Seasonal droughts induced by climate change pose a significant threat to the normal growth patterns of forests in the subtropical regions of southern China. Therefore, it is crucial to explore the response of tree water use patterns to seasonal drought to maintain tree physiological activities. However, it remains unknown whether changes in dry and wet seasons have an impact on the water use patterns of trees of different ages. In this study, a two-year experiment was conducted in Eucalyptus urophylla × Eucalyptus grandis (hereinafter referred to as Eucalyptus) plantations at three ages (4, 7, and 17 years). Specifically, the water use patterns of Eucalyptus in dry and wet seasons were calculated using hydrogen stable isotopes (including the isotopes in xylem water and 0–150 cm soil layers) coupled with MixSIAR. The results showed that there were notable variations in the proportions of water absorption from different soil layers by Eucalyptus during dry and wet seasons. During the dry season (April 2024), 4-year-old and 7-year-old Eucalyptus primarily utilized water from the 40–90 cm soil layer, while 17-year-old Eucalyptus mainly relied on deep soil water at depths of 60–150 cm, with a utilization ratio of 50.9%. During the wet season (August 2023), the depth of water uptake by Eucalyptus of different ages significantly shifted towards shallow layers, and the trees primarily utilized surface soil water from the 0–60 cm layer, with utilization ratios of 59.9%, 64.8%, and 61.6% for 4-year-old, 7-year-old, and 17-year-old Eucalyptus, respectively. The water sources of Eucalyptus during dry and wet seasons were variable, which allowed Eucalyptus to cope with seasonal drought stress. The differences in the water uptake strategies of Eucalyptus between dry and wet seasons can be attributed to their long-term adaptation to the environment. Our research revealed the differences in the water utilization of Eucalyptus with various ages between dry and wet seasons in subtropical China, providing new insights for a better understanding of the adaptive mechanisms of subtropical forests in response to alterations in water conditions caused by climate change.

The water cycle in forest ecosystems is a core component of ecosystem services, and it is crucial for revealing terrestrial hydrological processes and regulation mechanisms. Investigating water migration across interfaces of the soil–plant–atmosphere continuum (SPAC) has long been a hotspot in ecohydrological research. Eucalyptus stands are among the world’s major commercial timber species, and their hydrological effects have attracted growing public attention. Naturally occurring hydrogen and oxygen stable isotopes can effectively differentiate water sources, serving as powerful tracers widely applied in hydrological research. Nevertheless, comprehensive systematic reviews on SPAC water dynamics of Eucalyptus plantations using stable isotope quantification are still lacking. This study systematically reviews the application of hydrogen and oxygen stable isotopes in analyzing water transport and transformation processes at multiple interfaces of Eucalyptus plantation SPAC, covering atmospheric precipitation sources, soil and plant water migration, plant transpiration and groundwater exchange. Future research should strengthen in-situ continuous monitoring of isotopic signatures in diverse water bodies, facilitate the popularization of isotope-based models, and advance quantitative partitioning of evapotranspiration in Eucalyptus ecosystems. This review provides theoretical and technical support for exploring hydrological processes and mechanisms in forest ecosystems, and offers scientific references for improving forest water use efficiency and optimizing regional water resource allocation.