Water-use strategies of Chinese fir to simulated precipitation change in a subtropical region

Altered precipitation regimes due to climate change influence plant–water interactions through shifts in soil moisture dynamics, highlighting the need for a mechanistic understanding of diverse water-use strategies and plant adaptations. In this study, we adopted an integrated approach combining measurements of stable hydrogen and oxygen isotopes in soil, groundwater, and xylem water, alongside sap flow and tree growth using dendrometers, to investigate the water-use strategies of Chinese fir (Cunninghamia lanceolata) under varying drought intensities. The experimental design included a control (C) and three precipitation reduction treatments (−30%, −50%, and −80%). This study analyzed data collected from both the wet and dry seasons of 2022, with precipitation exclusion devices installed and functioning since October 2021. The results indicated that during the wet season, Chinese fir primarily used shallow soil water (0–20 cm), with uptake proportions of 54.30%, 87.90%, 86.00%, and 63.70% under the C, −30%, −50%, and −80% treatments, respectively. In the dry season, as shallow soil water became increasingly scarce, water uptake gradually shifted toward deeper soil layers (40–60 cm), accounting for 49.30%, 79.10%, 68.50%, and 32.40%, respectively, and to groundwater sources, with 37.60%, 6.90%, 21.30%, and 61.50%, respectively. As expected, all precipitation reduction treatments reduced growth and water consumption (transpiration) compared with the C group. Notably, Chinese fir under the extreme drought treatment maintained adequate transpiration by relying heavily on groundwater throughout both seasons. This enabled increased growth during the wet season, though it also induced early growth cessation during the dry season. These findings suggest that Chinese fir exhibits substantial plasticity in its water acquisition strategies, allowing dynamic adjustment of water uptake between soil layers and groundwater sources depending on moisture availability. Our 1-year study demonstrates that Chinese fir can regulate water use and maintain radial growth under varying precipitation reduction treatments and seasonal conditions. Continuous long-term monitoring is essential to assess the sustained effects of drought on these ecohydrological processes.