The bonding strength of glued wood is strongly related to wood type and environmental conditions, and this relationship is a dynamic variable. It is thus important to investigate the effect of wood type and relative humidity (RH) on the bonding strength using dynamic tests. Two-layered plywood was lab produced, and two sides of the bonding interphase were latewood and earlywood. After conditioning the samples at 60%, 80%, and 95% RH, the bonding strength was measured with a universal mechanical test machine, and strain distribution was recorded simultaneously with digital image correlation (DIC). The results show that bonding strength and deformation increased with an increase in RH. The moisture content (MC) in the latewood was more than that in the earlywood. This decreased the difference in stiffness between latewood and earlywood and led to a more homogeneous strain distribution. This can protect earlywood from failure caused by larger local structural ruptures. Large strain in latewood contributed to high bonding strength. For all samples, failure occurred in the glue line or on the side with earlywood only. For samples failing in the glue line, structural ruptures occurred in the bonding interphase and the wood next to the bonding interphase.

Wood displays three-dimensional characteristics at both macroscopic and microscopic scales. Accurately reconstructing its 3D structure is vital for a deeper understanding of the relationship between its anatomical characteristics and its physical and mechanical properties. This study aims to apply X-ray micro-computed tomography (XμCT) for the high-resolution, non-destructive visualization and quantification of softwood anatomical features. Six typical softwood species—Picea asperata, Cupressus funebris, Pinus koraiensis, Pinus massoniana, Cedrus deodara, and Pseudotsuga menziesii—were selected to represent a range of structural characteristics. The results show that a scanning resolution of 1–2 μm is suitable for investigating the transition from earlywood to latewood and resin canals, while a resolution of 0.5 μm is required for finer structures such as bordered pits, ray tracheids, and cross-field pits. In Pinus koraiensis, a direct 3D connection between radial and axial resin canals was observed, forming an interconnected resin network. In contrast, wood rays were found to be distributed near the surface of axial resin canals but without forming interconnected structures. The three-dimensional reconstruction of bordered pit pairs in Pinus massoniana and Picea asperata clearly revealed interspecific differences in pit morphology, distribution, and volume. The average surface area and volume of bordered pit pairs in Pinus massoniana were 1151.60 μm² and 1715.35 μm³, respectively, compared to 290.43 μm² and 311.87 μm³ in Picea asperata. Furthermore, XμCT imaging effectively captured the morphology and spatial distribution of cross-field pits across species, demonstrating its advantage in comprehensive anatomical deconstruction. These findings highlight the potential of XμCT as a powerful tool for 3D analysis of wood anatomy, providing deeper insight into the structural complexity and interconnectivity of wood.

Thermal modification can increase the physical stability and impact the mechanical strength of wood. It is necessary to understand the effects of modifications on the compressive stress of wood. In this study, Douglas fir (Pseudotsuga menziessi) blocks were modified at 180 °C (TM-180 °C) and 210 °C (TM-210 °C). The compressive stress of pure earlywood (EW), pure latewood (LW), and combined earlywood and latewood (ELW) specimens was measured. The specimens were compressed at 30% of their original thickness, and during the compression test the strain distribution of the ELW was recorded. In addition, the microstructures before and after compression were investigated, complemented with SEM to understand the structural changes taking place. The results showed that the compressive stress of the TM-180 °C specimens was the highest because the thermal modification increased the stiffness of cell walls and the homogenized strain distribution in the ELW specimens. The control specimens had a higher compression set recovery rate than the thermally modified specimens. The tracheid cell walls in the EW and LW specimens were flattened and buckled, respectively, due to compression. In the thermally modified materials, cell wall fissures and wood ray fractures in the EW and LW specimens, respectively, were observed. For the ELW specimens, the structural changes in the latewood were not obvious and the structural changes in the earlywood were less significant than in the full EW specimens. Compared to the EW specimens, the earlywood in the ELW specimens showed higher compression set recovery rates. It seems that structural failure in earlywood is limited when used in combination with latewood, resulting from the homogenized strain distribution in earlywood.