Abstract: As xylem components, vessels and fibers govern water transport and mechanical support. How their long-term trade-off is modulated by climate and radial growth constraints remains insufficiently understood. We integrated wood anatomy with dendrochronology to quantify 58-yr dynamics of xylem traits in Betula ermanii across three treeline sites in northeastern China (FHS: colder; LTD: warmer; LGS: intermediate). We introduce a trade-off index, the ratio of theoretical xylem-specific hydraulic conductivity to fiber cell wall percentage (K_s/RWTA), where K_s is estimated from vessel diameter and vessel density using the Hagen–Poiseuille approach, and RWTA is the mean percentage of fiber cell wall area within xylem. Under warm-drying phase (1988–2001) and at the colder site (FHS), vessel density and vessel percentage increased significantly (P < 0.05), resulting in increased theoretical xylem-specific hydraulic conductivity (P < 0.05). In contrast, at the warmer site (LTD) and during 2002–2020, vessel percentage and theoretical xylem-specific hydraulic conductivity significantly decreased, while fiber cell wall percentage increased significantly (P < 0.05). Mean vessel area, vessel density and theoretical xylem-specific hydraulic conductivity were strongly associated with temperature and the Palmer Drought Severity Index (PDSI), whereas fiber cell density and wall thickness showed weaker responses and were primarily associated with wind speed and sunshine duration. Over 58 yrs, vessels exhibited higher temporal plasticity, while fibers maintained mechanical stability. Climate-driven resource variation modulated the K_s/RWTA index across sites and years, balancing hydraulic efficiency against mechanical support. These findings elucidate climatic adaptation in B. ermanii and provide parameterization for climate–growth modeling.

Key words: Xylem anatomy, fiber–vessel trade-offs, hydraulic conductivity, xylem fiber allocation, treeline