Xylem anatomical and hydraulic responses to light conditions guide regeneration management in tropical tree species

Light limits juvenile tree performance in tropical forests, yet we still lack anatomical criteria to predict hydraulic function across contrasting light environments. Understanding how xylem structure adjusts is essential for identifying hydraulic traits that indicate species with higher regeneration potential. We examine how xylem cells and physiological leaf traits vary along a light gradient and affect growth in five Amazonian species: Bertholletia excelsa , Carapa guianensis , Hymenaea courbaril , Cedrela fissilis and Tabebuia rosea . The experiment was conducted in a silvicultural enrichment system realized with three canopy-openness levels in a secondary forest of Central Amazon. Five years after planting, we measured growth rate, wood traits and leaf gas exchange in five individuals per species randomized across five blocks. Cell fraction varied with light conditions, with greater investment in fiber walls in gaps and in parenchyma in the understory reflecting the conflicting demand between structural reinforcement and storage-oriented tissues. Hydraulic diameter and vessel density exhibited a strong species × light interaction. Only gap conditions positively affected the relationship between hydraulic diameter and stomatal conductance, and no other physiological trait showed a detectable association. Height and diameter growth rates were related to hydraulic diameter in all light conditions. Variation in the plasticity of hydraulic and anatomical traits explained the observed growth patterns. Together, these results indicate that species with greater anatomical plasticity can maintain hydraulic function and growth, which increase with greater light availability. This could support management strategies that prioritize species selected for plasticity in traits, including wood traits such as cell fraction and vessel density, rather than the traditional pioneer vs. non‑pioneer dichotomy.