Weakening Correlation and Delaying Response Time of Ecosystem Water Use Efficiency to Drought

Abstract. Ecosystem water use efficiency (WUE), defined as the ratio of carbon gain to water loss, is significantly affected by drought. Elucidating the coupling relationship between WUE and drought is essential for understanding the carbon–water trade-off strategies of vegetation under drought stress. Most existing studies mainly evaluated coupling relationship using correlation coefficients or regression slopes. However, the optimal drought timescale governing WUE responses to drought has not yet been systematically investigated. To fill these gaps, this study investigated the spatiotemporal patterns of the WUE–drought coupling relationship (characterized by the maximum correlation coefficient, R_max, and optimal lag time, T_opt) across global terrestrial ecosystems from 1982 to 2018, and further explored the potential causal mechanisms. The results revealed a delaying of the drought-response timescale of WUE, accompanied by a weakening in the WUE–drought correlation at the optimal timescale, as evidenced by a decrease in R_max at a rate of -0.0003/year and an increase in T_opt at a rate of 0.0155 months/year, indicating a globally weakened coupling relationship. Moreover, pronounced heterogeneity in the changes of coupling relationships changes was observed across different drought gradients and vegetation types. Attribution analysis indicated that CO₂ fertilization was the primary factor contributing to the weakening of the coupling relationship. Surface soil moisture (SMsurf) was the most critical hydrothermal driver, exhibiting nearly opposite effects and significant threshold effects on R_max and T_opt. Causality diagnosis was further employed to construct direct and indirect causal networks of hydrothermal factors affecting R_max and T_opt across different vegetation types and drought gradients. This study highlights the weakened coupling between WUE and drought, suggesting that vegetation's carbon-water trade-off is evolving toward drought adaptation, which is crucial for understanding the adaptive strategies of vegetation in response to climate change.