Covariation between leaf hydraulics and biomechanics is driven by leaf density in Mediterranean shrubs

Abstract

Leaf density links the resistance to mechanical and hydraulic stress in Mediterranean shrubs as it is associated with the water potential at turgor loss and the moduli of elasticity and strength. Understanding the patterns of hydraulic and mechanical trait variation in vascular plants is critical to predicting species’ stress tolerance. Although previous work has shown that hydraulic and mechanical traits are decoupled in stems, there is little information available for leaves, which are organs more diversified in structure, function, and possibly drought tolerance strategies across habitats. We tested for coordination between leaf hydraulic traits related to drought tolerance and the mechanical resistance of leaves, for 17 shrub species from the arid and semiarid vegetation of the California Floristic Province. Bayesian and phylogenetic correlations showed that across species, hydraulic and mechanical traits both had strong associations with the water potential at turgor loss, and with leaf tissue density. However, leaf maximum hydraulic conductance and the water potential at 50% and 80% loss of hydraulic conductance were statistically independent of two key mechanical traits, the leaf modulus of elasticity and leaf structural strength. Our results suggest that leaf biomechanical traits, which reflect construction costs and contribute to leaf longevity, are decoupled from hydraulic capacity and safety. The independence of hydraulic and mechanical protection in leaves enables a wide range of trait combinations in leaves, which would increase their adaptive potential across ecosystems.