Divergent Responses of Photosynthesis and Respiration Underlie the Nonlinear Response of Carbon Use Efficiency to Temperature in Sub-Frigid Forest in China

Forest ecosystems in the sub-frigid zone play a crucial role in global carbon cycle and exhibit distinct responses to climate change. The carbon use efficiency (CUE) of vegetation serves as a critical indicator for assessing the carbon sequestration potential of an ecosystem. Previous studies have predominantly examined long-term trends in CUE under climate change; therefore the mechanisms governing responses to short-term climate variability remain poorly understood. In this study, we investigated monthly spatiotemporal variations in vegetation CUE in sub-frigid zone ecosystems and elucidated its mechanistic responses to climate warming. The results demonstrated a regional mean CUE of 0.80 (± 0.04), which was significantly higher than values reported for other ecosystem types. CUE exhibited a bimodal temporal distribution, with peaks occurring in April and September, coupled with a distinct west-to-east spatial gradient characterized by relatively high values in the western regions. The CUE–temperature relationship conformed to a quadratic function, where CUE initially increased with increasing temperature, attained an optimum temperature threshold (TMPopt), and subsequently decreased. This pattern arose from the different temperature response rates between plant respiration and gross primary production. Precipitation emerged as the dominant controller of TMPopt variability, with TMPopt demonstrating significant quadratic dependence on precipitation. This study advances our understanding of the impacts of climate change on carbon cycling of the sub-frigid zone, providing a mechanistic framework for refining global carbon cycle models and adopting strategies for mitigating climate change.