4. Discussion
In the present study, we compared hydraulic and anatomical traits of woody species from different biomes, tested the vulnerability segmentation hypothesis (i.e., plant branches are more resistant to cavitation than their roots, P 50 root - branch > 0), and analyzed the coordination between P 50 root - branch and environmental variables, so as to provide new insights into how environment factors drive the vulnerability segmentation between branches and roots. Our results showed that P 50 root - branch > 0 was common across different biomes (tropical seasonal forest, temperate seasonal forest, woodland/shrubland, and desert), which are consistent with the vulnerability segmentation hypothesis. Moreover, the degree of vulnerability segmentation between roots and branches increased with environmental aridity, which may be attributed to the decoupling of root and branch xylem anatomical traits, e.g., hydraulically weighted diameters (D h) increased significantly with aridity in branches but not in roots.