Influence of the slope of terrain on the spatial variability of the wood density within Eucalyptus trees

The aim of this study was to understand how contrasting environments influence the wood formation in Eucalyptus clones and the effect on wood density and spatial variability. Wood density was assessed in clonal tests represented by 150 Eucalyptus urophylla x grandis hybrids with 6-year-old growing under different conditions. The main difference among the sites was the slope of the terrain: the clonal tests were replicated at plan site (0° of inclination), at site with 20°, and 40° of inclination. In order to provide experimental data to perform this study, gravimetric (reference) method and near infrared (NIR) spectroscopy were combined for assessing the wood density in a large sampling of Eucalyptus wood. Hence, regression model based on NIR spectra was developed for estimating such wood traits from NIR spectra recorded at different radial and longitudinal positions along the height of the tree. This approach allows the examination of the patterns of spatial variation of wood density within Eucalyptus trees. Variations in wood density along the stem are less consistent than those in the radial direction, especially close the base of the tree. Overall, the wood density strongly varied from pith (460 kg m-3) to bark (600 kg m-3) at the base. The radial variation in wood density at the base was about 140 kg m-3 while the radial variation at 25% of stem height was slightly low (~130 kg m-3). At 50% of height the trait also increased radially (~104 kg m-3), but in relative low magnitude. The density slightly increased from pith to bark at 75% of height (~50 kg m-3) and at the top of the tree the variation was of lower magnitude (~20 kg m-3). The radial variation at the base take into account the wood formed from the first to the sixty year of growth while the variation in the top of the tree refers to the wood developed with few months of difference. The pith to bark variations in wood density were higher in the trees from the site presenting 40° of inclination. At 25% of the tree height, the radial variation was 104 kg m-3 in the site plan (0°), 133 kg m-3 in the site presenting inclination of 20°, and 157 kg m-3 in the site with 40° of inclination. In conclusion, the higher the inclination of the terrain, the greater the magnitude of wood density variation. Sloped terrains induce formation of reaction wood influencing the radial variation in wood traits. (Résumé d'auteur