Résumé
The leaf economic spectrum is a powerful framework to understand the diversity of leaf structures and functions. However, this framework fails to integrate leaf size, especially leaf area, although this is subject to a variation of 6 orders of magnitude across species. Thus, we still do not know what are functional constraints prevailing on the shaping of leaf size diversity. We evaluated vasculature, hydraulic, and mechanical constraints on leaf size, by investigating shoots and leaves of 42 tropical tree species, covering a large range of blade surface area. Based on an anatomical approach, we measured tissue area, vasculature, and theoretical conductivity on shoots and leaves. We also measured stem modulus of elasticity and flexural stiffness. The leaf area-stem area relationship was allometric with higher leaf area per stem cross-sectional area on increasing leaf size. Large leave was characterized by (i) a higher leaf area-to-xylem area ratio and (ii) higher xylem-specific conductivity (Ks), driven by vessel size. We do not observe any loss of water supply per leaf area on large leaves thanks to the increasing Ks. Mechanical measurements suggest a more efficient way to achieve stiffness, due to geometry rather than the material, and driven by the pith proportion.