Carbon flux dynamics and carbon use efficiency and their key environmental drivers in a climate-sensitive Picea crassifolia forest.

Qinghai spruce (Picea crassifolia), a dominant forest ecosystem on the Tibetan Plateau, plays a critical role in regulating regional and global carbon cycles. However, the variations in carbon fluxes-particularly carbon use efficiency (CUE) and their responses to key environmental drivers across multiple time scales remain poorly understood for the Qinghai spruce forests of the Qilian Mountains, on the northeastern margin of the Plateau. Using five years (2020-2024) of eddy covariance (EC) data from a central Qilian Mountains spruce forest, this study examines the dynamics of carbon fluxes and CUE across multiple temporal scales. We employed Pearson correlation, regression analysis, random forest (RF), and structural equation modeling (SEM) to identify key environmental drivers and underlying causal pathways. Results demonstrated that the Qinghai spruce acted as a strong and stable carbon sink, with mean annual net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), and gross primary productivity (GPP) of -457.00 ± 3.35, 258.76 ± 20.06, and 715.76 ± 21.80 g C m-2 year-1, respectively. The mean annual CUE was 0.64 ± 0.02. SEM revealed that air temperature (Ta), and precipitation (P) directly regulated interannual variations in NEE and GPP, while Reco was primarily influenced by photosynthetically active radiation (PAR), soil temperature (Ts), and P. CUE is primarily regulated by the interaction between P and Ts. The differential responses of Reco and GPP to temperature and moisture govern forest carbon sequestration capacity. Our findings emphasize the key role of temperature and precipitation in shaping carbon flux patterns and enhance the understanding of alpine forest ecosystem responses to climate change, offering insights for climate-adaptive forest management.