Drought-driven declines in water-use efficiency reshape carbon dynamics of a subtropical forest

Abstract Background Subtropical forests play a critical role in global carbon cycling but are highly sensitive to climate-driven precipitation and temperature variability. At China's Dinghushan Biosphere Reserve, observed warming (+ 1.5 °C since 2002) and precipitation declines (8.2 ± 1.3 mm/yr) exceed global subtropical averages, yet their combined effects on water-use efficiency (WUE) and carbon exchange remain poorly quantified. Methods Using 21 years of eddy covariance and meteorological data (2002–2022), we analyzed WUE (defined as NEE/ET; g C kg −1 H 2 O) responses to drought events classified via the standardized precipitation index (SPI). Results The reserve experienced increasing drought frequency, with 7 extreme droughts (SPI ≤ – 2.0) post-2010 versus 2 pre-2010. Soil moisture extremes (< 0.15 m 3 m −3 ) reduced WUE by 2.35 g C kg –1 H 2 O. Carbon fluxes showed differential sensitivity: ecosystem respiration (RE) declined most sharply (– 71.9 × 10 –5 g C m −2 s –1 ) during extreme droughts, while gross primary productivity (GPP) exhibited non-linear reductions below a 1,200 mm yr –1 precipitation threshold. Late-successional forests maintained 0.94 g C kg –1 H 2 O higher WUE than mid-successional stands during droughts, linked to deeper rooting systems (120 ± 15 cm). The identified 3-year recovery lag suggests targeted reforestation during wet intervals (SPI ≥ 1.5) could enhance resilience. Conclusion By establishing quantitative drought thresholds and revealing hydraulic versus non-hydraulic limitations, our findings provide actionable strategies for subtropical forest conservation under climate change.