Variance components of tree height (HT) and stem diameter at 1.3 m above the ground (DBH) were investigated for the eight open-pollinated families of Zelkova serrata (Thumb.) Makino planted with three different initial planting spacings in a progeny test site, Chiba, Japan. Parent–offspring correlations were also evaluated by using these families and their mother trees. The smallest values of HT and DBH were observed in the narrowest initial planting spacing (1.10 x 1.10 m) compared to those in medium (1.30 x 1.36 m) and wide (2.00 x 1.80 m) spacings, suggesting that adverse effects of competition with neighboring trees occurred on both height and radial growth. Similar to HT and DBH, the initial planting spacings also affected the genetic parameter estimates: the ratio of family variance component to total phenotypic variance showed the highest value in narrow initial planting spacing for both HT and DBH. Thus, family variance component might include competition effects, leading to biased genetic parameter estimates. In contrast, parent–offspring correlation coefficients showed the highest value in wide initial planting spacing where competition effect might be smaller. Therefore, the growth traits of Z. serrata might be inherited from the parent to the offspring when competition effect was small.
