Pre-existing embolism causes reverse vulnerability segmentation in Populus

Abstract Using the optical vulnerability method, we evaluated leaf and stem embolism resistance across three Populus species (P. trichocarpa, P. deltoides, and P. grandidentata) grown under field and glasshouse conditions to explore the mechanisms of segmentation between organs. Classical vulnerability segmentation occurs when leaf xylem is more vulnerable to embolism than stems, serving as hydraulic “fuses” that protect the integrity of stem tissue during drought. Recent evidence suggests that reverse vulnerability segmentation—i.e., when leaves have higher embolism resistance than stems—may occur in Populus species. We observed reverse segmentation exclusively in field-grown older stems, while no segmentation was found in glasshouse-grown or newly formed stems. Micro-computed tomography and hydraulic measurements confirmed that the more vulnerable field-grown older stems had significant native embolism (>25% loss of conductivity). These findings support the hypothesis that reverse segmentation arises not from inherent xylem properties, but from the accumulation of native embolism likely induced by winter freeze–thaw cycles or stem age. Our results provide a mechanistic explanation for reverse segmentation and suggest that its occurrence in Populus may be a byproduct of environmental and life history rather than an adaptive trait in xylem architecture.