Evaluation and selection of hybrid aspen (Populus tremuloides x Populus tremula) clones | Hibrīdapšu (Populus tremuloides x Populus tremula) klonu salīdzināšana un atlase

Short rotation plantations of Populus species or there hybrids are grown in many countries. Hybrid between American aspen (Populus tremuloides Mich.) and common aspen (Populus tremula L.) and that demonstrates heterosis effect in first generation is most appropriated for Latvian conditions. Demand for aspen wood increased in mid-nineties of previous century, due to development of paper-industry. Newly created chemically-mechanical paper production technology was more nature-friendly and suitable for high-quality paper production. It required, however, raw material with certain properties and quantities that could be produced by specially selected clones in hybrid aspen plantations. This development triggered hybrid-aspen breeding activities in Latvia. Selection of hybrid aspen clones is a 3 step process: 1) evaluation of growth and quality traits in trials; 2) evaluation of wood properties of fast growing clones; 3) estimation of in vitro propagation coefficient of otherwise superior clones. Results of first step are demonstrated based on data from 3 trials, where tree height and diameter has been measured, stem straightness, branch thickness and angle visually assessed (in 3 grades) and number of spike knots noted. Highest heritability values was found for tree height (H**2=0,29-0,47), therefore this trait were chosen as main selection criteria. Selection diff erential above 10% was noticed for several clones and was especially high for clone Nr. 9 (2,1 m above average in experiment – 15,1 m). Clone Nr. 9 is among 5 best in ranking also according to stem volume and breast height diameter. Evaluation of wood properties (second selection step) reveal that wood density can be used as a selection criteria, since it has high correlation bending and compression strength (r= 0,91 and r=0,94 respectively) as well as with wood calorific value. Selection differential of cellulose and lignin content reached 10-15%, demonstrating notable differences in suitability of clones for paper-industry. In vitro proliferation coefficient at least 5 is required for efficient industrial propagation, which is rather high in comparison to average and stipulates high selection intensity in third selection step. In the register of forest reproductive material 18 hybrid aspen clones have been included. These clones are selected according all four criteria – productivity, stem and branch quality, wood properties and coefficient of in vitro propagation, and are therefore suitable for establishment of plantations.