关键词: 民勤沙井子, 荒漠植被, 变迁

Abstract: Shajingzi in Minqin of Gansu Province is a place where the Tengger Desert and Badain Jaran Desert meet, and locates on the alluvial plain of Da Xihe river and Xiao Xihe river which are two branches of Shi Yang river. The average precipitation is 113.2 mm, average evaporation is 2 604.3 mm. The supply water source comes from Shi Yang river besides precipitation. Before 1950s, there were season lakes. Then the water from upper reaches was annually becoming less and less influenced by natural and artificial factors. The level of groundwater fell to less than 13 m of present from 1~3 m of 1950s, a decending rate of 20~50 m/a. So the species and vegetation types of eremophytes have transformed. The Populus euphratica trees at beach of desert zone extinct in nature except artificial forestry. The desert meadow plants die out; some annual and perennial herbage are becoming less and less. The artificial vegetation by air planting and man planting declined along with groundwater level lowering; the flat of forestry of Elaeagnus angustifolia, Haloxylon ammodendron and Hedysarum scoparium die. The shrub vegetation has gradually transformed from saline vegetation to xerophytic vegetation. Nitraria tangutorum became the main edificator plant. It likes to be covered by sand, can sprout many adventitious roots and absorb water in sand. When environment became more arid, the ability of Nitraria tangutorum to adapt to arid environment enhanced by itself strongly. The Nitraria tangutorum has great vitality and plays a big role in fixing moving sand. With plant species reducing, relevant changes of vegetation types have also happened at Shajingzi zone. In desert vegetation of shrub and undershrub, the Nitraria tangutorum formation exist strongly stable, Achnatherum splendens, Kalidium foliatum and Karelinia caspia of meadow and saline plants elapse, and association of Tamarix ramosissima and relevant plants can not exist, so the formation of Tamarix ramosissima have experienced a big change. With groundwater level lowering and soil saline degree reducing, saline shrub formation of Reamuria soongorica, Kalidium foliatum left small pieces. In this vegetation type, artificial vegetation accounts for fixed ratio; many associations are formed in their formation. Specially, the association of Haloxylon ammodendron have been declined, but they have fixed great pieces of moving sands and improved ecological environment of the zone. In undershrub and hemifrutex vegetation has been strongly declining, now there are only small pieces of Ephedra przewalskil, Calligonum mongolicum and so on. In desert meadow vegetation type the most meadow plants have seceded with the environment change. There is only Phragmites communis, but it grows bad. Under the succession of xerophytic series or hygrophytic series at Shajingzi of Minqin zone, the sand mounds of Nitraria tangutorum and Tamarix ramosissima are the last landscape. The Nitraria tangutorum will develop into the prosperity period under trend of succession, and it will grow well and exist in a long time without great affect from action of people. The natural sand mound of Nitraria tangutorum must be protected for persisting a long time action to fix sands.

Key words: Shajingzi area in Minqin, desert vegetation, change