Genetic variation in a 26‐year old hybrid aspen trial in southern Sweden

A 26‐year‐old hybrid aspen trial has been evaluated for growth capacity, survival, quality and damage. The trial was situated in southern Sweden and included pure Populus tremuloides MICHX. and Populus tremula L. and both kinds of hybrids between these species. The major part of the trial consisted of tremula × tremuloides hybrids and the analysis was focused on an 11 × 7 factorial containing 37 hybrid families. Among the tremula × tremuloides hybrids the volume production per hectare varied between half sib families from 12 to 21 m3 ha‐1 year‐1 as an average over the 26 years. The average density was 380 kg m‐3. For height, diameter, and wood density we found heritabilities between 0.2 and 0.3 and there seemed not to be any tendency for the trees with high volume production to have low wood density or poor survival. The stems has considerable quality defects. Twenty‐seven percent of the trees had sinuous bends on the lower part of the trunk and 19 percent had long bends in the same region. Hypoxylon canker and bark stripping caused considerable damage. The mortality from age 12 to 26 is slightly more than one third. Survival and bark stripping were genetically controlled with heritabilities of 0.3 and 0.5, respectively. This study does not give any evidence that the geographic origin within Sweden for the P. tremula female and within north eastern USA or southern Canada for the P. tremuloides has to be considered when screening for well growing hybrids. The parent species were not found to differ from the hybrid aspens as regards occurrence of damage but they were inferior in growth. It is warned against a large scale reintroduction of hybrid aspen because of the high level of damage found in this trial and because of the bad experience internationally of Hypoxylon mammatum in the popular section which P. tremula and P. tremuloides belong.