Vertical Stratification Drives Divergent Spatial Trade-Offs Among Xylem Cell Types in Angiosperm Trees of a Mountain...

Vertical stratification in forests acts as an ecological filter, driving woody plants to evolve specialized survival strategies. Angiosperms, in particular, develop secondary xylem with three interdependent functions-water transport, mechanical support, and storage. Trade-offs between these functions vary with resource heterogeneity and environmental pressures. Balancing these functions is based on trade-offs in xylem structure, particularly in the xylem space allocation of vessels, fibers, and parenchyma fractions. However, how plants optimize these trade-offs along forest vertical strata remains unexplored. Anatomical methods were used to determine the fractions of vessels, fibers, and parenchyma in the secondary xylem of 119 individuals within a multilayered forest in eastern China. Ternary plots and standardized major axis analyses were employed to evaluate variations in trade-offs between vessel and fiber fractions, and between parenchyma and fiber fractions across different vertical strata. We found that trade-offs in spatial allocation among cell types occur in all vertical strata. For the fiber-vessel trade-off, canopy and understory trees followed a similar pattern, but canopy trees consistently maintained a higher vessel fraction. In contrast, the fiber-parenchyma trade-off was markedly stronger in understory trees. Our results illustrate that forest vertical stratification significantly influences trade-offs in xylem cell allocation, suggesting functional trade-offs of xylem depend on forest strata. These findings will help clarify how trees adapt to stresses associated with vertical forest strata.