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).