Abstract: Aims Examining how water use efficiency (WUE) in plantations responds to environmental factors is crucial for understanding vegetation's water strategies and assessing ecosystems' carbon and water recycling capacity. Methods Utilizing eddy covariance flux data alongside concurrent meteorological data from the National Posi-tioning Observation and Research Station of Eucalypt Plantation Ecosystem, spanning the years 2018 to 2022, this study conducted an analysis of the temporal variations and seasonal differences in WUE within the Eucalyptus plantation ecosystem. The analytical methods employed included stepwise regression analysis, path analysis, and redundancy analysis. An investigation was conducted to examine the relationship between WUE and various envi-ronmental factors, with a particular focus on the distinctions between drought and rainy seasons, as well as the primary factors influencing these variations. Important findings (1) Throughout the observation period, the mean WUE of the Eucalyptus plantation ecosystem was recorded at 3.85 g C·kg–1 H2O during the dry season (November to April) and 2.61 g C·kg–1 H2O during the rainy season (May to October). The annual mean WUE was determined to be 3.22 g C·kg–1 H2O. (2) WUE exhibited distinct diurnal variation patterns, with similar trends observed during both the dry and rainy seasons. A significant negative correlation was identified between diurnal WUE and variables such as vapor pressure deficit (VPD), air temperature (Tair), soil temperature (Tsoil), photosynthetically active radiation (PAR), and soil moisture (SM). Furthermore, stepwise regression analysis indicated that VPD serves as the primary environmental factor influencing the diurnal variation of WUE. (3) The temporal variation of daily scale WUE exhibited an inverse relationship with gross primary productivity (GPP) and evapotranspiration (ET). Path analysis indicated that WUE is predominantly influenced by direct negative impacts from VPD and Tsoil, in addition to indirect negative influences from PAR. The responses of WUE to VPD and Tsoil demonstrated significant discrepancies between dry and rainy seasons, while the seasonal responses to PAR were found to be consistent. (4) On a monthly basis, WUE was predominantly influenced by temperature and VPD during the dry season, while PAR played a significant role in the rainy season. The vari-ations in WUE across different seasons were primarily influenced by alterations in ET, which were governed by temperature, VPD, and PAR. The WUE of the Eucalyptus plantation ecosystem exhibited marked variations between the drought and rainy seasons, with the primary controlling factors differing according to the temporal scale.

Key words: Eucalyptus plantation, eddy covariance, water use efficiency, environmental factors