Properties of Catalpa bungei (Bignoniaceae) earlywood and latewood in the same growth ring during moisture adsorption/desorption: swelling and shrinkage at the cell level

Summary In this study, Dynamic Vapor Sorption Resolution combined with a Dino-Lite Edge Digital Microscope was used to document in real-time the swelling/shrinkage behavior of earlywood and latewood cells (wood fiber and vessel) on homogeneous samples of Catalpa bungei C.A.Mey. at the microscopic scale. The results showed that the lumen and cell wall of wood fibers swelled with the increase of relative humidity and shrank with the decrease of relative humidity in the sorption cycle of 0∼95% relative humidity condition. However, vessels showed an opposite trend to the behaviors mentioned above. In any region of both types of cells, latewood cell size (area and diameter) changed more significantly. For diameter change, the change of tangential diameter was larger than that of radial diameter. In addition, in the sorption cycle, the size (area and diameter) change of any region of wood fiber and vessel showed swelling hysteresis. Compared with earlywood cells, latewood cells had a more pronounced swelling hysteretic value, and the swelling hysteretic value in the tangential direction was higher than that in the radial direction in terms of diameter. Sorption hysteresis might be one of the contributing factors to swelling hysteresis. Finally, it was found that under any relative humidity, the dimensional changes of the wood fibers and the vessels were immediately stabilized when the moisture content reached its equilibrium.