The mechanical resistance of leaves is critically related to plant performance within and across habitats. A new understanding of leaf biomechanics will clarify our understanding of plant-animal relations, resource-use strategies, and ecosystem function. Even though leaf biomechanics is central to leaf carbon balance and economics, little is known about the underlying mechanisms for leaf mechanical resistance and their linkage with other traits and with environmental variation. We tested three major hypotheses relating to leaf mechanical resistance. First, within given species, leaf mechanical resistance should vary with leaf size due to correlation with mass proportions and density of the leaf tissue components. Second, leaf mechanical resistance should differ across species according to variation in midrib and lamina functional traits. Third, leaf mechanical traits should vary across habitats, corresponding to selection for leaf lifespan and drought tolerance. Here, we explored the relationships between leaf biomechanics and leaf functional traits using 21 species distributed in Chaparral, Mojave Desert and Coastal Sage communities of Southern California, USA. We quantified 17 leaf traits relating to the composition of leaf lamina and midrib, and 16 biomechanical traits, including tensile moduli of elasticity, tensile strength and flexural stiffness, in axial and lateral directions for the whole leaf, for midribs and for leaf lamina.
We found strong variation across communities and environments. Species of Chaparral had higher values than species of Coastal Sage for TME and TS, LMA, and leaf lamina density, with Mojave Desert values generally intermediate. The TME and TS were higher for the midribs than for leaf laminas and, with exception of the TS and TME of the whole leaf in lateral direction, most mechanical moduli were highly correlated between them, and with LMA and leaf density within and across species. The fraction of dry mass invested in leaf support was negatively correlated with TME and TS across species, and was not correlated with flexural stiffness. Our results indicate the role of leaf size and composition as a mechanism underlying differences in leaf stiffness and strength within and across species. The correlations between leaf and twig size are pervasive, even at intra-leaf scales. The dramatic correspondence of leaf biomechanics with plant community origin has impacts on community-level differences in carbon economics, resistance to drought, and herbivory.