Effects of site, silviculture and seedlot on wood density and estimated wood stiffness in radiata pine at mid-rotation

To understand the underlying control of patterns of important wood properties is fundamental to silvicultural control of wood quality and genetic selection. This study examines the influences of site, silviculture and seedlot on diameter growth, wood density and estimated wood stiffness in mid-rotation radiata pine (Pinus radiata D Don) stands across New Zealand. Selected treatment combinations were assessed across five sites in a 17-year-old experiment comparing silvicultural treatments and improved breeds of radiata pine. Diameter at breast height (DBH), and stress-wave velocity (an indicator of wood stiffness) and outerwood (outermost five growth rings) basic density at breast-height were assessed for ten trees from each plot in the experiment. There were large differences in DBH and wood properties between sites. Silviculture (stand density) had a stronger influence than seedlot on DBH and stress-wave velocity, while the converse applied to outerwood density. There was a positive relationship between stand density and both stress-wave velocity and outerwood density. Trees in the un-pruned 500 stems ha(-1) treatment had larger DBH, lower outerwood density and lower stress-wave velocity than trees in the 400 stems ha(-1) pruned treatment. This suggests that silvicultural manipulation (pruning) of green crown length is important for controlling both growth and wood properties. Results from this study support previous research which indicates that thinning, and to a lesser extent pruning, have a strong impact on DBH, stress-wave velocity and outerwood density. Increasing stand density is consistently associated with stiffer and denser outerwood.