Position-dependent variations in cell wall orientation of hardwood branches: Evidence from wide-angle X-ray diffraction

The secondary cell wall is a highly optimised structure and large repository of carbon, making wood an efficient tool for carbon sequestration in long-lasting materials. However, the prospective material use of branch wood is not realised, as natural variability is poorly understood. To address this, we analysed the structural orientation of the secondary cell wall in radial wood strips from branches and stem wood of beech, oak and poplar using X-ray diffraction. Our position-resolved results revealed that the flat branches of beech and oak exhibited greater asymmetry, with the highest microfibril orientation on the upper side, associated with the architectural tree design. Steep branches and poplar samples showed a more uniform, stem-like structure. Herman's orientation factor showed a pith-to-bark trend with the lowest orientation in the pith region, but was interrupted by tension wood regions reaching local extremes, identified as interesting tissue for future material designs due to high cell wall quantity and orientation. Our study advances the understanding of the relationship between cell wall orientation and tree architecture, which can now be determined using non-invasive laser scanning, thus enabling the search for materials that meet certain quality criteria and help to foster the new material use of branch wood.