2.1. Study area
Miyun County is located in the Northeast of Beijing, with geographical coordinates of 116°39′–117°30′ E and 40°13′–40°47′ N. It is flanked by the Pinggu, Shunyi, and Huairou Counties of Beijing in the Southeast to Northwest regions and Hebei Province in the North and East regions and has a total area of approximately 2229.45 km2, making it the largest district in Beijing.
Miyun belongs to the Yanshan Mountains and North China Plain junction, and mountains surround the terrain of the entire area on three sides (East, North, and West). The Miyun area has a warm temperate monsoon continental semi-humid and semi-arid climate with four seasons and apparent changes in dry, wet, cold, and warm conditions. The annual average temperature is 6–19 °C; the frost-free period is about 150 d; sunshine is sufficient; and precipitation is mainly concentrated from June to August. The precipitation distribution generally decreases from Southeast to Northwest, giving an average annual of 300–700 mm.
In 2020, the main ecosystem types in Miyun County were forests (41.6%) and shrubs (22.9%), followed by farmlands (21.9%) and wetlands (6.6%). Natural ecosystems (forests, shrubs, grasslands, and wetlands) account for 72.4% of the total area of Miyun County. The quality of the ecosystem in Miyun County is good, and natural resources are abundant.