Pre-existing embolism causes reverse vulnerability segmentation in <i>Populus</i>

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 vulnerability segmentation between organs. Classical vulnerability segmentation occurs when leaf xylem is more vulnerable to embolism than stem xylem, with leaves serving as hydraulic 'fuses' that protect the hydraulic integrity of the stem during drought. Recent evidence suggests that reverse vulnerability segmentation-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. X-ray 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, probably 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 environment and life history rather than an adaptive trait of xylem architecture.