Effects of precipitation, soil water content and soil crust on artificial Haloxylon ammodendron forest
降水、土壤水分和结皮对人工梭梭(Haloxylon ammodendron)林的影响

Based on the large-scale survey and positioning observation of Haloxylon ammodendron forest with various ages growing under different site conditions in Shajingzi region at the downstream of Shiyang River, this paper studied the effects of ecological factors as precipitation, soil water content and soil crust on the artificial H. ammodendron population and communities. Result shows that H. ammodendron is not restricted by the minimum threshold of precipitation, while restricted by the maximum threshold of precipitation, 180mm. At present, artificial H. ammodendron in the downstream of Shiyang River is basicly relying on precipitation. However, the annual discontinuous rainfall with similar intensity will not apparently affect the growth of artificial H. ammodendron population, whereas will significantly affect species composition, quantity of plant population and biomass of herbs in H. ammodendron forest. Soil water content is the essential ecological factor affecting artificial H. ammodendron forest. H. ammodendron will die when water content in aeolian sandy soil is less than 0.824%; and degenerate when water content is between 0.824% to 1.30%; and grow normally when water content is above 1.30%. The growth of H. ammodendron, restricted by soil water content, will aggravate the loss of soil water. Soil water content reduced to 1.3% after 8 years of afforestation of H. ammodendron; reduced to less than the wilting coeffcient 14 years later; but increased to 1.30% 30 years later, when the remaining H. ammodendron population can grow normally. Soil crust, especially physical crust, played an important role on fixing sand and protecting H.ammodendron from erosion, but simultaneously, it restricts infiltration, which will result in soil drought, and accelerate degeneration of artificial H. ammodendron. With undermined soil crust, soil water content will increase gradually. Comparing with bare mobile sand dunes, soil crust has significantly changed the composition and the quantity of species in H. ammodendron forest.