Heterosis and genotype x environment interactions of juvenile aspens in two contrasting sites

Aspen seedlings from a combined interspecific mating of Populus tremuloides Michx. and Populus tremula L. and intraspecific polycross of P. tremuloides were grown under two contrasting environments, one being uniform farmland, and the other, a typical cutover aspen site. Juvenile growth traits, height, diameter, and volume, were measured to examine the heterosis of hybrids, genotype x environment interaction of the inter- and intra-specific crosses, and selection response in each environment. Interspecific hybrids showed superior growth over the intraspecific crosses on both sites, but the heterosis was much greater for all traits on the farmland than cutover site, 190% vs. 75% for 3-year volume, respectively. Highly significant genotype x environment interaction was found for all traits across the two sites. The hybrids with higher heterozygosity were not more stable across sites than intraspecific crosses with lower heterozygosity. The P. tremula seems to contribute more to variation for diameter and volume than P. tremuloides. Strong heterosis of the hybrids in stem volume may be largely due to heterotic gene action contributed from alleles of P. tremula. Selection based on interspecific full-sib family means is a more effective strategy to maximize the growth on the uniform farmland than selection based on intraspecific half-sib means.