We choose the arid land of northwestern China as our study area because of its extensive area, specific geographical location, dry climate, varied topography, vulnerable ecosystems and long-term human disturbance, as well as the existence of current conflicts between economic development and environmental protection. Based on the literature, we choose ten desert species with clear and relatively accurate geographical distribution ranges to model their distributions:Anabasis brevifolia, Atraphaxis frutescens, Calligonum mon-golicum, Ephedra przewalskii, Gymnocarpos przewalskii, Haloxylon ammodendron, Haloxylon persicum, Ka-lidium cuspidatum, Salsola laricifolia, and Sympegma regelii.

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Basesd on the precipitation difference in Tsaidam Basin of Qinghai Province, three sampling areas, including Geremu, Delinghan and Doulan, were selected, and then three sapmling plots in each sampling area were selected to study the characteristics of plant community which dominated by Ceratoides lateens, Nitraria tangutorum, Tamarix chinensis, Kalidium gracile, Haloxylon ammodendron and Salsola arbuscula respectively. At the same time, the morphological and physiological features of above-mentioned 6 species were measured for probing their drought tolerance. By using the subordinative function method, grey correlative degree analyse and no-weight index comprehensive evaluation method, their drought tolerance were compared as well. The results were showed as follows: The main xeric vegetation types in the desert steppe of Tsaidam Basin were Nitraria tangutorum community, Tamarix chinensis community, Haloxylon ammodendron community, Kalidium gracile+Salsola arbuscula community, Ceratoides lateens+ Haloxylon ammodendron+Kalidium gracile community.

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Preference Salsola and mugwort sort of grass.

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And the leaf succulence of Salsola implicata and Suaeda linifolia changes drastically, strong drought resistance is indicated, but that of Halocnemum strobilaceum and Kalidium foliatum is not

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The relationships between root length of Salsola implica, Halocnemum strobilaceum and Kalidium foliatum seedlings and soil salt concentration are significant, but the relationship between the number of lateral roots of Suaeda linifolia seedlings is significantly negative, the tall, fresh weigh and the number of leaves of Suaeda linifolia and Salsola implicata are almost normal distribution in the range of soil salt concentration, the optimum salt concentration is 0.27-0.89% and 0.16-0.36% respectively; The growth of Halocnemum strobilaceum and Kalidium foliatum seedlings are poor in high salt soil.

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In Salsola passerina -Reaumuria soongorica community, the first ordination axes explains the salinization gradient, along the order of Caragana tibetica community,Salsola passerina-Oxytropis aciphylla community,and Reaumuria soongorica-Salsola passerine community,soil alkalization increases. The second ordination axis explains soil structure gradient, along the order of Caragana tibetica community, Reaumuria soongorica-Salsola passerina community and Salsola passerina community, soil texture becomes coarser. In the Stipa breviflora-Stipa grandis community, the first ordination axis indicates the soil water gradient, and the second ordination axes explains hydrothermal coupling gradient. In the Prunus mongolica-Ulmus glaucescens community, the first ordination axis explains the soil pH gradient, along the order of Ulmus glaucescens-Prunus mongolica community, Prunus mongolica-Potentilla fruticosa community, and Potentilla fruticosa-Prunus mongolica community, soil pH value reduces. The second ordination axis explains soil structure gradient, along the order Prunus mongolica-Ulmus glaucescens community, and Prunus mongolica-Potentilla fruticosa community, the contents of silt and clay increase, and soil texture suggests a fine trend.

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In Salsola passerina-Reaumuria soongoriea community, the first ordination axes explains the salinization gradient. along the order of Caragana tibetica community, Salsola passerina-Oxytropis aciphylla community, and Reaumuria soongorica-Salsola passerine community, soil alkalization increases. The second ordination axis explains soil structure gradient, along the order of Caragana tibetica community, Reaumuria soongorica-Salsola passerina community and Salsola passerina community, soil texture becomes coarser. In the Stipa breviflora-Stipa grandis community, the first ordination axis indicates the soil water gradient, and the second ordination axes explains hydrothermal coupling gradient. In the Prunus mongolica-Ulmus glaucescens community, the first ordination axis explains the soil pH gradient, along the order of Ulmus glaucescens-Pnuius mongolica community, Prunus mongolica-Potentilla fruticosa community, and Potentilla fnuicosa-Prunus mongolica community, soil pH value reduces. The second ordination axis explains soil structure gradient, along the order Prunus mongolica-Ulmus glaucescens community, and Prunus mongolica-Potentilla fruticosa community, the contents of silt and clay increase, and soil texture suggests a fine trend.

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Geographical Distribution of Salsola L.

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Grandis had the higher caloric values that were 19.03 kJ/g and 19.28 kJ/g respectively in two communities. The species with the low value were Axyris amaranthoides and Chenopodium glaucum in L. chinensis community (15.04 kJ/g), and Salsola collina in S. grandis community (12.75 kJ/g).

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Based on these, this study was to determine the relationship between standing vegetation and soil seed bank. The main results are as follows: 1 Effects of grazing on plant communities The grazing successions were Stipa klemenzii → Salsola collina + Stipa klemenzii → Salsola collina community in the desert steppe; Stipa grandis or Leymus chinensis → Cleistogenes squarrosa + Stipa krylovii or Stipa grandis → Potentilla acaulis + Cleistogenes squarrosa community in the typical steppes and Leymus chinensis + forbs →Leymus chinensis + Stipa baicalensis community in the meadow steppe.

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