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

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.