Decoding spatiotemporal changes in carbon-water coupling and their drought response in China’s tea ecosystems since the early 21st century

China possesses the largest tea cultivation area globally, constituting a distinctive ecosystem that plays a critical role in regional carbon-water cycles and socio-economic development. However, the spatiotemporal variation characteristics of water use efficiency (WUEeco) across the entire tea ecosystems in China and their response to drought remain underexplored due to the scarcity of field observation sites (e.g., eddy covariance flux towers). To address this gap, we adopted remotely sensed and simulated carbon-water flux datasets (i.e., gross primary productivity (GPP) and actual evapotranspiration (ETc act)), multi-scale SPEI drought indices, and a high-precision tea plantation map to comprehensively estimate the WUEeco dynamics and their drought responses across China’s tea ecosystems from 2000 to 2023. The results reveal that the mean annual WUEeco of China’s tea ecosystems is 2.34 g C kg −1 H 2 O, exhibiting a significant increasing trend of 0.012 g C kg −1 H 2 O year −1 ( p < 0.05). Despite this overall improvement, approximately 32.5% of tea plantation areas are projected to experience declining WUEeco trends in the future. Moreover, WUEeco showed a significant positive correlation with SPEI, implying that WUEeco decreased as drought intensified. Crucially, increasing drought severity led to a widespread reduction in WUEeco, with the affected tea plantation areas surging from 63.86% under mild drought to 94.69% under severe drought, and the average loss rate intensifying from −2.99% to −7.57%, highlighting the severe threat of drought to the WUEeco of tea ecosystems. Additionally, our results further reveal that drought exerted notable lagged and cumulative effects on WUEeco, lasting an average of 6.13 and 4.61 months, respectively. In conclusion, to the best of our knowledge, this study presents the first national-scale assessment of WUEeco across China’s tea ecosystems and provides novel insights into its spatiotemporal characteristics across such a large region. Further, our findings highlight the urgent need to develop targeted water management strategies to prevent and mitigate the serious threats posed by droughts to tea plantations.