Genotypic variation and heritability of growth and adaptive traits, and adaptation of young poplar hybrids at northern margins of natural distribution of Populus nigra in Europe

The study aimed at estimating the adaptation, genotypic variation and heritability of growth and other adaptive traits of poplar hybrids at the northern margins of the natural distribution of Populus nigra in Europe, and to compare with the performance of native aspen hybrids from the middle of their geographic ranges. A set of 105 clones of poplar hybrids originated from crossing Populus deltoides, P. nigra, Populus trichocarpa, Populus maximowiczii and Populus balsamifera and aspen hybrids obtained from crossing Populus tremula, Populus tremuloides and Populus alba were studied in two juvenile clonal trials in Lithuania. ANOVA revealed a significant hybrid effect for growth traits and bud flush phenology. P. maximowiczii × trichocarpa had the highest height increment followed by P. tremuloides × tremula. Most adaptive traits were under strong genetic control, with 0.61-0.94 heritability. Genotypic variation for height increment ranged 19.40-58.10%. Hybrid × site interaction was significant for height growth. The highest plasticity was found in P. balsamifera × trichocarpa and P. nigra, with ecovalences reaching 21.4-24.7%. The low inter-site (B-type) genotypic correlations and significant clone × site interaction found for most growth traits and autumn phenology indicated a presence of genotypic variation in plasticity/response of clones. Negative correlations between autumn leaf shed phenology and tree condition and survival indicate that clones with late growth cessation suffer from adaptation problems. Thus poplar breeding in Northern Europe has to incorporate hardy species from harsher climates of North America to maximize both tree growth and survival to match phenology to local climates.