Growth and wood properties of a 38-year-old Populus simonii × P. nigra plantation established with different densities in semi-arid areas of northeastern China

To characterize the effects of plantation densities on the growth characteristics (diameter at breast level, tree height and volume) and the common wood properties, 38-year-old Populus simonii × P. nigra clones planted with four levels of spacing (2 m × 2 m, 3 m × 3 m, 4 m × 4 m, and 5 m × 5 m) in a semi-arid area in northeastern China were examined. The results of ANOVA showed significant differences (P < 0.01) for all the investigated growth traits and wood properties under different plantation densities, except for the chemical composition of wood. The repeatability and phenotypic variations of all the traits varied from 0.34 to 0.99 and from 13.45 to 59.65%, respectively. Except for wood density, which was significantly negatively correlated with the growth traits, a positive correlation was observed between the growth traits and all the other wood mechanical properties. However, most of the correlations between the growth traits and the chemical composition of the wood were not significant. The path analysis for the wood mechanical characteristics and the growth in the prediction of volume were significant and ranged from 0.18 to 0.72 for wood density and diameter at breast height, respectively, while those for the chemical composition of wood ranged from 0.001 to 0.336, which showed a low impact on the volume. The highest stand volume (610 m3) per hectare was observed with the 2 m × 2 m spacing, which consequently provided a high total price and income, while a high individual volume growth per tree was observed with the 5 m × 5 m spacing. The results suggested that for the poplar trees younger than 40 years in a semi-arid area in China, 2 m × 2 m spacing is suitable for obtaining a high volume per hectare, while 5 m × 5 m spacing is best for obtaining a high individual volume per tree.