Leaf-branch vulnerability segmentation occurs all year round for three temperate evergreen tree species

Vulnerability segmentation (VS) and Hydraulic segmentation (HS) hypotheses propose higher hydraulic resis-tance and vulnerability to embolism in leaves than in branches, respectively. The VS and HS are suggested as an acclimation strategy of trees to drought stress, but whether they occur during freezing stress has rarely been explored. We measured the leaf and branch hydraulic traits of three temperate evergreen tree species [Picea koraiensis (Korean spruce), Pinus koraiensis (Korean pine), and Pinus sylvestris var. mongolica (Mongolian pine)] during four seasons (winter, spring, summer, and autumn) across the year. We assessed the applicability of VS and HS all year round, particularly in winter. The water potential at which leaf hydraulic conductance lost 50% (P50L), was more negative in winter than in summer, while higher leaf mass per area was obtained in winter. These results suggest that these species invest more carbon into leaf (including hydraulic systems) to acclimate to winter frost drought. Leaf and branch hydraulic conductance (KmL and KmB) were lower, and the percentage loss of branch hydraulic conductance (PLCB) was higher in spring than in autumn. These results were probably because of more freeze-thaw cycles in spring (69 cycles) than in autumn (37 cycles). The water potential at which branch hydraulic conductance lost 50%, P50B, was more negative than P50L across the year. The values of VS (P50L minus P50B) were positive, i.e. leaf was more vulnerable than the branch in all species and across seasons, with higher values occurring in spring or autumn. However, KmL positively correlated with KmB, suggesting hydraulic coordination between leaf and branch, but did not support HS. Our findings indicate that leaf-branch vulnera-bility segmentation can occur all year round, including freezing stress, to protect branches from hydraulic failure in temperate evergreen conifers.

Automated summary

The Vulnerability segmentation (VS) and Hydraulic segmentation (HS) hypotheses propose that leaves are more vulnerable to hydraulic resistance and embolism than branches. However, whether these hypotheses hold true during freezing stress has been rarely investigated. This study measured the hydraulic traits of leaf and branch in three temperate evergreen tree species across four seasons and found that vulnerability segmentation can occur all year round, including freezing stress, to protect branches from hydraulic failure.