Historically, how much cropland was on the Qinghai–Tibetan Plateau? Case study of the Tubo Kingdom and the Yuan Dynasty

The Qinghai–Tibetan Plateau is one of the least hospitable areas for agriculture. Nevertheless, humans have long conducted agricultural activities on the Qinghai–Tibetan Plateau, although the amount and extent of historical cropland remains unclear. We focused on the Qinghai–Tibetan Plateau and selected the Tubo Kingdom (AD 633–842) and the Yuan Dynasty (AD 1271–1368) as study periods to explore the human–Earth relationship in high–cold plateau areas. First, we estimated the population size of the Tubo Kingdom and extracted the household data of the Yuan Dynasty from historical literature. Then, to estimate the total cropland area of the Qinghai–Tibetan Plateau, we proposed a comprehensive method for cropland estimation due to the specificity of historical literature. Finally, we obtained the Qinghai–Tibetan Plateau cropland cover by allocating the cropland area into 5'×5' grids. Finally, the main findings were as follows. (1) Total cropland area was 3.87 × 103 and 1.34 × 103 km2 during the Tubo Kingdom and Yuan Dynasty, respectively. (2) In the Tubo Kingdom, the fraction of cropland was highest in the Three-River Region (Yarlung Zangbo River, Lhasa River, and Nianchu River), with a gradual decline in the northeastward direction. During the Yuan Dynasty, cropland was concentrated in the Three-River Region and scattered in the valleys of the Nujiang and Lancang rivers. (3) Our reconstructions were highly consistent with regional development history, and addressed the gap in current global land cover datasets regarding the Qinghai–Tibetan Plateau. Thus, this study is vital to support prediction and simulation of regional and global environmental change.