Abstract: 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 (TMP_opt), 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 TMP_opt variability, with TMP_opt 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.

Key words: sub-frigid zone, forest ecosystems, carbon use efficiency, global climate warming