Growth and phenology of hybrid aspen clones (Populus tremula L. x Populus tremuloides Michx.)

Height, basal diameter, diameter at breast height, bud burst, and leaf deve lopment were recorded in a 5-year-old hybrid aspen clonal trial. The field trial consisted of four aspen hybrid clones (Populus tremula x Populus trem uloides) and one local P. tremula seedling source. Phenological traits were observed in the 3rd year. Growth patterns were recorded during the 3rd and 4th years. Phenological traits were explored in relation to hybrid vigor e xpressed as growth traits. Differences were observed for phenological and g rowth traits among hybrid clones and P. tremula. The growth period varied f rom 143-158 days for the four hybrid clones, and was 112 days for P. tremul a. The correlation between growth period and yield was highly significant. The annual growth rate of height for the hybrids was 4.2 cm per 7 days (2.4 for P. tremula) in the 3rd year and 6.4 cm per 7 days (2.9 for P. tremula) in the 4th year. After 5 years, mean estimated stem volume of the hybrids was 3.9 times that of P. tremula. Significant clone by year interaction was observed for height, diameter, and volume growth. The hybrid vigor seems t o be mainly attributable to a longer growth period.