The effects of genotype and spacing on Pinus radiata [D. Don] corewood stiffness in an 11-year old experiment

The influence of initial stand density and genetic population on corewood dynamic stiffness of 11-year-old Pinus radiata [D. Don] was investigated at a field trial in Canterbury, New Zealand. Corewood dynamic stiffness was determined on standing trees using the stress wave method over the lower stem (0.2–2.0 m) of 182 trees, from an experiment which included three contrasting genetic populations (GF1, GF27 and clonal) grown at 833 and 2500 stems ha −1. Stiffness was significantly influenced by planting density ( P < 0.001), and genetic population ( P < 0.01). Planting density had the largest influence on stiffness, with values in the high-density plots exceeding values in the low-density plots by on average 1.7 GPa or 34%. Gains in stiffness attributable to genetic population averaged 0.8 GPa or 15%. Stiffness was not significantly influenced by the interaction between planting density and genetic population. There was a significant ( P < 0.01) negative relationship between tree diameter at breast height (DBH) and stiffness, for all genetic populations, which explained 57%, 56% and 14% of the variation in stiffness for the clones, GF27 and GF1, respectively. Although correction for variation in DBH reduced variation in stiffness, residual variation in stiffness between planting densities and genetic population were still significant after the effect of DBH had been removed. These findings highlight the importance of planting density in regulating stiffness, and strongly suggest that effects of planting density and genetic population on stiffness are independent.