Whole-plant water hydraulic integrity to predict drought-induced Eucalyptus urophylla mortality under drought stress

Drought-driven tree mortality has been occurring globally and been investigated by many researchers. Yet, while subtropical forests of southern China are predicted to experience an increased frequency and intensity of climate change-induced drought events in the near future, there still a big knowledge gap in our understanding of the physiological vulnerability of Eucalyptus urophylla S.T. Blake under drought stress. In this study, E. urophylla was selected both for field and greenhouse experiments under controlled soil water conditions. We investigated whole-plant hydraulic conductivity (K-plant), sap flow density (J(s)), leaf photosynthetic characteristics and nonstructural carbohydrates (NSC) to explore how E. urophylla species responded to drought from the perspective of hydraulic and carbon dynamics. Results showed that the percentage loss of stem xylem hydraulic conductance (PLC) caused by drought was up to 84.4%. Both K-plant and J(s) significantly decreased with the decline of the soil water availability. Also, the interaction of hydraulic conductivity to stomatal conductance was more sensitive under drought. In addition, drought-induced decline of total NSC content in leaf and stem tissue was not distinct. A conversion of starch to soluble sugar was found under drought stress. Our findings suggested that hydraulic failure could be a major threat to the survival of E. urophylla species under drought stress due to a high xylem embolism vulnerability, which improves prediction of the response of E. urophylla species to future climate changes.