Effects of precipitation, soil water content and soil crust on artificial Haloxylon ammodendron forest

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 such as precipitation, soil water content and soil crust on the artificial H. ammodendron population and communities. Results show that H. ammodendron is restricted by the maximum threshold of precipitation, 180 mm, rather than by the minimum threshold of precipitation. Soil water content is the essential ecological factor affecting artificial H. ammodendron forest. H. ammodendron dies when water content in aeolian sandy soil is less than 0.824%, degenerates when water content is between 0.824% and 1.30%, and grows normally when water content is above 1.30%. The growth of H. ammodendron, restricted by soil water content, aggravates the loss of soil water. Soil water content reduced to 1.3% 8 years after afforestation of H. ammodendron, reduced to less than the wilting coefficient 14 years later, but increased to 1.30% 30 years later when the remaining H. ammodendron population grew normally. Soil crust restricts rainwater infiltration, which will result in soil drought and accelerate degeneration of artificial H. ammodendron forest. Soil water content will increase gradually with undermined soil crust. Compared with bare mobile sand dunes, soil crust has significantly changed the composition and quantity of species in H. ammodendron forest.