Objective This study aimed to elucidate the physiological and ecological mechanisms underlying decline and mortality of elm (Ulmus pumila L.) trees.

Method Differences in leaf traits, hydraulic traits, and non-structural carbon (NSC) were measured among healthy, declining, and dying trees. Based on these data, the key factors driving elm trees decline were identified and quantified their relative importance.

Result Soil water content at 10～40 cm depth around dying trees was significantly lower than that around healthy trees. Dying trees exhibited significantly reduced leaf area and lower C:N ratio, but higher leaf density and leaf mass per area, indicating a shift toward a conservative resource-use strategy under drought stress. Midday water potential in branches and leaves of dying trees decreased by 36.92% and 40.51%, respectively, compared with healthy trees, while the percentage loss of hydraulic conductivity (PLC) of the branches increased by 59.35%, with significant differences. Additionally, levels of soluble sugars and NSC in dying trees were markedly lower than in healthy ones, particularly starch content in branches, trunks, and roots decreased by 27.59%, 31.30%, and 31.33%, respectively, suggesting that dying trees mobilized stored starch reserves to maintain survival. Quantitative analyses identified PLC, mid-layer soil moisture, and midday leaf water potential as the primary factors driving elm trees decline and mortality. Starch contributed 8.92%, indicating that carbon reserves also play an important role.

Conclusion Overall, hydraulic failure is the primary driver of elm trees decline, while depletion of NSC in stems and roots exacerbates deterioration. These findings clarify the physiological and ecological mechanisms leading to elm trees decline in the Three-North regions, providing a scientific basis for species selection, shelterbelt construction, and forest structure optimization in the region.