Mapping China’s planted forests using high resolution imagery and massive amounts of crowdsourced samples

Abstract

Tree planting has been suggested as a potentially effective solution for mitigating climate change. China has implemented the world’s largest afforestation and reforestation project since the 1970s, but high-resolution maps of China’s planted forests remain unavailable. In this study, we explored the use of multi-source remote sensing images and crowdsourced samples to produce the first high-resolution (30-m) map of China’s planted forests. We constructed a Google Earth Engine (GEE)-based mapping framework using spectral, temporal, structural, textural and topographic features derived from Landsat and Sentinel-1 time series imagery, Digital Elevation Model (DEM) and Chinese Forest Canopy Height (CFCH) data. Over 300,000 high-quality crowdsourced samples were collected for training the mapping pipeline. Validation against independent field samples indicated an accuracy of 84.93 % and an F1 score of 0.85. The uncertainty map of each pixel was also constructed and showed that the areas of low and medium uncertainties accounted for 38.27 % and 50.98 % of the total area, respectively, indicating the high estimation reliabilities of the planted forest map. We show that China’s planted forests in the year of 2020 had a total area of 769853.01 km², accounting for 31.30 % of the world’s total planted forests. The majority (77.45 %) of China’s planted forests were located in the Eastern, Center-South, and Southwestern regions. By further assessing the performance of the image features used to map the planted forests, we found that temporal features are key to identifying the planted forests in East and Center-South of China, where they are mainly timber plantations. However, structural and textural features were more useful for locating the planted forests in North and Northeast of China, where are dominated by planted shelterbelts. Our study demonstrated that combining crowdsourced samples with high-resolution satellite images allows mapping planted forests with unprecedented resolution (30-m) across large areas. Our map could contribute to the sustainable management of China’s forests and a more accurate quantification of the carbon balance of China’s natural ecosystems.