Estimating Aboveground Biomass for Sparse Tree-shrub Mixed Forest Using Muti-scale Optical Remote Sensing data in the Dryland Ecosystem

Sparse mixed forest with trees, shrubs, and green herbaceous vegetation is a typical landscape in the afforestation areas in northwestern China. It is a great challenge to accurately estimate the woody aboveground biomass (AGB) of a sparse mixed forest with heterogeneous woody vegetation types and background types. In this study, a novel woody AGB estimation methodology (VI-AGB model stratified based on herbaceous vegetation coverage) using a combination of Landsat-8, GaoFen-2, and unmanned aerial vehicle (UAV) images was developed. The results show the following: 1) The woody and herbaceous canopy can be accurately identified using the object-based support vector machine (SVM) classification method based on UAV red-green-blue (RGB) images, with an average overall accuracy and kappa coefficient of 93.44% and 0.91, respectively. 2) Compared with the estimation uncertainties of the woody coverage-AGB models without considering the woody vegetation types (RMSE=14.98 t∙ha^-1 and rRMSE=96.31%), the woody coverage-AGB models stratified based on five woody species (RMSE=5.82 t∙ha^-1 and rRMSE=37.46%) were 61.1% lower. 3) Of the six VIs used in this study, the near-infrared reflectance of pure vegetation (NIRv)-AGB model performed best (RMSE=7.91 t∙ha^-1 and rRMSE=50.89%), but its performance was still seriously affected by the heterogeneity of the green herbaceous coverage. The normalized difference moisture index (NDMI)-AGB model was the least sensitive to the background. The stratification-based VI-AGB models considering the herbaceous vegetation coverage derived from GaoFen-2 and UAV images can significantly improve the accuracy of the woody AGB estimated using only Landsat VIs, with the RMSE and rRMSE of 6.6 t∙ha^-1 and 42.43% for the stratification-based NIRv-AGB models. High spatial–resolution information derived from UAV and satellite images has a great potential for improving the woody AGB estimated using only Landsat images in sparsely vegetated areas. This study presents a practical method of estimating woody AGB in sparse mixed forest in dryland areas.