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.

973I.973II.974III.976IV.978V.981VI.982VII.989989References990 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 (AA). However, the controls on AA 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 AA is surprisingly similar in different plant functional types, they differ fundamentally in the share of the controls on AA by constituent traits. Species with high rates of canopy development and leaf turnover, exhibiting highly dynamic light environments, actively change AA by nitrogen reallocation among and partitioning within leaves. By contrast, species with slow leaf turnover exhibit a passive AA 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.

Summary• 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.