Drought-driven declines in water-use efficiency reshape carbon dynamics of a subtropical forest
Brian Njoroge Mwangi, Wyckliffe Ayoma, Yuelin Li, L. A. Bruijnzeel, Juxiu Liu, Jun Zhang, Dennis Otieno, Lindsay Sikuku, Mengmeng Yang, Vincent Suba, Oluoch Emily Achieng, Iheme Okechukwu, Ayesha Akter, Sohel Rana, Guowei Chu, Qianmei Zhang, Xuli Tang, Ze Meng, Deqiang Zhang
Abstract
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.