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

Area of Science:

• Plant Physiology
• Forest Ecology
• Drought and Frost Acclimation

Background:

• Vulnerability segmentation (VS) and Hydraulic segmentation (HS) hypotheses suggest leaves are more vulnerable to embolism than branches, aiding drought acclimation.
• The occurrence of VS and HS during freezing stress remains underexplored in temperate evergreen conifers.

Purpose of the Study:

• To investigate the seasonal dynamics of leaf and branch hydraulic traits in temperate evergreen conifers.
• To assess the applicability of VS and HS hypotheses across all seasons, especially during winter freezing stress.
• To determine if leaf-branch hydraulic vulnerability segmentation occurs year-round and its implications for drought and frost acclimation.

Main Methods:

• Seasonal measurement of leaf and branch hydraulic traits (conductance, vulnerability curves) in three temperate evergreen conifer species.
• Calculation of P50L (leaf) and P50B (branch) to assess vulnerability.
• Analysis of leaf mass per area and correlation between leaf and branch hydraulic conductance (KmL and KmB).

Main Results:

• Leaf hydraulic conductance (P50L) became more negative in winter, indicating increased vulnerability, alongside higher leaf mass per area, suggesting carbon investment for frost drought acclimation.
• Higher percentage loss of branch hydraulic conductance (PLC B) and lower KmL and KmB were observed in spring compared to autumn, likely due to freeze-thaw cycles.
• P50B was consistently more negative than P50L across seasons, resulting in positive VS values, indicating leaves were more vulnerable than branches.
• Leaf and branch hydraulic conductance were positively correlated (KmL with KmB), supporting hydraulic coordination but not HS.

Conclusions:

• Leaf-branch vulnerability segmentation is a year-round phenomenon in temperate evergreen conifers, including during freezing stress.
• This segmentation strategy protects branches from hydraulic failure, contributing to overall tree survival under seasonal stress.
• Species acclimate to winter frost drought through increased carbon investment in leaf structures, including hydraulic systems.