A worldwide analysis of within-canopy variations in leaf structural, chemical and physiological traits across plant functional types

Extensive within-canopy light gradients importantly affect the photosynthetic productivity of leaves in different canopy positions and lead to light-dependent increases in foliage photosynthetic capacity per area (A(A)). However, the controls on A(A) variations by changes in underlying traits are poorly known. We constructed an unprecedented worldwide database including 831 within-canopy gradients with standardized light estimates for 304 species belonging to major vascular plant functional types, and analyzed within-canopy variations in 12 key foliage structural, chemical and physiological traits by quantitative separation of the contributions of different traits to photosynthetic acclimation. Although the light-dependent increase in A(A) is surprisingly similar in different plant functional types, they differ fundamentally in the share of the controls on A(A) by constituent traits. Species with high rates of canopy development and leaf turnover, exhibiting highly dynamic light environments, actively change A(A) by nitrogen reallocation among and partitioning within leaves. By contrast, species with slow leaf turnover exhibit a passive A(A) acclimation response, primarily determined by the acclimation of leaf structure to growth light. This review emphasizes that different combinations of traits are responsible for within-canopy photosynthetic acclimation in different plant functional types, and solves an old enigma of the role of mass- vs area-based traits in vegetation acclimation.