Weathering induced microscopic and color changes in Japanese fast-growing hardwoods

Fast-growing hardwoods are increasingly used in Japan. However, their early deterioration under outdoor weathering conditions remains poorly understood. This study investigates the initial microscopic anatomical changes and surface color variations in Toona sinensis, Choerospondias axillaris, and Alnus japonica after outdoor exposure. Weathering was conducted from September 28 to December 11, 2020, with specimens installed facing south at a 45° angle. Low-vacuum scanning electron microscopy (LV-SEM) was used to observe transverse, radial, and tangential surfaces. Color changes were quantified using CIELAB parameters (L*, a*, b*), while chemical and structural changes were evaluated by ATR-FTIR and X-ray diffraction. The key novelty is the LV-SEM-based comparative observation of the same anatomical positions before and after exposure, without coating or ion sputtering, thereby avoiding high-vacuum artifacts. Results showed consistent surface deterioration across species, with species-dependent differences in cell separation, deformation, and micro-cracking. ATR-FTIR and XRD analyses confirmed chemical degradation of amorphous cell-wall components. All species exhibited a decrease in L*, while variations in a* and b* reflected differing chemical degradation pathways. These findings clarify species-specific early-stage weathering mechanisms and provide baseline information for future long-term outdoor durability studies.