2.1 Study area
The sandy land in Horqin is part of north-eastern China’s Inner Mongolia desert zone, from the North-East China Plain to the transition zone of the Inner Mongolia plateau (42°41′–45°15′ N,118°35′–123°30′ E, 178.5–631.9 m above the sea level), and is spread over approximately 51.75 km². The climate ranges from temperate semi-arid to semi-humid continental monsoon. The study site was at the Daqinggou Ecological Station, Institute of Applied Ecology, Chinese Academy of Science. Average annual temperature in the area is 3–7 °C (ranging from −12.6 °C to −16.2 °C, in January, to 20.3–23.9 °C, in July), with 140–160 frost-free days a year. Average annual precipitation is 343–500 mm, about 70% of which is received between June and August. Spring is fairly dry with hardly any rainfall in most places. Average annual evaporation is 1500–2500 mm; average annual wettability is 0.3–0.5; the drying coefficient ranges from 1 to 1.8; cumulative active temperature (≥10 °C) is 3032–3168 °C; and annual daily mean sunshine hours are 2837–2892, with 120 kcal/cm² of global radiation. Annual average wind speed is 3.4–4.4 m/s (4.2–5.9 m/s in spring); for 210–310 days a year, and even up to 330 d in some years, the wind is laden with sand and its speed is more than 5 m/s. Each year also sees 25–40 gale days (wind speeds exceeding 17.2 m/s), including 10–15 days, mainly in the spring, of sand storms. The annual sunshine hours are 2900–3100, maximum in May and minimum in December. The soils are mainly aeolian sandy soils and meadow soils. Substratum mobility varies from fixed sand dunes, semi-fixed sand dunes, and mobile dunes. The zonal soils in the Horqin sandy lands are dark brown, chestnut, and loess soils whereas the non-zonal soils are mainly sandy, meadow, and saline. Due to desertification, the zonal soils are gradually turning into sandy soils. Sandy plains are widely distributed, consisting mainly of sandy soils, and, following the standard soil classification, can be divided into raw grass sandy soils, flowing sand soils, and aeolian sandy chestnut soils. The area is a typical agro-pastoral transition zone in northern China and comprises many different ecosystems including natural deciduous broad-leaved forests, forest plantations, grasslands, wetlands, and farmlands. The vegetation consists of mainly psammophytes of the Mongolian and Huabei floras.