Variations in bark structural properties affect both water loss and carbon economics in neotropical savanna trees in the Cerrado region of Brazil

Even after complete stomatal closure, plants lose water through the leaf cuticles and bark. This residual water conductance of leaves (g(leaf-res)) and stems (g(bark)) can negatively impact plant water balance and affect plant survival in seasonally dry environments. However, little is known about the costs and benefits associated with such water leaks, especially on stem level. Here, we characterized the structural and functional determinants of the variability in g(bark) across tropical savanna species to elucidate how variations in this trait are related to contrasting growth strategies. The high variability in g(bark) across species was associated with morphoantomical properties of the outer bark (thickness, density and lenticel investment), and such characteristics influenced both stem transpiration and respiration, suggesting the existence of a trade-off between water conservation and oxygen permeability, which reflected contrasting growth and dehydration tolerance strategies. For instance, species with higher g(bark) and g(leaf-res) presented a fast resource acquisition strategy but were more prone to drought-induced mortality by hydraulic failure. However, model simulations revealed that the relative contribution of g(leaf-res) and g(bark) to overall water balance depended on whether leaves were less or more resistant to cavitation than the stems. Synthesis. By combining correlative studies, experimental results and a modelling exercise, we provide a new understanding of the costs and benefits associated with the variability in g(bark) across tropical savanna species and a new perspective for studies of water relations and carbon economics in species from a hyperdiverse savanna.