Fundamental plant traits such as support, anchorage, and protection against environmental stress depend substantially on biomechanical design. The costs, subsequent trade-offs, and effects on plant performance of mechanical traits are not well understood, but it appears that many of these traits have evolved in response to abiotic and biotic mechanical forces and resource deficits. The relationships between environmental stresses and mechanical traits can be specific and direct, as in responses to strong winds, with structural reinforcement related to plant survival. Some traits such as leaf toughness might provide protection from multiple forms of stress. In both cases, the adaptive value of mechanical traits may vary between habitats, so is best considered in the context of the broader growth environment, not just of the proximate stress. Plants can also show considerable phenotypic plasticity in mechanical traits, allowing adjustment to changing environments across a range of spatial and temporal scales. However, it is not always clear whether a mechanical property is adaptive or a consequence of the physiology associated with stress. Mechanical traits do not only affect plant survival; evidence suggests they have downstream effects on ecosystem organization and functioning (e.g., diversity, trophic relationships, and productivity), but these remain poorly explored.

Summary• Although sclerophylly is defined by textural properties, its adaptive significance has been debated without a strong base of mechanical data. We measured a wide range of mechanical properties across a diverse range of species and leaf forms, including highly scleromorphic leaves, and compared these with sclerophylly indices to determine the mechanical properties of sclerophylls.• Fracture and flexure tests were used to determine leaf strength, toughness (work to fracture) and flexural stiffness ('structural' properties), and specific strength, specific toughness and Young's modulus of elasticity ('material' properties, i.e. normalized per unit leaf thickness).• Leaves varied considerably in all properties tested, and in the way they combined various 'structural' and 'material' properties. However, on average, highly scleromorphic leaves were stronger, tougher and stiffer than soft leaves. 'Structural' properties correlated more strongly with sclerophylly than 'material' properties, and the ratio of stiffness to strength and toughness increased in sclerophyllous species.• Of the structural properties, strength, toughness and flexural stiffness each made substantial independent contributions to the variation in sclerophylly indices, but the best individual explanators were flexural stiffness and strength, with the best predictive model being a combination of these two properties. This model should now be tested on leaves from contrasting environments.

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