Are leaves more vulnerable to cavitation than branches?

1. Vulnerability segmentation hypothesis proposes that plant branches are more resistant to cavitation than their terminal leaves, namely, the difference in vulnerability to cavitation between branches and their terminal leaves is positive (P50leaf-branch>0). This allows leaves to act as safety valves to protect hydraulic pathway from dysfunction. 2. Recent studies showed that several species from humid regions have negative values of P50leaf-branch, indicating a lack of vulnerability segmentation in these species (LVS species). 3. We hypothesize that compared with species from humid regions, vulnerability segmentation is critical and necessary for arid species to survive droughts, but LVS species may take compensatory hydraulic strategies to maintain safety margins during drought periods. 4. We compile branch and leaf hydraulic trait data of 69 broadleaved woody species belonging to different biomes with contrasting climatic conditions as indicated by variable aridity index (AI), and we find that P50leaf-branch becomes small and even negative with the increase of AI. Although leaves of LVS species may lose their function as safety valves, their deep rooting, great hydraulic conductivity, and/or high stem capacitance features are critical for them to maintain hydraulic safety. 5. The results indicate that vulnerability segmentation is evidently common for species from arid regions, but not necessarily the case for some species from humid regions. With robust cavitation-resistant leaves and effective hydraulic compensatory strategies, LVS species can maintain water supply and the functionality of leaves, thus giving them a hydraulic advantage over counterparts in the face of potential drought stress. A is available for this article.