Abstract: Though coverage has been regarded as the key indicator of ecological functions and status of desert vegetation for a long time,it cannot reflect the real situation of plant cover surface because of its very irregular branches;besides,generally there is no corresponding relationship between cover and the projector cover,then the vegetation cover cannot characterize the ecological functions and status of the plant,and it also often resulted in overlook of the ecological function of plant height. As an example,the ecological functions of 5 kinds of main plant communities were analyzed by using the layered projection cover degrees with locating plots observation data from 2002 to 2013 years in Minqin desert area,Gansu Province,China. The results show that cover and projection cover of desert vegetation was quite different,e.g. the average projected coverage of Nitraria tangutorum communities was only 28.6% of crown cover. From large to small,the vegetation cover of the 5 kinds of plant communities in 2002-2013 were ranked as:Nitraria tangutoru,Haloxylon ammodendron+Nitraria tangutorum, Artemisia arenaria,Ephedra przewalskii and Haloxylon ammodendron;for projection cover,they were ranked as:Haloxylon ammodendron+Nitraria tangutorum,Nitraria tangutorum,Ephedra przewalskii,Artemisia arenaria and Haloxylon ammodendron;and for ecological functions,they were ranked as:Nitraria tangutorum,Ephedra przewalskii,Haloxylon ammodendron+Nitraria tangutorum,Haloxylon ammodendron and Artemisia arenaria,indicating that the vegetation cover,projector cover and ecological functions did not correspond with each other. When taking the standard deviation of interannual variation of vegetation cover and projected cover as the representatives for the community stability,the communities with lower coverage showed higher stability,while those with higher coverage showed relatively lower stability,e.g. vegetation cover of Ephedra przewalskii community was relatively low,but the community was most stable;vegetation cover of Haloxylon ammodendron+Nitraria tangutorum was relatively high,but stability of community was the worst. Accumulating sand to dune is an important manifestation of plant functions for sand prevention and sand fixation,and results show that the closer leaves were to the ground,the more conducive the plants accumulate sand to mound,e.g. the Nitraria tangutorum nebkha was conducive to accumulation sand to dune,but the Haloxylon ammodendron was not because its branches mainly distributed in the range of 60-210 cm height,and there was a taproot above the ground diameter. The ecological functions of the height for desert plants mainly reflected in:first,the height range to prevent wind and fix sand is larger;second,the sparse plant density,sparse plant shoots and the relatively high latitude make every layer of the desert plants can conduct photosynthesis,absorb and consume solar energy,and further regulate temperature. Innovation points of the article are:calculating the ecological functions with hierarchical cover of vegetation communities,considering the ecological functions of plant height,and finally proposing the projection cover of vegetation. It is recommended that the layered cover must be calculated dominantly with the projected cover in future function analysis of desert plants.

Key words: plant communities, layered projection cover, determination, ecological functions, Minqin desert area