Shifts in leaf and stem hydraulic traits across aridity gradients in eastern Australia

Premise of research. Plants are faced with a challenge across all climates they inhabit-they must transport water to their leaves so that photosynthesis can take place. Although this is simple in concept, it can be achieved by different arrangements of root, stem, and leaf traits. The hydraulic functioning of species across aridity gradients is determined by the coordination of these traits. Nevertheless, we have an imperfect understanding of which trait shifts are favored across aridity gradients as well as the alignment of trait shifts with climate. Methodology. We measured hydraulic traits relating to Darcy's law for 120 angiosperm species across a broad range of climates in eastern Australia; nearly one-third of all biome space on Earth was represented. We then determined which hydraulic trait shifts have been favored across aridity gradients and which climate characteristics these trait shifts aligned with. Pivotal results. Increasing aridity, from climates with similar precipitation and evaporation to climates where precipitation was only a third of evaporation, was associated with a 4.8-fold decrease in plant height, a 3.0-fold decrease in leaf area-to-sapwood area ratio, and a 3.3-fold decrease in leaf water potential. However, sapwood-specific conductivity decreased by 5.9-fold, more than any other hydraulic trait. Greater sapwoodspecific conductivity (decreasing resistance) at wet sites compensated for increasing resistance and hydraulic demand that was associated with taller plants and leafier shoots. All hydraulic traits were strongly correlated with growth season aridity (r > 0.82; P < 0.05) but were not correlated with maximum aridity. This suggests that plant hydraulic traits are most responsive to water availability and evaporative demand present during the most suitable months for growth rather than the driest months. Conclusions. We suggest that evolution has equipped plants with various mechanisms to avoid desiccation during the dry season while optimizing hydraulic traits for carbon gain during the growth season.10 page(s