LAUSR

Abstract Alpine grassland ecosystem on the Tibetan Plateau (TP) is highly sensitive to climate change and plays a critical role in terrestrial carbon cycle. However, the long-term responses of alpine grassland ecosystem carbon fluxes to climate change at different growth stages and their controlling mechanisms have not been well understood. In this study, we quantified the spatial and temporal variations of gross primary productivity (GPP) and net ecosystem productivity (NEP) of alpine grassland ecosystems from 1982 to 2015 based on an improved Biome-BGC model, and then we examined how different climate factors (i.e., soil moisture, temperature, vapor pressure deficit (VPD), and solar shortwave radiation) affected GPP and NEP dynamics at different growth stages (i.e., the early rapid growth, physiological maturity, and senescence stages) using partial correlation analysis. Furthermore, the relative contributions of different climate factors on GPP and NEP dynamics were also analyzed using path analysis model. The results showed that both GPP and NEP of alpine grassland ecosystems increased significantly during the past 34 years, and the responses of GPP and NEP to climate change and their dominant climate factors significantly varied at different growth stages. Specifically, temperature severely limited the productivity of most alpine grasslands at all growth stages, and soil moisture played an increasingly important role in vegetation carbon fixation before senescence stage, while the effects of VPD and solar shortwave radiation on vegetation carbon fixation were relatively weak. Our study suggested that a finer temporal scale (e.g., the growth stage basis) should be considered to reveal the complex relationships between climate change and the variations of ecosystem carbon fluxes, which could provide a new sight to understand the mechanisms of terrestrial ecosystem's responses to global climate change.