Macro- and micro-scale free shrinkage characteristics of Eucalyptus urophylla × Eucalyptus grandis

Wood shrinkage during drying is strongly affected by moisture migration and anatomical structure, especially in collapse-prone hardwoods such as Eucalyptus urophylla × Eucalyptus grandis. However, the relationship between shrinkage at different structural scales and its effect on the fibre saturation point (FSP) remains unclear. This study investigated the free shrinkage behaviour of E. urophylla × E. grandis at macro - and micro-scales using 2 mm specimens and 0.06 mm ultra-thin longitudinal sections, combined with in situ microscopic observation. Results showed that “premature shrinkage” was evident when the moisture content exceeded FSP, accompanied by cell collapse. A clear scale effect was found; tangential shrinkage of the 2 mm specimens (8.4%) was higher than that of the 0.06 mm sections (6.7%), due to the combined effects of drying stress and capillary tension. At the microscopic level, wood fibres contributed more to deformation than vessels, with areal shrinkage nearly twice as high. The apparent FSP shifted upward to 29.7% – 35.8% because of early collapse. These results indicate that drying deformation is controlled by multi-scale interactions among moisture migration, capillary tension, and anatomical structure.