Effect of Long-Term Outdoor Storage on the Multi-Scale Structural and Mechanical Properties of Oriental Beech (Fagus orientalis Lipsky) and European Hornbeam (Carpinus betulus L.) Wood

This study examines the effects of nearly six years of outdoor storage on the chemical, thermal, structural, micromorphological, and mechanical properties of European hornbeam (Carpinus betulus L.) and Oriental beech (Fagus orientalis Lipsky) wood. Differences between fresh (control) and stored samples were characterised using attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy, thermogravimetric/derivative thermogravimetric analysis (TGA/DTG), X-ray diffraction (XRD), scanning electron microscopy (SEM), and testing of compressive strength parallel to the grain. ATR-FTIR analysis points to alterations in carbohydrate- and lignin-related absorption bands after storage. TGA/DTG profiles suggest that hornbeam largely maintains its thermal degradation behaviour, whereas beech exhibits more pronounced shifts in thermal stability. XRD results indicate a reduction in crystallinity index (CrI) for both species. SEM observations of beech wood revealed filamentous structures within cell lumens, together with localised cell-wall disruption and microcracks, indicating more extensive microstructural modification than in hornbeam. Mechanical testing shows significant reductions in modulus of elasticity (MOE), with decreases of 32.2% in hornbeam and 28.4% in beech. Statistical evaluation confirms the significance and effect sizes of these changes. Overall, the findings demonstrate that long-term outdoor storage induces species-dependent degradation across molecular, microstructural, and macroscopic scales, ultimately compromising wood performance.