Reasons for the degeneration Of Haloxylon ammodendron population in the western part of Gurbantonggut Desert in mid-asia

Haloxylon ammodendron is constructive species, which had degeneration in large areas in the western part of Gurbantonggut Desert, the second largest desert of China. Therefore, we investigated the spatial distribution of dead plants, the age structure of both live and dead plants in the population, the depth of groundwater and soil quality. The results showed that: (1) the dead plants of H. ammodendron showed very significantly cluster distribution in 15 plots among all observation 19 plots, which indicated that it was not self-thinning within species. (2) The degeneration population could hardly renew. The relation is significantly negative (R² = 0.374, P <;; 0.01) between electrical conductivity of soil (0~30cm) and the numbers of H. ammodendron seedling (height <;; 40cm). There were dead plants in all ages of H. ammodendron population, which indicated that there were no relationship between plants age and degeneration. (3) The relationship between survival index of H. ammodendron population and groundwater level were significantly negative (R² = 0.364;P <;; 0.01). (4) Compared soil quality, the electrical conductivity of plain and the small dune area where H. ammodendron severely degenerated was significantly larger than the big dune where those grew normally. The infiltration of soil water was difficult and the numbers of lateral roots in the depth of 0~90cm in the degeneration area is fewer than that of big dunes. In sum, slow infiltration of soil water and excessive soil salinity, little distribution of lateral roots and deep groundwater, restricted utilizing water of H. ammodendron in the degeneration area, a great number of H. ammodendron plants perish due to water shortage. The renewal of population was restricted by excessive soil salinity, so there were few seedlings of H. ammodendron to supply population.