Modelling the variation in wood density of New Zealand-grown Douglas-fir

Abstract

Background: Wood density is an important property that affects the performance of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) timber. In order to develop strategies to achieve certain end-product outcomes, forest managers and wood processors require information on the variation in wood density across sites, among trees within a stand and within trees. Therefore, the aim of this study was to develop models explaining the variation in outerwood density among sites and among trees within a stand, and the radial and longitudinal variation of wood density within a tree. Methods: An extensive dataset was assembled containing wood density measurements from historical studies carried out over a period spanning more than 50 years. The dataset contained breast height outerwood density cores from approximately 10,800 individual trees sampled from 312 stands throughout New Zealand, pith-to-bark radial density profiles from 515 trees from 47 stands, and discs taken from multiple heights in 172 trees from 21 stands. Linear and non-linear mixed models were developed using these data to explain the variation in inter-and intra-tree variation in Douglas-fir wood density. Results: Breast height outerwood density was positively related to mean annual air temperature in stands planted after 1969. This relationship was less apparent in older North Island stands, possibly due to the confounding influences of genetic differences. After adjusting for age and temperature, wood density was also positively related to soil carbon (C) to nitrogen (N) ratio in South Island stands where data on soil C:N ratio were available. There was only a minimal effect of stand density on breast height outerwood density, and a weak negative relationship between wood density and tree diameter within a stand. Within a stem, wood density decreased over the first seven rings from the pith and gradually increased beyond ring ten, eventually stabilising by ring 30. Longitudinal variation in wood density exhibited a sigmoidal pattern, being fairly constant over most of the height but increasing in the lower stem and decreasing in the upper stem. Conclusions: The study has provided greater insight into the extent and drivers of variation in Douglas-fir wood density, particularly the relative contributions of site and silviculture. The models developed to explain these trends in wood density have been coupled together and linked to a growth and yield simulator which also predicts branching characteristics to estimate the impact of different factors, primarily site, on the wood density distribution of log product assortments. Further work is required to investigate the impacts of genetic and soil properties on wood density, which may improve our understanding of site-level variation in wood density.

The effect of thinning on mechanical properties of Douglas fir, Norway spruce, and Sitka spruce

“…Various other factors can also impact the properties of wood in trees, including site, soil, or climate (Zobel and van Buijtenen 1989). A review paper focused on Sitka spruce by Macdonald (2002) found that wood density decreases with increasing site productivity; this was recently confirmed in radiata pine (Kimberley et al 2015) and Douglas fir (Kimberley et al 2017) as well. The effect of site on dynamic MOE in young radiata pine was also found to be substantial (Watt et al 2006;Lasserre et al 2008).…”

“…Most authors seem to agree that those changes are a consequence of wider tree rings from accelerated growth after the thinning. A small number of studies reported no change in density after early respacing or thinning-in Sitka spruce , Douglas fir (Kimberley et al 2017), or black spruce (Picea mariana (Mill.) BSP) (Vincent et al 2011).…”

Models for Predicting Specific Gravity and Ring Width for Loblolly Pine from Intensively Managed Plantations, and Implications for Wood Utilization

“…), and black spruce (Picea mariana (Mill.) B.S.P) all exhibit SG radial trends where SG is high near the pith, decreases in the transition wood, and then increases in the outerwood [19,40,41]. For loblolly pine, the reason for the low SG near the pith is due to the low proportion of latewood to earlywood tracheids and reduced cell wall thicknesses, as well as lower latewood SG [13,42].…”

Prediction of Douglas-Fir Lumber Properties: Comparison between a Benchtop Near-Infrared Spectrometer and Hyperspectral Imaging System

“…In the southern pines, SG increases from pith to bark, while at the same time, microfibril angle decreases from pith to bark [70,71]. For Douglas-fir, the microfibril angle trend is the same as the southern pines, but SG decreases from pith to bark for approximately 10 years, then it gradually increases [67]. Thus, the relationships between SG and MOE and MOR is not as strong as what was reported previously for southern pine, and they mirror results others have found for Douglas-fir [72].…”

“…For both models, the quadratic model form instead of a linear model was used because of the differences in the radial variation trends for wood properties of Douglas-fir. Wood density decreases for approximately 8 years from pith to bark and then increases, whereas stiffness and strength increases from pith to bark because of decreasing microfibril angle [67]. Thus, for samples near the pith, a lower density value does not necessarily mean a decrease in wood stiffness or strength.…”