Declining Sensitivity and Increasing Resistance Time of Ecosystem Water Use Efficiency to Meteorological Drought

Abstract

Drought is a dominant factor influencing terrestrial ecosystem water-use efficiency (WUE). However, the coupling relationship between WUE and drought remains insufficiently understood. Currently, the coupling relationship is primarily assessed using correlation coefficients or linear regression slopes. However, the optimal drought timescale at which WUE responds to drought has largely been overlooked. Therefore, this study investigated the spatiotemporal patterns of the WUE – meteorological drought coupling relationship across global terrestrial ecosystems from 1982 to 2018 with satellite- derived and model-simulated WUE, together with the Standardized Precipitation-Evapotranspiration Index (SPEI), and explored the potential causal mechanisms. Within the framework of WUE-SPEI coupling, the maximum correlation coefficient between WUE and SPEI represents the sensitivity of WUE to meteorological drought (Rmax), whereas the corresponding optimal drought timescale represents its resistance time (Topt). The results indicated that the sensitivity of WUE to meteorological drought decreased at a rate of -0.0003 yr−1 (p< 0.01), while the resistance time increased at a rate of 0.0155 month yr−1 (p< 0.01), indicating a weakening of the coupling between WUE and meteorological drought. Attribution analysis indicated that CO2 fertilization was the primary factor contributing to the weakening of the coupling relationship. Surface soil moisture was the most critical hydrometeorological driver, exhibiting nearly opposite effects and significant threshold effects on Rmax and Topt. Peter & Clark Momentary Conditional Independence (PCMCI+) algorithm was further employed to construct a causality diagnosis framework for identifying the relationships between WUE-drought coupling and temperature, precipitation, radiation, wind speed, vapor pressure deficit, and surface and root-zone soil moisture. The results showed that the decrease in the Rmax had direct negative causal effects on precipitation, temperature, and radiation. In contrast, the increase in the Topt was primarily driven by a negative causal effect of radiation. This study highlights the weakened coupling between WUE and meteorological 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.