Estimating Aboveground Biomass of Alpine Grassland During the Wilting Period Using In Situ Hyperspectral, Sentinel-2, and Sentinel-1 Data

Accurately estimating of grassland above-ground biomass (AGB) during the wilting period is vital in the dynamic monitoring of vegetation survey, carbon storage research, and grazing livestock supplementation. However, previous studies on grassland AGB during the wilting period have rarely involved the integration of ground-based in situ hyperspectral data and satellite images. In this study, we proposed a multisource remote sensing monitoring approach for grassland AGB based on the differential fusion of satellite–ground spectral data from 139 sample sites collected during the grassland’s wilting period (September–November) on the northeastern Tibetan Plateau. First, the in situ hyperspectral data and Sentinel-2 images were differentiated fusion by using the nonnegative matrix factorization (NMF) method. Then, the Sentinel-1 synthetic aperture radar (SAR) images were further integrated to develop the random forest (RF) model for estimating AGB in the grassland’s wilting period. The results showed that: 1) the NMF-based differentiated fusion model ( $R^{2}$ = 0.60 and root-mean-square error (RMSE) = 586.56 kg/ha) effectively improved the estimation accuracy of AGB for the grassland wilting period compared with the Sentinel-2 satellite model ( $R^{2}$ = 0.54 and RMSE = 627.53 kg/ha); 2) the vegetation indices (VIs) derived from short-wave infrared (SWIR) bands are sensitive to variations of grassland AGB during wilting, which have great potential in the estimation of grassland AGB; and 3) the grassland AGB model’s performance is only slightly improved by adding Sentinel-1 SAR data and no more significantly positive synergistic effect on the model performance was observed. Overall, this study’s proposed satellite–ground collaborative monitoring method integrates the advantages of multisource remote sensing data and is expected to further improve the large-scale and high-accuracy monitoring capability for alpine grassland AGB during the wilting period.