Leaf shape and venation pattern alter the support investments within leaf lamina in temperate species:: a neglected source of leaf physiological differentiation?

1. A trade-off between the investments in functional and support structures is a major determinant of leaf physiological activities. A variety of leaf shapes and venation densities occur in coexisting vegetation, but the costs and benefits of various leaf shapes and venation architectures are poorly understood. As the lever arms (location of leaf mass centre) become effectively longer as the maximum of lamina mass distribution shifts farther from the lamina base, we hypothesized that the fraction of lamina biomass in the mid-rib (F-MR) is larger in leaves in which the centroid of lamina mass is located at, or greater than, half-leaf length ('elliptic' leaves) compared with leaves having the centroid of lamina mass located closer to the leaf base ('ovate' leaves). We further hypothesized that minor vein density (rho(V)) is larger in leaves with lower F-MR, compensating for lower investments in central support. Finally, we predicted that rho(V) is lower in parallel/palmate-veined than in pinnate-veined leaves, due to a more uniform distribution of large veins in parallel/palmate-veined leaves. 2. F-MR and rho(V) were studied in 44 herbs and woody seedlings with an overall variation in lamina fresh mass (M-FL) of more than five orders of magnitude, and a sixfold variation in leaf longevity. Species were separated between pinnate-veined elliptic and ovate leaves, and parallel- or palmate-veined elliptic and ovate leaves. 3. Contrary to the hypothesis, support investment in the mid-rib was similar among leaf shapes, and scaled positively with leaf size and negatively with leaf longevity. However, F-MR and rho(V) were negatively associated. Fractional biomass investment in the mid-rib scaled with lamina size (fresh and dry mass and area), but at a common lamina size F-MR was larger in pinnate-veined elliptic than in parallel/palmate-veined elliptic leaves. In addition, rho(V) was larger in pinnate than in parallel/palmate-veined leaves, and the differences in lamina carbon content further suggested an overall greater investment of lamina biomass in the minor veins of pinnate-veined leaves. 4. These data demonstrate that the effect of leaf shape on biomass investments in central support is less significant than predicted by biomechanical models, partly because of the trade-off between the biomass investments in central support and minor veins, which compensate for differences in lamina shape. These data collectively indicate that leaf size, longevity, shape and venation pattern can importantly modify the distribution of foliage biomass between support and functional tissues, and thus can alter foliage physiological activity and leaf functioning in environments with different resource availability.