Hygrothermal coupling-induced shrinkage-swelling evolution of two types of vascular bundles in moso bamboo

Hygrothermal coupling significantly governs bamboo dimensional variation. Variable temperature and humidity facilitate the standardized shaping of curved bamboo strips but also induce structural defects such as cracking and warping. Bamboo is a low-carbon, eco-friendly biomaterial featuring outstanding mechanical strength, which primarily consists of vascular bundles and parenchyma cells. As the fundamental structural units of bamboo, vascular bundles feature a unique plum‑blossom radial distribution with outward increasing density, which prominently intensifies moisture-induced dimensional changes. Focusing on this typical hierarchical structure, this study systematically investigated the size and positional evolution of different vascular bundles and their internal fiber sheaths under dynamic hygrothermal conditions. The results demonstrated that vascular bundles with different phenotypic structures underwent more significant area variations during the hygroscopic stage than the desorption stage. At a relative humidity above 50%, bamboo specimens exhibited larger dimensional areas at 30 °C than at 20 °C. Furthermore, the Center‑X positions of all phenotypic vascular bundles migrated toward the bamboo outer side, and such positional displacement was positively correlated with the spatial distribution of vascular bundles. At different ambient temperatures, the normalized area variation rate of vascular bundles reached its maximum value at RH = 50%, while the radius ratio of open vascular bundles (KF) remained steadily stable at approximately 1.2. Semi‑open vascular bundles (BKF) presented a narrower relative area variation range for F1 and F2 fiber sheaths at 30 °C, with most variations concentrated within ±0.2%.