented by two ramets in each of two blocks in a randomized complete block design. Three-year height and diameter growth displayed the largest amount of variation at the clonal level, followed by lesser amounts at the family and population levels. A significant block × clone interaction, combined with some responses contrasting with earlier studies on a different site, suggests that the interaction of genotype with environment is important for this species. Weak clinal trends were observed, despite a large degree of within-population variation. Narrow-sense heritability estimates and their standard errors were 0.10 ± 0.08 and 0.13 ± 0.09 for 3rd-year height and diameter, respectively. Significant trait correlations were obtained between volume and branch characteristics. The implications of these results and their relationship to other ongoing work in black cottonwood are discussed.