Hydraulic vulnerability segmentation between branches and roots increases with environmental aridity

The hydraulic vulnerability segmentation hypothesis proposes that plant branches are more resistant to cavitation than roots, namely, the difference in vulnerability to cavitation between branches and roots is positive ( P > 0). However, it is not clear how this phenomenon can vary along environmental aridity gradients. We compiled the above hypothesis with 105 woody species from four biomes with increasing aridity, by compiling functional traits related to hydraulic properties and anatomical structures of branches and roots. We investigated the relationships between P and several environmental factors that are associated with aridity. We found a positive P across species, which supported the hydraulic vulnerability segmentation hypothesis, and P increased significantly with environmental aridity. Branch xylem hydraulic conductivity changed from “more efficient” (e.g., wider conduit, higher hydraulic conductivity) to “safer” (e.g., narrower conduit, more negative P) in response to increased aridity, while root xylem hydraulic conductivity remained unchanged across aridity gradients. Our results demonstrated that hydraulic vulnerability segmentation is more pronounced for species from arid regions. Changes in branch traits may be responsible for hydraulic vulnerability segmentation between branches and roots.