Genetic variation and productivity of Populus trichocarpa and its hybrids. II. Biomass production in a 4-year plantation

Productivities of 5 black cottonwood clones from each of 10 source populations (50 clones in total) were compared at one plantation site in western Washington. The source populations are located west of the Cascade Mountains, between central Oregon and southern British Columbia. Most source stands represented populations in major river valleys. Individual clones were selected for superior form and growth rate. Spacing was 1.2 × 1.2 m (6944 stems•ha −1 ) and trees were harvested after 4 years. Considerable variation in mortality, height, and productivity were found among clones within individual stands and among stands. Southwestern clones were generally more productive. Heights of individual clones averaged between 8.5 and 11.8 m at 4 years. Average dry weight production of black cottonwood clones was 12.5 Mg•ha −1 •year −1 and varied between 5.2 and 23.1 Mg•ha −1 •year −1 for individual clones. Three new Populustrichocarpa × P. deltoides hybrids planted in the trial were generally more productive than black cottonwood or the 'Robusta' hybrid, with average heights varying from 11.5 to 12.2 m and dry weight production ranging from 15.6 to 27.8 Mg•ha −1 •year −1 for an average of 23.6 Mg•ha −1 •year −1 . The high yields in this experiment are attributed to favorable climate (1940 degree-days•year −1 ) 3 , cultural treatments (irrigation, N fertilization) and genetic constitution of certain clones. The data promise substantial gains in short-rotation productivity from combining clonal selection with interspecific hybridization.