The growth period varied from 143–158 days for
the four hybrid clones, and was 112 days for P. tremula. The correlation between growth
period and yield was highly signifi cant. 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. Signifi cant clone by year interaction was observed for
height, diameter, and volume growth. The hybrid vigor seems to be mainly attributable
to a longer growth period.