Hygroscopicity and liquid water absorption of earlywood and latewood in radiata pine heartwood within the same growth ring

The significant difference in resin content between earlywood (EW) and latewood (LW) in radiata pine heartwood is a key factor governing their distinct water-related properties. In this study, dynamic vapor sorption apparatus and immersion methods were employed to investigate the hygroscopicity and liquid water absorption of EW and LW within the same growth ring, and the underlying mechanisms were elucidated by examining chemical composition, microstructure, pore structure, and surface wettability. Results showed that EW had only 3.3% resin content, while that of LW was 23.7%. The high resin content in LW not only reduced the content of cell wall polymers that provide accessible hydroxyl groups but also weakened their water vapor sorption capacity, thereby leading to an equilibrium moisture content of EW during adsorption being 3.04–11.97 percentage points higher than that of LW at 10%–95% relative humidity. Furthermore, the accumulated resin severely blocked the resin canals and cell lumens of LW, reducing its porosity by 25.1% and its average pore size by 89.1%, which endowed LW with water-resistant performance. Liquid water absorption tests revealed an initial absorption rate of 3.96%/min for EW, in contrast to only 0.83%/min for LW.