Spatio-temporal patterns of NEE based on upscaling eddy covariance measurements in the alpine grassland of the Tibetan Plateau

By integrating data from 25 flux observation sites in the alpine grasslands of the Tibetan Plateau with corresponding remote sensing and reanalysis data, a data-driven Extremely Randomized Trees regression (ETR) was used to estimate the NEE of alpine grasslands on the Tibetan Plateau from 1982-2018. The spatial and temporal variation patterns of the NEE were also analyzed. The results show that the annual mean NEE of alpine meadows on the Tibetan Plateau from 1982 to 2018 was -35.59 g C m^-2 yr^-1, and showed a significant decreasing trend at -0.78 g C m^-2 yr^-1; on the spatial scale, the alpine meadows in the relatively wet eastern and northeastern parts of the Tibetan Plateau were strong carbon sinks with the intensity around -150 ~ -100 g C m^-2 yr^-1. From the alpine meadows in the east to the semi-arid or arid alpine grasslands in the west and north, the carbon sink intensity gradually decreased along the longitudinal gradient and becomes a weak carbon sink or a weak carbon source (0 ~ ± 20 g C m^-2 yr^-1). The sensitivity analysis showed that precipitation and mean temperature contributed significantly to the interannual trend variation of NEE in grasslands on the Tibetan Plateau during the 1982-2018. The contribution of precipitation was large in the alpine steppe region in the western and northwestern part of the plateau, while the contribution of mean temperature was highest in the alpine meadow region in the east and south, where precipitation dominated 84% of the interannual NEE variation of the entire alpine steppe region, while mean temperature accounts for 55% of that of the alpine meadow region. In general, the interannual variability of NEE in the alpine steppe region tended to be dominated by precipitation, while the alpine meadow region tended to be regulated by temperature. In addition, the NEE of alpine meadow region showed a significant decreasing trend with -0.91 and -0.67 g C m^-2 yr^-1 during 1982-1999 and 2000-2018, respectively, while the alpine steppe region showed a non-significant decreasing trend change with -0.37 and -0.19 g C m^-2 yr^-1, respectively. The different changes of NEE in different vegetation type regions at different time periods were mainly caused by the changes of temperature and precipitation.