Divergent photosynthesis and leaf respiration responses of four common subtropical tree species to modest warming

Subtropical forests are vital to global carbon pools, and their responses to increasing warming may significantly influence carbon sequestration. However, how subtropical tree species adjust the photosynthesis and respiration process in response to climatic warming through phenotypic plasticity is still unclear which is critical for predicting future forest carbon dynamics. A two-year warming experiment was conducted using open-top chambers (OTCs) in a subtropical forest in South China, increasing ambient temperature by approximately 1.5 ± 0.5 °C. Measurements included photosynthetic rate, stomatal conductance, chlorophyll fluorescence (Fv/Fm), night respiration, predawn and midday water potential, and key leaf structural traits across ten individuals of Schima superba, Ormosia pinnata, Pinus massoniana, and Castanopsis hystrix. Schima superba, Ormosia pinnata, and Pinus massoniana exhibited increased photosynthesis, stomatal conductance, leaf area, and biomass under warming, indicating strong physiological plasticity. In contrast, Castanopsis hystrix showed reduced gas exchange, growth, and stomatal traits because it may lack adaptive traits critical for warming resilience, indicating a divergent response to the same environmental condition. All species exhibited reduced PSII efficiency (Fv/Fm) and more negative water potentials under warming. Several structural traits, including stomatal density and specific leaf area, were positively correlated with improved physiological performance. Subtropical tree species exhibit distinct thermal response strategies, with some benefiting from modest warming and others displaying signs of stress. Species with high physiological plasticity may better maintain function and productivity under warming. These findings highlight the importance of accounting for species-specific traits in predicting forest responses to climate change and inform forest management under future warming scenarios.