2.1. Study area
The LP covers 62.4×104 km2 (34–45°5’N, 101–114°33’N) and is located in the upper and middle reaches of the Yellow River in the central-northern part of China. The LP is surrounded by complicated mountain conditions, with the Yin Mountain in the north, the Qinling Mountain in the south, the Riyue Mountain in the west and the Taihang Mountain in the east. The terrain over the LP gradually declines from northwest to southeast, with a unique topography composed of abysses, ridges, gullies and hills. The LP lies in the continental monsoon climate zone, spanning 750 km from north to south and 1000 km from east to west, so the LP exhibits various climate characteristics (Zhang, Wu et al. 2014, Liu, Wang et al. 2016, Zhao, Zhai et al. 2017). The LP covers a wide temperature gradient decreasing from south to north and a large precipitation gradient decreasing from southeast to northwest.
2.2. Data