Summary: 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. Contents Summary 973 I. Introduction 973 II. Defining the structural, chemical and partitioning controls on foliage photosynthetic potentials 974 III. Construction of a global database on within‐canopy variation in leaf structural, chemical and photosynthetic characteristics 976 IV. Methodology for analysis of within‐canopy leaf trait variation: concepts and standardizations 978 V. Global variation in leaf characteristics through the canopies 981 VI. Trait scaling with light, plasticity and quantitative limitations 982 VII. Conclusions: the economics spectrum for the within‐canopy plasticity 989 Acknowledgements 989 References 990

Foliage photosynthetic and structural traits were studied in 15 species with a wide range of foliage anatomies to gain insight into the importance of key anatomical traits in the limitation of diffusion of CO₂ from substomatal cavities to chloroplasts. The relative importance of different anatomical traits in constraining CO₂ diffusion was evaluated using a quantitative model. Mesophyll conductance (g_m) was most strongly correlated with chloroplast exposed surface to leaf area ratio (S_c/S) and cell wall thickness (T_cw), but, depending on foliage structure, the overall importance of g_m in constraining photosynthesis and the importance of different anatomical traits in the restriction of CO₂ diffusion varied. In species with mesophytic leaves, membrane permeabilities and cytosol and stromal conductance dominated the variation in g_m. However, in species with sclerophytic leaves, g_m was mostly limited by T_cw. These results demonstrate the major role of anatomy in constraining mesophyll diffusion conductance and, consequently, in determining the variability in photosynthetic capacity among species.

• Leaf-level determinants of species environmental stress tolerance are still poorly understood. Here, we explored dependencies of species shade (T_shade) and drought (T_drought) tolerance scores on key leaf structural and functional traits in 339 Northern Hemisphere temperate woody species. • In general, T_shade was positively associated with leaf life-span (L_L), and negatively with leaf dry mass (M_A), nitrogen content (N_A), and photosynthetic capacity (A_A) per area, while opposite relationships were observed with drought tolerance. Different trait combinations responsible for T_shade and T_drought were observed among the key plant functional types: deciduous and evergreen broadleaves and evergreen conifers. • According to principal component analysis, resource-conserving species with low N content and photosynthetic capacity, and high L_L and M_A, had higher T_drought, consistent with the general stress tolerance strategy, whereas variation in T_shade did not concur with the postulated stress tolerance strategy. • As drought and shade often interact in natural communities, reverse effects of foliar traits on these key environmental stress tolerances demonstrate that species niche differentiation is inherently constrained in temperate woody species. Different combinations of traits among key plant functional types further explain the contrasting bivariate correlations often observed in studies seeking functional explanation of variation in species environmental tolerances.

Vegetation indices are calculated from reflectance data of discrete spectral bands. The reflectance signal in the visible spectral range is dominated by the optical properties of photosynthetic pigments in plant leaves. Numerous spectral indices have been proposed for the estimation of leaf pigment contents, but the efficacy of different indices for prediction of pigment content and composition for species-rich communities is unknown. Assessing the ability of different vegetation indices to predict leaf pigment content we identify the most suitable spectral indices from an experimental dataset consisting of field-grown high light exposed leaves of 33 angiosperm species collected in two sites in Mallorca (Spain) with contrasting leaf anatomy and pigment composition. Leaf-level reflectance spectra were recorded over the wavelength range of 400 – 900 nm and contents of chlorophyll a, chlorophyll b, total carotenoids, and anthocyanins were measured in 33 species from different plant functional types, covering a wide range of leaf structures and pigment content, five-fold to more than 10-fold for different traits. The best spectral region for estimation of leaf total chlorophyll content with least interference from carotenoids and anthocyanins was the beginning of near-infrared plateau well beyond 700 nm. Leaves of parallel-veined monocots and pinnate-veined dicots had different relationships between vegetation indices and pigments. We suggest that the nature and role of “far-red” chlorophylls which absorb light at longer wavelengths than 700 nm constitute a promising target for future remote sensing studies.