Background Under global climate change, forest carbon-water fluxes and water use efficiency (WUE) are highly sensitive to climate fluctuations, but their response mechanisms differ across ecosystem types. Southwest Zhejiang’s mid-altitude coniferous-broadleaf mixed forests are representative of the southeastern monsoon region, where temperature, radiation and precipitation show marked seasonal variations. However, the key environmental drivers of their carbon-water coupling processes and the causes of WUE’s temporal variability remain unclear, which impedes scientific management for improving soil and water conservation and carbon sink functions.

Methods This study selected a typical mid-mountain coniferous-broadleaf mixed forest in Jingning She Autonomous county, Lishui city as the research site. The site is located at N 27.91°, E 119.67° with an altitude of 1201.5 m. From August 2023 to July 2024, an eddy covariance system mounted on a 48 m flux tower was used to monitor carbon-water fluxes and environmental factors at 30-minute intervals. The monitored carbon-water fluxes included CO₂ turbulent flux and latent heat flux while the environmental factors covered air temperature (T_a), soil temperature (T_s), solar radiation (R_t), precipitation (P_r), and so on. After data quality control involving invalid data elimination and interpolation via marginal distribution sampling, correlation analysis and structural equation modeling (SEM) were applied to analyze the dynamic characteristics of carbon-water fluxes and WUE as well as their response to environmental factors.

Results The results show that 1) The ecosystem mainly functioned as a carbon sink with a cumulative carbon sequestration of 705.0 g CO₂/m². The daily average net ecosystem exchange (NEE) in the growing season was -2.3 g/(m²·d), which was 64.3% higher in carbon absorption intensity than that in the dormant season where the daily average NEE was −1.4 g/(m²·d). The cumulative gross primary productivity (GPP) reached 3130.0 g/m². The daily average GPP in the growing season was 11.3 g/(m²·d), 2.35 times that of the dormant season where the daily average GPP was 4.8 g/(m²·d). 2) The main influencing factors of GPP are T_a and R_t, with a significant positive correlation (P < 0.01), but it was inhibited by P_r in the growing season. The cumulative evapotranspiration (ET) was 1148.4 mm, and 888.7 mm of this amount (accounting for 77.4%) was concentrated in the growing season.The main influencing factors of ET are T_s and vapor pressure deficit (V_PD) with a significant positive correlation (P < 0.01), and it was supplemented by P_r in the growing season. 3) The WUE during the entire period was 2.70 g CO₂/mm H₂O. It showed large monthly fluctuations ranging from 1.97 to 3.22 g CO₂/mm H₂O but small seasonal differences. The main reason is the interaction between the impacts of short-term climate fluctuations and long-term seasonal change trends on GPP and ET. In addition, it is slightly inhibited by R_t and soil moisture in the growing season.

Conclusions Our study clarifies the key environmental drivers of carbon-water coupling and WUE variability in Southwest Zhejiang’s mid-altitude coniferous-broadleaf mixed forests, resolving the unclear issue of how the ecosystem’s carbon-water processes respond to climate factors. It confirms that GPP and ET are core drivers of WUE, with their regulation by T_a, R_t, T_s and P_r varying seasonally. These findings support optimizing local forest management (e.g., adjusting stand structure) to enhance soil-water conservation and carbon sink functions.