European beech resistance to false spring is associated with small-vessel xylem conductivity

Climate warming advances spring phenology in temperate forests, increasing exposure of early leaf-out species such as European beech ( Fagus sylvatica L.) to late frost defoliations (LFDs). Leaf‑out timing, a key factor in determining the exposure to spring frosts, may be partially associated with wood anatomical traits. Trees with a higher fraction of narrow conduits exhibit limited xylem embolism during winter, which would promote high xylem conductance in spring, enabling trees to rapidly develop a new canopy. This study aims to determine whether intraspecific variation in xylem vessel traits formed during the preceding growing season predicts individual level susceptibility to spring‑frost growth reductions in European beech. Four stands were sampled along elevation gradients: two in the Apennines (Italy) and two in the Pyrenees (Spain). Tree-ring analyses identified negative pointer years linked to validated LFDs. Frost resistance was related to a set of vessel traits: theoretical hydraulic conductivity (Ks), conductivity from small (18-30 µm wide) vessels (Ks 18–30 ), mean hydraulic diameter (Dh), and relative conductive area (RCA). Despite representing only ≈0.6% of total Ks, Ks 18–30 was the only xylem trait significantly associated with higher vulnerability to LFD in the linear mixed-effects models. Most variation was attributable to site-, elevation-, and event-specific factors, while Ks 18–30 explained only a limited portion of the residual individual-level variability. This result was consistent across alternative model formulations and sensitivity analyses, suggesting that Ks 18–30 may represent a robust anatomical correlate of spring-frost vulnerability and a potential link between xylem anatomy and tree phenological responses to extreme spring climatic events.