Background Terracing is an important measure for soil and water conservation, in the southern region of China, the ancient terraces have become a model for the harmonious development of human activities and the natural landscape structure of the mountains during the development of farming culture for hundreds and thousands of years. And the water and soil conservation role of terraces and the sustainable development of ecosystems are of great significance to maintain the sustainable development of mountain agriculture in China.

Methods Taking 12 typical ancient terraces in southern China as the research objects, we combined Landsat 8 OLI image data and DEM data, visually interpreted and extracted the landscape elevation data of each ancient terrace and the slope and slope direction data of the terrace subsystem using ArcGIS technology, and calculated the landscape pattern index of each ancient terrace using the principle of landscape ecology and Fragstats software to analyze its distribution characteristics and landscape features.

Results 1) The ancient terraces in southern China are mainly located in Yunnan province, Sichuan province, Guizhou province, Guangxi Zhuang autonomous region, Hunan province, Jiangxi province, Fujian province and Zhejiang province, with a total area of over 2 600 km².Representative famous terraces include: Hani Terraces, Longji Terraces, Ziquejie Terraces, Shangbao Terraces, Yunhe Terraces, etc. 2) The ancient terraces in southern China are located in the subtropical mountainous and hilly areas between E 101°48'-119°29' and N 22°33'-29°46'. The altitude range is about 150-3 000 m.Moreover, they are most distributed on the sunny slope, second on the semi-sunny slope and semi-shady slope, and least on the shady slopein the range of 6°-25°. 3) The ancient terraced ecosystems in southern China mainly consist of forest land and cultivated land (terraces), with the percentage of area occupied by forest land from 53.77% to 86.36% and the percentage of area occupied by cultivated land (terraces) from 6.59% to 35.66, and the ratio of forest to field area is 1.62-13.42. Forests provide a constant source of water for the terraces. 4) The vertical landscape structure of ancient terraces in southern China is dominated by "four-elements isomorphism" and "multi-elements isomorphism" from the mountain top to the river valley, in which the landscape structure of "four-elements isomorphism" is characterized as follows: Forest-village-terraces-river. The landscape structure of "multi-elements isomorphism" is characterized as: Forest-village-terraces-town-river. The unique landscape structure has formed the self-flow irrigation system from the forest at the top of the mountain to the village on the mountainside and the terraces to the river valley and the nutrient input system of the terraces with the help of the irrigation canal system, which guarantees the longevity of the terraces.

Conclusions The study of ancient terraces in southern China clarifies their distribution, landscape structure and common features of their formation.It provides a basis for improving the soil and water conservation function of terraces, promoting the sustainable development of mountain agriculture and the construction of modern terraces.