Abstract: Warm temperate forests have the large potential to sequester atmospheric carbon dioxide (CO₂), while the interannual variability (IAV) of net forest ecosystem carbon exchange (NEE) in the global carbon cycle is still not fully understood. In this study, we conducted eddy-covariance measurement to investigate the IAV of carbon fluxes and concurrent influencing factors in a warm temperate natural oak forest from 2017 to 2022. Our results showed the natural oak forest was a strong CO₂ sink with an increase of 27.79 g C m⁻² a⁻¹ in annual carbon sequestration, resulting from a larger increase in annual gross primary production (GPP) than that of annual ecosystem respiration (Re). Precipitation in spring (PPT_spring) negatively influenced annual GPP, soil water content in spring (SWC_spring) negatively influenced annual Re, while the water conditions had lesser effect on annual NEE attributing to the synchronous changes of annual GPP and annual Re. Increase of temperature in autumn (Ta_autumn) delayed the end date of the growing season, leading to the increase in annual carbon sequestration. In addition, carbon fluxes did not significantly decrease under dramatic reduction of summer precipitation, indicating that warm temperate natural oak forest had a high resistance to seasonal drought. Our study helped us to better understand the mechanisms underlying forest carbon fluxes in response to drought in the context of future climate change.

Key words: carbon fluxes, eddy covariance, environment factors, phenology, interannual variability