Reasons For The Degeneration Of Haloxylon Ammodendron Populations In The Western Part Of Gurbantunggut Desert

Haloxylon ammodendron is constructive species of Gurbantonggut Desert, the second largest desert of China, In the sand-fixing, soil and maintenance of the oasis-desert ecological balance has an irreplaceable role. But the survey found it has degenerated in large areas of plains and small dunes (height<5m). Meanwhile in the large dunes (height>10m) nearby, the plants grow normally. There is a large number of dead plants, limiting the renewal of these populations and the degenerated area of about 40 million hm2. The reason of degradation urgently requires analysis.In this paper, H. ammodendron forest of a serious degradation in the plains and small dunes habitats for the study, the grow well in the big sand dune habitat for the comparison. From the characteristics of H.ammodendron population, climatic factors, the depth of groundwater and water quality, climatic factors, ground water level and water quality, contrast seed germination and seedling survival of H. ammodendron and the pH值ysical and chemical soil properties in 3 habitats of the plain, small and large sand dune and other aspects of comprehensive analysis the causes of H. ammodendron degradation of Gurbantunggut Desert. 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 inner species. There were dead plants in all ages of H.ammodendron population, which indicated that there were no relationship between plants age and degeneration.2) Annual precipitation in the study area did not change significantly the past 20, while the average annual potential evapotranspiration moderate increasing trend; indicates that precipitation is not a major factor in H. ammodendron degradation.3) The relationship between survival index of H. ammodendron population and groundwater level were significantly negative(R2=0.364; P<0.01), groundwater depth and groundwater high concentrations of salt combined effect groundwater use of H.ammodendron.4) The 100 seed weight with winged perianth, winged perianth diameter and 100 seed weight with winged perianth are significant difference in three habitats. H.ammodendron seed germination rate was not significant at the non-salt threaten condition, even all decreases with the salt concentration increasing, but the decrease of seed germination rate is more significant in normal area, indicating that characteristics of H.ammodendron seed is not the main factor affecting regeneration ability of H.ammodendron population. 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), the high salinity severely of degenerated area limit the survival rate of H.ammodendron seedling.5) 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. Analysis showed that the high electrical conductivity of soil in degradation areas severely affected survival of H.ammodendron plants, slow infiltration of soil water and excessive soil salinity restricted the distribution of lateral root of H.ammodendron.6) In sum, groundwater level and water quality restricted utilizing groundwater of H. ammodendron, excessive soil salinity and slow infiltration of soil water, little distribution of lateral roots restricted utilizing precipitation of H. ammodendron, the renewal of population was restricted by excessive soil salinity, the end result is a large area of H.ammodendron forest degraded.