4.3 Increasing P50 root - branch with increasing aridity
We found that precipitation-related environmental variables (e.g., AI, Pwet, MAP and Ps) showed more explanatory power than others, indicating the importance of moisture in driving the shift ofP 50 root - branch, with AI that reflects evapotranspiration and precipitation status of the study sites, being the major environmental factor in explaining the variation ofP 50 root - branch. Moreover, we found notable changes in P 50 root - branch (on average) across different biomes (Figure 4a). In particular, P 50 root - branch decreased or even became negative with increasing AI, which is consistent with the pattern observed between branches and leaves in a previous study (Zhu, et al ., 2016). In general, the vulnerability segmentation is more pronounced for species from arid regions compared to humid habitats (i.e., showing lowerP 50 root - branch values). Notably, the vulnerability segmentation might be advantageous for woody species from arid environments, where survival during protracted drought may depend on isolating a part of roots from drying soils (Johnson et al ., 2016). By contrast, woody species in wetter environments displayed a relatively lower degree of vulnerability segmentation, because abundant water availability in wetter environments might have relieved the vulnerability segmentation pressure. A relatively higher hydraulic conductivity both in branches and roots for species from humid environments might help them to absorb and transport water more efficiently, hence keeping water balance of the whole plant. Besides, a greater plant hydraulic conductivity might contribute to lower water potential gradients from soil to the terminal roots, resulting in a lower risk of cavitation in both roots and branches (Wang et al ., 2022).