Seasonal drought leads to contrasting nonstructural carbohydrate dynamics but stable phenolic defense in tall-canopy and short-understory tropical trees

Carbon allocation plays an important role in determining tree productivity and survival under environmental change. However, our understanding of how allocation patterns and their responses to drought vary among diverse functional types in tropical forests remains limited. In a tropical forest equipped with an 80-m canopy crane, we measured leaf gas exchange and water status, leaf nonstructural carbohydrates (NSCs) and phenolics, stem growth, and crown characteristics of mature trees from 18 species spanning different canopy positions, water-use and growth strategies during both the wet and dry seasons. The results show that tall canopy trees experienced stronger VPD and water stress and greater reductions in leaf gas exchange than short understory trees, leading to declines in NSCs (particularly starch) in canopy trees but increases in understory trees in the dry season. Despite changes in carbon supply, leaf phenolic levels remained remarkably stable across species, with species-specific variation explained by tree height and herbivory. With increasing height, both whole-tree leaf phenolics and NSCs increased whereas stem growth varied among canopy species. We highlight that canopy position–driven differences in resource availability and environmental stress are key for understanding and predicting carbon balance and allocation strategies in tropical forests experiencing seasonal droughts.