The Qinling () or Qin Mountains, formerly known as the Nanshan ("Southern Mountains"), are a major east–west mountain range in southern Shaanxi Province, China. The mountains mark the divide between the drainage basins of the Yangtze and Yellow River systems, providing a natural boundary between North and South China and support a huge variety of plant and wildlife, some of which is found nowhere else on earth. To the north is the densely populated Wei River valley, an ancient center of Chinese civilization. To the south is the Han River valley. To the west is the line of mountains along the northern edge of the Tibetan Plateau. To the east are the lower Funiu and Dabie Mountains, which rise out of the coastal plain. The northern side of the range is prone to hot weather, however the physical barrier of the mountains mean that the land to the North has a semi-arid climate, with the lack of rich, fertile landscape that can not support a wealth of wildlife. The mountains also acted as a natural defense against nomadic invasions from the North, as only four passes cross the mountains. In the late 1990s a railway tunnel and a spiral was completed, thereby easing travel across the range. The highest mountain in the range is Mount Taibai at , which is about west of the ancient Chinese capital of Xi'an. Three culturally significant peaks in the range are Mount Hua (), Mount Li (), and Mount Maiji (). Environment, flora and fauna The environment of the Qin Mountains is a deciduous forest ecoregion. The Qin Mountains form the watershed between the Yellow River basin, which was historically home to deciduous broadleaf forests, and the Yangtze River basin, which has milder winters and more rainfall, and was historically home to warm temperate evergreen broadleaf forests. Therefore the Qin Mountains is commonly used as the line separating northern and southern China. The low-elevation forests of the foothills are dominated by temperate deciduous trees like oaks (Quercus acutissima, Q. variabilis), elm (Ulmus spp.), common walnut (Juglans regia), maple (Acer spp.), ash (Fraxinus spp.) and Celtis spp. Evergreen species of these low-elevation forests include broadleaf chinquapins (Castanopsis sclerophylla), ring-cupped oaks (Quercus glauca) and conifers like Pinus massoniana. At the middle elevations, conifers like Pinus armandii are mixed with broadleaf birch (Betula spp.) oak (Quercus spp.) and hornbeam (Carpinus spp.). From elevation, these mid-elevation forests give