Vapor pressure deficit shapes the distributions of carbon use efficiency across Siberia

As a key indicator of terrestrial carbon cycle, carbon use efficiency (CUE) represents the efficiency of carbon fixation and carbon allocation strategy, shaping ecosystem function and services. Siberia, as a major carbon sink, is experiencing rapid and dramatic climate change, which triggers complex ecological responses and leads to uncertainties in its carbon balance evaluations. However, the spatiotemporal variations of CUE and the underlying mechanisms remain rarely studied, limiting our understanding of Siberian carbon cycle under climate change. This study investigates the spatial variations of CUE across Siberian ecosystem and employs random forest and SHapley Additive exPlanation (SHAP) to analyze the intrinsic driving mechanisms from 2001 to 2020. The results indicate that the annual mean CUE over Siberia from 2001 to 2020 is 0.60 ± 0.07, with notable ecosystem-specific variations ranging from grasslands (GRA) (0.64 ± 0.08) to closed shrublands (CSH) (0.52 ± 0.06). Vapor pressure deficit (VPD) primarily shapes the CUE spatial variations, and its interactions with mean annual temperature (MAT) and shortwave radiation (SW) synergistically distribute CUE over Siberia. Rather than non-significant trends from 2001 to 2020, CUE exhibits significant decreasing trends over Siberian regions under both SSP1-2.6 and SSP5-8.5 from 2021 to 2100, with substantial fluctuations within this period. Moreover, under SSP5-8.5, CUE experiences twice the decreasing rate of that from SSP1-2.6, indicating more vulnerable responses of Siberian ecosystems to climate change under a higher warming projection. Our results provide valuable insights into the dynamics of CUE in Siberia and offer scientific guidance for climate adaptation strategies in this region.